Pacemaker Timing and Intervals Objectives l l l
Pacemaker Timing and Intervals
Objectives: l l l Describe expected pacemaker function based on the NBG code Interpret intervals comprisingle and dual chamber timing Recognize various modes of dual chamber device operation from lower to upper rate behaviors Calculate upper rate behavior based on programmed parameters Identify therapy specific device operations when presented on patient ECG
Timing Intervals Are Expressed in Milliseconds l One millisecond = 1 / 1, 000 of a second
Converting Rates to Intervals and Vice Versa Rate to interval (ms): u 60, 000/rate (in bpm) = interval (in milliseconds) u Example: 60, 000/100 bpm = 600 milliseconds l Interval to rate (bpm): u 60, 000/interval (in milliseconds) = rate (bpm) u Example: 60, 000/500 ms = 120 bpm l
NBG Code Review I Chamber Paced II Chamber Sensed III Response to Sensing V: Ventricle T: Triggered P: Simple programmable P: Pace A: Atrium I: Inhibited M: Multiprogrammable S: Shock D: Dual (A+V) D: Dual (T+I) C: Communicating D: Dual (P+S) O: None R: Rate modulating O: None S: Single (A or V) O: None IV Programmable Functions/Rate Modulation O: None V Antitachy Function(s)
Single-Chamber Timing
Single Chamber Timing Terminology l l Lower rate Refractory period Blanking period Upper rate(in rate responsive mode)
Lower Rate Interval l Defines the lowest rate the pacemaker will pace Lower Rate Interval VP VP VVI / 60
Refractory Period Interval initiated by a paced or sensed event l Designed to prevent inhibition by cardiac or non-cardiac events l Lower Rate Interval VP Refractory Period VP VVI / 60
l l l During refractory periods, the pacemaker “sees” but is unresponsive to any signals. This is designed to avoid restarting the lower rate interval in the event of oversensing. T-wave oversensing in VVI and AAI modes will occur if refractory periods are too short. In the AAI mode, the pacemaker may even sense the QRS complex (“far-field R wave”) if the refractory period is not long enough. Events that fall into the refractory period are sensed by the pacemaker (the marker channel will display a “SR” denoting ventricular refractory or atrial refractory in single chamber systems) but the timing interval will remain unaffected by the sensed event. A refractory period is started by a non-refractory paced, or sensed event.
Blanking Period The first portion of the refractory period l Pacemaker is “blind” to any activity l Designed to prevent oversensing pacing stimulus l Lower Rate Interval VP Blanking Period Refractory Period VP VVI / 60
A paced or sensed event will initiate a blanking period. l Blanking is a method to prevent multiple detection of a single paced or sensed event by the sense amplifier (e. g. , the pacemaker detecting its own pacing stimuli or depolarization, either intrinsic or as a result of capture). l During this period, the pacemaker is "blind" to any electrical activity. l A typical blanking period duration in a singlechamber mode is 100 msec*. l
Upper Sensor Rate Interval Defines the shortest interval (highest rate) the pacemaker can pace as dictated by the sensor (AAIR, VVIR modes) l The upper sensor rate interval in single chamber pacing is available only in rate-responsive modes. The upper rate defines the limit at which sensor-driven pacing can occur. l Lower Rate Interval Upper Sensor Rate Interval VP Blanking Period Refractory Period VP VVIR / 60 / 120
Single Chamber Mode Examples
VOO Mode l Asynchronous pacing delivers output regardless of intrinsic activity Lower Rate Interval VP Blanking Period VOO / 60 VP
VOO mode paces in the ventricle but will not sense and, therefore, has no response to cardiac events. l Pacemakers programmed to the VVI, VVIR, and VDD modes will revert to VOO mode upon magnet application. l In this example, an intrinsic beat occurs, but it has no effect on the timing interval and another ventricular pace is delivered at the programmed rate. l No sensing occurs, thus, the entire lower rate interval is unresponsive to intrinsic activity. l
VVI Mode l Pacing inhibited with intrinsic activity l In inhibited modes (VVI/AAI), intrinsic events that occur before the lower rate interval expires will reset the lower rate interval, as shown in the example above. As with paced events, sensed events will also initiate blanking and refractory periods. Lower Rate Interval { VP Blanking/Refractory VVI / 60 VS VP
VVIR l Pacing at the sensor-indicated rate Lower Rate Upper Rate Interval (Maximum Sensor Rate) VP VP Refractory/Blanking VVIR / 60/120 Rate Responsive Pacing at the Upper Sensor Rate
Single chamber rate-responsive pacing is identical to non-rate responsive pacing operation, with the exception that the pacing rate is driven by a sensor. l The sensor determines whether or not a rate increase is indicated, and adjusts the rate accordingly. l The highest rate that the pacemaker is allowed to pace is the upper rate limit or interval. l In this example, the pacemaker is pacing at the maximum sensor indicated rate of 120 ppm. l
AAIR l Atrial-based pacing allows the normal A-V activation sequence to occur Lower Rate Interval Upper Rate Interval (maximum sensor rate) AP Refractory/Blanking AAIR / 60 / 120 (No Activity) AP
l l l Although this mode is seldom used , AAI/R pacing is a mode which, unlike VVI/R, allows for normal AV conduction to occur. AAI/R is not often used because of the risk of development of AV block which can occur over time. In this example, the patient received a single chamber device programmed to the AAIR pacemaker mode due to sick sinus syndrome and chronotropic incompetence. Presently the patient is at rest, so the sensor is at the programmed lower rate. An atrial event (paced or sensed) will initiate a refractory period including a blanking period. In AAI/R, the refractory period must be long enough so that the far-field R and T waves are ignored. Therefore, the refractory period must be longer in the AAI/R mode than in the VVI/R mode—typically 400 msec. Atrial events sensed during the refractory period in AAI/R are marked with an "SR" on the marker channel.
