Appendix D: Theory of Breath Delivery
217
Figure 166: Volume Control Breath Waveform
Inspiratory phase is terminated when set tidal volume (VT) is delivered. If Limit
Pressure is set lower than Alarm Pressure, tidal volume may be decreased, when
Limit Pressure is reached (c in the figure above). If the Adaptive Flow is selected,
the inspiratory time is increased to reach the set tidal volume (d in the figure
above).
PRESSURE CONTROL BREATH
A pressure control breath is a mandatory breath in which the ventilator provides
a constant pressure set during the inspiratory phase. The inspiratory pressure,
inspiratory time, rate and maximum delivered tidal volume values are set by the
operator. This breath type can be initiated both by the ventilator (ventilator
pressure control breath) and by the patient (assist pressure control breath). The
inspiratory phase is terminated by the ventilator when inspiration time is elapsed
or set limit for tidal volume is delivered prior to the end of the inspiratory phase.
The figure below demonstrates how pressure and flow behave when the
ventilator delivers a pressure control breath. When a pressure control breath is
initiated, the ventilator delivers the maximum possible flow until the patient
airway pressure exceeds the set level. Once this pressure level is exceeded, the
ventilator adjusts the flow to whatever rate is required to maintain the airway
pressure between the target pressure and a value, which is about 2 cmH2O
lower. At the end of the inspiratory phase, the ventilator allows the patient to
exhale.
Pressure control breaths terminate when set inspiratory time elapses.
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