The Cardiac Pacemaker - Using Event-B for Critical Device Software Systems - page 178

Fig. 9.2 Cardiac pacemaker sensors and actuators

Ta b l e 9 . 1 Bradycardia operating modes of pacemaker system

Category Chambers paced Chambers sensed Response to sensing Rate modulation

Letters O —None O —None O —None R —Rate modulation

A —Atrium A —Atrium T —Triggered

V —Ventricle V —Ventricle I —Inhibited

D —Dual (A + V) D —Dual (A + V) D —Dual (T + I)

9.2.3 Bradycardia Operating Modes

In order to understand the language of pacing, it is necessary to comprehend the
coding system that produced by a combined working party of the North American
Society of Pacing and Electrophysiology (NASPE) and the British Pacing and Elec-
trophysiology Group (BPEG) known as NASPE/BPEG generic (NBG) pacemaker
code [ 15 ]. This is a code of five letters of which the first three are most often used.
The code provides a description of the pacemaker pacing and sensing functions. The
sequence is referred to as bradycardia operating modes (see Table 9.1 ). In practice,
only the first three or four-letter positions are commonly used to describe brady-
cardia pacing functions. The first letter of the code indicates which chambers are
being paced; the second letter indicates which chambers are being sensed; the third
letter of the code indicates the response to sensing and the final letter, which is
optional indicates the presence of rate modulation in response to the physical ac-
tivity measured by the accelerometer. An accelerometer is an additional sensor in
the pacemaker system that detects a physiological result of exercise or emotion, and
increases the pacemaker rate on the basis of a programmable algorithm. “X” is a
wildcard used to denote any letter (i.e. “O”, “A”, “V” or “D”). Triggered (T ) refers
to deliver a pacing stimulus and Inhibited (I ) refers to an inhibition from further
pacing after sensing of an intrinsic activity from the heart chambers.