Other Single Chamber Operations
Hysteresis l Allows the rate to fall below the programmed lower rate following an intrinsic beat Lower Rate Interval-60 ppm VP VP Hysteresis Rate-50 ppm VS VP
Hysteresis allows the sensed intrinsic rate to decrease to a value below the programmed lower rate before pacing resumes. l Hysteresis provides the capability to maintain the patient's own heart rhythm as long as possible, while pacing at a faster rate if the intrinsic rhythm falls below the hysteresis rate. l The hysteresis rate is always < the lower rate limit. l The lower rate limit is initiated by a paced event, while the hysteresis rate is initiated by a non-refractory sensed event. l l In the example above, the lower rate limit is 60 ppm (1000 ms), while the hysteresis rate is 50 ppm (1200 ms). The patient is paced at 60 ppm until an intrinsic event occurs, and an interval of 1200 ms is started. This patient did not have another sensed event, so a ventricular pace was delivered. However, if another sensed event had occurred, the pacemaker would again have extended the interval to 1200 ms.
Noise Reversion l Continuous refractory sensing will cause pacing at the lower or sensor driven rate Lower Rate Interval Noise Sensed VP VVI/60 SR SR VP
The portion of the refractory period after the blanking period ends is commonly called the "noise sampling period. “ l This is because a sensed event in the noise sampling period will initiate a new refractory period and blanking period. l If events continue to be sensed within the noise sampling period causing a new refractory period each time, the pacemaker will asynchronously pace at the lower rate since the lower rate timer is not reset by events sensed during the refractory period. This behavior is known as "noise reversion. " l Note: In rate-responsive modes, noise reversion will cause pacing to occur at the sensor-driven rate. l
Benefits of Dual Chamber Pacing l l l Provides AV synchrony Lower incidence of atrial fibrillation Lower risk of systemic embolism and stroke Lower incidence of new congestive heart failure Lower mortality and higher survival rates
Dual-Chamber Timing
Benefits of Dual-Chamber Pacing Study Results Higano et al. 1990 Improved cardiac index during low level exercise (where most patient activity occurs) Gallik et al. 1994 Increase in LV filling Santini et al. 1991 30% increase in resting cardiac output Rosenqvist et al. 1991 Decrease in pulmonary wedge pressure Increase in resting cardiac output Sulke et al. 1992 Increase in resting cardiac output, especially in patients with poor LV function Decreased incidence of mitral and tricuspid valve regurgitation
Four “Faces” of Dual Chamber Pacing l Atrial Pace, Ventricular Pace (AP/VP) AV AP V-A VP Rate = 60 bpm / 1000 ms A-A = 1000 ms AV AP VP V-A
Four “Faces” of Dual Chamber Pacing l Atrial Pace, Ventricular Sense (AP/VS) AV AP V-A VS Rate = 60 ppm / 1000 ms A-A = 1000 ms AV AP VS V-A
Four “Faces” of Dual Chamber Pacing l Atrial Sense, Ventricular Pace (AS/ VP) AV AS VP V-A AV AS Rate (sinus driven) = 70 bpm / 857 ms A-A = 857 ms VP
Four “Faces” of Dual Chamber Pacing l Atrial Sense, Ventricular Sense (AS/VS) AV AS V-A VS Rate (sinus driven) = 70 bpm / 857 ms Spontaneous conduction at 150 ms A-A = 857 ms AV AS VS V-A
Dual Chamber Timing Parameters l l l Lower rate AV and VA intervals Upper rate intervals Refractory periods Blanking periods
Lower Rate l The lowest rate the pacemaker will pace the atrium in the absence of intrinsic atrial events Lower Rate Interval AP DDD 60 / 120 VP AP VP
In order to provide optimal hemodynamic benefit to the patient, dual-chamber pacemakers strive to mimic the normal heart rhythm. l In dual-chamber pacemakers, the lower rate is the rate at which the pacemaker will pace the atrium in the absence of intrinsic atrial activity. l Similar to single-chamber timing, the lower rate can be converted to a lower rate interval (A-A interval), or the longest period of time allowed between atrial events. l
AV Intervals l Initiated by a paced or non-refractory sensed atrial event u Separately programmable AV intervals – SAV /PAV Lower Rate Interval PAV 200 ms AP DDD 60 / 120 VP SAV 170 ms AS VP
l l l The SAV is usually programmed to a shorter duration than the PAV to allow for the difference in interatrial conduction time between intrinsic and paced atrial events. difference in the activation sequence between a cycle initiated with an intrinsic atrial event versus a paced atrial event. The cycle starting with the intrinsic atrial event will use the normal conduction pathways between the right atrium and the left atrium. The cycle starting with the paced atrial beat will not use the normal interatrial conduction pathways but will instead use muscle tissue, which takes a little longer to reach the left atrium and causing it to contract. If the AV interval is timed to allow the appropriate amount of time for left ventricular filling when the cycle is initiated with a sensed atrial event, the same duration for the PAV may not be the appropriate amount of time to allow for left ventricular filling when the cycle is initiated by a paced atrial event. Proper LA-LV timing promotes left ventricular filling ("atrial kick") and prevents regurgitant flow through an open mitral valve. Therefore, it is beneficial to have separately programmable PAV and SAV intervals. In this example, the lower rate interval is terminated by a sensed atrial event, which initiates a SAV interval (and restarts the lower rate interval).
