The Cardiac Pacemaker - Using Event-B for Critical Device Software Systems - page 185

Fig. 9.7 The DDDR pacing scenarios

should be ignored. Sensed atrial activity is called a P wave, and sensed ventricular
activity is called a QRS complex. A T wave follows a QRS complex and represents
the recovery of the ventricles [ 57 ]. There are four possible scenarios for pacing and
sensing activities, which are given in Fig. 9.7 .
• Scenario A—shows a situation in which the pacemaker paces after a standard

time interval in both chambers. This is the reaction when no intrinsic heart activity
is detected.

• Scenario B—shows a situation in which the pacemaker paces in the atrial cham-

ber after a standard interval, while the ventricular pacing is inhibited due to a
sensing of intrinsic activity from the ventricle.

• Scenario C—shows a situation in which intrinsic atria activity is sensed, pacing

inhibited in the atrial chamber but occurs in the ventricular chamber after AVI
(due to a lack of intrinsic ventricular activity).