The Cardiac Pacemaker - Using Event-B for Critical Device Software Systems - page 177

abnormal rate or rhythm or when normal conduction pathways are interrupted, and
a different part of the heart takes over control of the rhythm. An arrhythmia can
involve an abnormal rhythm increase (tachycardia; > 100 bpm) or decrease (brady-
cardia; < 60 bpm), or may be characterised by an irregular cardiac rhythm, e.g.
due to asynchrony of the cardiac chambers. The irregularity of the heartbeat, called
bradycardia and tachycardia. The bradycardia indicates that the heart rate falls be-
low the expected level while in tachycardia indicates that the heart rate goes above
the expected level of the heart rate. An artificial pacemaker can restore synchrony
between the atrial and ventricles. In an artificial pacemaker system, the firmware
controls the hardware such that an adequate heart rate is maintained, which is nec-
essary either because the heart's natural pacemaker is insufficiently fast or slow or
there is a block in the heart's electrical conduction system [ 4 , 14 , 22 , 35 , 37 , 39 ].
Beats per minute (bpm) is a basic unit to measure the rate of heart activity.
9.2.2 The Pacemaker System
The basic elements of the pacemaker system [ 4 , 14 ]are:
1. Leads : One or more flexible coiled metal wires, normally two, that transmit elec-
trical signals between the heart and the pacemaker. The same lead incorporate
sensors, which are able to detect the intrinsic heart activity.
2. The Pacemaker Generator : This is both the power source and the brain of the
artificial pacing and sensing systems. It contains an implanted battery and a con-
troller.
3. Device Controller-Monitor ( DCM ) or Programmer : An external unit that inter-
acts with the pacemaker device using a wireless connection. It consists of a hard-
ware platform and the pacemaker application software.
4. Accelerometer ( Rate Modulation Sensor ): An electromechanical device inside
the pacemaker that measures the body motion and acceleration of a body in or-
der to allow modulated pacing. In the rate adaptive mode, a cardiac pacemaker
automatically calculates the desire rate of the heart through the physical activities
of the patient [ 31 ]. The rate modulation sensor is used to capture these physical
activities and adjust the timing requirements for pacing.
The specification document [ 7 ] of our case study describes all possible operating
modes that are controlled by the different programmable parameters of the pace-
maker. All the programmable parameters are related to the real-time and action-
reaction constraints that are used to regulate the heart rate.
Figure 9.2 depicts a basic block diagram of the cardiac pacemaker and shows the
sensors and actuators that will be monitored and controlled in the design presented
in the remainder of this chapter.