5.8 Acute Respiratory Distress Syndrome & Mechanical Ventilation
Key Takeaways
- Acute respiratory distress syndrome and management of ventilators are separately enumerated blueprint subsections under Pulmonary Disease.
- Lung-protective ventilation using roughly 4 to 8 mL per kilogram of predicted body weight with a plateau pressure at or below 30 cm of water reduces mortality.
- Predicted body weight, calculated from height and sex, must be used for tidal volume rather than actual body weight.
- Prone positioning improves survival in severe acute respiratory distress syndrome with a markedly reduced oxygenation ratio.
- Daily spontaneous awakening and spontaneous breathing trials shorten ventilation duration and are the standard weaning approach.
Critical care pulmonology and pleural diseases encompass life-threatening conditions where rapid diagnosis and protocolized, evidence-based management directly determine patient survival. The ABIM examination frequently tests mechanical ventilation physiology, ARDS clinical trial parameters, weaning indices, pleural fluid biochemical interpretation using Light's criteria, and the emergent management of complicated effusions and tension pneumothorax.
1. Acute Respiratory Distress Syndrome (ARDS)
ARDS is a catastrophic form of acute, diffuse, inflammatory lung injury resulting in increased alveolar-capillary membrane permeability, loss of aerated lung tissue (non-cardiogenic pulmonary edema), surfactant inactivation, and severe intrapulmonary shunt physiology.
The Berlin Definition of ARDS
To establish a diagnosis of ARDS, a patient must meet all 4 of the following criteria:
- Timing: Acute onset within 1 week of a known clinical insult (sepsis, severe pneumonia, aspiration, major trauma, pancreatitis, TRALI) or new/worsening respiratory symptoms.
- Chest Imaging: Bilateral opacities on chest radiograph or CT scan not fully explained by pleural effusions, lobar/lung collapse, or pulmonary nodules.
- Origin of Edema: Respiratory failure not fully explained by cardiac failure or fluid overload. If no clear ARDS risk factor is present, objective assessment (e.g., transthoracic echocardiogram) is required to exclude hydrostatic/cardiogenic pulmonary edema.
- Oxygenation Impairment (measured with PEEP >= 5 cm H2O):
- Mild ARDS: 200 mmHg < PaO2/FiO2 <= 300 mmHg
- Moderate ARDS: 100 mmHg < PaO2/FiO2 <= 200 mmHg
- Severe ARDS: PaO2/FiO2 <= 100 mmHg
Evidence-Based ARDS Management Protocols
| Therapeutic Strategy | Trial & Evidence Base | Exact Clinical Parameters & Targets |
|---|---|---|
| Low Tidal Volume Ventilation | ARMA Trial (ARDSNet): Lower tidal volumes reduced 28-day mortality by 22% (31% vs 39.8%) compared to traditional 12 mL/kg ventilation by preventing volutrauma and barotrauma. | - Initial tidal volume: 6 mL/kg of Predicted Body Weight (PBW) (range: 4–8 mL/kg PBW).<br/>- PBW Formula (Men): $50 + 0.91 \times (\text{Height in cm} - 152.4)$<br/>- PBW Formula (Women): $45.5 + 0.91 \times (\text{Height in cm} - 152.4)$<br/>(Never use actual body weight, which leads to fatal overventilation). |
| Plateau Pressure Limitation | ARDSNet Protocol: Measured during an end-inspiratory occlusion pause (0.5 sec) in a passive patient. | - Target Plateau Pressure ($P_{plat}$) <= 30 cm H2O.<br/>- If $P_{plat} > 30$, decrease tidal volume in 1 mL/kg steps down to 4 mL/kg PBW. |
| Driving Pressure Optimization | Amato et al. (NEJM 2015): $\Delta P = P_{plat} - PEEP$. Driving pressure is the single strongest ventilator variable correlating with survival in ARDS. | - Target Driving Pressure ($\Delta P$) <= 15 cm H2O. |
| Permissive Hypercapnia | Prevents lung injury by tolerating moderate respiratory acidosis rather than escalating tidal volume or rate. | - Target arterial pH >= 7.20 (PaCO2 50–70 mmHg tolerated).<br/>(Contraindicated in severe metabolic acidosis, increased ICP, or severe right heart failure). |
| Prone Positioning | PROSEVA Trial: In moderate-to-severe ARDS (PaO2/FiO2 < 150 mmHg with PEEP >=10 and FiO2 >=0.6), early prone positioning reduced 28-day mortality from 32.8% to 16.0% ($p < 0.001$). | - Must be maintained for >= 16 consecutive hours per day.<br/>- Mechanisms: Homogenizes transpulmonary pressure gradients, recruits dorsal atelectatic alveoli, improves V/Q matching, decreases ventral overdistension. |
