Mechanical Ventilation Modes Reference

Description: The ventilator delivers a full-volume (or full-pressure) breath for every respiratory effort the patient makes, plus guarantees a minimum mandatory rate. If the patient breathes above the set rate, each breath receives full ventilator support. If the patient fails to trigger, the ventilator initiates breaths at the set rate.

Volume AC (VAC): Each breath delivers a set tidal volume regardless of pressure generated. Peak inspiratory pressure is a dependent variable — monitor for excessive pressures.

Pressure AC (PAC): Each breath delivers a set pressure; tidal volume varies with lung compliance. Safer for ARDS (lung-protective) — Vt is monitored as the dependent variable.

Basic indications: Acute respiratory failure, post-operative patients requiring full support, patients who cannot sustain any spontaneous breathing work, or those requiring strict control of ventilation (elevated ICP, severe ARDS).

Nursing note: If the patient triggers excessive breaths above the set rate, they may develop respiratory alkalosis. Monitor RR, pH, PaCO₂, and patient-ventilator synchrony.

Description: The ventilator delivers a set number of mandatory breaths per minute (synchronized with patient effort when possible). Between mandatory breaths, the patient can breathe spontaneously — these spontaneous breaths receive no volume or pressure support unless pressure support is added.

Basic indications: Transitional support mode during weaning from full ventilation, patients with intact respiratory drive who need partial support, and as a bridge to extubation.

Weaning in SIMV: The mandatory rate is progressively reduced to increase patient work. Respiratory rate orders may change frequently during active weaning — monitor closely and communicate changes.

Nursing note: Spontaneous breaths between mandatory breaths may be unsupported. If the patient fatigues or tachypneic, notify the provider — increased pressure support or mode change may be indicated.

Description: Patient-triggered only — no mandatory breaths. Each patient-initiated inspiratory effort receives a set pressure boost to augment tidal volume. The patient controls the respiratory rate, timing, and inspiratory flow.

Basic indications: Weaning from mechanical ventilation, spontaneous breathing trials (SBTs), compensation for ETT resistance during partial weaning. Low PS (5–8 cmH₂O) simulates breathing through an ETT alone and is often used during extubation readiness assessment.

No backup rate: PSV provides no mandatory breaths. If the patient becomes apneic (sedation, neurological change), ventilation stops until the ventilator triggers an apnea alarm and switches to backup mode. This is a key safety difference from AC mode.

Nursing note: Monitor tidal volume closely — actual Vt depends on patient effort plus lung compliance. Ensure apnea alarm settings are appropriate.

Description: Delivers a constant positive airway pressure throughout the entire respiratory cycle. No mandatory breaths, no pressure support assistance — the patient does all the breathing work. CPAP recruits alveoli and maintains FRC (functional residual capacity) to improve oxygenation.

Basic indications (invasive): Final stage before extubation — the patient breathes spontaneously through the ETT while PEEP is maintained for oxygenation. Also used as a spontaneous breathing trial modality.

Non-invasive CPAP (mask): Used for obstructive sleep apnea, acute cardiogenic pulmonary edema, and hypoxemic respiratory failure as an alternative to intubation. Delivered via nasal or full-face mask.

Nursing note: CPAP does not assist ventilation (CO₂ removal) — it only maintains positive pressure for oxygenation. If the patient is hypoventilating, CPAP alone will not correct hypercapnia. Escalation to a mode with ventilatory support is needed.