DDD pacemakers: Basic Functions
VSP
ventricular safety pacing *does not prevent crosstalk but does prevent V inhibition *causes a characteristic shortening of AV interval *only occurs after paced atrial event *normally 100-110ms and includes PAVB
Prevention of Crosstalk
*reduce atrial output *reduce ventricular sensitivity *use bipolar leads *prolong the PAVB (normally 10-60ms) *turn VSP ON!
4 Fundamental Timing Cycles of DDD pacer
1. LRI: longest interval b/t paced or sensed V event and succeeding V paced event w/o intervening sensed events 2. VRP: interval initiated by V event during which a new LRI cannot be initiated
PVAB
1. if too SHORT: you get over sensing of atrial stim on vent channel and 'self-inhibition' 2. if too LONG: you get under sensing which can miss a PVC in the PVAB leading to stim on T wave - DANGEROUS
To test for crosstalk
1. set lower rate above patient's own rate 2. shorten AV delay to < PR interval 3. program atrial output to max 4. program vent sensitivity to most sensitive **this sequence will help unmask problems as the large atrial spike may be sensed by the ventricular channel and cause crosstalk (initiating VSP or inhibition if no VSP is programmed)
AEI: Atrial Escape Interval
AEI = LRI - AVI should be constant in pacers that use ventricular based lower rate timing
4 Fundamental Timing Cycles of DDD pacer: AVI
Atrioventricular interval: interval b/t an atrial event and the scheduled delivery of a ventricular stimulus * electronic analog of PR interval
PVARP
Interval after a V paced or sensed event during which an atrial event cannot initiate a new AVI *avoids inappropriate atrial sensing of V events *eliminates sensing of retrograde P waves from V-A conduction
PVAB
Postatrial Ventricular Blanking *removes influence of atrial stim on ventricular channel *no PVAB after atrial SENSING *usually 10-60ms
AV Crosstalk
Sensing of atrial stim by vent channel *can lead to ventricular asystole if no underlying rhythm *can lead to inhibition of v stim causing a longer AVI (sensed)
TARP: total atrial refractory period
TARP = AVI + PVARP * if P rate is such that p waves fall into the TARP then every other beat will be non-sensed. So....TARP effectively becomes the upper rate interval
