10 things you need to know on 2:1 block during exercise Copy Copy

  1. the sum of the sensed AV delay and the PVARP defines the total atrial refractory period (TARP); all atrial events sensed outside the TARP are able to initiate an AV delay followed by ventricular pacing
  2. TARP is a very important timing cycle since it defines the total amount of time that the pacemaker will not respond to events on the atrial channel; the duration of the TARP determines when 2:1 multiblock occurs
  3. TARP is not a programmable setting as an independent value on the programmer screen but some programmers will however display the point at which 2:1 block is reached
  4. the rate at which 2:1 block occurs can be easily predicted by dividing TARP into 60,000; if the sensed AV delay is set to 150 ms and the PVARP to 280 ms, the TARP equals 430 ms and 2:1 block occurs at atrial rates of about 140 beats per minute
  5. when the cycle of the intrinsic atrial rate is shorter than the TARP, 2:1 AV block is installed, every other atrial event falls within the PVARP and is followed neither by an AV delay nor by ventricular pacing; ventricular tracking occurs on an alternate basis, one P wave falls within the TARP while the next P wave falls outside the TARP and is tracked; the AV delay always remains constant
  6. the ventricular pacing rate falls precipitously since 2:1 block cuts the pacing rate in half in one beat; the abrupt rate transition can be associated with very disabling symptoms
  7. the 2:1 point must be set as high as possible to enable the 1:1 tracking of the sinus P waves over the entire range of rates that might be observed in any given patient; the TARP duration(AV delay + PVARP) must, therefore, be shortened during exercise; raising the 2:1 block rate can be achieved by programming a rate adaptive AV delay (automatic shortening with faster ventricular rates); rate-adaptive or dynamic PVARP is also available in some devices
  8. in some patients with repetitive episodes of PMT, minimizing TARP may be difficult since PVARP has to be programmed longer than the retrograde conduction time that can sometimes exceed 350 or 400 ms
  9. when 2:1 block occurs, the decrease in heart rate may be mitigated by various rhythms stabilizing functions or smoothing
  10. it is usual to program a 2:1 atrial tracking rate above the maximal tracking rate (TARP < upper rate limite interval) to allow a zone of Wenckebach behavior before development of 2:1 block and to avoid precipitous fall in heart rate