Atrial Escape Interval (V-A Interval) l The interval initiated by a paced or sensed ventricular event to the next atrial event Lower Rate Interval 200 ms AV Interval AP DDD 60 / 120 PAV 200 ms; V-A 800 ms VP 800 ms VA Interval AP VP
Atrial escape interval (AEI)– V-A interval l l The A-V interval is employed to allow the appropriate amount of time to optimize ventricular filling and mimic the activation sequence of the normal heart. Knowing the lower rate interval and the PAV interval (AV interval after a paced atrial event), the V-A interval can be found: V-A interval = lower rate interval minus PAV interval. The V-A interval is the longest period that may elapse after a ventricular event before the atrium must be paced in the absence of atrial activity. The V-A interval is also commonly referred to as the atrial escape interval.
Upper Activity (Sensor) Rate l In rate responsive modes, the Upper Activity Rate provides the limit for sensor-indicated pacing Lower Rate Limit Upper Activity Rate Limit PAV DDDR 60 / 120 A-A = 500 ms AP VP V-A PAV AP VP V-A
This upper rate is defined as the upper activity rate, also known as the upper sensor rate or maximum sensor rate. l Before mode switching was available, pacemakers utilized a separate activity/sensor rate and upper tracking rate to limit the rate to which the patient could track (e. g. , in the presence of SVTs), but allow the patient to pace to higher rates if they were exercising. l
Upper Tracking Rate l The maximum rate the ventricle can be paced in response to sensed atrial events Lower Rate Interval { Upper Tracking Rate Limit SAV AS VA VP DDDR 60 / 100 (upper tracking rate) Sinus rate: 100 bpm SAV AS VP VA
l l The sequence of an atrial intrinsic event being sensed, starting an SAV interval, timing out the SAV interval, and pacing in the ventricle can be referred to as "tracking. " If the atrial rate begins to increase and continues to increase, it is not desirable to let the ventricle "track" to extremely high rates. to limit the rate at which the ventricle can pace in the presence of high atrial rates. This limit is called the upper tracking rate.
Refractory Periods l VRP and PVARP are initiated by sensed or paced ventricular events u The VRP is intended to prevent self-inhibition such as sensing of T-waves u The PVARP is intended primarily to prevent sensing of retrograde P waves A-V Interval (Atrial Refractory) Ventricular Refractory Period (VRP) AP Post Ventricular Atrial Refractory Period (PVARP) VP
l l l The Post-Ventricular Atrial Refractory Period (PVARP) is the period of time after a ventricular pace or sense when the atrial channel is in refractory. In other words, atrial senses outside of blanking that occur during this period are "seen" (and marked “AR) on the marker channel), but do not initiate an AV interval. The purpose of PVARP is to avoid allowing retrograde P waves, far-field R waves, or premature atrial contractions to start an AV interval which would cause the pacemaker to pace in the ventricle at a high rate. The refractory period after a ventricular event (paced or sensed) is designed to avoid restarting of the V-A interval due to a T wave. Ventricular sensed events occurring in the noise sampling portion of the ventricular refractory period are "seen" but will not restart the V-A interval. The atrial channel is refractory following a paced or sensed event during the AV interval. This allows atrial senses occurring in the AV interval to be "seen" but not restart another AV interval.
Blanking Periods l First portion of the refractory period-sensing is disabled AP AP VP Atrial Blanking (Nonprogrammable) Post Ventricular Atrial Blanking (PVAB) Post Atrial Ventricular Blanking (Nonprogrammable)
l DDD/R modes have four types of blanking periods: u A non-programmable atrial blanking period (varies from 50 -100 msec) is initiated each time the atrium paces or senses. u This is to avoid the atrial lead sensing its own pacing pulse or P wave (intrinsic or captured). In Thera and Kappa devices, this blanking period is dynamic, depending on the strength of the paced/sensed signal. u The PVAB-(Post-Ventricular Atrial Blanking Period) is initiated by a ventricular pace or sensed event (nominally set at 220 msec) to avoid the atrial lead sensing the far-field ventricular output pulse or R wave. u In dual-chamber timing, a non-programmable ventricular blanking period occurs after a ventricular paced or sensed event to avoid sensing the ventricular pacing pulse or the R wave (intrinsic or captured). This period is 50 -100 msec in duration and is dynamic, based on signal strength.
l l l There also is a ventricular blanking period after an atrial pacing pulse in order to avoid sensing the far-field atrial stimulus (crosstalk). This period is programmable (nominally set at 28 msec). This blanking period is relatively short because it is important not to miss ventricular events (e. g. , PVCs) that occur early in the AV interval. Ventricular blanking does not occur coincident with an atrial sensed event. This is because the intrinsic P wave is relatively small and will not be far-field sensed by the ventricular lead. A note of caution in programming long ventricular blanking periods after an atrial pace should be mentioned. If the ventricular blanking period after an atrial pace is excessively long, conducted ventricular events may go unsensed and cause the pacemaker to pace in the ventricle after the AV interval expires. This pace could occur before the ventricle has recovered from depolarization and may induce a ventricular arrhythmia (R on T phenomena).
