METHODS OF VENTILATORY SUPPORT
- Continous mandatory ventilation
Breaths are delivered at preset intervals, regardless of patient effort. This mode is used most often in the paralyzed or apneic patients because it can increase the work of breathing if respiratory effort is present. Continuous mandatory ventilation has given way to assist-control mode because assist-control with the apneic patient is tantamount to continuous mandatory ventilation. Many ventilators do not have a true continuous mandatory ventilation mode and offer assist-control instead.
- Assist-control ventilation
The ventilator delivers preset breaths in coordination with the respiratory efforts of patient. With each inspiratory efforts, the ventilator delivers a full assisted tidal volume. Spontaneous breathing independent of the ventilator between assist-control breaths is not allowed. As might be expected, this mode is better tolerated than continuous mandatory ventilation in patients with intact respiratory effort.
- Intermittent mandatory ventilation
With intermittent mandatory ventilation, breath 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.
- Synchronous intermittent mandatory ventilation
The 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 patients’s spontaneous breath attempts to limit barotrauma that may occur with intermittent mandatory ventilation when a preset breath is delivered to a patient who is already maximally inhaled (breath stacking) or is forcefully exhaling. One disadventage of synchronous intermittent mandatory ventilation is increased work of breathing, though this may be mitigated by adding pressure support on top of spontaneous breath.
The initial choice of ventilation mode is institution and practitioner dependent. Assist-contrl ventilation, as in continuous mandatory ventilation, 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 carbon dioxide production of the respiratory muscles. A potential drawback of assist-control 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.