13
Figure 7. A simulated pumping mechanism in a CADD‑Prizm
®
PCS II pump.
Lock Button
Latch Button
Pressure Plate
Inlet Valve
Outlet Valve
Pump Tubing
Cassette Hinge
Occlusion Sensor
Expulsor
Pump Housing
Camshaft
Motor
Pumping Mechanism
The pumping mechanism is linear peristaltic
with two active valves. Pumping occurs when
the expulsor presses on the reservoir pump
tubing in sequence with the inlet and outlet
valves. At rest, the outlet valve is pressing down
fully on the tubing and the expulsor and inlet
valve are retracted. (See Figure 7.)
When the microprocessor commands the
mechanism to pump, the camshaft begins to
rotate, thus controlling the following pump cycle:
1. The inlet valve closes.
2. In synchrony with the expulsor moving down
to compress the tubing, the outlet valve
opens, expelling 0.050 ml of fluid.
3. The outlet valve closes.
4. The inlet valve opens as the expulsor is
retracted, causing fluid from the reservoir to
again fill the pump tubing segment.
5. The camshaft rotation stops after half a
revolution and the cycle is completed.
Pumping Characteristics
If the fluid path to the patient becomes blocked,
the pump tubing will expand as pumping
occurs. When there has been an amount of
inflation corresponding to 124 ± 62 kPa
(1.24 ± 0.62 bar, 18 ± 9 psi), the occlusion analog
sensor trips, whereupon the microprocessor
stops the pump mechanism and issues visual
and audible alarms. Thus the maximum
pressure which can be developed is 186 kPa
(1.86 bar, 27 psi).
To deliver the amount of drug specified by the
parameter settings, the pump’s microprocessor
causes the pump mechanism to deliver 0.05 ml
fluid “pulses” timed according to the desired
rate. At rates higher than 3 ml/hr, 2 pulses in
succession will be given. Thus, to deliver 20
ml/hr, for example, the microprocessor solves
these equations:
Mechanism activations per hr:
= 20 ml per hr/0.1 ml per activation
= 20/0.1
= 200
Time (seconds) between activations:
= 3600 sec per hr/number of activations per hr
= 3600/200
= 18
The microprocessor uses its timer circuits to
accurately time the 18 seconds (in this example)
between mechanism activations. The timebase
accuracy is ultimately determined by the 3.6864
MHz quartz crystal oscillator.
Rate
Volume
(ml/hr)
Resolution (ml)
Cassette or
0 ‑ 3
0.050
Admin Set
3.1 ‑ 125
0.100
