CARDIAC PACING AND DEFIBRILLATION - Design and Development of Medical Electronic Instrumentation - page 378

4. Refractory period ([R Time Out])
5. Completely alert period ([A Time Out])
6. Go to 1.

cation of a real pacemaker's state machine because it does not
consider the following situations that occur in clinical practice:

This is an oversimpli fi

• Atrial tachycardia: an inappropriately high atrial rate caused by such a simpli fi ed

state machine
• Noise detected by the sense ampli

ers
• Need for atrial refractory of duration di

fi

ff erent from that of the ventricular refractory

As such, this state machine is presented for illustration purposes only. The description
of the complete state machine of a modern pacemaker is outside the scope of this topic.

PROGRAMMABLE PACEMAKERS

Modern pacemakers make it possible to program the various timeouts and pacing pulse
parameters. This allows therapy to be tuned to the speci

c requirements of the patient.
Possible programmable parameters for a microprocessor implementation of the simpli

fi

fi ed

dual-chamber state machine described above are:

Timeouts

• [A Time Out] Period during which the device is able to detect the heart's intrinsic

activity

• [R Time Out] Refractory period

Programmability of these parameters is accomplished by maintaining the programmed val-
ues in variables that can be handled by the device's firmware. Physicians are asked to enter

desired values for the minimal heart rate (in beats per minute) and the refractory period (in
milliseconds). The alert period is then calculated as

fi

60,000 (ms/min)
heart rate (beats/min)

[A Time Out](ms) refractory period (ms)

Pacing Stimulus Parameters

• Pacing pulse width: duration of the pacing pulse, can be implemented in the same
way as timeouts
• Pacing pulse amplitude: initial voltage of the pacing pulse; requires the hardware to
enable the

fi firmware to adjust the pacing voltage to the desired level

Sensing Parameters

• Atrial sensing sensitivity: threshold voltage level (in millivolts) that the atrial elec-
trogram signal must reach for the sense ampli fi er to report the occurrence of intrinsic

atrial activity as an atrial sense event
• Ventricular sensing sensitivity: same as above, but for the ventricle