Pulmonary Disorders and Mechanical Ventilation
The core framework
Almost every ventilator question reduces to two ideas: protect the lung with small tidal volumes based on predicted body weight, and match the mode/settings to the underlying physiology (obstructive versus restrictive).
- ARDS: 6 mL/kg predicted body weight, plateau pressure ≤30 cmH2O, driving pressure under 15, PEEP titrated to oxygenation, permissive hypercapnia, prone positioning 12–16 hours/day when P/F is under 150, conservative fluid strategy.
- Obstructive disease (asthma, COPD): low respiratory rate, high inspiratory flow, long expiratory time to limit auto-PEEP; watch for dynamic hyperinflation causing hypotension.
- Weaning: spontaneous awakening and breathing trials, rapid shallow breathing index, cuff leak, post-extubation support (HFNC or NIV) in high-risk patients.
Traps
Tidal volume set from actual body weight; confusing peak with plateau pressure (peak also reflects resistance); missing auto-PEEP in a hypotensive ventilated asthmatic; using NIV in an unprotected airway, vomiting patient, or facial trauma. Also know that steroids are indicated early in asthma exacerbation but not routinely in COPD without exacerbation, and that a normal SpO2 does not exclude carbon monoxide poisoning.
Worked example
Male, 70 inches tall: predicted body weight = 50 + 2.3 × (70 − 60) = 73 kg. Initial tidal volume = 6 × 73 = 438 mL, so set 440 mL. For a female, use 45.5 + 2.3 × (inches over 60).
Rule of thumb
Predicted body weight (male) = 50 + 2.3 × (height in inches − 60); females 45.5 + 2.3 × (inches − 60). Never use the bed scale.