Pacemaker Timing Cycles

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1 : Pacemaker Timing Cycles R.X. Stroobandt*, A.F. Sinnaeve**, S.S. Barold *** Department of Cardiology*, University Hospital Ghent, Belgium Technical University**, Ostend, Belgium University of South Florida***, College of Medicine and Tampa General Hospital, Tampa, Florida, USA
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3 : Atrial Events and IntervalsAbbreviations and Defintions of Terms
4 : Atrial Events and Intervals (1) Ap: Atrial paced event As: Atrial sensed event Ar: Atrial refractory sensed event AVI: Atrioventricular Interval The interval between an atrial event (either sensed or paced) and the scheduled delivery of a ventricular stimulus. pAVI: Paced Atrioventricular Interval The time from a paced atrial event to the succeeding ventricular paced event. sAVI: Sensed Atrioventricular Interval The time from a sensed atrial event to the succeeding ventricular paced event.
5 : Atrial Events and Intervals (3) PVAB: Postventricular Atrial Blanking First part of the PVARP initiated by a ventricular event during which the atrial sensing amplifier is turned-off in antibradycardia pacemakers. PVAB < PVARP. BPVAB: Brady-PVAB PVAB for the antibradycardia function of dual chamber defibrillators. This term is used only with ICDs to differentiate it from the tachy-PVAB as indicated below. TPVAB: Tachy-PVAB Short BP for the tachycardia function of dual chamber defibrillators that terminates before the BPVAB times out. It starts with either a paced or ventricular event. In some devices it starts only with a paced ventricular event. This term is used only with ICDs to differentiate it from the brady-PVAB above. TPVAB < BPVAB.
6 : Atrial Events and Intervals (2) AVE: AV Extension Increment in the sAVI during upper rate function of pacemakers with a Wenckebach blocking response based on the relationship: upper rate interval (URI) > total atrial refractory period (TARP) > spontaneous P–P interval. The maximum increment is equal to URI – TARP. AB: First portion of the AVI during which the detection of all signals is blocked. A paced event initiates the pAB and a sensed event initiates the sAB. An atrial event sensed in the refractory period (Ar) usually initiates the same sAB. AVI-U: Unblanked second part of the AVI after AB when P waves can be sensed and used in the mode switching algorithm, although they are unable to trigger a new ventricular stimulus.
7 : Atrial Events and Intervals (4) PVARP: Postventricular Atrial Refractory Period Interval after a ventricular paced or sensed event during which the atrial channel is refractory. PVARP = PVAB + PVARP-U In some pacemakers the duration of the PVARP (but not the PVAB) may vary according to sensor input or the atrial rate. In some AMS algorithms the PVARP duration is mandated by the programmed settings. The DDI(R) destination mode during AMS may not have the same PVARP as the DDD(R) mode before AMS. PVARP-U: Unblanked Postventricular Atrial Refractory Period Second part of the PVARP during which the atrial channel can sense but cannot initiate an AV delay.
8 : Atrial Events and Intervals (5) TAB: Total Atrial Blanking Period. AB + PVAB. For sensing regular SVT, the functional TAB = AVI + PVAB if AVI < atrial cycle when the AVI is partially blanked. TARP: Total Atrial Refractory Period TARP = AVI + PVARP The AVI may be pAVI, sAVI or AVE.
9 : Atrial Events and Intervals (6) OAI: Open Atrial Interval A sensing interval during which a sensed atrial event will trigger a ventricular stimulus according to the programmed AV delay. During the OAI, a pacemaker with atrial-based lower rate timing will reset the lower rate interval upon atrial sensing or pacing. The OAI starts at the end of PVARP and ends with the next paced or sensed atrial event unless interrupted by a ventricular sensed event that starts the PVARP. OAI = Lower rate interval – PVARP
10 : Ventricular Events and IntervalsAbbreviations and Defintions of Terms
11 : Ventricular Events and Intervals (1) Vp: Ventricular paced event Vs: Ventricular sensed event PAVB: Postatrial Ventricular Blanking Interval A brief interval initiated by an atrial output pulse when the ventricular sensing amplifier is switched off. It prevents AV crosstalk or sensing of the atrial stimulus by the ventricular channel. VB: Ventricular Blanking Period Free-standing or first portion of the ventricular refractory period initiated by a ventricular paced or sensed event during which the detection of all signals is blocked.
