# Pacemaker Timing Part I

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Pacemaker Timing Part I
Welcome to Pacemaker Timing, a course module in CorePace. The Pacemaker Timing module addresses single and dual chamber pacing operations, timing intervals, upper rate behavior and therapy-specific device operations.

Pacemaker Timing Welcome to Pacemaker Timing, a course module in CorePace. The Pacemaker Timing module addresses single and dual chamber pacing operations, timing intervals, upper rate behavior and therapy-specific device operations.

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

Timing Intervals Are Expressed in Milliseconds
One millisecond = 1 / 1,000 of a second Part of understanding timing intervals requires an acquaintance with milliseconds. Many Healthcare professionals are accustomed to measuring intervals in seconds. Timing intervals in pacing, however, are always measured in milliseconds. The exception to this is lower and upper rates, which are usually expressed in beats per minute/bpm. The graphic above shows intervals in milliseconds of a normal sinus beat. The entire graph represents 1000 milliseconds or one second of time. The smallest box on the ECG represents 40 milliseconds or .04 seconds. The medium box represents 200 milliseconds or .2 seconds.

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 The way to convert bpm to milliseconds is to divide the rate into 60,000 (the number of milliseconds in one minute). Converting an interval in milliseconds to a rate in bpm is done by dividing 60,000 by the millisecond interval.

NBG Code Review P: Simple programmable V: Ventricle V: Ventricle
II III IV V Chamber Chamber Response Programmable Antitachy 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 (A+V) D: Dual (T+I) C: Communicating D: Dual (P+S) O: None O: None O: None R: Rate modulating O: None S: Single (A or V) S: Single (A or V) O: None

Single-Chamber Timing

Single Chamber Timing Terminology
Lower rate Refractory period Blanking period Upper rate Single chamber timing has three components: Lower rate interval Refractory period Blanking period Single chamber devices that are programmed to a rate responsive mode add a fourth component, the upper rate interval.

Lower Rate Interval Defines the lowest rate the pacemaker will pace
The lower rate defines the lowest rate that the pacemaker will pace. For example, if the lower rate is programmed to 60 ppm in the VVI mode, the pacemaker is required to pace at a rate of 60 ppm if the patient's intrinsic ventricular rate is less than 60 bpm. A paced or non-refractory sensed event restarts the rate timer at the programmed rate. VP VVI / 60

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

Blanking Period The first portion of the refractory period
Pacemaker is “blind” to any activity Designed to prevent oversensing pacing stimulus Lower Rate Interval 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*. Note: In Thera and Kappa devices, nonprogrammable blanking parameters are dynamic (ranging from ms) depending on the strength/duration of the paced or sensed signal. VP VVI / 60 Blanking Period Refractory Period

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

Single Chamber Mode Examples

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

{ VVI Mode Pacing inhibited with intrinsic activity
Lower Rate Interval 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. VP VS VP Blanking/Refractory VVI / 60

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

AAIR Atrial-based pacing allows the normal A-V activation sequence to occur Lower Rate Interval Upper Rate Interval (maximum sensor rate) Although this mode is seldom used (particularly in the USA) , AAI/R pacing is a mode which, unlike VVI/R, allows for normal AV conduction to occur. Single-chamber, atrial inhibited pacing is selected only for those patients in whom the bradyarrhythmia is a sinus mechanism and AV block is not a problem.1 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. As previously stated, 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. Moses HW et al. A Practical Guide to Cardiac Pacing. 4th ed. Boston: Little, Brown and Company, Page 91. AP Refractory/Blanking AAIR / 60 / 120 (No Activity)

Other Single Chamber Operations

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

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

Dual-Chamber Timing

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 Studies have been done that demonstrate the differences in outcome, hemodynamic improvement, and quality of life assessment by using AV synchronous, or "atrial-based," pacing modes instead of VVI/R. Some of the benefits of using an atrial-based pacing mode include: AV synchrony–Clinical benefits such as increased cardiac output, augmentation of ventricular filling (especially important for the majority of the pacing population with LVD and reduced compliance from effects of aging). Providing AV synchrony minimizes valvular regurgitation, and preserves atrial electrical stability. In the Framingham Study, the development of chronic AF was associated with a doubling of overall mortality and of mortality from cardiovascular disease (Kannel, 1982) The following emphasize the importance of preventing atrial fibrillation: Patients with AF unrelated to rheumatic or prosthetic valvular disease have a risk of ischemic stroke about five times higher than those with normal sinus rhythm. AF is associated with over 75,000 cases of stroke per year. See bibliography for listing of studies cited.

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 level exercise (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, especially in patients with poor LV function Decreased incidence of mitral and tricuspid valve regurgitation Included is a summary of some studies depicting long-term results of AV synchronous (atrial based) and non-synchronous (VVI/R) pacing. Higano, et al. Hemodynamic importance of atrioventricular synchrony during low levels of exercise. PACE, 1990; 13:509 Abstract. Gallik DM, et al. Comparison of ventricular function in atrial rate adaptive versus dual chamber rate adaptive pacing during exercise. PACE, 1994; 17(2): Santini, et al. New Perspectives in Cardiac Pacing. Mount Kisco, NY: Futura Publishing, 1991. Rosenquist M, et al. Relative importance of activation sequence compared to atrioventricular synchrony during low levels of exercise. AM J Cardiology, 1991;67: Sulke N, et al. “Subclinical pacemaker syndrome: A randomized study of symptom free patients with ventricular demand (VVI) pacemakers upgraded to dual chamber devices. Brit Heart J, 1992; 67(1):57-64.

