Continuous mandatory ventilation

Breaths are delivered at preset intervals, regardless of patient effort. This mode is used most often in the paralyzed or apneic patient because it can increase the work of breathing if respiratory effort is present. Continuous mandatory ventilation (CMV) has given way to assist-control (A/C) mode because A/C with the apneic patient is tantamount to CMV. Many mechanical ventilators do not have a true CMV mode and offer A/C instead.

Assist-control ventilation

The mechanical ventilator delivers preset breaths in coordination with the respiratory effort of the patient. With each inspiratory effort, the mechanical ventilator delivers a full assisted tidal volume. Spontaneous breathing independent of the ventilator between A/C breaths is not allowed. As might be expected, this mode is better tolerated than CMV in patients with intact respiratory effort.

Intermittent mandatory ventilation

With intermittent mandatory ventilation (IMV), breaths are delivered at a preset interval, and spontaneous breathing is allowed between ventilator-administered breaths. Spontaneous breathing occurs against the resistance of the airway tubing and ventilator valves, which may be formidable. This mode has given way to synchronous intermittent mandatory ventilation (SIMV).

Synchronous intermittent mandatory ventilation

The mechanical ventilator delivers preset breaths in coordination with the respiratory effort of the patient. Spontaneous breathing is allowed between breaths. Synchronization between preset mandatory breaths and the patient's spontaneous breaths attempts to limit barotrauma that may occur with IMV when a preset breath is delivered to a patient who is already maximally inhaled (breath stacking) or is forcefully exhaling. One disadvantage of SIMV is increased work of breathing, though this may be mitigated by adding pressure support on top of spontaneous breaths.

The initial choice of ventilation mode (eg, SIMV, A/C) is institution and practitioner dependent. A/C ventilation, as in CMV, is a full support mode in that the ventilator performs most, if not all, of the work of breathing. These modes are beneficial for patients who require a high minute ventilation. Full support reduces oxygen consumption and CO2 production of the respiratory muscles. A potential drawback of A/C ventilation in the patient with obstructive airway disease is worsening of air trapping and breath stacking.

When full respiratory support is necessary for the paralyzed patient following neuromuscular blockade, no difference exists in minute ventilation or airway pressures with any of the above modes of ventilation. In the apneic patient, A/C with a respiratory rate (RR) of 10 and a TV of 500 mL delivers the same minute ventilation as SIMV with the same parameters.