ARAP-RCS-MV-LANTAKA.pdf

# Principles of mechanical ventilation Mechanical ventilation is a life-support system used when a patient cannot ventilate or oxygenate adequately on their own. It aims to improve gas exchange, relieve respiratory distress, improve pulmonary mechanics, permit lung healing, and avoid complications [4](#page=4). ## 1. Pathophysiological factors affecting ventilation and oxygenation Several pathophysiological factors can lead to the need for mechanical ventilation: ### 1.1 Increased airway resistance Airway resistance is defined as airflow obstruction within the airways. It is influenced by the airway's length, size, and patency, as well as the endotracheal tube and ventilator circuit. The unit of measurement is cm H2O/L/sec. Factors affecting airway resistance include [5](#page=5): * Changes inside the airway (e.g., secretions, bronchospasm) [6](#page=6). * Changes in the wall of the airway (e.g., inflammation, edema) [6](#page=6). * Changes outside the airway (e.g., external compression) [6](#page=6). Common causes of increased airway resistance include: * Obstructive lung diseases: Chronic Bronchitis, Emphysema, Bronchiectasis, Chronic Asthma, Cystic Fibrosis [7](#page=7). * Mechanical conditions: Post-intubation obstruction, Foreign body aspiration [7](#page=7). * Infectious processes: Laryngotracheobronchitis (croup), Epiglottitis, Bronchiolitis [7](#page=7). According to Poiseuille’s Law, a reduction in airway radius significantly increases the driving pressure required for airflow. In healthy adults, normal airway resistance is between 0.5 and 2.5 cm H2O/L/sec but it is higher in intubated patients due to the smaller diameter of the endotracheal tube. High airway resistance increases the work of breathing (WOB) which can lead to respiratory muscle fatigue, ventilatory failure, and subsequently oxygenation failure if not addressed. Airflow resistance can be monitored using a Pressure-Volume (P-V) loop; increased bowing suggests elevated resistance [10](#page=10) [11](#page=11) [8](#page=8) [9](#page=9). ### 1.2 Changes in compliance Lung compliance is defined as the volume change (lung expansion) per unit pressure change. It reflects the lungs' and chest wall's elastic recoil and is measured in mL/cmH2O [12](#page=12) [14](#page=14). * **Low Compliance (High Elastance):** Means a small volume change occurs with a given pressure change. The lungs are stiff and noncompliant. This increases the work of breathing and is often associated with conditions like Acute Respiratory Distress Syndrome (ARDS) [13](#page=13). * **High Compliance:** Means a large volume change occurs with a given pressure change. This can lead to incomplete exhalation due to reduced elastic recoil, as seen in emphysema [14](#page=14). #### 1.2.1 Static vs. Dynamic Compliance * **Static Compliance (Cstat):** Calculated by dividing the volume by the plateau pressure (measured when airflow is momentarily stopped). It reflects the elastic resistance of the lung and chest wall. Normal values are 40-60 mL/cmH2O [15](#page=15). * **Dynamic Compliance (Cdyn):** Calculated by dividing the volume by the peak inspiratory pressure (measured when airflow is present). It reflects airway resistance and the elastic resistance of the lung and chest wall. Normal values are 30-40 mL/cmH2O [16](#page=16) [21](#page=21). An increase in both Plateau and Peak Inspiratory Pressure suggests a decrease in both static and dynamic lung compliance, indicative of conditions like atelectasis. An increase in Peak Inspiratory Pressure with unchanged Plateau Pressure suggests a decrease in dynamic compliance only, indicating increased airway resistance [19](#page=19) [20](#page=20). Compliance is inversely related to the work of breathing; decreased compliance increases WOB, potentially leading to hypoventilation and ventilatory failure [25](#page=25). ### 1.3 Hypoventilation Hypoventilation is characterized by a reduction in alveolar ventilation (VA) and an increase in arterial carbon dioxide tension (PaCO2). It leads to ventilatory failure, defined as the inability of the pulmonary system to maintain proper removal of carbon dioxide, with hypercapnia being the key feature [32](#page=32) [33](#page=33). * **Minute Alveolar Volume = (VA x F)** [33](#page=33). * Mechanical ventilation adjustments for hypoventilation include increasing tidal volume (VT) or respiratory rate (RR), and removing dead space in Volume Controlled Ventilation. In Pressure Controlled Ventilation, increasing peak inspiratory pressure and respiratory rate are options [34](#page=34). ### 1.4 V/Q mismatch The ventilation/perfusion (V/Q) ratio represents ventilation relative to pulmonary blood flow. A mismatch occurs when ventilation and perfusion are not adequately matched [35](#page=35). * Causes of decreased ventilation include airway obstruction [35](#page=35). * Causes of decreased pulmonary perfusion include pulmonary embolism and blood loss [35](#page=35). * V/Q mismatch is a primary cause of hypoxemia. Mechanical ventilation adjustments include increasing RR, VT, and FiO2 [35](#page=35). ### 1.5 Intrapulmonary shunting Intrapulmonary shunting refers to perfusion in excess of ventilation ("wasted" perfusion). It causes refractory hypoxemia. Mechanical ventilation adjustment for shunting primarily involves increasing Positive End-Expiratory Pressure (PEEP) [36](#page=36) [38](#page=38). ### 1.6 Diffusion defects Diffusion defects involve impaired gas exchange across the alveolar-capillary membrane, primarily dependent on gas pressure gradients [39](#page=39). ## 2. Classification of mechanical ventilation Mechanical ventilators can be classified based on control and phase variables. ### 2.1 Control variables These are the primary variables a mechanical ventilator controls during inspiration: pressure, volume, flow, and time [52](#page=52). * **Pressure Controller:** The ventilator controls the transrespiratory system pressure. Positive pressure ventilators apply pressure inside the chest, while negative pressure ventilators apply subatmospheric pressure outside the chest [52](#page=52). * **Volume Controller:** The ventilator ensures a constant volume delivery, allowing pressure to vary with changes in resistance and compliance. The formula for volume is: Volume (L) = Flow (L/sec) $\times$ Inspiratory Time (sec) [54](#page=54). * **Flow Controller:** The ventilator directly measures and controls flow, allowing pressure to vary with changes in patient compliance and resistance [54](#page=54). * **Time Controller:** The ventilator measures and controls inspiratory and expiratory time, allowing pressure and volume to vary [54](#page=54). ### 2.2 Phase variables A ventilator breath consists of four phases: expiration to inspiration, inspiration, inspiration to expiration, and expiration. The phase variables are [56](#page=56): * **Trigger Variable:** Initiates the breath. Can be time-triggered (ventilator-initiated after a set time interval) pressure-triggered (patient effort creates a pressure gradient) or flow-triggered [57](#page=57) [58](#page=58). * **Limit Variable:** The maximum value a variable (pressure, flow, or volume) can reach during inspiration before inspiration is limited [59](#page=59). * **Cycle Variable:** Ends inspiration. Can be pressure-cycled, volume-cycled, flow-cycled, or time-cycled [60](#page=60). * **Baseline Variable:** Controlled during the expiratory phase, such as Positive End-Expiratory Pressure (PEEP) or Continuous Positive Airway Pressure (CPAP) [61](#page=61). ## 3. Operating modes of mechanical ventilation Mechanical ventilation modes are categorized by the type of support provided. ### 3.1 Negative pressure ventilation This type creates a transairway pressure gradient by decreasing alveolar pressure below airway opening pressure. Examples include the Iron Lung and Chest Cuirass [70](#page=70). ### 3.2 Positive pressure ventilation This is achieved by applying positive pressure at the airway opening [72](#page=72). ### 3.3 Types of mechanical ventilation based on control * **Spontaneous:** Patient controls timing and tidal volume; patient-triggered, patient-cycled [74](#page=74). * **Mandatory:** Ventilator controls timing, tidal volume, or inspiratory pressure; machine-triggered, machine-cycled [74](#page=74). * **Assisted:** Patient-triggered, machine-cycled [74](#page=74). ### 3.4 Specific