Respiratory Disorders and Mechanical Ventilation
Core concepts
Master the difference between oxygenation (reflected by PaO2/FiO2 and managed with PEEP and FiO2) and ventilation (reflected by PaCO2 and managed with rate and tidal volume). Know ARDS by the Berlin definition: onset within 1 week, bilateral opacities, not primarily cardiogenic, with PEEP ≥5 and P/F ≤300. Understand lung-protective ventilation (6 mL/kg predicted body weight, plateau ≤30 cm H2O), permissive hypercapnia, and driving pressure. Be fluent in obstructive disease (dynamic hyperinflation, auto-PEEP, prolonged expiration, low respiratory rate to allow exhalation) versus restrictive disease. Recognize ventilator asynchrony, intrinsic PEEP, and the work of spontaneous breathing trials.
Traps
Questions test predicted body weight (height-based), not actual weight. Do not chase a normal PaCO2 in a severe COPD patient - target their baseline. High peak pressure with normal plateau suggests airway resistance; high plateau suggests poor compliance. Remember that recruitment maneuvers and prone positioning improve oxygenation in severe ARDS but do not replace low tidal volume.
Worked example
A patient on volume control has peak pressure 45 and plateau 20 cm H2O. The gradient (peak minus plateau) is 25, indicating high airway resistance (secretions, bronchospasm, kinked tube) rather than parenchymal stiffness. Suction, bronchodilate, and check the circuit before adjusting tidal volume. Rule of thumb: if plateau is ≤30, the lung is being protected; investigate the resistance side of the equation.