Initiation of mechanical insufflation

A mechanical breath is initiated (triggered) either automatically by the machine or in synchrony with spontaneous inspiration by the patient. In the first option, a timing mechanism automatically initiates a mandatory breath at intervals independent of spontaneous breathing. A control device regulates the number of mechanical breaths occurring per minute, and this rate establishes the duration of the ventilatory cycle. Alternatively, a mechanical breath is triggered when a gas flow or pressure sensor signals the electronic logic indicating that spontaneous inspiration has occurred.

Gas flow triggering system

The inspiratory valve(s) generate a selected baseline gas flow (usually 5-20 l/min) through the patient circuit. The baseline gas flow vents to ambient via the expiratory valve and is quantified by a sensor. The microprocessor calculates the disparity between baseline gas flow and gas flow exhausted from the patient circuit. The disparity in gas flow necessary to initiate a mechanical breath may be set from 1 l/min to 50 per cent of the baseline rate.

Pressure triggering system

The sensing site for the pressure signal feedback to the microprocessor is near the gas outflow port within the ventilator chassis, or the interface between patient circuit and artificial airway, or the exhalation valve. Regardless of sensing site, the gas pressure transducer is inside the ventilator chassis and the 'pneumatic' signal travels through the sampling tubing at the speed of sound (0.33 m/s at sea level). During pressure-triggered events, the inspiratory and expiratory valves are closed; thus a nearly equivalent pneumatic signal is transmitted regardless of sensor location. Trigger delay includes the time between the start of inspiratory effort and detection of effort by electronic logic and the time from detection of inspiratory effort to initiation of gas flow by the inspiratory valve(s).

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