Pressure-controlled mechanical ventilation rapidly achieves a fixed pressure throughout the breath by delivering a decelerating inspiratory flow pattern (Figure 3). The result is a tidal volume that varies with lung compliance and resistance. For example, if there is an increase in airway resistance, or reduction in lung compliance, the delivered tidal volume decreases and hypoventilation results. Pressure-controlled ventilation is usually closely monitored with alarms set for a minimal acceptable tidal and/or minute volume. Volume-controlled and pressure-controlled ventilation are compared in Figure 4.
Interaction between ventilated breaths and the patient’s inspiratory efforts –
The common modes are as follows.
Controlled mandatory ventilation (CMV) with no allowance for spontaneous breathing is the most common mode used in the operating theatre during routine anaesthesia.
In synchronized intermittent mandatory ventilation (SIMV) controlled breaths (volume- or pressure-controlled) are delivered to a preset respiratory rate separate from the spontaneous breaths.
In assist-controlled ventilation, triggered spontaneous breaths are assisted identically to the controlled breaths.
In pressure-support ventilation, spontaneous patient breaths trigger a set amount of pressure to assist the breath.
Biphasic positive airway pressure (BIPAP) is a mixture of spontaneous breathing and time-driven, pressure-controlled ventilation (Figure 5). This system alternates between two adjustable pressure levels of continuous positive airway pressure (CPAP). Spontaneous breaths are possible at both pressure levels at all times. Cycling between the two levels produces gas flow and a resulting mechanical breath.
© 2003 The Medicine Publishing Company Ltd