| Neuromuscular Blockade | ACURASYS Trial: Early continuous IV infusion of Cisatracurium besylate for 48 hours. | - Indicated for severe ARDS (PaO2/FiO2 < 150) with significant patient-ventilator dyssynchrony, breath-stacking, or persistently elevated plateau pressures. |
| Conservative Fluid Strategy | FACTT Trial: Protocolized fluid restriction and diuresis to achieve a neutral-to-negative fluid balance once shock has resolved. | - Increases ventilator-free days and ICU-free days without worsening non-pulmonary organ failure or renal replacement therapy rates. |
| Veno-Venous (VV) ECMO | EOLIA Trial & CESAR Trial: Extracorporeal membrane oxygenation for refractory severe hypoxemic ARDS. | - Indicated when PaO2/FiO2 < 80 mmHg for >6 hours or PaO2/FiO2 < 100 for >12 hours, or pH < 7.15 despite lung-protective ventilation and prone positioning. |
2. Mechanical Ventilation Weaning Protocols
Daily assessment of readiness for liberation from mechanical ventilation accelerates extubation and prevents ventilator-associated pneumonia and vocal cord injury.
Protocolized Weaning Assessment
- Spontaneous Awakening Trial (SAT): Daily interruption of all sedatives and opioids in stable patients.
- Spontaneous Breathing Trial (SBT): Performed when the patient passes SAT, meets oxygenation stability criteria (PaO2/FiO2 >= 150–200 on PEEP <= 5–8 cm H2O and FiO2 <= 0.4–0.5), has stable hemodynamics without high-dose vasopressors, and demonstrates spontaneous respiratory effort.
- SBT Technique: Low-level pressure support (PSV 5 cm H2O / PEEP 5 cm H2O) or T-piece trial for 30 to 120 minutes.
- Rapid Shallow Breathing Index (RSBI / Tobin Index):
- Calculated during 1 minute of unassisted room air / T-piece breathing:
- RSBI < 105 breaths/min/L strongly predicts successful extubation.
- RSBI > 105 breaths/min/L indicates respiratory muscle fatigue and high likelihood of extubation failure.
- Cuff Leak Test: Deflating the endotracheal tube cuff to verify air movement around the tube. Indicated in patients with risk factors for post-extubation stridor/laryngeal edema (prolonged intubation >6 days, traumatic intubation, female sex). If cuff leak volume is low (<110 mL or <24% of inspired volume), administer IV Methylprednisolone 20–40 mg every 4–6 hours for 12–24 hours prior to re-evaluating extubation.
3. Pleural Effusions: Diagnostic Evaluation & Light's Criteria
A pleural effusion represents an abnormal accumulation of fluid in the pleural space resulting from altered hydrostatic/oncotic forces (transudate) or increased capillary permeability/lymphatic obstruction (exudate).
Diagnostic Thoracentesis Indications
- Indicated for any new, unexplained pleural effusion >= 10 mm in thickness on lateral decubitus radiograph or bedside ultrasound.
- Exception: Bilateral effusions with classic clinical signs of decompensated Congestive Heart Failure (CHF) do not require immediate thoracentesis unless atypical features are present (fever, pleuritic chest pain, asymmetric effusion size, or lack of response to diuresis).
A 52-year-old woman with severe intra-abdominal sepsis following a perforated diverticulum is admitted to the intensive care unit. On hospital day 2, she develops acute hypoxemic respiratory failure requiring endotracheal intubation. Her height is 163 cm (5 ft 4 in; predicted body weight [PBW] = 55 kg) and her actual weight is 88 kg. Mechanical ventilation is initiated on volume-assist/control mode with a tidal volume of 550 mL, respiratory rate 18 breaths/min, PEEP 12 cm H2O, and FiO2 0.80. Arterial blood gas 1 hour later reveals: pH 7.32, PaCO2 48 mmHg, PaO2 71 mmHg (PaO2/FiO2 ratio = 89 mmHg). A chest radiograph demonstrates diffuse, confluent bilateral infiltrates with normal cardiac silhouette, and bedside echocardiogram shows normal LV systolic function without left atrial enlargement. Inspiratory hold reveals a plateau pressure (Pplat) of 34 cm H2O. Which of the following initial ventilatory management strategies is most appropriate?