ventilation modes #### 3.4.1 Continuous Mandatory Ventilation (CMV) / Control Mode Also known as Continuous Mandatory Ventilation or Control Mode. The ventilator delivers a preset tidal volume at a time-triggered frequency, controlling tidal volume and respiratory rate. The patient cannot change the ventilator frequency or breathe spontaneously. Indications include patients who are heavily sedated or paralyzed, fighting the ventilator, or in conditions requiring complete rest for the respiratory system. A complication is diaphragm atrophy due to complete dependence on the ventilator [93](#page=93) [95](#page=95) [97](#page=97). #### 3.4.2 Assist Control (AC) Mode The patient can increase ventilator frequency (assist) in addition to the preset mechanical frequency (control). Each breath, whether control or assist, delivers a preset tidal volume. This mode provides full ventilatory support and is used for patients with a stable respiratory drive who can trigger the ventilator. The work of breathing is minimal in this mode. A complication is alveolar hyperventilation (respiratory alkalosis) if the patient has an inappropriately high respiratory drive [100](#page=100) [98](#page=98). #### 3.4.3 Intermittent Mandatory Ventilation (IMV) The ventilator delivers control (mandatory) breaths, and the patient can breathe spontaneously between these breaths. A primary complication is breath stacking, which occurs when a spontaneous breath and a time-triggered mandatory breath overlap . #### 3.4.4 Synchronized Intermittent Mandatory Ventilation (SIMV) Mandatory breaths are synchronized with the patient's spontaneous breathing efforts to avoid breath stacking. Spontaneous breaths in SIMV are truly spontaneous. The primary indication is partial ventilatory support, allowing the patient to contribute to minute ventilation. Advantages include maintaining respiratory muscle strength, reducing V/Q mismatch, and facilitating weaning. A disadvantage is weaning failure if the patient is weaned too rapidly, leading to increased work of breathing and muscle fatigue . #### 3.4.5 Pressure Support Ventilation (PSV) This mode augments a patient's spontaneous effort with positive pressure to lower the work of breathing and increase spontaneous tidal volume. It is often used with SIMV to facilitate weaning. PSV helps overcome the increased airway resistance and work of breathing imposed by the endotracheal tube. Benefits include increased spontaneous tidal volume, decreased spontaneous frequency, and decreased work of breathing . #### 3.4.6 Adaptive Support Ventilation (ASV) ASV is a dual control mode that provides a mandatory minute ventilation by adjusting mandatory breaths and pressure support based on the patient's breathing pattern. The ventilator uses dynamic compliance and expiratory time constants to achieve a target minute ventilation . #### 3.4.7 Airway Pressure Release Ventilation (APRV) APRV uses two CPAP or pressure levels: a high pressure (Phigh) and a low pressure (Plow or PEEP). Patients can breathe spontaneously without restriction. Release from Phigh to Plow stimulates mechanical exhalation. It provides partial ventilatory support with lower peak airway pressures than PSV and SIMV, and is an alternative to conventional volume-controlled ventilation, aiming to avoid barotrauma . #### 3.4.8 Inverse Ratio Ventilation (IRV) IRV uses an inspiratory to expiratory (I:E) ratio between 2:1 and 4:1, meaning inspiratory time is longer than expiratory time. It is indicated to improve oxygenation in patients with ARDS, particularly those with refractory hypoxemia. IRV improves oxygenation by reducing intrapulmonary shunting, improving V/Q mismatch, and decreasing dead space ventilation. It leads to increased mean airway pressure (mPaw), which helps reduce shunting, and causes Auto-PEEP. An adverse effect is an increased incidence of barotrauma due to increased mean alveolar pressure . #### 3.4.9 High-Frequency Oscillatory Ventilation (HFOV) HFOV delivers extremely small volumes at very high frequencies (1 Hz = 60 cycles/min). Its primary goal is to minimize lung injury while providing ventilation. Primary settings include Mean Airway Pressure, Frequency, Percent inspiration, and Inspiratory bias flow. It is often used in neonatal populations and for patients failing conventional ventilation. Ventilation can be increased by decreasing frequency or increasing amplitude/inspiratory flow/bias flow. Oxygenation can be increased by increasing mean airway pressure and FiO2 . ### 3.5 Positive End-Expiratory Pressure (PEEP) PEEP increases the end-expiratory airway pressure above atmospheric pressure. It is used to improve oxygenation, especially in hypoxemia refractory to high FiO2. Indications include intrapulmonary shunting, refractory hypoxemia, decreased Functional Residual Capacity (FRC), and low lung compliance. Complications include decreased venous return, barotrauma, and increased intracranial pressure [75](#page=75) [77](#page=77). * **Extrinsic PEEP:** Applied by the ventilator [77](#page=77). * **Intrinsic PEEP (Auto-PEEP):** Occurs when exhalation is incomplete, common in obstructive lung disorders [77](#page=77) [78](#page=78). * **Total PEEP:** Extrinsic PEEP + Intrinsic PEEP [77](#page=77). * **Optimal PEEP:** The level that improves lung compliance without compromising cardiac function, often assessed by changes in PvO2 . ### 3.6 Continuous Positive Airway Pressure (CPAP) CPAP is PEEP applied to a spontaneously breathing patient. Indications are similar to PEEP, with the requirement of adequate lung function for eucapnic ventilation. Patient interfaces include face masks, nasal masks, or endotracheal tubes. It is used to treat oxygenation issues and refractory hypoxemia [79](#page=79). ### 3.7 Bilevel Positive Airway Pressure (BiPAP) BiPAP allows independent positive airway pressures for inspiration (IPAP) and expiration (EPAP). IPAP controls ventilation, while EPAP controls oxygenation. Pressure Support is the difference between IPAP and EPAP. Indications include preventing intubation in end-stage COPD, chronic ventilatory failure, restrictive chest wall disease, neuromuscular disease, and nocturnal hypoventilation. Adjustments involve increasing IPAP to improve CO2 washout and increasing EPAP (while maintaining pressure support) to improve oxygenation [80](#page=80) [81](#page=81) [82](#page=82). ## 4. Initiation of mechanical ventilation Mechanical ventilation is initiated when there is a failure or impending failure of the respiratory system. ### 4.1 Goals of mechanical ventilation * Improve gas exchange . * Relieve respiratory distress . * Improve pulmonary mechanics . * Permit lung and airway healing . * Avoid complications . ### 4.2 Indications for mechanical ventilation * **Acute Ventilatory Failure:** Sudden increase in PaCO2 > 50 mmHg with respiratory acidosis (pH < 7.30). This includes Type I (Hypoxemic Respiratory Failure) and Type II (Acute Hypercapneic Respiratory Failure) . * **Impending Ventilatory Failure:** The patient can maintain marginally normal blood gases only at the expense of significantly increased work of breathing . * **Severe Hypoxemia:** Hypoxemia not responding to supplemental oxygen, often assessed by PaO2 or the alveolar-arterial oxygen pressure gradient (P(A-a)O2). The P/F ratio is also used to assess hypoxemia severity . * Alveolar oxygen pressure (PAO2) calculation: $PAO_2 = (P_B - P_{H_2O}) \times F_{IO_2} - (PaCO_2 / R)$ . * $P_B$: Barometric pressure (typically 760 mmHg) . * $P_{H_2O}$: Water vapor pressure (47 mmHg) . * $F_{IO_2}$: Fraction of inspired oxygen (as a decimal) . * $R$: Respiratory quotient (typically 0.8) . * Alveolar-arterial oxygen pressure gradient: $P(A-a)O_2 = PAO_2 - PaO_2$ . * **Prophylactic Ventilatory Support:** Provided when the risk of pulmonary complications, ventilatory failure, or oxygenation failure is high, aiming to minimize hypoxia and reduce cardiopulmonary stress . * **Hyperventilation Therapy:** Short-term therapy used to control PaCO2, especially in acute head injury to reduce intracranial pressure . ### 4.3 Contraindications for mechanical ventilation * Untreated pneumothorax (> 20%) . * Patient's informed request . * Medical futility . * Reduction or termination of patient pain and suffering . ### 4.4 Initial ventilator settings Key initial settings include: * **Mode:** Full or partial ventilatory