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Pacemaker Timing and Intervals

Pacemaker Timing and Intervals. Objectives:. Describe expected pacemaker function based on the NBG code Interpret intervals comprising single and dual chamber timing Recognize various modes of dual chamber device operation from lower to upper rate behaviors

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Pacemaker Timing and Intervals

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  1. Pacemaker Timing and Intervals

  2. Objectives: • Describe expected pacemaker function based on the NBG code • Interpret intervals comprising single 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

  3. Timing Intervals Are Expressed in Milliseconds • One millisecond = 1 / 1,000 of a second

  4. Converting Rates to Intervals and Vice Versa • Rate to interval (ms): • 60,000/rate (in bpm) = interval (in milliseconds) • Example: 60,000/100 bpm = 600 milliseconds • Interval to rate (bpm): • 60,000/interval (in milliseconds) = rate (bpm) • Example: 60,000/500 ms = 120 bpm

  5. NBG Code Review I II III IV V Programmable Antitachy Chamber Chamber Response Paced Sensed to Sensing Functions/Rate Function(s) Modulation P: Simple programmable V: Ventricle V: Ventricle T: Triggered P: Pace M: Multi- programmable A: Atrium A: Atrium I: Inhibited S: Shock D: Dual (A+V) D: Dual (T+I) D: Dual (P+S) D: Dual (A+V) C: Communicating O: None O: None O: None O: None R: Rate modulating S: Single (A or V) S: Single (A or V) O: None

  6. Single-Chamber Timing

  7. Single Chamber Timing Terminology • Lower rate • Refractory period • Blanking period • Upper rate(in rate responsive mode)

  8. VP VP Lower Rate Interval • Defines the lowest rate the pacemaker will pace Lower Rate Interval VVI / 60

  9. VP VP Refractory Period Refractory Period • Interval initiated by a paced or sensed event • Designed to prevent inhibition by cardiac or non-cardiac events Lower Rate Interval VVI / 60

  10. 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.

  11. VP VP Blanking Period • The first portion of the refractory period • Pacemaker is “blind” to any activity • Designed to prevent oversensing pacing stimulus Lower Rate Interval VVI / 60 Blanking Period Refractory Period

  12. A paced or sensed event will initiate a blanking period. • 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). • During this period, the pacemaker is "blind" to any electrical activity. • A typical blanking period duration in a single-chamber mode is 100 msec*.

  13. VP VP Upper Sensor Rate Interval • Defines the shortest interval (highest rate) the pacemaker can pace as dictated by the sensor (AAIR, VVIR modes) • 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. Lower Rate Interval Upper Sensor Rate Interval VVIR / 60 / 120 Blanking Period Refractory Period

  14. Single Chamber Mode Examples

  15. VP VP Blanking Period VOO Mode • Asynchronous pacing delivers output regardless of intrinsic activity Lower Rate Interval VOO / 60

  16. VOO mode paces in the ventricle but will not sense and, therefore, has no response to cardiac events. • Pacemakers programmed to the VVI, VVIR, and VDD modes will revert to VOO mode upon magnet application. • 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. • No sensing occurs, thus, the entire lower rate interval is unresponsive to intrinsic activity.

  17. VP VS VVI Mode • Pacing inhibited with intrinsic activity • 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

  18. VP VP VVIR • Pacing at the sensor-indicated rate Lower Rate Upper Rate Interval (Maximum Sensor Rate) Refractory/Blanking VVIR / 60/120 Rate Responsive Pacing at the Upper Sensor Rate

  19. 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. • The sensor determines whether or not a rate increase is indicated, and adjusts the rate accordingly. • The highest rate that the pacemaker is allowed to pace is the upper rate limit or interval. • In this example, the pacemaker is pacing at the maximum sensor indicated rate of 120 ppm.

  20. AP AP AAIR • Atrial-based pacing allows the normal A-V activation sequence to occur Lower Rate Interval Upper Rate Interval (maximum sensor rate) Refractory/Blanking AAIR / 60 / 120 (No Activity)

  21. 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.

  22. Other Single Chamber Operations

  23. Hysteresis • Allows the rate to fall below the programmed lower rate following an intrinsic beat Lower Rate Interval-60 ppm Hysteresis Rate-50 ppm VP VP VP VS

  24. Hysteresis allows the sensed intrinsic rate to decrease to a value below the programmed lower rate before pacing resumes. • 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. • The hysteresis rate is always < the lower rate limit. • The lower rate limit is initiated by a paced event, while the hysteresis rate is initiated by a non-refractory sensed event. • 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.

