Andersons pediatric cardiology 564

variations of a single complete pacing cycle In each mode a pacing output is delivered at the end of the escape interval that will correspond with the programmed lower rate limit If there is a sensed event, the pacing output is inhibited and the escape interval is reset The escape interval is the longest time between any two atrial or ventricular events To sense and pace the same chamber, refractory periods must be considered Directly following a paced event, will be a blanking period of about 50 to 100 ms During the blanking period, no event is sensed This feature is designed to prevent oversensing of the afterpotentials Following the blanking period is the refractory period The generator can sense the event, but this event will not reset the timing cycle Blanking and refractory periods help to decrease the risk of oversensing afterpotentials, such as near-field T-wave oversensing, far-field atrial or ventricular activity, myopotentials, electromagnetic interference, lead noise from a possible insulation defect, or header connection problems Oversensing happens when signals other than the local cardiac activation at the lead electrode site is counted as a cardiac signal FIG 22.36 (A) Tracing showing a dual chamber pacemaker with appropriate atrial tracking (AS-VP) (B) Tracing demonstrating a dualchamber pacemaker with atrioventricular sequential pacing (AP-VP) Mode switch is designed to prevent undesirable rapid ventricular pacing caused by tracking atrial arrhythmias during DDD operation, but it can add to the complexity of interpretation If atrial arrhythmias are detected, the result may be reversion to a nontracking mode (DDI or VVI) Undersensing occurs when the device fails to sense an intrinsic event and therefore delivers a paced beat Programmable Features Knowing that default settings are designed for the adult population, a detailed understanding of all the possible programmable features is paramount when programming the device for a pediatric patient Programming the lower rate limit determines the lowest acceptable rate before a pacing stimulus will occur If rate modulation is programmed, the rate is determined by sensor activity If there is an attempt to minimize ventricular pacing, the hysteresis rate can be programmed Hysteresis allows for a longer escape interval from the last ventricular sensed event to the first ventricular paced event Programming a hysteresis rate permits a lower intrinsic heart rate before pacing stimulation For example, if the device is programmed with a lower rate limit of 60 beats/min (technically the device is programmed at ppm not beats per minute) and a hysteresis rate of 50 beats/min, the device will not pace as long as the intrinsic rate is above 50 beats/min However, if the rate drops below 50 beats/min, the device will pace at the programmed lower rate limit of 60 beats/min Another option to discourage pacing is by programming the sleep mode Because cardiac demand during sleep is less than the chronotropic need while awake, some devices have a sleep mode option Sleep mode has the ability to determine a programmed window of time that will decrease the lower rate limit to the programmed rate during those hours For example, if the lower rate limit is set for 60 beats/min, one can program nighttime hours to pace at 50 beats/min The AV interval (AVI) is a programmed interval that determines the maximum time after an atrial event in which a ventricular event can happen before delivering a ventricular paced stimulus Programming of the pacemaker AVI optimizes hemodynamics by permitting coordinated atrial and ventricular contractions that mimic the intrinsic PR interval The challenge is to achieve a short enough AVI to promotes AV synchrony and a long enough AVI to allow time for intrinsic activation to occur The postventricular atrial refractory period (PVARP) is a programmable parameter that occurs after a sensed or paced ventricular event in which atrial sensed events do not result in a corresponding AV interval for the programmed time period PVARP settings can be helpful in preventing sensing of retrograde P waves after a ventricular paced or sensed event Without this programmed feature, the atrial channel may sense a retrograde atrial signal as an intrinsic event and stimulate a ventricular paced event that also conducts retrograde This recurring cycle is known as pacemaker-mediated tachycardia (PMT) If PMT is present on interrogation of the patient's device, you will commonly find the pacemaker pacing at or below the MTR To prevent PMT, the PVARP is often programmed to be longer than the retrograde conduction time Extending the PVARP too long can lead to an absence of atrial tracking at rates lower than the upper rate limit (Fig 22.37) FIG 22.37 Pacemaker-mediated tachycardia is an endless-loop tachycardia that is sustained by the pacemaker and requires retrograde ventriculoatrial activation Extension of the postventricular atrial refractory period places the atrial beat in refractory mode and successfully terminates the tachycardia The PVARP period prevents sensing retrograde P waves and/or tracking farfield ventricular repolarization on the atrial channel  ... understanding of all the possible programmable features is paramount when programming the device for a pediatric patient Programming the lower rate limit determines the lowest acceptable rate before a pacing stimulus will occur