support (e.g., CMV, AC, SIMV, BiPAP, PSV) . * **Frequency (f):** Typically set between 10-12 breaths/min to achieve eucapnic ventilation; higher rates (>20/min) can cause Auto-PEEP. Adjusting rate affects expiratory time, I:E ratio, and minute ventilation (VE) . * **Tidal Volume (VT):** Usually set at 6-8 mL/kg of predicted body weight (PBW), or 4-6 mL/kg for ARDS. COPD patients may benefit from reduced VT to allow longer expiratory time . * Ideal body weight formula: Male: $106 + [6 \times (\text{height in inches} - 60)]$. Female: $105 + [5 \times (\text{height in inches} - 60)]$ . * **Peak Inspiratory Pressure (PIP):** Should be set to achieve the target VT, aiming to avoid excessive pressure . * **Minute Ventilation (VE):** Normal range is 5-10 L/min. VE = VT $\times$ RR. ABGs are used to assess effectiveness: increase VE if PaCO2 > 45 mmHg, decrease if PaCO2 < 45 mmHg . * **Inspiratory Flow:** Normal setting is 40-80 L/min, typically set to deliver inspiration in approximately 1 second, yielding an I:E ratio of 1:2 or 1:3 . * **I:E Ratio:** The ratio of inspiratory time to expiratory time, usually 1:2 to 1:4. It can be manipulated by adjusting flow rate, inspiratory time, frequency, or minute volume . * Formulas: * Inspiratory Time ($I$ time) = $\frac{VT (L)}{FLOW RATE (L/sec)}$ . * Total Cycle Time ($TCT$) = $\frac{60}{RR}$ . * Expiratory Time ($E$ time) = $TCT - I \text{ time}$ . * I:E ratio = $\frac{I \text{ time}}{I \text{ time}}: \frac{E \text{ time}}{I \text{ time}}$ . * **Flow Patterns:** Square, Accelerating, Decelerating, Sine wave. Decelerating flow patterns can produce high initial inspiratory pressure . * **Fractional Inspired Oxygen (FiO2):** Initially 100% for severe hypoxemia, then reduced to < 50% to avoid oxygen-induced lung injuries . * **PEEP:** Used to maintain positive pressure after a ventilator breath. Contraindications include hypotension, elevated intracranial pressure, and uncontrolled pneumothorax . * **Sensitivity:** Determines patient effort needed to trigger inspiration, typically -1.0 to -2.0 cm H2O . ## 5. Non-invasive ventilation (NIV) NIV is a strategy that delivers ventilatory support without an endotracheal tube, using devices that connect to the patient's face . ### 5.1 Interfaces These devices connect the ventilator tubing to the patient's face. Common types include : * Nasal Mask: Covers only the nose, less claustrophobic, but can have mouth leaks . * Nasal Pillows: Small cushions that fit under the nose, allowing for speech and eating but can cause air leaks and nasal irritation . * Hybrid Oronasal Mask: Covers the mouth with a small mask and seals the nares . * Full Face Mask: Covers the entire face, providing better control of mouth leaks and suitable for mouth breathers, but can increase aspiration risk and cause claustrophobia . ### 5.2 Clinical indications for NIV * **COPD Exacerbation:** First-line therapy for hypercapneic respiratory failure secondary to COPD exacerbation, especially with severe dyspnea and signs of respiratory muscle fatigue . * **Acute Cardiogenic Pulmonary Edema:** CPAP and BiPAP are equally effective; BiPAP is preferred for hypercapneic respiratory failure. CPAP of 8-12 cm H2O with 100% O2 is a first-line treatment . * **Pickwickian Syndrome (Obesity Hypoventilation Syndrome):** NIV improves daytime hypercapnia and nocturnal hypoventilation; it's a first-line treatment . * **Prevention of Reintubation:** NIV after extubation in high-risk patients, particularly those with COPD and CHF, shows lower reintubation rates . ### 5.3 Contraindications for NIV * Need for emergent intubation . * Hemodynamic instability . * Inability to protect the airway or clear secretions . * Severely impaired consciousness (GCS < 8) . * Facial surgery or trauma . * Prolonged MV anticipated . --- # Ventilatory and oxygenation failure Ventilatory and oxygenation failure occur when the cardiopulmonary system cannot adequately remove carbon dioxide or supply sufficient oxygen for metabolism, respectively [10](#page=10). ### 2.1 Mechanisms of failure Failure of the pulmonary system to maintain adequate gas exchange can stem from several underlying pathophysiologic factors [4](#page=4): * Increased airway resistance [10](#page=10) [4](#page=4). * Changes in compliance [4](#page=4). * Hypoventilation [4](#page=4). * Ventilation/perfusion (V/Q) mismatch [4](#page=4). * Intrapulmonary shunting [4](#page=4). * Diffusion defects [4](#page=4). * Reduction of inspired oxygen tension ($P_I O_2$) [32](#page=32). ### 2.2 Ventilatory failure Ventilatory failure is characterized by the inability of the pulmonary system to effectively remove carbon dioxide from the body, leading to hypercapnia (an increase in arterial carbon dioxide tension, $Pa_{CO_2}$). Five primary mechanisms can lead to ventilatory failure: hypoventilation, persistent V/Q mismatch, persistent intrapulmonary shunting, persistent diffusion defects, and persistent reduction of inspired oxygen tension [32](#page=32). #### 2.2.1 Hypoventilation Hypoventilation is defined as a reduction in alveolar ventilation ($V_A$) which results in an elevated $Pa_{CO_2}$. This can be caused by central nervous system (CNS) depression, neuromuscular disorders, or airway obstruction [33](#page=33). > **Tip:** The formula for alveolar ventilation ($V_A$) is related to minute alveolar volume and frequency (F) [33](#page=33). When hypoventilation is present and mechanical ventilation is initiated, adjustments can be made. In volume-controlled ventilation, this includes increasing tidal volume ($V_T$) or respiratory rate (RR), and removing dead space if present. In pressure-controlled ventilation, adjustments involve increasing peak inspiratory pressure and respiratory rate [34](#page=34). #### 2.2.2 Ventilation/perfusion (V/Q) mismatch The ventilation/perfusion (V/Q) ratio represents the proportion of ventilation relative to pulmonary blood flow (perfusion). A V/Q mismatch occurs when there is an imbalance between these two factors. Decreased ventilation can be caused by airway obstruction, while decreased perfusion can result from conditions like pulmonary embolism or blood loss. V/Q mismatch is a significant contributor to hypoxemia [35](#page=35). Mechanical ventilation adjustments for V/Q mismatch include increasing frequency (RR), tidal volume, and fraction of inspired oxygen ($FiO_2$) [35](#page=35). #### 2.2.3 Intrapulmonary shunting Intrapulmonary shunting describes a situation where pulmonary blood flow (perfusion) exceeds ventilation, leading to "wasted" perfusion. This condition is a primary cause of refractory hypoxemia [36](#page=36). Mechanical ventilation adjustments for intrapulmonary shunting primarily involve increasing positive end-expiratory pressure (PEEP) and the use of continuous positive airway pressure (CPAP) [38](#page=38). #### 2.2.4 Diffusion defects A diffusion defect impairs the movement of gases, such as oxygen and carbon dioxide, across the alveolar-capillary membrane. This process is largely dependent on the pressure gradients of the gases involved [39](#page=39). #### 2.2.5 Reduced inspired oxygen tension ($P_I O_2$) A reduction in the partial pressure of inspired oxygen can contribute to ventilatory failure by not providing sufficient oxygen to the alveoli for diffusion into the blood [32](#page=32). ### 2.3 Oxygenation failure Oxygenation failure is defined by severe hypoxemia that does not improve with moderate to high concentrations of supplemental oxygen. Hypoxemia refers to a reduced level of oxygen in the blood. It is commonly assessed using the partial pressure of arterial oxygen ($Pa_{O_2}$) from arterial blood gases. However, $Pa_{O_2}$ only reflects dissolved oxygen and not the portion carried by hemoglobin. Therefore, total arterial oxygen content ($CaO_2$) is a more comprehensive measure [40](#page=40). The formula for total arterial oxygen content is: $$ CaO_2 = (\text{Hb} \times 1.34 \times Sa_{O_2}) + (Pa_{O_2} \times 0.003) $$ [40](#page=40). The units for $CaO_2$ are typically milliliters per deciliter ($mL/dL$) [40](#page=40). > **Example:** Given Hb = 14 g/dL, $Sa_{O_2}$ = 95%, and $Pa_{O_2}$ = 88 mmHg, the total arterial O2 content would be calculated as: > $CaO_2 = (14 \times 1.34 \times 0.95) + (88 \times 0.003) \approx 17.96 + 0.264 \approx 18.224 \text{ mL/dL}$ [42](#page=42). Hypoxia, a broader term for reduced oxygen in body