Upper Rate Behavior
Upper Rate Behaviors – Wenckebach and 2: 1 Block Ventricular rate Upper rate Atrial tracking Lower rate Atrial rate
When the intrinsic atrial rate approaches (and exceeds) the programmed upper rate (assuming the TARP is less than the upper rate interval), pacemaker operations will change from 1: 1 tracking operations to blocking operations, which are designed to prevent tracking atrial arrhythmias which are too fast, and will likely cause patients to become symptomatic. l The jagged line represents Wenckebach operation, characterized by a lengthening of the A-V interval which occurs as the atrial rate exceeds the upper rate limit. If the atrial rate continues to increase, 2: 1 block will occur, which means that every other P wave will fall into refractory and will not be sensed. l The ventricular paced rate will typically be half the atrial rate. l
Total Atrial Refractory Period (TARP) Sum of the AV Interval and PVARP l The Total Atrial Refractory Period (TARP) is equal to the SAV interval plus the PVARP. The TARP is important to understand as it defines the highest rate that the pacemaker will track atrial events before 2: 1 block occurs. l Lower Rate Interval Upper Tracking Rate SAV = 200 ms PVARP = 300 ms Thus TARP = 500 ms (120 ppm) AS AS VP SAV PVARP TARP { DDD LR = 60 ppm (1000 ms) UTR = 100 bpm (600 ms) Sinus rate = 66 bpm (900 ms) P Waves Blocked VP SAV PVARP
Wenckebach Operation l Prolongs the SAV until upper rate limit expires u Produces gradual change in tracking rate ratio Lower Rate Interval { Upper Tracking Rate P Wave Blocked (unsensed or unused) AS AS AR VP VP PVARP SAV TARP DDD Sinus rate = 109 bpm (550 ms) SAV = 200 ms AP VP PAV PVARP TARP LR = 60 bpm (1000 ms) UTR = 100 ppm (600 ms) PAV = 230 ms PVARP = 300 ms
l l l Pacemaker Wenckebach has the characteristic Wenckebach pattern of the PR (AV) interval gradually extending beat-to-beat until an atrial event falls into the PVARP and cannot restart an AV interval. In effect, a ventricular beat is “dropped”. In this graphic, starting from the left side of the ECG, the pacemaker senses an atrial beat and starts an SAV. Because no ventricular event occurs by the end of the SAV, a ventricular pace is delivered. Now the pacemaker is looking for a sensed atrial beat. An atrial beat is sensed outside of the PVARP and starts an SAV. This time, when the SAV times out, the upper rate interval has not yet expired. Since the pacemaker can never violate the upper tracking rate, the ventricular pace has to be delayed until the end of the upper rate interval, at which time a ventricular pace is delivered. This pattern of sensing a P wave, starting an SAV, waiting for the upper rate interval to time out, and pacing in the ventricle repeats until a P wave falls into the PVARP and does not start an SAV. The amount of delay created by the time from the sensed P wave until the upper rate interval expires is a little longer each time, producing the gradually lengthening of the P wave to ventricular pace intervals. Once a P wave falls into the PVARP and does not initiate an SAV, the pacemaker looks for the next sensed P wave and the pattern starts all over again. This is how the classic Wenckebach pattern develops. The rate at which the pacemaker will exhibit Wenckebach behavior is at the upper tracking rate (or upper rate if the pacemaker does not have a separate upper tracking rate and upper activity rate).
Wenckebach Operation DDD / 60 / 120 / 310
2: 1 block Pacemaker 2: 1 block is characterized by two sensed P waves per paced QRS complex. This pattern develops because every other P wave falls into PVARP. l The rate at which the pacemaker will exhibit a 2: 1 block pattern is determined by the SAV and the PVARP (or the TARP). l Atrial rates with a P-P coupling interval shorter than the TARP will result in 2: 1 block. l To determine at what rate the pacemaker will go into 2: 1 block, the TARP is simply converted from an interval to a rate. Therefore, the rate the pacemaker will go into 2: 1 block is: 60, 000/TARP. l
2: 1 Block Every other P wave falls into refractory and does not restart the timing interval l Starting on the left side of this ECG, the sequence begins with a sensed P wave. This P wave initiates a SAV, followed by a paced ventricular event. l The next P wave falls into the PVARP, started by the ventricular pace, so no SAV is initiated. The following P wave is sensed outside of the PVARP, so a SAV is started. Again, no ventricular event occurs during the SAV, so the pacemaker paces in the ventricle. In this manner, a 2: 1 block pattern is created l Lower Rate Interval { Upper Tracking Limit AS AR VP { Sinus rate = 150 bpm (450 ms) PVARP = 300 ms SAV = 200 ms Tracked rate = 66 bpm (900 ms) AV PVARP TARP P Wave Blocked AS AR VP AV PVARP TARP
2: 1 Block DDD / 60 / 120 / 310
Wenckebach vs. 2: 1 Block If the upper tracking rate interval is longer than the TARP, the pacemaker will exhibit Wenckebach behavior first… l If the TARP is longer than the upper tracking rate interval, then 2: 1 block will occur l
Wenckebach vs. 2: 1 Block – What Will Happen First? What will the upper rate behavior of this pacemaker be? Lower rate = 60 ppm Upper tracking rate = 120 ppm PAV = 230 ms SAV = 200 ms PVARP = 350 ms
Wenckebach vs. 2: 1 Block – Solution Upper tracking rate = 120 ppm PVARP = 350 ms SAV = 200 ms Upper tracking rate interval = 60, 000/120 ppm = 500 ms l 2: 1 block interval = TARP = SAV + PVARP (200 ms + 350 ms = 550 ms) l TARP is greater than the upper tracking rate interval Thus, 2: 1 block will be in effect l
Wenckebach vs. 2: 1 Block – What Will Happen First? What will the upper rate behavior of this pacemaker be? Lower rate = 60 ppm Upper tracking rate = 110 ppm PAV = 150 ms SAV = 120 ms PVARP = 350 ms
Wenckebach vs. 2: 1 Block – Solution Upper tracking rate = 110 ppm PVARP = 350 ms SAV = 120 ms Upper tracking rate interval = 60, 000/110 = 545 ms l 2: 1 block interval = TARP = SAV + PVARP (120 ms + 350 ms = 470 ms) l Upper tracking rate interval is greater than the TARP Thus, Wenckebach will be in effect l
