Figure 1-5. Mark 2 servo.
(a) When pressure in chamber B overcomes spring tension, valve
number 2 opens, venting chamber A. The tension of the spring closes valve number 1
and terminates gas flow from the outlet, ending the inspiratory phase. As this occurs,
valve number 3 opens due to the loss of gas pressure from valve number 1 to its
diaphragm, and allows chamber B to vent to the atmosphere, which causes valve
number 2 to close.
(b) Adjustment of the expiratory time control determines the length of
time required to fill chamber A, or the expiratory time. Adjustment of the inspiratory time
control sets the length of time required to fill chamber B, or the inspiratory time. To
summarize the function of the Mark 2 servo, it is on in inhalation of CIMV and it is off in
exhalation of CIMV.
(5) Inspiratory-interrupter switch. A fourth outlet from the inlet manifold
delivers source gas to the inlet of the inspiratory-interrupter switch (A in figure 1-6),
which consists of a spring-loaded plunger and diaphragm within a closed housing.
During the expiratory phase of controlled ventilation and during spontaneous breathing,
the interrupter switch performs two functions when source gas flow through: the first is
to load the diaphragms of the compound-lockout cartridge and the second is to power
the jet of the expiratory venturi located in the breathing circuit.