CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 381

EXTERNAL VVI PACEMAKER

Figure 8.9 shows a simple implementation of a VVI pacemaker which will be presented as
an example of the techniques involved in the design of pacemakers. The device is repre-
sentative of a simple external pacemaker and is switch-programmable as follows:

• Mode: VVI, VOO
• Rate: 40, 50, 55, 60, 65, 70, 80, 90, 100, 110, 120, 140 beats/min
• Refractory period: 250, 350 ms
• Pacing pulse amplitude: battery voltage (e.g., 3 V) and two times battery voltage
(e.g., 6 V)
• Pacing pulse width: 0.125 to 1.5 ms in 0.125-ms steps
• Ventricular sensing sensitivity: 1.0 to 6.0 mV in 1.0-mV steps

Although functional, the design is given only to illustrate the basic implementation of
a pacemaker's circuits and state machines. The design does not include a number of impor-
tant features found in modern temporary pacemakers, such as:

• Low battery detection
• “Runaway” detection (which provides protection against clock speedup which can
result in pacing rates beyond 180 ppm)
• Regulated power supply
• Protection against EMI, electrosurgery, and de fi brillation

As shown in Figure 8.10, the state machines that make up this pacemaker's logic run on
a Microchip PIC16C76 microcontroller. This device spends the vast majority of time in the
“sleep” mode and is awaken only after timeout of its counters or when a ventricular sense
event is communicated via a high logic state on the circuit's input line, RB0. The timer is
driven through a crystal-based oscillator running at 32.768 kHz. However, a 4-MHz RC
clock is used to run the CPU (not the timer) once the microcontroller wakes up. The RC
oscillator is preferable over a crystal oscillator because the former starts oscillation and
stabilizes much more quickly once the microcontroller is awaken.

Microprocessor

V+

V+

Stimulus
Generator

PIC
16C 76

Sensing

V- V-

Sensitivity

Other
Parameters

Figure 8.9 Block diagram of a simple external (temporary) VVI pacemaker circuit. Operating
parameters are programmed through switches.