12 : Ventricular Events and Intervals (2) VRP: Ventricular refractory period VRP = VB + VRP-U VRP-U: Unblanked Ventricular Refractory Period Interval after VB when the ventricular channel can sense signals but cannot initiate a lower rate or atrial escape interval. Devices without a VRP-U such as defibrillators deal with oversensing of the QRS complex by auto adjusting sensitivity. OVI: Open ventricular interval A sensing interval during which a sensed ventricular event initiates the lower rate or atrial escape interval in devices with ventricular- based lower rate timing.
13 : Crosstalk Ideally, for optimal Automatic Mode Switching (AMS) function, the atrial channel of the pacemaker should operate at maximal sensitivity and look for atrial activity through the entire pacing cycle. However the atrial channel must be insensitive during certain parts of the pacing cycle to avoid two potential problems: 1. double (near-field) sensing of atrial activity 2. crosstalk which is the inappropriate detection by the channel of one chamber, of an event originating in the other chamber a phenomenon also known as far-field sensing (see Fig. 1).
14 : Crosstalk 1. AV Crosstalk AV crosstalk refers to the detection of atrial signals by the ventricular channel. 2. VA Crosstalk Ventriculoatrial (VA) or reverse crosstalk refers to far-field sensing that occurs when ventricular signals (usually from a paced ventricular beat) in the atrial electrogram are sensed by the atrial channel in the PVARP beyond the postventricular atrial blanking period (Fig. 1). VA crosstalk can also involve the spontaneous QRS complex within the terminal portion of the AVI.
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16 : Far-Field sensing during AV Interval by the atrial channel The spontaneous QRS complex generates an intracardiac electrogram capable of being sensed by the ventricular channel as a near-field signal and the atrial channel as a far-field event. Because of its high sensitivity (0.5 mV), the atrial channel senses the smaller far-field signal before the ventricular channel programmed with a lower sensitivity (4 mV) can sense the near-field signal (see Fig.3).
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18 : Brady- and Tachycardia Postventricular Atrial Blanking Period At pAVI = 200ms the paced atrial blanking period (pAB) occupies the entire AV interval (see Fig.4: Medtronic® GEM DR II ICD). There are 2 postventricular atrial blanking periods initiated simultaneously and coincidentally with release of the ventricular stimulus. - The 30ms period is for both the tachycardia and bradycardia functions and is labeled the tachycardia postventricular atrial blanking period (TPVAB). - The second one (150 ms) is only for the bradycardia pacing function (BPVAB). An atrial event can be sensed in the BPVAB (and used for tachycardia discrimination) and depicted as an Ar event but it will be ignored by the pacing component of the system. The ventricular channel shows a postatrial ventricular blanking period (PAVB) and a ventricular blanking period (VB). Note that there is a relative refractory period in the atrial channel (unblanked part of PVARP) but not in the ventricular channel.
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20 : Retriggerable Atrial Refractory Periods All the atrial events are detected by the atrial channel (see Fig. 5) outside the blanking periods which are not shown. There is an atrial tachycardia and the first atrial event is sensed (As). The next atrial signal (Ar) falls in the unblanked portion of the PVARP and is sensed. This signal does not start an AVI but it initiates a new total atrial refractory period (TARP). Continual retriggering of the TARP by the rapid atrial rate, causes operation of the pacemaker in the DVI mode at the lower rate interval (LRI). Note that the emission of an atrial stimulus also reinitiates the TARP.
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22 : Influence of Blanking Periods and AV intervalon Atrial Sensing during Atrial Tachycardia Panel A: The second and fifth P waves fall in the PVAB and are unsensed. sAVI + PVAB > P–P interval. A shorter PVARP would have caused 2:1 atrial sensing if sAVI PVARP < 2 atrial cycles (see Fig. 6) Panel B: All the P waves are detected when the sAVI is shortened to 50ms so that sAVI + PVAB < P–P interval. Panel C: All the P waves are sensed by prolonging the sAVI so that the sAVI > P–P interval when the sAVI is not totally blanked
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24 : Blanked Atrial Flutter Search Algorithm(Medtronic® Kappa 700) The algorithm (see Fig. 8) prolongs the PVARP for one beat (arrow) after 8 cycles whenever twice the P-P interval is less than twice (sAVI + PVAB). PVARP prolongation allows a P wave to occur within the unblanked part of the PVARP. This detected P wave in the PVARP-U, together with the succeeding and sensed P wave (outside the atrial refractory period) reveals the true P–P interval of the tachycardia whereupon AMS occurs. AVI bottom right refers to the AVI during DDI pacing but it is not initiated by As.
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