Ellenbogen Pacing

ingICD06

12/17/04

06:16 PM

Page 265

Pacemaker Timing Cycles

David L. Hayes and Paul A. Levine

6

Understanding various pacing modes and paced electrocardiograms (ECGs)

requires a thorough understanding of pacemaker timing cycles. Pacemaker timing cycles include all potential variations of a single complete pacing cycle— the time from paced ventricular beat to paced ventricular beat; the time from paced ventricular beat to an intrinsic ventricular beat, whether it is a conducted R wave or a premature ventricular contraction (PVC); the time from paced atrial beat to paced atrial beat; the time from intrinsic atrial beat to paced atrial beat; the time from intrinsic ventricular beat to paced ventricular beat; and so forth. These cycles include events sensed, events paced, and periods when the sensing circuit or circuits are refractory. Each portion of the pacemaker timing cycle should be considered in milliseconds and not in pulses per minute (ppm). Although thinking of the patient’s pacing rate in paced beats per minute may be easier, portions of the timing cycle are too brief to consider in any unit but milliseconds. Knowledge of the relation between elements of the paced ECG enhances understanding of pacemaker rhythms. Although multiple unknown factors may affect a native rhythm, each timing circuit of a pacemaker can function in only one of two states. A given timer can proceed until it completes its cycle; completion results in either the release of a pacing stimulus or the initiation of another timing cycle. Alternatively, a given timer can be reset, at which point it starts the timing period again. To make this chapter more readable and to facilitate clarity, a series of abbreviations is used to designate native and paced events and portions of the timing cycle. These abbreviations are listed in Box 6.1. P indicates native atrial depolarization, A an atrial paced event, R native ventricular depolarization, and V a ventricular paced event. I...