organs and tissues, can be categorized into several types [44](#page=44): * **Hypoxic hypoxia:** Low inspired oxygen (e.g., high altitude) [44](#page=44). * **Anemic hypoxia:** Reduced or dysfunctional hemoglobin (e.g., anemia, blood loss, carbon monoxide poisoning) [44](#page=44). * **Circulatory/stagnant hypoxia:** Perfusion defects (e.g., decreased cardiac output) [44](#page=44). * **Histotoxic hypoxemia:** Tissue dysfunction (e.g., cyanide poisoning) [44](#page=44). #### 2.3.1 Signs of oxygenation failure and hypoxia Key clinical signs of oxygenation failure and hypoxia include hypoxemia ($Pa_{O_2}$ < 40 mmHg not responding to 50-100% supplemental oxygen), dyspnea, tachypnea, tachycardia, cyanosis, shortness of breath, and disorientation [45](#page=45). #### 2.3.2 Assessment of hypoxemia Severe hypoxemia can be assessed by measuring $Pa_{O_2}$ or the alveolar-arterial oxygen pressure gradient ($P(A-a)O_2$) . The formula for the alveolar-arterial oxygen gradient is: $$ P(A-a)O_2 = PAO_2 - Pa_{O_2} $$ . The alveolar oxygen pressure ($PA_{O_2}$) can be calculated using the alveolar air equation: $$ PA_{O_2} = (P_B - P_{H_2O}) \times FiO_2 - \frac{Pa_{CO_2}}{R} $$ . Where: * $P_B$ is the barometric pressure . * $P_{H_2O}$ is the water vapor pressure, typically 47 mmHg . * $FiO_2$ is the fraction of inspired oxygen, converted to a decimal . * $Pa_{CO_2}$ is the partial pressure of arterial carbon dioxide . * $R$ is the respiratory quotient, typically 0.8 . > **Example:** To calculate $PA_{O_2}$ for a patient breathing room air ($FiO_2$ = 0.21) at sea level ($P_B$ = 760 mmHg) with a $Pa_{CO_2}$ = 40 mmHg and $R$ = 0.8: > $PA_{O_2} = (760 - 47) \times 0.21 - \frac{40}{0.8}$ > $PA_{O_2} = 713 \times 0.21 - 50$ > $PA_{O_2} = 149.73 - 50 = 99.73 \text{ mmHg}$ . The PaO2/FiO2 (P/F) ratio is also used to assess the severity of hypoxemia in critically ill patients . > **Tip:** When interpreting hypoxemia, consider the A-a gradient. A normal gradient is typically 8-12 mmHg. An elevated gradient (>12 mmHg) suggests intrapulmonary issues like V/Q mismatch, shunting, or diffusion defects, while a normal gradient might point to hypoventilation or decreased $P_I O_2$ . ### 2.4 Clinical conditions leading to mechanical ventilation Mechanical ventilation may be indicated in various clinical scenarios, including acute ventilatory failure, impending ventilatory failure, severe hypoxemia, and as prophylactic support . * **Acute ventilatory failure:** This is a primary indication for mechanical ventilation. It is characterized by a sudden increase in $Pa_{CO_2}$ greater than 50 mmHg with accompanying respiratory acidosis (pH < 7.30). It can be classified as Type I (Hypoxemic Respiratory Failure) due to issues like V/Q mismatch, diffusion defects, alveolar hypoventilation, or decreased inspired oxygen, or Type II (Acute Hypercapneic Respiratory Failure) caused by CNS disorders, reduced respiratory drive, depressant drugs, or neuromuscular disorders . * **Impending ventilatory failure:** This occurs when a patient can maintain marginally normal blood gases only by significantly increasing their work of breathing. The $Pa_{CO_2}$ may be normal or low initially. Early mechanical ventilation is crucial to correct acidosis and hypoxemia . * **Severe hypoxemia:** When hypoxemia is severe and does not respond to oxygen therapy, mechanical ventilation may be necessary to support oxygenation deficits. Conditions like Acute Lung Injury (ALI), Acute Respiratory Distress Syndrome (ARDS), pulmonary edema, and carbon monoxide poisoning often require ventilatory support primarily for oxygenation . * **Prophylactic ventilatory support:** This is provided when there is a high risk of pulmonary complications, ventilatory failure, or oxygenation failure. It aims to minimize hypoxia to vital organs like the brain and myocardium, reduce cardiopulmonary stress, and decrease the risk of pulmonary complications in conditions such as prolonged shock, head injury, or smoke inhalation . * **Excessive ventilatory workload:** When the work of breathing exceeds a patient's capacity, ventilatory and oxygenation failure can occur, necessitating mechanical ventilation. This can arise from a failure of the ventilatory pump, which involves the lung parenchyma, respiratory muscles, and thoracic skeletal structures, leading to increased work of breathing [48](#page=48) [49](#page=49). * **Hyperventilation therapy:** Mechanical ventilation can be used for short-term therapy to control and manipulate $Pa_{CO_2}$ to lower than normal levels, particularly in cases of acute head injury with increased intracranial pressure (ICP). Normal ICP is typically 10-15 mmHg, while levels above 20 mmHg may warrant hyperventilation . > **Tip:** The failure of the ventilatory pump can lead to increased work of breathing and eventual ventilatory and oxygenation failure [48](#page=48). --- # Modes and settings of mechanical ventilation This section provides a detailed overview of the fundamental components and various modes of mechanical ventilation, including control and phase variables, specific ventilation modes, initial settings, and troubleshooting strategies [51](#page=51) [52](#page=52). ### 3.1 Control variables Mechanical ventilators can control four primary variables during inspiration: pressure, volume, flow, and time [52](#page=52). #### 3.1.1 Pressure controller A pressure controller classifies a ventilator that manages the transrespiratory system pressure (airway pressure minus body surface pressure). Positive pressure ventilators apply pressure inside the chest to expand it, while negative pressure ventilators use subatmospheric pressure outside the chest to inflate the lungs [52](#page=52). #### 3.1.2 Volume controller A volume controller allows pressure to fluctuate with changes in resistance and compliance, while maintaining a constant delivered volume. The relationship between these variables is expressed as [54](#page=54): $$ \text{Volume (L)} = \text{Flow (L/sec)} \times \text{Inspiratory Time (sec)} $$ #### 3.1.3 Flow controller Flow controllers directly measure and control flow, allowing pressure to vary based on the patient's compliance and resistance [54](#page=54). #### 3.1.4 Time controller Time controllers measure and regulate inspiratory and expiratory times, allowing pressure and volume to vary with changes in pulmonary compliance and resistance [54](#page=54). ### 3.2 Phase variables A ventilator-supported breath can be divided into four phases: expiration to inspiration, inspiration, inspiration to expiration, and expiration. The key phase variables are the trigger, limit, cycle, and baseline variables [56](#page=56). #### 3.2.1 Trigger variable The trigger variable initiates inspiration. It can be: * **Time-triggered:** The ventilator initiates and delivers a breath when a preset time interval has elapsed. The frequency control on the ventilator is an example of time-triggering [57](#page=57). * **Pressure-triggered:** The ventilator initiates a breath when it senses the patient's spontaneous inspiratory effort, which generates a pressure gradient [57](#page=57). * **Flow-triggered:** The breath is initiated based on a detected flow change [58](#page=58). #### 3.2.2 Limit variable The limit variable is a parameter (pressure, flow, or volume) that is not allowed to exceed a preset value during inspiration [59](#page=59). #### 3.2.3 Cycle variable The cycle variable determines when inspiration ends. This can be: * **Pressure-cycled:** Inspiration stops when a set pressure is reached [60](#page=60). * **Volume-cycled:** Inspiration stops when a set volume is delivered [60](#page=60). * **Flow-cycled:** Inspiration stops when inspiratory flow drops to a preset level [60](#page=60). * **Time-cycled:** Inspiration stops after a preset inspiratory time has elapsed [60](#page=60). #### 3.2.4 Baseline variable The baseline variable is controlled during the expiratory phase or expiratory time. Positive End-Expiratory Pressure (PEEP) and Continuous Positive Airway Pressure (CPAP) are examples that increase functional residual capacity (FRC) to improve gas distribution and oxygenation [61](#page=61). ### 3.3 Terminology of ventilation modes Different modes of ventilation offer distinct approaches to supporting a patient's breathing [63](#page=63). #### 3.3.1 Volume-controlled ventilation (VCV) VCV allows clinicians to