Respiratory Failure and Critical Care: Oxygenation, Ventilation, and Lung Protection
Classify the failure
Type 1 (hypoxemic) failure has a normal or low PaCO2; type 2 (hypercapnic) has a PaCO2 >45 mmHg. The alveolar gas equation (PAO2 = 150 − PaCO2/0.8) and the A-a gradient separate hypoventilation from a true gas-exchange defect: a hypoxemic patient with a normal gradient has hypoventilation, whereas a widened gradient means V/Q mismatch, shunt, or diffusion limitation. The gradient rises with age, roughly by age/4 plus 4.
ARDS and mechanical ventilation
Berlin criteria require onset within one week, bilateral opacities not explained by heart failure, and P/F ratio ≤300 on PEEP ≥5. Severity is graded by P/F: mild 201–300, moderate 101–200, severe ≤100. Lung-protective ventilation means tidal volume 6 mL/kg predicted body weight (not actual weight - a classic trap in obese patients), plateau pressure <30 cmH2O, and permissive hypercapnia. Prone positioning improves survival in severe ARDS; neuromuscular blockade is reserved for selected cases.
Noninvasive support
NIV reduces intubation and mortality in COPD exacerbation with hypercapnia and pH 7.25–7.35, and in cardiogenic pulmonary edema. It is not a rescue for altered mental status, vomiting, facial trauma, or hemodynamic instability - those need intubation.
Liberation
Daily sedation interruption, spontaneous breathing trials, and early mobilization shorten ventilation. Remember that rising PaCO2 after extubation with stridor suggests laryngeal edema, treated with nebulized epinephrine and steroids rather than immediate reintubation if mild.
Worked rule
On room air, PaO2 55 and PaCO2 55: alveolar oxygen ≈ 150 − 68.75 = 81; A-a gradient ≈ 26 - modest for the degree of hypercapnia.