so that the pressure is applied to each of the transducers 70 72 74 which can be calibrated to read 34 0 34 when the midchest pressure is applied fig9 illustrates the construction of one of the valves of the apparatus the particular valve 30 being the arterial interface valve the valve 30 includes a stationary member 90 and a pair of movable members 92 94 pivotably mounted about the axis 96 on the stationary member 90 the stationary member has two through holes 98 100 each of which can be aligned with holes 102 104 on the pivotal members 92 94 in order to move the valve 30 to the conditions shown in fig3 the pivotal member 92 is pivoted so its hole is at position 102a wherein it is aligned with the through hole 100 in the stationary member also the other pivotal member 94 is left in a position where its hole 104 is aligned with the hole 98 this allows fluid to flow from port 28 to port 40 or from port 32 to port 36 the pivotal members 92 94 are pivoted by air operated actuators not shown which are fed air pulses from circuitry in a circuit portion 18 on the apparatus of fig1 operation of a corresponding button on the control panel 16 will cause all of the appropriate valves to shift to the required position so that a clinician does not have to separately operate many valves fig3 shows a hydraulic circuit 18a of the circuit portion of the apparatus which is connected by air lines 108 112 to various valves to properly position them for the first arterial sample step however in case of power failure each of the valves can be.