set the tidal volume delivered with each breath. Pressure will vary depending on the patient's pulmonary compliance and airway resistance [63](#page=63). * **Advantage:** Regulates both Tidal Volume (VT) and Minute Ventilation (VE) [63](#page=63). * **Formula:** $ \text{VE} = \text{VT} \times \text{Frequency (RR)} $ [63](#page=63). #### 3.3.2 Pressure-controlled ventilation (PCV) PCV allows clinicians to set a peak inspiratory pressure for each mechanical breath. Since pressure is constant, volume and minute ventilation vary with changes in the patient's pulmonary compliance or airway resistance [64](#page=64). * **Advantage:** Protects lungs from excessive pressures, preventing Ventilator-Induced Lung Injury (VILI) [64](#page=64). * If compliance decreases ($C_L \downarrow$) or resistance increases ($R_{aw} \uparrow$), VT and VE decrease [64](#page=64). #### 3.3.3 Intermittent Mandatory Ventilation (IMV) IMV allows spontaneous breathing between time-triggered ventilator breaths, which can be volume or pressure-controlled. Spontaneous breaths can be augmented with pressure support to increase tidal volume and reduce the work of breathing associated with endotracheal tube resistance [65](#page=65). #### 3.3.4 Pressure Support Ventilation (PSV) PSV augments a patient's spontaneous effort with positive pressure. The patient must trigger each breath, which is typically pressure or flow-triggered. This mode increases tidal volume by applying adjustable pressure [66](#page=66) [68](#page=68). * **Indications:** Increases spontaneous tidal volume, decreases spontaneous frequency, and decreases the work of breathing. It is often used with SIMV to facilitate weaning in difficult-to-wean patients by overcoming the resistance of the endotracheal tube . ### 3.4 Operating modes of mechanical ventilation Mechanical ventilation can be categorized into negative and positive pressure ventilation [70](#page=70) [72](#page=72). #### 3.4.1 Negative pressure ventilation Negative pressure ventilation creates a transairway pressure gradient by decreasing alveolar pressure below airway opening pressure. Examples include the "Iron lung" and the "Chest cuirass" [70](#page=70). #### 3.4.2 Positive pressure ventilation Positive pressure ventilation is achieved by applying pressure greater than atmospheric pressure at the airway opening [72](#page=72). ### 3.5 Types of mechanical ventilation Ventilation can be spontaneous, mandatory, or assisted [74](#page=74). * **Spontaneous:** Patient controls timing and tidal volume (Patient-triggered, Patient-cycled) [74](#page=74). * **Mandatory:** Ventilator controls timing, tidal volume, or inspiratory pressure (Machine-triggered, Machine-cycled) [74](#page=74). * **Assisted:** Patient-triggered, Machine-cycled [74](#page=74). ### 3.6 Positive End-Expiratory Pressure (PEEP) and Continuous Positive Airway Pressure (CPAP) Positive End-Expiratory Pressure (PEEP) increases the end-expiratory airway pressure above atmospheric pressure [75](#page=75). * **Indications:** Intrapulmonary shunting, refractory hypoxemia, decreased FRC and lung compliance, and auto-PEEP unresponsive to settings [75](#page=75). * **Complications:** Decreased venous return, barotrauma, and increased intracranial pressure [77](#page=77). * **Types:** Extrinsic PEEP, Intrinsic PEEP (AutoPEEP), Total PEEP, and Optimal PEEP [77](#page=77). * **Intrinsic PEEP (AutoPEEP):** Occurs in obstructive lung disorders due to insufficient expiratory time. Troubleshooting involves increasing flow, adding PEEP, decreasing VT, and decreasing RR [78](#page=78). Continuous Positive Airway Pressure (CPAP) is PEEP applied to a spontaneously breathing patient. Indications are similar to PEEP, requiring adequate lung function for eucapnic ventilation. CPAP primarily treats oxygenation and refractory hypoxemia [79](#page=79). ### 3.7 Bilevel Positive Airway Pressure (BiPAP) BiPAP allows independent positive airway pressures for inspiration (IPAP) and expiration (EPAP) [80](#page=80). * **IPAP:** Controls ventilation [80](#page=80). * **EPAP:** Controls oxygenation [80](#page=80). * **Pressure Support:** The difference between IPAP and EPAP ($ \text{IPAP} - \text{EPAP} $) [80](#page=80). * **Indications:** Preventing intubation in end-stage COPD, chronic ventilatory failure, restrictive chest wall disease, neuromuscular disease, and nocturnal hypoventilation [81](#page=81). * **Initial Settings:** IPAP 8 cm H2O, EPAP 4 cm H2O, Frequency (S/T mode) 2-5 breaths below patient's spontaneous frequency [81](#page=81). * **Adjustments:** * To increase delivered volume (CO2 washout), increase IPAP [82](#page=82). * To improve oxygenation, increase EPAP and maintain IPAP to keep pressure support consistent [82](#page=82). * When IPAP = EPAP, it functions as CPAP [82](#page=82). * **ABG Interpretation for Adjustment:** * Interpret ABGs first to determine if the problem is ventilation (PaCO2) or oxygenation (PaO2) related [84](#page=84). * Normal values: pH 7.35-7.45, PaCO2 35-45 mm Hg, HCO3 22-26 meq/L, PaO2 80-100 mm Hg [86](#page=86). * For stable COPD: pH normal but acidic side, PaCO2 >50 mm Hg, PaO2 50-65 mm Hg [86](#page=86). ### 3.8 Controlled Mandatory Ventilation (CMV) Also known as Continuous Mandatory Ventilation (CMV) or Control Mode, CMV delivers preset tidal volumes at a set frequency, with the ventilator controlling tidal volume and frequency (RR) or minute ventilation. The patient cannot initiate spontaneous breaths [93](#page=93). * **Indications:** Patients adequately medicated with sedatives, respiratory depressants, and neuromuscular blockers; patients fighting the ventilator; tetanus; seizures; complete rest (e.g., 24 hours); and flail chest injuries [95](#page=95). * **Complications:** Patient dependence on the ventilator, diaphragm muscle atrophy, diaphragmatic oxidative injury with prolonged use, and reduced diaphragm function [97](#page=97). ### 3.9 Assist Control (AC) Mode In AC mode, the patient can increase ventilator frequency (assist) beyond the preset mechanical frequency (control). Each control breath is time-triggered, and each assist breath is patient-triggered, with both delivering a preset tidal volume. The patient cannot take spontaneous breaths in this mode [98](#page=98). * **Indications:** Full ventilatory support for patients newly placed on a ventilator or those with a stable respiratory drive (spontaneous frequency of at least 10-12/min) who can trigger inspiration [100](#page=100). * **Advantages:** Low work of breathing, allows patient to control frequency and minute volume to normalize PaCO2 . * **Complications:** Alveolar hyperventilation (respiratory alkalosis) if the patient has an excessively high respiratory drive. Solutions include adding mechanical dead space or switching to SIMV . ### 3.10 Intermittent Mandatory Ventilation (IMV) IMV delivers control (mandatory) breaths and allows spontaneous breathing between these breaths . * **Primary Complication:** Breath stacking, which occurs when a spontaneous breath coincides with a time-triggered mandatory breath . ### 3.11 Synchronized Intermittent Mandatory Ventilation (SIMV) SIMV synchronizes mandatory breaths with the patient's spontaneous breathing efforts to prevent breath stacking. Mandatory breaths can be time-triggered or patient-triggered. Spontaneous breaths taken in SIMV are truly spontaneous . * **Indications:** Partial ventilatory support, allowing the patient to contribute to minute volume . * **Advantages:** Maintains respiratory muscle strength, reduces V/Q mismatch, decreases mean airway pressure, and facilitates weaning . * **Complications:** High work of spontaneous breathing and muscle fatigue if weaning is too rapid . ### 3.12 Adaptive Support Ventilation (ASV) ASV is a dual-control mode that provides a mandatory minute ventilation by adjusting the number of mandatory breaths and pressure support based on the patient's breathing pattern. It measures dynamic compliance and expiratory time constants to adjust mechanical VT and frequency for a target minute ventilation . ### 3.13 Airway Pressure Release Ventilation (APRV) APRV utilizes two CPAP or pressure levels: high pressure (Phigh or Pinsp) and low pressure (Plow or PEEP). Spontaneous breathing is unrestricted. Release from Phigh to Plow stimulates mechanical exhalation . * **Indications:** Similar to pressure control, as an alternative to