Gas flow pattern during mechanical insufflation

Gas flow during volume-controlled insufflation can be set as either constant or non-constant (e.g. accelerating, decelerating, or positive-phase sinusoidal). Depending on the design of the ventilator, the peak gas flow or the inspiratory time will change when the gas flow pattern is changed for a given tidal volume. During time-cycled ventilation, in which both tidal volume and inspiratory time are selected, the mean gas flow required to produce a constant pattern is computed by the microprocessor as

where Vmean is the mean gas flow in liters per second, VT is tidal volume in liters, and is the inspiratory time in seconds. Therefore, to preserve inspiratory time, peak gas flow will be higher when non-constant flow patterns are used. During volume-cycled ventilation, the peak gas flow is selected. Thus the inspiratory time required for insufflation varies with the mean gas rate and tidal volume:

Mean gas flow is less during non-constant flow than during constant flow. Therefore inspiratory time is longer when tidal volume is delivered by means of a non-constant gas flow.

Some ventilators are designed to permit gradual increases in inspiratory gas flow to a selected or computed final value during volume-controlled ventilation. The time during which the gas flow increases from the onset of mechanical insufflation to the final gas flow can be set from zero to 10 per cent of the total respiratory cycle. At zero per cent the mean gas flow computed by the electronic logic ( VTIJ\) is immediately generated and sustained throughout mechanical insufflation, thus producing a constant gas flow. As the rise time is increased to 10 per cent, the gas flow is accelerated to its final value.

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