On initiating mechanical ventilation, the physician must choose between pressure-targeted ventilation ('pressure control') or flow-controlled volume-cycled ventilation
('volume control') (Fig 1). The fundamental difference is implicit in their names; pressure-targeted modes guarantee pressure at the expense of variable tidal volumes;
volume-targeted modes guarantee flow, and consequently the volume provided to the circuit in the allowed inspiratory time (tidal volume), at the expense of variable airway pressures (Table 1). In a well-monitored apneic patient, pressure- and volume-targeted modes can be used with virtually identical effect. If pressure control is used, the targeted inspiratory pressure (above positive end-expiratory pressure (PEEP)) and inspiratory time must be selected, usually with consideration of the desired tidal volume. If volume-cycled ventilation is used, the physician may select (depending on the ventilator) either tidal volume and flow delivery pattern (waveform and peak flow) or flow delivery pattern and minimum minute ventilation (with tidal volume the resulting ratio of minute ventilation to back-up frequency). Once chosen, flow is inflexible to changing inspiratory flow demands.
Fig. 1 Airway pressure and flow waveforms during pressure-controlled time-cycled ventilation and during flow-controlled volume-cycled ventilation delivered with constant and decelerating flow profiles. (Reproduced with permission from MariniaDdWheeleillSSZ])
Table 1 Pressure-controlled versus volume-controlled ventilation
Flow and tidal volumes are important variables to monitor in pressure targeting, while pressure is the monitored variable of parallel importance in volume targeting. Volume-targeted modes provide a preset volume unless a specified pressure limit is exceeded. The major advantages of volume targeting are the capacity to deliver non-varying tidal volumes (except in the presence of a gas leak or when the pressure limit is exceeded), flexibility of flow and volume adjustments, and ability to ventilate difficult patients. Unlike pressure-targeted ventilation, volume-targeted modes do not ventilate consistently unless the airway is well sealed. Furthermore, once the flow rate is set, the inflation time of the machine remains unresponsive to the patient's native cycling rhythm.
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