l Remember: u 1: 1 tracking occurs whenever the patient’s atrial rate is below the upper tracking rate limit (assuming the TARP is less than the upper tracking rate limit) u Wenckebach will occur when the atrial rate exceeds the upper tracking rate limit (and is longer than the TARP) u Atrial rates greater than TARP cause 2: 1 block
What Can We Do to Make Wenckebach Occur First? Going to 2: 1 block first without a Wenckebach period may not be the optimal situation because many patients do not tolerate a precipitous drop in ventricular rate well l Shorten or reduce the TARP by: u Shortening the PVARP u Shortening the SAV u Programming Rate Adaptive-AV (RA-AV) l
Rate-Adaptive AV (RA-AV) is a shortening of the A-V interval in the presence of an increased atrial rate—be it intrinsic or sensor driven l Pacemaker shortens AV intervals as atrial rates increase u Shortened SAV intervals increase the programmable tracking range u Shortened PAV intervals increase the programmable upper activity rate range u Both permit a longer atrial sensing window l
l l l Both the PAV and the SAV shorten with increasing rates. For the PAV, the adaptation is based on the sensor-indicated rate. For the SAV, the adaptation is based on the intrinsic atrial rate. RA-AV has three programming requirements, in addition to the SAV and the PAV (at lower rates): A start rate, which determines when RA-AV operation begins A minimum AV interval, which is the shortest allowable SAV or PAV value A stop rate, which determines the rate where the minimum SAV and PAV is reached
Rate-Adaptive AV 240 220 200 Programmed PAV 180 Programmed SAV AV Interval (ms) 160 Ra te A da p 140 da p tiv e 120 PA V SA V 100 80 Minimum PAV Minimum SAV 60 40 20 0 50 80 100 150 Rate (ppm) Start Rate Stop Rate 180
Rate-Adaptive AVI Mimics Intrinsic Response to Increasing Heart Rates l In the normal heart, AV conduction times shorten as heart rates increase; RA-AV mimics this physiologic response AS AR VP AV PVARP AV = 200 ms TARP = 500 ms PVARP = 300 ms Atrial rate = 450 ms (133 bpm) Without RA-AV 2: 1 block occurs AS VP AV PVARP AV = 100 ms TARP = 400 ms PVARP = 300 ms Atrial rate = 450 ms (133 bpm) 1: 1 tracking with RA-AV “on”
Wenckebach vs. 2: 1 Block – What Will Happen First? What will the upper rate behavior of this pacemaker be? Lower rate = 70 ppm Upper tracking rate = 130 ppm Upper activity rate = 130 ppm PAV = 180 ms SAV = 150 ms RA-AV = ON Start rate = 80 ppm Stop rate = 120 ppm Minimum SAV = 100 ms PVARP = 320 ms
Wenckebach vs. 2: 1 Block – Solution Upper tracking rate = 130 ppm SAV = 150 ms RA-AV = ON Start rate = 80 ppm Stop rate = 120 ppm Minimum SAV = 100 ms PVARP = 320 ms l Upper tracking rate interval = 60, 000/130 ppm = 462 ms l TARP = 100 ms + 320 ms = 420 ms l TARP is less than the upper tracking rate limit interval Thus, Wenckebach will be in effect
Rate Responsive Pacing l Pacing in DDDR mode can prevent precipitous drops in heart rate due to upper rate behavior Lower Rate Upper Activity (Sensor) Rate Limit AS VP AR AP SAV PVARP DDDR 60 / 120 upper activity rate Sensor-indicated rate = 100 ppm (600 ms) VP PAV PVARP
A-A Timing In A-A timing, if a conducted ventricular event occurs during the AV interval, the ventricular pace is inhibited but the A-A interval remains consistent and does not exhibit the same shortening in the presence of AV conduction that V-V timing does. The goal of A-A timing is to provide for consistent A-A intervals, regardless of ventricular conduction. A to A = 1000 ms A-A Timing AV = 200 V-A = 800 A to A = 1000 ms AV = 150 V-A = 850 Atrial rate = 60 ppm Ventricular rate = 63 ppm (first interval); 60 ppm AV = 200
V-V Timing In V-V timing, if a conducted ventricular event occurs during the AV interval, the ventricular pace is inhibited and a ventricular escape Interval (V-A interval) is immediately started. This effective shortening of the AV interval causes the entire V-V interval to be shortened. Therefore, it is possible to be pacing in the atrium in DDD mode and be at a rate slightly faster than the programmed lower rate. A to A = 1000 ms V-V Timing AV = 200 V-A = 800 A to A = 950 ms AV = 150 V-A = 800 Atrial rate = 60; 63 ppm Ventricular rate = 63; 60 ppm AV = 200
A-A vs. V-V timing A to A = 1000 ms A-A Timing AV = 200 V-A = 800 A to A = 1000 ms AV = 150 V-A = 850 AV = 200 Atrial rate is held constant at 60 ppm A to A = 1000 ms V-V Timing AV = 200 V-A = 800 A to A = 950 ms AV = 150 V-A = 800 Atrial rate varies with intrinsic ventricular conduction AV = 200
l l In this graphic, we can see the difference between A-A timing and V-V timing schemes. In V-V timing, if a conducted ventricular event occurs during the AV interval, the ventricular pace is inhibited and a ventricular escape Interval (V-A interval) is immediately started. This effective shortening of the AV interval causes the entire V-V Interval to be shortened. Therefore, it is possible to be pacing in the atrium in DDD mode and be at a rate slightly faster than the programmed lower rate. In A-A timing, if a conducted ventricular event occurs during the AV interval, the ventricular pace is inhibited but the A-A interval remains consistent and does not exhibit the same shortening in the presence of AV conduction that V-V timing does. The goal of A-A timing is to provide for consistent A-A intervals, regardless of ventricular conduction. A-A timing is most important at higher rates. Imagine a pacemaker programmed to an upper rate of 130 ppm (interval of approximately 460 msec). Now let's say that there is ventricular conduction and the difference between the programmed PAV and the ventricular conduction time is 30 msec. That means that if the pacemaker were operating under V-V timing rules, the entire V-V interval at the upper rate would be shortened by 30 msec–equating to a rate of 140 ppm! This is quite a difference from the intended programmed upper rate of 130 ppm. If the pacemaker were operating under A-A timing rules, the entire AV interval would time out regardless of ventricular conduction, maintaining the intended upper rate of 130 ppm.