Four “Faces” of Dual Chamber Pacing
Atrial Pace, Ventricular Pace (AP/VP) AV V-A AV V-A Knowing the basic A-V and V-A intervals will help in understanding the four modes or “faces” of dual chamber pacing. In the first example, the pacemaker is pacing in both the atrium and the ventricle–most likely a patient with sinus node dysfunction and AV block. AP VP Rate = 60 bpm / 1000 ms A-A = 1000 ms

Four “Faces” of Dual Chamber Pacing
Atrial Pace, Ventricular Sense (AP/VS) AP VS V-A AV In this example, the atrium is being paced, but AV conduction is intact, so the ventricular output is inhibited by a sensed ventricular event. Rate = 60 ppm / 1000 ms A-A = 1000 ms

Four “Faces” of Dual Chamber Pacing
Atrial Sense, Ventricular Pace (AS/ VP) V-A AV In this example, the atrial rate is driving the ventricular rate–also called atrial tracking. This patient has adequate sinus node function with AV block. AS AS VP VP Rate (sinus driven) = 70 bpm / 857 ms A-A = 857 ms

Four “Faces” of Dual Chamber Pacing
Atrial Sense, Ventricular Sense (AS/VS) V-A AV AS VS In this example, the patient has adequate sinus node function and intact AV conduction, but may experience little to no increase in sinus rate with activity and/or AV block that occurs at increased rates. At appropriate rates, it is best to try and utilize the patient’s intrinsic rhythm when possible. Rate (sinus driven) = 70 bpm / 857 ms Spontaneous conduction at 150 ms A-A = 857 ms

Dual Chamber Timing Parameters
Lower rate AV and VA intervals Upper rate intervals Refractory periods Blanking periods Dual-chamber pacing requires attention to these parameters: Lower rate AV and V-A intervals Upper rates Refractory periods Blanking periods

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

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

Atrial Escape Interval (V-A Interval)
Lower rate interval – AV interval 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.

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

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 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. DDDR 60 / 120 A-A = 500 ms AP VP

DDDR 60 / 100 (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 VA SAV VA 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, is it desirable to let the ventricle "track" to extremely high rates? No. It is desirable 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. AS VP DDDR 60 / 100 (upper tracking rate) Sinus rate: 100 bpm

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 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" (and marked “VR” on the marker channel) 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 . AP A-V Interval (Atrial Refractory) Post Ventricular Atrial Refractory Period (PVARP) VP Ventricular Refractory Period (VRP)

Blanking Periods First portion of the refractory period-sensing is disabled AP AP VP Atrial Blanking (Nonprogrammable) Post Ventricular Atrial Blanking (PVAB) DDD/R modes have four types of blanking periods: A non-programmable atrial blanking period (varies from 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 msec in duration and is dynamic, based on signal strength. 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. The issue of ventricular safety pacing and cross-talk will be addressed later on in the presentation. 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). Ventricular Blanking (Nonprogrammable) Post Atrial Ventricular Blanking

General Medtronic Pacemaker Disclaimer
INDICATIONS Medtronic pacemakers are indicated for rate adaptive pacing in patients who may benefit from increased pacing rates concurrent with increases in activity (Thera, Thera-i, Prodigy, Preva and Medtronic.Kappa 700 Series) or increases in activity and/or minute ventilation (Medtronic.Kappa 400 Series). Medtronic pacemakers are also indicated for dual chamber and atrial tracking modes in patients who may benefit from maintenance of AV synchrony. Dual chamber modes are specifically indicated for treatment of conduction disorders that require restoration of both rate and AV synchrony, which include various degrees of AV block to maintain the atrial contribution to cardiac output and VVI intolerance (e.g., pacemaker syndrome) in the presence of persistent sinus rhythm. 9790 Programmer The Medtronic 9790 Programmers are portable, microprocessor based instruments used to program Medtronic implantable devices. 9462 The Model 9462 Remote Assistant™ is intended for use in combination with a Medtronic implantable pacemaker with Remote Assistant diagnostic capabilities. CONTRAINDICATIONS Medtronic pacemakers are contraindicated for the following applications: ·       Dual chamber atrial pacing in patients with chronic refractory atrial tachyarrhythmias. ·       Asynchronous pacing in the presence (or likelihood) of competitive paced and intrinsic rhythms. ·       Unipolar pacing for patients with an implanted cardioverter-defibrillator because it may cause unwanted delivery or inhibition of ICD therapy. ·       Medtronic.Kappa 400 Series pacemakers are contraindicated for use with epicardial leads and with abdominal implantation. WARNINGS/PRECAUTIONS Pacemaker patients should avoid sources of magnetic resonance imaging, diathermy, high sources of radiation, electrosurgical cautery, external defibrillation, lithotripsy, and radiofrequency ablation to avoid electrical reset of the device, inappropriate sensing and/or therapy. Operation of the Model 9462 Remote Assistant™ Cardiac Monitor near sources of electromagnetic interference, such as cellular phones, computer monitors, etc. may adversely affect the performance of this device. See the appropriate technical manual for detailed information regarding indications, contraindications, warnings, and precautions.  Caution: Federal law (U.S.A.) restricts this device to sale by or on the order of a physician.

Medtronic Leads For Indications, Contraindications, Warnings, and Precautions for Medtronic Leads, please refer to the appropriate Leads Technical Manual or call your local Medtronic Representative. Caution: Federal law restricts this device to sale by or on the order of a Physician. Note: This presentation is provided for general educational purposes only and should not be considered the exclusive source for this type of information. At all times, it is the professional responsibility of the practitioner to exercise independent clinical judgment in a particular situation.

Continued in Pacemaker Timing Parts II and III