conventional VCV, to avoid barotrauma and excessive peak airway pressure . ### 3.14 Inverse Ratio Ventilation (IRV) IRV uses an inspiratory to expiratory (I:E) ratio between 2:1 and 4:1, often in conjunction with pressure-controlled ventilation. Conventional ventilation typically has an I:E ratio of 1:1.5 to 1:3 . * **Indications:** To improve oxygenation in patients with Acute Respiratory Distress Syndrome (ARDS) . * **Physiology:** Improves oxygenation by reducing intrapulmonary shunting, improving V/Q mismatch, and decreasing dead space ventilation. It increases mean airway pressure (mPaw), which helps reduce shunting. Auto-PEEP is present due to the longer inspiratory time . * **Adverse Effects:** Increased incidence of barotrauma due to increased mean alveolar pressure and volume . ### 3.15 High-Frequency Oscillatory Ventilation (HFOV) HFOV delivers extremely small volumes at very high frequencies (1 Hz = 60 cycles/minute). It aims to minimize lung injury during mechanical ventilation . * **Primary Settings:** Mean Airway Pressure, Frequency, Mean Airway Pressure, Percent inspiration, Inspiratory bias flow, FiO2 . * **Conversions:** Hertz (Hz) to cycles: $ \text{cycles} = \text{Hz} \times 60 $; Cycles to Hertz (Hz): $ \text{Hz} = \text{Cycles} / 60 $ . * **Mechanism:** Delivers constant bias flow and oscillates using a piston pump at frequencies from 3 Hz to 15 Hz (180 to 900 breaths/min). Adult patients are typically sedated . * **Indications:** Primarily used in the neonatal population, failing conventional ventilation, increasing ventilation requirements, hyaline membrane disease, and pulmonary hypertension ($OI \ge 15$) . * **Ventilation Adjustments:** * Increase ventilation by decreasing frequency or increasing amplitude, inspiratory flow, and bias flow . * Decrease ventilation by increasing frequency or decreasing amplitude, inspiratory flow, and bias flow . * **Oxygenation Adjustments:** * Increase oxygenation by increasing mean airway pressure (PIP, PEEP, inspiratory time, inspiratory hold) and increasing FiO2 . * Decrease oxygenation by decreasing mean airway pressure and decreasing FiO2 . ### 3.16 Initial ventilator settings Key initial settings for mechanical ventilation include mode, tidal volume, frequency, trigger, FiO2, I:E ratio, and alarm limits . #### 3.16.1 Mode Modes are selected based on whether full or partial ventilatory support is required . * **Full ventilatory support:** Modes that assume nearly all the work of breathing (e.g., CMV, AC) . * **Partial ventilatory support:** Modes providing less than total work of breathing (e.g., SIMV < 12 BPM, BiPAP, PSV) . #### 3.16.2 Frequency The initial frequency is set to achieve eucapnic ventilation (patient's normal PaCO2), typically between 10 and 12 breaths/min. Frequencies of 20/min or higher should be avoided due to the risk of Auto-PEEP. Adjusting rate controls expiratory time, thus altering the I:E ratio . * Increasing rate decreases expiratory time and minute ventilation, leading to decreased PaCO2 (CO2 washout) . * Decreasing rate increases expiratory time and minute ventilation, leading to increased PaCO2 (CO2 retention) . #### 3.16.3 Tidal Volume / Peak Inspiratory Pressure Initial tidal volume is usually set between 6 to 8 mL/kg of predicted body weight. For ARDS, lower tidal volumes of 4 to 6 mL/kg are recommended. Peak inspiratory pressure (PIP) should be set to achieve the target exhaled VT. COPD patients may benefit from reduced tidal volumes to allow sufficient expiratory time and prevent air trapping . * **Ideal Body Weight (IBW) Formula:** * Male: $ 106 + [6 \times (\text{height in inches} - 60)] $ . * Female: $ 105 + [5 \times (\text{height in inches} - 60)] $ . * Convert pounds to kilograms by dividing by 2.2 . #### 3.16.4 Minute Ventilation (VE) Minute ventilation is adjusted by respiratory rate and tidal volume to achieve an acceptable level, typically in the range of 5 to 10 L/min . * **Formula:** $ \text{VE} = \text{VT} \times \text{RR} $ . * Adjustments based on ABG: * If $ \text{PaCO}_2 > 45 \text{ mm Hg} $: Increase VE . * If $ \text{PaCO}_2 < 45 \text{ mm Hg} $: Decrease VE . #### 3.16.5 Inspiratory Flow Normal inspiratory flow settings range from 40 to 80 L/min. A starting point is to set flow to deliver inspiration in approximately 1 second (0.8 to 1 sec), resulting in an I:E ratio of 1:2 or 1:3 . * Increasing flow rate decreases inspiratory time . * Decreasing flow rate increases inspiratory time . #### 3.16.6 I:E Ratio The I:E ratio is the relationship between inspiratory time and expiratory time, typically maintained between 1:2 and 1:4 . * **Normal ratios:** Adult 1:2, Infant 1:1 . * **Special considerations:** COPD 1:3 or 1:4, MAS 1:4, RDS 1:3, ARDS (inverse ratio) . * The I:E ratio can be altered by adjusting tidal volume, flow rate, or respiratory rate . * **Formulas:** * Inspiratory Time: $ \text{I time} = \frac{\text{VT (L)}}{\text{FLOW RATE (Liters per second)}} $ . * Total Cycle Time: $ \text{TCT} = 60 / \text{RR} $ . * Expiratory Time: $ \text{E time} = \text{TCT} - \text{I time} $ . * To correct an inverse I:E ratio alarm: decrease RR, decrease VT, or increase flow . #### 3.16.7 Inspiratory Flow Patterns Modern ventilators offer various inspiratory flow patterns: square (constant), accelerating (ascending), decelerating (descending), and sine wave . * **Square:** Provides even, constant peak flow . * **Accelerating:** Improves ventilation distribution in partial airway obstruction . * **Decelerating:** Produces high initial inspiratory pressure . * **Sine wave:** Considered physiologic, similar to natural breathing patterns . #### 3.16.8 Fractional Inspired Oxygen (FiO2) Initially, 100% FiO2 may be given for severe hypoxemia, abnormal cardiopulmonary function, post-resuscitation, smoke inhalation, and ARDS. After stabilization, aim to keep FiO2 < 50% to avoid oxygen-induced lung injuries like absorption atelectasis and O2 toxicity . #### 3.16.9 Positive End-Expiratory Pressure (PEEP) PEEP maintains positive airway pressure after a ventilator breath . * **Optimal PEEP:** The PEEP level that improves lung compliance without cardiac compromise. It can be assessed using mixed venous PO2 (PvO2) . * **Normal PvO2:** 35 to 45 mm Hg . * **Contraindications:** Hypotension, elevated intracranial pressure, and uncontrolled pneumothorax . #### 3.16.10 Sensitivity Sensitivity determines the patient effort needed to trigger inspiration, typically set between -1.0 and -2.0 cm H2O . * If the ventilator auto-triggers, sensitivity is too high; decrease sensitivity . * If it requires more than -2 cm H2O to trigger, increase sensitivity . ### 3.17 Troubleshooting and management of mechanical ventilation The primary goals of mechanical ventilation are to improve ventilation and oxygenation. Common settings include Frequency (f), Tidal Volume (VT), FiO2, PEEP, Pressure Support Ventilation (PSV), and Pressure Gradient (DP) . #### 3.17.1 Strategies to improve ventilation Strategies focus on managing gas exchange, particularly carbon dioxide levels . #### 3.17.2 Strategies to improve oxygenation These strategies aim to enhance oxygen delivery to the tissues . #### 3.17.3 Mechanical ventilation based on ABG interpretation * **Respiratory Acidosis:** * **Main cause:** Hypoventilation . * **Troubleshooting:** Increase ventilation by increasing VT/PIP (6-8 mL/kg), increasing RR (f), adding/increasing PSV, or removing mechanical dead space . * **Respiratory Alkalosis:** * **Main cause:** Hyperventilation (decreased PaCO2) . * **Troubleshooting:** Decrease ventilation by decreasing VT/PIP (6-8 mL/kg), decreasing RR (f), decreasing PSV, or adding mechanical dead space. Increase FiO2 if hypoxemia is also present . * **Metabolic Acid-Base Abnormalities:** Ventilation adjustments should not be made to compensate for metabolic issues. Metabolic acidosis may be managed with HCO3- infusion, and metabolic alkalosis with KCl infusion . #### 3.17.4 Troubleshooting of common ventilator alarms and events * **Low Pressure/Volume Alarm:** * **Low Pressure:** Set 10-15 cm H2O below observed PIP . * **Low Expired Volume:** Set 100-150 mL or 10-15% lower than expired mechanical VT . * **High Pressure Alarm:** * Set 10-15 cm H2O above observed PIP . * **Causes:** * Increased airflow resistance ($PIP \uparrow, P_{plat}$ unchanged) . * Decreased lung/chest wall compliance ($PIP \uparrow, P_{plat} \uparrow$) . * **Apnea