Other Dual Chamber Modes
VDD l Provides atrial synchronous pacing u System utilizes a single pass lead Lower Rate Interval { Upper Tracking Limit AS AS VP VP VDD LR = 60 ppm UTR = 120 ppm Spontaneous A activity = 700 ms (85 ppm); followed by pause VP
l l l In the VDD mode, the pacemaker will pace only in the ventricle and will sense in both chambers. In response to sensing in the ventricle, the pacemaker will inhibit. If a P wave is sensed, an SAV will be triggered. There is no PAV in the VDD mode because the pacemaker will not pace in the atrium. Since the VDD mode does not have the capability to pace in the atrium, the pacemaker will operate as if in the VVI mode in the absence of atrial activity faster than the programmed lower rate. Therefore, this mode is only appropriate for patients with a normally functioning, chronotropically competent sinus node and second- or third-degree heart block. In this example, a P wave is sensed and initiates an SAV. Since no ventricular activity is sensed during the SAV, the pacemaker paces in the ventricle. The V-A interval is then initiated, followed by another sensed atrial and paced ventricular event. Following this VA interval, no atrial activity is sensed, and a ventricular pace is delivered at the end of the V-A interval (lower rate interval).
DDI/R l A non-tracking mode u Provides AV sequential pacing at lower or sensor indicated rate Lower Rate VA Interval Lower Rate Interval AP AP VP PAV PVARP DDI 60 AV = 200 ms PVARP = 300 ms AS VP PAV PVARP AP VP
This is an example of normal DDI/R operation. In the DDI/R mode, the pacemaker will pace in both chambers and sense in both chambers. l In response to sensing, the pacemaker will inhibit, but a sensed P wave will not trigger an AV Interval (therefore, there is no SAV Interval in the DDI/R mode). DDI/R pacing can be thought of as AAI/R with VVI/R backup. l In this example, since the atrium is paced, a PAV is initiated. Since no intrinsic ventricular activity occurs, a ventricular pace is delivered, and a V-A interval is initiated. l This cycle repeats itself. An intrinsic atrial event occurs. Since no SAV is initiated, the pacemaker is simply looking for any ventricular activity to occur in the escape interval-thus, the sensed atrial event is not tracked. The pacemaker finally delivers a ventricular pace as the V-A expires (at the programmed lower rate). l
Additional Device Therapies Issues and Solutions
Additional Device Therapies l l l l Ventricular Safety Pacing Mode Switching/DDIR with Sensor Varied PVARP PVC Response and PMT Intervention Non-Competitive Atrial Pace Rate Drop Response Sinus Preference Sleep Function
Issue: Crosstalk l Crosstalk is the sensing of a pacing stimulus delivered in the opposite chamber, which results in undesirable pacemaker response, e. g. , false inhibition DDD / 70 / 120
l l l Cross talk Crosstalk is a phenomenon that occurs when one chamber senses the output pulse of the other chamber. Crosstalk can become a problem when one chamber senses the output of the other chamber and is inhibited. If the ventricular chamber is inhibited by the atrial pacing pulse, as seen in the third complex above, the ventricular output is withheld. In this particular example, crosstalk inhibition is intermittent but the outcome could be disastrous if it occurred with every paced atrial beat. If the ventricular lead is "blanked" for an adequate period of time after the atrial pacing pulse to avoid seeing the atrial pacing pulse, crosstalk inhibition will not occur. Programmable ventricular blanking after an atrial pace is one method used to address the problem of crosstalk. Another solution is ventricular safety pacing.
Solution: Ventricular Safety Pacing l Following an atrial paced event, a ventricular safety pace interval is initiated u If a ventricular sense occurs during the safety pace window, a pacing pulse is delivered at an abbreviated interval (110 ms) PAV Interval Post Atrial Ventricular Blanking Ventricular Safety Pace Window
One method to manage crosstalk is to program Ventricular Safety Pacing (VSP) ON. If VSP is programmed ON, a ventricular safety pace window opens up for 110 msec after an atrial pace. The first portion of this window (about 28 msec) is blanked. l After the blanking period ends, if a ventricular event is sensed within 110 msec after the atrial pace, the pacemaker will pace at the end of the 110 msec window. l The logic here is that it is assumed that if a sensed event happens within 110 msec of an atrial pace, it may not have happened as a result of conduction to the ventricle (i. e. , it is not physiologic), and it may be crosstalk or noise. l Rather than inhibit the ventricular pace and risk having no ventricular support, the pacemaker will pace. By pacing at the end of 110 msec, if the event was truly physiologic, the pace will fall into the absolute refractory period of the ventricular muscle tissue. l
l l l Ventricular Safety Pacing is characterized by short (110 msec) AV intervals. On the marker channel, the VSP will be marked by two downward spikes–one for the ventricular sense and one for the ventricular pace. Ventricular Safety Pacing is designed to minimize the effects of cross-talk, but it can also occur under other circumstances. If a ventricular sensed event (e. g. , a PVC or a conducted ventricular event) falls within the first 110 msec after an atrial pace, the pacemaker may Ventricular Safety Pace. Also, if there is an atrial undersensing problem, ventricular safety pacing may be seen. This happens if a scheduled atrial pace is delivered shortly after this unsensed P wave. The scheduled atrial pace initiates a PAV. If the unsensed P wave conducts to the ventricle within the Ventricular Safety Pace window, a Ventricular Safety Pace will occur. Other names for Ventricular Safety Pacing are "non-physiologic AV delay" or "110 -msec phenomenon". When in effect, the AV interval will always be shortened.
