Mechanical Ventilation: Modes, Pressures, Alarms, and Weaning
Modes and initial settings
Mechanical ventilation supports or replaces spontaneous breathing. Volume control delivers a set tidal volume; pressure control delivers a set pressure with a variable volume; SIMV blends mandatory and spontaneous breaths; pressure support augments spontaneous effort; CPAP provides PEEP alone. Initial tidal volume is 6–8 mL/kg of ideal body weight - never actual weight - with PEEP about 5 cm H2O, FiO2 titrated to SpO2 88–95%, and a rate matched to needed minute ventilation.
Reading the pressures
Peak inspiratory pressure reflects resistance plus compliance; plateau pressure reflects compliance alone. A high peak with a normal plateau (<30 cm H2O) means increased resistance - secretions, biting, kinks, bronchospasm. Elevation of both means poor compliance - ARDS, pneumothorax, abdominal distension, pulmonary edema. Measure auto-PEEP with an expiratory hold in asthma or COPD.
Alarms and weaning
High-pressure alarms: the resistance and compliance causes above. Low-pressure alarms: disconnection, cuff leak, circuit leak. Wean FiO2 to 0.40 or less, then PEEP; a spontaneous breathing trial with an RSBI (rate ÷ tidal volume in liters) below 105 predicts success.
Worked example
Ideal body weight for a male = 50 + 2.3 × (height in inches − 60). A 70-inch man: 50 + 23 = 73 kg, so 6–8 mL/kg is about 440–580 mL. Setting 700 mL because he actually weighs 100 kg would overdistend the lungs and risk barotrauma.