PV Delay and Pacing Modes

The PV Delay

  • The PV Delay interval is a timing concept important for understanding certain exam questions.
  • Also known as P Tracking Interval and PV Interval.
  • PV Delay can be a fixed or variable value.
  • Modes that use the PV Interval: DDD(R), VDD(R), DDT(R), and VAT.
  • DDT and VAT modes are rare and won't be discussed.
  • The PV Delay interval begins when the atrial output escape interval (AEI) is interrupted by a sensed event on the atrial channel.
  • A sensed event, not just a P wave, triggers the PV Delay. Far-field ventricular events and noise can be sensed and start the PV Delay.
  • When an event is detected, the atrial channel becomes refractory, and after a specific duration (PV Delay), the ventricular output is generated.
  • An R wave or any perceived ventricular event will stop the PV interval and inhibit the ventricular output pulse.
  • On the exam, focus on the relationship between the P wave and the ventricular output spike.
  • The P to V interval should not lengthen as the rate increases.
  • Progressive lengthening of the PV Interval may indicate pacemaker Wenkebach.
  • Pacemaker Wenkebach is a NORMAL function in dual-chamber pacing. It happens when the sensed P rate is faster than the programmed maximum tracking rate (or upper rate interval).
  • In DDD pacing, P tracking occurs from rates slightly above the programmed base rate to the upper tracking rate limit.
  • Rates at or below the base rate should appear as AV or AR pacing.
  • VDD Timing Consideration
    • Unlike DDD timing, VDD mode has NO atrial output pulse.
    • If a rhythm strip shows P tracking above the base rate but also has episodes resembling VVI (non-P Synchronous) at low rates, suspect VDD mode.
  • In DDD mode, a ventricular output cannot occur unless an atrial event (sensed or paced) precedes it.
  • In VDD pacing, if the atrial rate drops below the base rate, the mode will appear to be VVI.
  • VDD pacing idiosyncrasy: below base rate ventricular pacing behavior when the P rate is very close to the programmed base rate.
    • Example: Device programmed to 60 ppm (1000 ms) with an AV delay of 200 ms, and the sinus rate is 61 bpm (984 ms).
    • The V - V interval will be 1184 ms (approximately 51 ppm).
    • This occurs because the AV Interval starts when the P wave is sensed, delaying the ventricular output.
    • V−V interval=AV Delay+Sinus RateV-V \ interval= AV\ Delay + Sinus\ Rate where AV DelayAV\ Delay and Sinus RateSinus\ Rate are expressed in milliseconds.
    • V−V interval=200 ms+984 ms=1184 msV-V \ interval= 200\ ms + 984\ ms = 1184\ ms
  • Below base rate ventricular pacing is NORMAL in VDD pacing.
  • Ventricular pacing may occur at or above the programmed rate.