🫁 Chapter 17: Respiratory Failure

Oxygen content · A-a gradient · V/Q matching · Lung mechanics · HFOV · ARDS pathophysiology · Ventilatory strategies · Clinical recognition and management

🔍 Core Concepts: Respiratory Failure

📈 Oxygen content & dissociation
CaO2 = 1.36×Hgb×SaO2 + 0.003×PaO2. Hypoxia defined as PaO2 <60 mmHg (SpO2 <90%). Right shift (acidosis, ↑DPG, ↑temp) → O2 unloading. Left shift (alkalosis, ↓temp) → tissue hypoxia.
📐 A-a gradient & V/Q
A-a gradient = (FiO2×(713) - PaCO2/0.8) - PaO2; normal 4-15 mmHg. Helps differentiate hypoventilation (normal A-a) from parenchymal disease (↑A-a). Low V/Q → shunt; High V/Q → dead space.
🫀 Oxygen delivery & uptake
DO2 = CO × CaO2. Critical DO2 → VO2 dependent. Resuscitation goal: match supply/demand. Lactate clearance, ScvO2 surrogate markers.
🫁 Lung mechanics
Compliance = ΔV/ΔP (normal 50-100 mL/cmH2O adult). Time constant = compliance × resistance. ARDS ↓ compliance → short TC → increase rate. Asthma ↑ resistance → long TC → low rate, prolonged expiration.
⚖️ Types of respiratory failure
Type I (hypoxemic): PaO2 <60, PaCO2 normal/low (pneumonia, ARDS). Type II (hypercapnic): PaCO2 >50 (drugs, neuromuscular, severe airway). pH + HCO3 differentiate acute vs chronic.
💡 Key ventilator principles
Low tidal volume (6-8 mL/kg) lung protection. PEEP improves FRC. Auto-PEEP in asthma: set PEEP 2/3 of measured auto-PEEP. Plateau pressure <30 cmH2O.

🩺 Stepwise Clinical Approach: Acute Respiratory Failure

1
Recognize & categorize
Assess work of breathing, SpO2, ABG. Determine Type I (hypoxemic) vs Type II (hypercapnic). Check A-a gradient; normal suggests hypoventilation, elevated suggests lung pathology.
2
Immediate oxygenation & support
Give O2 to maintain SpO2 >90% (88-92% if risk of hypercapnia). Use nasal cannula, mask, or high-flow. NIV (CPAP/BiPAP) for moderate distress with intact airway reflexes.
3
Assess need for intubation
Indications: altered sensorium, inability to maintain SpO2 >90% on 100% O2, failed NIV, hemodynamic instability, rising PaCO2 with acidosis, respiratory fatigue.
4
Optimize pre-intubation physiology
Preoxygenate (NIPPV + apneic oxygenation). For hypotensive: fluid bolus, vasoactive infusion. Choose ketamine/etomidate. Use modified RSI if hypoxemic.
5
Disease-specific ventilation
ARDS: low tidal volume (6-8 mL/kg), PEEP optimization, plateau <30 cmH2O, consider HFOV if refractory. Asthma: low rate, long expiratory time, permissive hypercapnia, set external PEEP 2/3 auto-PEEP.
6
Ongoing monitoring & rescue
Monitor SpO2, ETCO2, ABG, lung mechanics, hemodynamics. Refractory hypoxemia: prone positioning, inhaled nitric oxide, recruitment maneuvers, ECMO as salvage.