Alarm:** * Set for a maximum of 20 seconds or to prevent missing more than two breaths . * **Causes:** Apnea, circuit disconnection, CNS depressants, respiratory center dysfunction, respiratory muscle fatigue. Important for patients on CPAP, PSV, or low SIMV rates . * **High Frequency Alarm:** Set 10/min over observed frequency. Causes include distressed or agitated patients, or incorrect sensitivity settings (auto-triggering) . * **High/Low FiO2 Alarm:** High: 5-10% over analyzed FiO2. Low: 5-10% below analyzed FiO2 . * **High/Low PEEP Alarm:** * **High PEEP:** Set 3-5 cm H2O above PEEP. Causes: circuit obstruction, AutoPEEP. Troubleshooting: Increase flow, decrease VT, decrease RR, bronchodilators . * **Low PEEP:** Set 2-5 cm H2O below PEEP. Causes: circuit disconnection, leaks . ### 3.18 Ventilator strategies for specific conditions * **Chronic Obstructive Pulmonary Disease (COPD):** * **Problems:** Worsening oxygen saturation, increasing PCO2, Auto-PEEP . * **Solutions:** Restore baseline PaCO2 for COPD (55-65 mm Hg); manage Auto-PEEP by increasing flow, decreasing VT, and decreasing RR . * **Acute Respiratory Distress Syndrome (ARDS):** * **Problems:** Decreased lung compliance, refractory hypoxemia . * **Management (ARDSNet Protocol):** Low tidal volume ventilation (4-6 mL/kg), use PEEP, pressure limit of 30-35 cm H2O, Inverse Ratio Ventilation . * **"Three 3s":** VT: 300 mL, RR (f): 30 BPM, Pplat: <30 cm H2O . * **Permissive Hypercapnia:** A strategy to minimize ventilator-induced lung injuries by using low tidal volumes (4-6 mL/kg) . * **Permissive Hypocarbia ("Hyperventilation Test"):** Used for head injury (ICP > 20 mm Hg) to decrease intracranial pressure by maintaining PaCO2 between 30-40 mm Hg, causing cerebral vasoconstriction . * **Inverse Ratio Ventilation (IRV):** Used for ARDS patients with refractory hypoxemia, where inspiratory time is longer than expiratory time . --- # Weaning from mechanical ventilation This section details the process of transitioning a patient from mechanical ventilation to spontaneous breathing, covering success criteria, failure indicators, and the procedures involved . ### 4.1 Definitions * **Weaning success:** The ability of a patient to maintain spontaneous breathing for a prescribed period, typically leading to the termination of mechanical ventilation. Absence of ventilatory support for 48 hours following extubation is also considered success . * **Weaning failure:** Defined by the need to reintroduce mechanical ventilation, which can occur during a spontaneous breathing trial or within 48 hours following extubation . ### 4.2 Criteria for weaning To initiate a weaning attempt, patients must meet several criteria, focusing on their overall stability and respiratory function . #### 4.2.1 General criteria * The patient should be awake and alert . * Hemodynamic stability is essential . * The underlying disease or condition causing respiratory failure should be stable or improving . * The patient should not be receiving medications that could impair spontaneous ventilation . * There should be no life-threatening situations present . * Absence of anemia, fever, or electrolyte imbalances is required . #### 4.2.2 Respiratory parameters Several objective measurements are used to assess a patient's readiness for weaning : * **Vital Capacity (VC):** Should be greater than 10-15 mL/kg . * **Spontaneous Tidal Volume (VT):** Should be greater than 4 to 6 mL/kg . * **Spontaneous Minute Ventilation (VE):** Should be less than 10 to 15 L/min . * **Maximal Inspiratory Pressure (MIP):** Should be at least -20 cm H2O . * **Dead Space to Tidal Volume Ratio (VD/VT):** Should be less than 60% . * **Alveolar-Arterial Oxygen Gradient (P(A-a)O2):** Should be less than 350 mm Hg when on 100% oxygen . * **Rapid Shallow Breathing Index (RSBI):** Should be less than 105. The formula for RSBI is : $$ \text{RSBI} = \frac{\text{Respiratory Rate (RR)}}{\text{Tidal Volume (VT)}} $$ * **Respiratory Rate (RR):** Should be less than 35 breaths/min . * **Positive End-Expiratory Pressure (PEEP):** Should be less than or equal to 8 cm H2O . * **PaO2/FiO2 Ratio (P/F ratio):** Should be greater than 200 . > **Tip:** To effectively consider weaning, the patient should ideally be on an fraction of inspired oxygen (FiO2) of less than 0.40 to 0.50, maintaining an arterial oxygen saturation (SaO2) of at least 90% . ### 4.3 Weaning procedures Several methods can be employed to facilitate the weaning process, allowing the patient to gradually resume spontaneous breathing . * **Spontaneous Breathing Trial (SBT):** This is a critical component of weaning where the patient breathes spontaneously for a set period . * **T-Piece (Briggs Adaptor):** Allows for spontaneous breathing with humidified oxygen, interspersed with periods of mechanical ventilation . * **Synchronized Mandatory Ventilation:** Involves gradually reducing the number of ventilator-delivered breaths while allowing patient effort . * **Pressure Support Ventilation (PSV):** This mode helps reduce the work of breathing by assisting each spontaneous breath and compensating for airflow resistance from the endotracheal tube and ventilator circuit . * **Continuous Positive Airway Pressure (CPAP):** Weaning can also be achieved by progressively decreasing the level of CPAP . ### 4.4 Indications of weaning failure Early recognition of weaning failure is crucial to prevent patient decompensation and allow for prompt re-institution of ventilatory support . * **Tachypnea:** An abnormally rapid breathing rate . * **Dyspnea:** A sensation of shortness of breath . * **Use of accessory muscles:** Increased reliance on muscles of the neck and chest wall to breathe . * **Paradoxical abdominal movements:** The abdomen moves inward during inspiration instead of outward . > **Note:** If a patient does not tolerate the weaning procedure, they should be returned to full ventilatory support and allowed to rest before another attempt is considered . --- # Intubation procedure and complications This section outlines the sequential steps for endotracheal intubation and addresses potential complications, including ventilator-associated pneumonia (VAP) . ### 5.1 Intubation procedure The endotracheal intubation procedure involves a series of critical steps, from equipment preparation to confirmation of tube placement . #### 5.1.1 Equipment preparation The first step involves checking and assembling all necessary equipment for the intubation procedure. This includes : * Oxygen flowmeter and tubing . * Suction apparatus . * Bag-valve-mask (BVM) . * Laryngoscope with assorted blades . * Three sizes of endotracheal (ET) tubes . * Stylet . * Stethoscope . * Tape . * Syringe . * Sterile gloves . #### 5.1.2 Patient positioning The patient should be positioned in the "sniffing position" to facilitate intubation . #### 5.1.3 Pre-oxygenation Prior to intubation attempts, the patient must be pre-oxygenated with 100% oxygen. Each intubation attempt should not exceed 30 seconds. If intubation is unsuccessful, the patient should be ventilated with 100% oxygen for 3 to 5 minutes before another attempt is made . #### 5.1.4 Laryngoscope insertion The laryngoscope is inserted to visualize the vocal cords . * **Miller blade:** This is a straight blade, typically used for neonates and pediatrics. It is placed under the epiglottis and lifted upward and forward to expose the cords . * **Macintosh blade:** This is a curved blade used for adults. It is inserted between the epiglottis and the base of the tongue ( vallecula ) for indirect visualization . #### 5.1.5 Endotracheal tube insertion After visualization, the endotracheal tube is inserted. The depth of insertion can be estimated using the following guidelines : * **Male:** 21 to 23 cm . * **Female:** 19 to 21 cm . * **Pediatrics (formula):** Internal diameter x 3 . * **Newborns:** Gestational age divided by 10, or weight in kilograms plus 6 . #### 5.1.6 Confirmation of ET tube position Confirming the correct placement of the ET tube is crucial and can be achieved through several methods : * **Inspection:** Observing for bilateral chest rise during ventilation . * **Auscultation:** Listening to breath sounds over the chest . * **Chest X-ray:** A definitive imaging confirmation of tube placement . * **Tube location:** Noting the markings on the tube at the teeth . * **CO2 Detector:** Using