Mechanical Ventilation and Critical Care Respiratory Therapy
Modes and goals
Volume-control delivers a set VT; pressure-control delivers a set pressure and variable VT. SIMV and PSV are partial-support modes; PRVC targets a VT with variable pressure. Initial settings: VT 6–8 mL/kg predicted body weight, PEEP 5 cmH2O, RR 12–16, I:E about 1:2 to 1:3. In ARDS use 4–8 mL/kg PBW, plateau ≤30 cmH2O, driving pressure ≤15 cmH2O.
Monitoring and alarms
High peak pressure with normal plateau = airway resistance; high plateau = compliance. Auto-PEEP occurs with air trapping; measure with an expiratory hold. Low-pressure alarms suggest circuit leak, disconnection, or cuff leak. Weaning readiness: cause resolved, adequate cough, PaO2/FiO2 >150–200, stable hemodynamics; use SBT with PSV 5–7 or T-piece for 30–120 minutes.
Worked example
A 170-cm man has PBW = 50 + 0.91(170 − 152.4) = 66 kg. Lung-protective VT at 6 mL/kg = 396 mL, about 400 mL. If plateau is 32 cmH2O and PEEP 8, driving pressure is 24 cmH2O, above target; reduce VT by 1 mL/kg and reassess.
Traps
Do not set VT by actual body weight in obesity. A high peak pressure alone does not prove ARDS; check plateau. Do not mistake auto-PEEP for reduced compliance. Do not extubate solely because ABG values look acceptable.