Ventricular Safety Pace AP AP VP AV PVARP AP VS VP PVARP 110 ms VP AV PVARP
Ventricular Safety Pace Programmed parameters for this strip are: DDD; lower rate 60; upper rate 120; PAV 150 ms; SAV 150 ms. Ventricular Safety Pace (VSP) ON. VSP occurred due to a PVC falling in the AV interval. DDD 60 / 120
Other Methods for Managing Crosstalk Reduce atrial output (amplitude and/or pulse width) l Decrease (increase value) ventricular sensitivity l Program bipolar (if possible) l Increase the post -atrial ventricular blanking period l
Issue: Managing PSVTs l Patients with intermittent atrial arrhythmias may experience palpitations when episodes occur u In a tracking mode, high rate pacing will result DDD / 60 / 140
Some patients with dual-chamber pacemakers have intermittent paroxysmal supraventricular tachycardias (PSVTs) that are not desirable to track due to non-physiologic high rate pacing in the ventricle. l These patients have traditionally been managed by utilizing TARP (2: 1 block) and/or separately programmable upper rates (e. g. , upper tracking rate = 90 ppm, upper sensor rate = 120 ppm) to avoid tracking SVTs to excessively high rates in the ventricle. l Recently, the advent of mode switching has offered another alternative for the management of SVTs. l
Solution: Mode Switching l Indicated for patients with 3 rd degree heart block u Mode will switch from tracking mode (DDDR, DDD) to DDIR (non-tracking mode) when atrial arrhythmia is detected u Ventricular pacing is decoupled from atrial events, but rate responsive pacing is matched to metabolic needs
Mode Switch l The device detects an atrial arrhythmia by constantly comparing intervals with the programmed mode switch detection rate DDD / 60 / 120 Mode Switch ON
l l At the onset of an atrial arrhythmia, the pacemaker compares a mean atrial interval (which is a running index of the atrial rate) to the current A-A interval. If the A-A interval is shorter than the MAI, the MAI is shortened by 24 ms. If the A-A interval is longer than the MAI, the MAI is lengthened by 8 ms. When the MAI reaches the interval corresponding to the mode switch detection rate interval, the pacemaker switches from the DDDR mode to the DDIR mode.
DDIR with Sensor Varied PVARP Patients who have intact conduction are better served with the DDIR mode u Mode switch programmed ON will result in unnecessary ventricular pacing l Sensor Varied PVARP will vary the length of the PVARP based on the sensorindicated rate l
DDIR with Sensor Varied PVARP l l l Due to the shortening of the AV interval that is required in conjunction with Mode Switch, it is not optimal to use Mode Switch in patients with intact AV conduction. Using Mode Switch in these patients may force the ventricle to be paced (rather than conduct) which would almost universally be viewed as less hemodynamically effective. In addition to Mode Switch, another method of managing SVTs is to use the DDI/R mode. The DDI/R mode will pace in both the atrium and the ventricle, sense in both the atrium and the ventricle, and respond to sensing by inhibiting but not initiating an SAV. By using this mode, atrial arrhythmias are sensed but do not cause tracking to high pacing rates in the ventricle. In patients with intact AV conduction, whether the R waves are in a 1: 1 ratio with the P waves is a function of the AV node.
Sensor-Varied PVARP l PVARP will shorten as rate increases Long PVARP with little activity (Rate 63 ppm) Shorter PVARP with increased activity (Rate 86 ppm)
l l l In DDI/R with a fixed PVARP, as the sensor-indicated pacing rate increases, the atrial pacing output gets closer and closer to the PVARP and may even occur during PVARP. If an atrial sense occurs during the PVARP, it does not inhibit the scheduled atrial pace and competitive atrial pacing may ensue. If the scheduled atrial pace occurs before the atrium has recovered from depolarization, a pacemaker-induced atrial tachycardia may be initiated. Some pacemakers have a Sensor Varied PVARP (SV-PVARP) feature. SV-PVARP is intended to promote AV synchrony by preventing inhibition of atrial pacing by an atrial sense early in the V-A interval. It also reduces the likelihood of competitive atrial pacing at high sensor-indicated rates. SV PVARP creates a minimum 300 ms buffer period after the end of PVARP and before the next scheduled atrial pace in dual-chamber modes. At low rates, the SV-PVARP is limited to to 400 ms. At high rates, the PVARP can never be shorter than the programmed PVAB.
Issue: Pacemaker Mediated Tachycardia (PMT) l PMT is a paced rhythm, usually rapid, which is sustained by ventricular events conducted retrogradely (i. e. , backwards) to the atria l PMT can occur with loss of AV synchrony caused by: u PVC u Atrial non-capture u Atrial undersensing u Atrial oversensing
l l l Even patients who have complete antegrade block may have the ability to conduct retrograde. But having the ability to conduct retrograde is not enough. There must be a situation in which the conduction pathways have had a chance to recover when a ventricular contraction occurs. Basically, anything that causes a loss of AV synchrony may promote retrograde conduction and potentially a PMT. All of the above conditions (PVC, atrial noncapture, atrial undersensing, and atrial oversensing) cause a loss of AV synchrony and may promote a PMT.
PMT
PMT l l l A retrograde P wave occurs as a result of the PVC. This retrograde P wave is sensed outside of the PVARP and starts an SAV Interval. When the SAV Interval times out, if the Upper Tracking Rate has not yet expired so the SAV Interval is extended. A ventricular pace is delivered at the end of the upper tracking rate. The AV conduction pathways have recovered and the ventricular pace causes another retrograde P wave. The sequence continues resulting in a sustained Pacemaker Mediated Tachycardia (PMT).