a device that detects the presence of carbon dioxide, indicating tracheal placement . * **Capnometry:** Quantitative measurement of carbon dioxide . * **Colorimetry:** A color-changing device that indicates the presence of CO2 . ### 5.2 Complications of intubation and ventilation Potential complications can arise during and after intubation and mechanical ventilation, with Ventilator-Associated Pneumonia (VAP) being a significant concern . #### 5.2.1 Sources of Ventilator-Associated Pneumonia (VAP) The potential sources for VAP are multifaceted and include : * **The patient:** Particularly their oropharynx, which can harbor pathogens . * **Healthcare providers:** Their hands can transmit microorganisms . * **Equipment and supplies:** * Respiratory instruments . * Aerosol nebulizers . * Endotracheal tubes . * Nasogastric tubes . #### 5.2.2 VAP-bundle to prevent pneumonia A VAP-bundle is a set of evidence-based interventions aimed at reducing the incidence of VAP. Key components include : * Utilizing noninvasive positive-pressure ventilation when clinically appropriate . * Minimizing sedation and assessing readiness to extubate daily . * Performing spontaneous breathing trials with sedation discontinued . * Facilitating early patient mobility . * Using endotracheal tubes with subglottic secretion drainage for patients expected to require more than 48 to 72 hours of mechanical ventilation . * Changing ventilator circuits only if visibly soiled or malfunctioning . * Elevating the head of the bed to an angle of 30 to 45 degrees . --- ## Common mistakes to avoid - Review all topics thoroughly before exams - Pay attention to formulas and key definitions - Practice with examples provided in each section - Don't memorize without understanding the underlying concepts

| Term | Definition | |------|------------| | Mechanical Ventilation | A medical procedure where a machine, known as a ventilator, assists or replaces a patient's natural breathing process. It is used to help patients who cannot breathe adequately on their own due to illness or injury. | | Airway Resistance | The opposition to airflow within the respiratory system. High airway resistance can make it harder for air to move in and out of the lungs, increasing the work of breathing. | | Compliance (Lung Compliance) | A measure of how easily the lungs can expand. It is defined as the change in volume per unit of pressure change. Low compliance means the lungs are stiff and difficult to inflate. | | Static Compliance | Calculated by dividing the volume by the plateau pressure, which is measured when airflow is momentarily stopped. This measurement reflects the elastic resistance of the lung and chest wall without the influence of airway resistance. | | Dynamic Compliance | Calculated by dividing the volume by the peak inspiratory pressure when airflow is present. This measurement includes the effects of both airway resistance and the elastic properties of the lungs and chest wall. | | Deadspace Ventilation | Refers to ventilation that does not participate in gas exchange. This includes anatomic deadspace (airways that don't reach alveoli) and alveolar deadspace (alveoli that are ventilated but not perfused). | | Ventilatory Failure | A condition where the lungs are unable to adequately remove carbon dioxide from the body, leading to an increase in arterial carbon dioxide levels (hypercapnia). | | Oxygenation Failure | A condition characterized by severe hypoxemia, meaning there is a lack of adequate oxygen in the blood, which does not respond to high levels of supplemental oxygen. | | Hypoxemia | A condition of abnormally low oxygen levels in the blood, typically measured by the partial pressure of oxygen ($PaO_2$). | | Hypoxia | A condition of insufficient oxygen supply to the body's tissues and organs. | | Positive Pressure Ventilation | A type of mechanical ventilation where positive pressure is applied to the airways to inflate the lungs, typically delivered by a ventilator. | | Negative Pressure Ventilation | A method of mechanical ventilation that uses external negative pressure to expand the chest wall and facilitate lung inflation, mimicking normal breathing. Examples include iron lungs and chest cuirasses. | | PEEP (Positive End-Expiratory Pressure) | The pressure maintained in the airway at the end of exhalation, above atmospheric pressure. It helps to keep alveoli open, improving oxygenation. | | CPAP (Continuous Positive Airway Pressure) | A form of non-invasive ventilation that applies constant positive pressure to the airway throughout the respiratory cycle, used for patients who are breathing spontaneously. | | BiPAP (Bilevel Positive Airway Pressure) | A non-invasive ventilation mode that delivers two different levels of positive airway pressure: a higher pressure during inspiration (IPAP) and a lower pressure during expiration (EPAP). | | CMV (Controlled Mandatory Ventilation) | A mode of mechanical ventilation where the ventilator delivers all breaths at a preset volume or pressure and at a set rate. The patient does not initiate any breaths. | | AC (Assist Control) Mode | A mode of mechanical ventilation where the patient can trigger breaths, and each triggered breath is delivered by the ventilator at a preset tidal volume. The ventilator also delivers mandatory breaths at a set rate if the patient doesn't trigger. | | SIMV (Synchronized Intermittent Mandatory Ventilation) | A mode of mechanical ventilation that allows the patient to take spontaneous breaths between ventilator-delivered mandatory breaths. The mandatory breaths are synchronized with the patient's efforts. | | PSV (Pressure Support Ventilation) | A mode of spontaneous ventilation where the patient triggers each breath, and the ventilator delivers a preset level of pressure support during inspiration to reduce the work of breathing. | | HFOV (High-Frequency Oscillatory Ventilation) | A mode of mechanical ventilation that delivers very small tidal volumes at extremely high frequencies, used to minimize lung injury. | | Intubation | The insertion of a tube into the trachea (windpipe) to maintain an open airway or to administer oxygen, drugs, or anesthesia. | | VAP (Ventilator-Associated Pneumonia) | A type of pneumonia that develops in patients who are on mechanical ventilation, typically occurring 48 hours or more after intubation. | | NIV (Non-Invasive Ventilation) | A method of respiratory support that delivers positive pressure to the airways without the need for intubation, typically using a mask. | | Barotrauma | Lung injury caused by excessive pressure applied during mechanical ventilation. | | Auto-PEEP (Intrinsic PEEP) | Positive end-expiratory pressure that develops unintentionally during mechanical ventilation, typically in patients with obstructive lung disease, due to incomplete exhalation. | | I:E Ratio (Inspiratory:Expiratory Ratio) | The ratio of the duration of inspiration to the duration of expiration during breathing. In mechanical ventilation, this can be manipulated to optimize gas exchange and reduce lung injury. | | Tidal Volume (VT) | The volume of air inhaled or exhaled in a single breath. | | Peak Inspiratory Pressure (PIP) | The highest pressure reached in the airway during inspiration in mechanical ventilation. | | Plateau Pressure (Pplat) | The pressure measured in the airway during mechanical ventilation when airflow is temporarily stopped at the end of inspiration. It reflects the static compliance of the respiratory system. | | FiO2 (Fraction of Inspired Oxygen) | The percentage or fraction of oxygen in the air that a patient is breathing. | | Alveolar-Arterial Oxygen Gradient (P(A-a)O2) | A measure used to assess the difference between the partial pressure of oxygen in the alveoli and the partial pressure of oxygen in the arterial blood, indicating the efficiency of oxygen transfer across the alveolar-capillary membrane. | | Respiratory Quotient (R) | The ratio of carbon dioxide produced to oxygen consumed ($VCO_2 / VO_2$), used in gas exchange calculations. | | Weaning | The process of gradually withdrawing mechanical ventilatory support from a patient as their respiratory function improves. | | Spontaneous Breathing Trial (SBT) | A test performed to assess a patient's ability to breathe independently, typically involving a period of spontaneous breathing with minimal ventilator support. |