Figure 1-9. Outflow valve.
(b) The outflow valve provides peak inspiratory pressure limiting during
controlled IMV. When inspiratory venturi output pressure is directed to top of the outflow
valve diaphragm, a secondary closing force is developed: the venturi opposes the flow
of expelled cases from the patient circuit, so increased pressure against the outflow
valve diaphragm will increase the pressure of the expelled gases. Peak inspiratory
pressure limiting is adjusted by the inspiratory pressure relief control, normally between
10 and 60mmHg. The airBird and shuttle valve provide for backup manually-controlled
(7) Overpressure governor. The overpressure governor shown in figure
1-10 prevents the development of breathing circuit pressures in excess of 30cmH2O.
Overpressures are vented to the atmosphere through the governor. A mechanical alarm
is built into the governor. The alarm is a pneumatically-operated split reed which is
allowed to float off its seat as overpressures are vented to the atmosphere when the flow
setting on the reed is exceeded. As the reed vibrates, a tone sounds.
Figure 1-10. Overpressure governor.