Solution: PVC Response l A sensed ventricular event preceded by another ventricular event without an intervening atrial event is defined as a PVC u PVARP is extended to 400 ms Lower Rate Interval AV AV VA VA Restarts VA Interval PVC AV PVARP Retrograde P -Wave (unused) AV PVARP
One way to prevent sensing retrograde P waves when they happen due to a PVC is "PVC Response. " l pacemakers define a PVC as the second of any two consecutive ventricular events with no intervening atrial event. l When PVC Response is programmed ON, a pacemaker defined PVC starts an extended PVARP of 400 msec if the programmed PVARP is less than 400 msec. This extended PVARP allows retrograde P waves, should they occur, to fall within the refractory period and, therefore, does not initiate an SAV. l
PVC Response This ECG strip illustrates the PVARP extension of 400 ms following a PVC. DDD / 60 / 120 / 310
Solution: PMT Intervention l Designed to interrupt a Pacemaker. Mediated Tachycardia DDD / 60 / 120
l l l If a PMT is initiated, PMT Intervention may be able to stop the PMT cycle. If PMT Intervention is programmed ON, the pacemaker will monitor for a PMT by looking for eight consecutive VA Intervals that meet all of the following conditions: u Duration less than 400 msec u Start with a ventricular paced event u End with an atrial sensed event If PMT Intervention is ON and the above conditions are met, the PVARP will be forced to 400 msec after the ninth paced ventricular event. By extending the PVARP, the intent is to interrupt atrial tracking for one cycle and break the PMT. After an intervention, PMT Intervention is automatically suspended for 90 seconds before the pacemaker can monitor for a PMT again.
Issue: Atrial Arrhythmias Induced by Competitive Atrial Pacing If an atrial pace falls within the atrium's relative refractory period, an atrial tachycardia may be induced. l This can happen if a P wave falls during the PVARP (which will not inhibit the scheduled atrial pace) and then the scheduled atrial pace occurs shortly after the refractory sensed P wave and induces an atrial arrhythmia l
Solution: Non-Competitive Atrial Pacing (NCAP) l Refractory sensed atrial events initiate a 300 ms NCAP interval; no atrial pacing may occur within this window u PAV interval shortens to maintain a stable ventricular rate DDDR / 60 / 120 NCAP “ON”
Non competitive atrial pacing Non-Competitive Atrial Pacing (NCAP) can be used in an effort to prevent atrial pacing from occurring too close to a refractory sensed event. l If NCAP is programmed ON, the scheduled atrial pace will be delayed until at least 300 msec has elapsed since the refractory sensed P wave occurred. l The ensuing PAV may then be shortened to keep the ventricular rate from experiencing the same delay. l Note: The PAV may never be shorter than 80 ms. l
Issue: Neurocardiogenic Syncope Hypersensitive Carotid Sinus Syndrome l Vasovagal Syncope l
Solution: Rate Drop Response Therapy
l l l l Rate Drop Response therapy can be used for CSS and VVS patients for whom a permanent pacemaker is indicated. Rate Drop Response algorithms include three steps: (1) the pacemaker looks for a drop in heart rate (a detection cycle), (2) the pacemaker looks for that rate to remain low to confirm that it is indeed a rate drop episode (confirmation cycle), and (3) the pacemaker intervenes at a high pacing rate that is separate from the programmed Lower Rate or Upper Rate (intervention cycle). When an episodic drop in heart rate occurs and is detected and confirmed, Rate Drop Response (RDR) therapy provides an immediate increase in the pacing rate for a specified period, and then gradually slows pacing to resynchronize to the sinus rate. RDR makes it possible for the pacemaker to differentiate between episodic drops in heart rate and the slowing of the heart rate after exercise or circadian slow down as bedtime approaches.
Issue: Pacing in the Presence of Appropriate Sinus Rhythm l The best “sensor” for determining metabolic need and heart rate is a properly functioning sinus node
Solution: Sinus Preference l l l Sinus Preference is a programmable feature in the Kappa 400 devices. Sinus Preference proactively searches for sinus activity when sensor-driven pacing overrides the sinus node, and allows for Pwave tracking even when the sinus rate falls below the sensor rate. Sinus Preference is best suited for patients with Chronotropic Incompetence, with sinus rates that may occasionally exceed and/or lag closely behind the sensor-indicated rate during exercise. In this graphic, Sinus Preference shows how sinus activity is detected below the sensor rate during a Search Episode. A Sinus Preference Zone (programmable feature) is selected to determine if the intrinsic rate lags behind the sensor rate, while the Search Interval (also programmable) determines how often the search process will occur. After 8 consecutive paced atrial events at the lower limit of the Sinus Preference Zone, the paced rate will gradually increase until the sensor-indicated rate is reached.
Solution: Sinus Preference l Intrinsic atrial rhythms slower than the sensor-indicated rate can be tracked
Sinus Preference l l l This graphic illustrates Intrinsic Episode, which permits tracking of sinus rates that rise above, then drift below the sensor rate, provided they remain within a preselected range or zone. Termination is the same as with Search Episode. It should also be noted that there is a physiologic rate range above the sensor-indicated rate that is equal to the Sinus Preference Zone. Atrial events that are faster than this range will disable Sinus Preference until the next Search Interval occurs. In the event that this occurs, the rate will fall back to the sensor-indicated rate following termination of tracking of high atrial events. In addition to the benefit of utilizing natural heart rate reserves, programming Sinus Preference ON may increase device longevity by pacing less frequently.
Sinus Preference
Sleep Function The sleep function suspends the programmed lower rate and replaces it with a sleep rate (slower than the lower rate) during a specified sleep period. The slower pacing rate during the sleep period is intended to reduce the paced rhythm during sleep for patient comfort. 30 mins. Rate Lower Rate Sleep Rate Bed Time Wake Time
Summary l l l Review of NBG codes Single and dual chamber timing intervals Device operations from lower to upper rate behavior Calculate Wenckebach or 2: 1 block Therapy specific device operations
l. Thank you
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