  25. Noise Reversion • Continuous refractory sensing will cause pacing at the lower or sensor driven rate Lower Rate Interval Noise Sensed SR SR SR SR VP VP VVI/60

  26. The portion of the refractory period after the blanking period ends is commonly called the "noise sampling period.“ • This is because a sensed event in the noise sampling period will initiate a new refractory period and blanking period. • 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." • Note: In rate-responsive modes, noise reversion will cause pacing to occur at the sensor-driven rate.

  27. Benefits of Dual Chamber Pacing • 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

  28. Dual-Chamber Timing

  29. Benefits of Dual-Chamber Pacing Study Results Higano et al. 1990 Gallik et al. 1994 Santini et al. 1991 Rosenqvist et al. 1991 Sulke et al. 1992 Improved cardiac index during low levelexercise (where most patient activity occurs) Increase in LV filling 30% increase in resting cardiac output Decrease in pulmonary wedge pressure Increase in resting cardiac output Increase in resting cardiac output, especiallyin patients with poor LV function Decreased incidence of mitral and tricuspidvalve regurgitation

  30. AP AP VP VP Four “Faces” of Dual Chamber Pacing • Atrial Pace, Ventricular Pace (AP/VP) V-A AV AV V-A Rate = 60 bpm / 1000 ms A-A = 1000 ms

  31. AV V-A AV V-A AP AP VS VS Four “Faces” of Dual Chamber Pacing • Atrial Pace, Ventricular Sense (AP/VS) Rate = 60 ppm / 1000 ms A-A = 1000 ms

  32. V-A AV V-A AV Four “Faces” of Dual Chamber Pacing • Atrial Sense, Ventricular Pace (AS/ VP) AS AS VP VP Rate (sinus driven) = 70 bpm / 857 ms A-A = 857 ms

  33. AV V-A AV V-A AS AS VS VS Four “Faces” of Dual Chamber Pacing • Atrial Sense, Ventricular Sense (AS/VS) Rate (sinus driven) = 70 bpm / 857 ms Spontaneous conduction at 150 ms A-A = 857 ms

  34. Dual Chamber Timing Parameters • Lower rate • AV and VA intervals • Upper rate intervals • Refractory periods • Blanking periods

  35. Lower Rate • The lowest rate the pacemaker will pace the atrium in the absence of intrinsic atrial events Lower Rate Interval AP AP VP VP DDD 60 / 120

  36. In order to provide optimal hemodynamic benefit to the patient, dual-chamber pacemakers strive to mimic the normal heart rhythm. • 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. • 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.

  37. SAV PAV 200 ms 170 ms AP AS VP VP AV Intervals • Initiated by a paced or non-refractory sensed atrial event • Separately programmable AV intervals – SAV /PAV Lower Rate Interval DDD 60 / 120

  38. 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 the lower rate interval).

  39. AP AP VP VP Atrial Escape Interval (V-A Interval) • The interval initiated by a paced or sensed ventricular event to the next atrial event Lower Rate Interval 800 ms 200 ms VA Interval AV Interval DDD 60 / 120 PAV 200 ms; V-A 800 ms

  40. Atrial escape interval (AEI)– V-A interval • 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 (A-V 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.

  41. AP AP VP VP Upper Activity (Sensor) Rate • In rate responsive modes, the Upper Activity Rate provides the limit for sensor-indicated pacing Lower Rate Limit Upper Activity Rate Limit PAV V-A PAV V-A DDDR 60 / 120 A-A = 500 ms

  42. This upper rate is defined as the upper activity rate, also known as the upper sensor rate or maximum sensor rate. • 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.

  43. AS AS VP VP Upper Tracking Rate • The maximum rate the ventricle can be paced in response to sensed atrial events { Lower Rate Interval Upper Tracking Rate Limit SAV SAV VA VA DDDR 60 / 100 (upper tracking rate) Sinus rate: 100 bpm

  44. 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.

  45. Refractory Periods • VRP and PVARP are initiated by sensed or paced ventricular events • The VRP is intended to prevent self-inhibition such as sensing of T-waves • The PVARP is intended primarily to prevent sensing of retrograde P waves AP A-V Interval (Atrial Refractory) Post Ventricular Atrial Refractory Period (PVARP) VP Ventricular Refractory Period (VRP)

  46. 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 .

  47. Blanking Periods • First portion of the refractory period-sensing is disabled AP AP VP Atrial Blanking (Nonprogrammable) Post Ventricular Atrial Blanking (PVAB) Ventricular Blanking (Nonprogrammable) Post Atrial Ventricular Blanking

  48. DDD/R modes have four types of blanking periods: • A non-programmable atrial blanking period (varies from 50-100 msec) is initiated each time the atrium paces or senses. • 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. • 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. • 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.

  49. 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).

  50. Upper Rate Behavior

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