Postventricular Atrial Blanking

Fig. 9. Schematic of a lithium iodide battery. This is the most common chemistry used in modern pacemakers.

Fig. 9. Schematic of a lithium iodide battery. This is the most common chemistry used in modern pacemakers.

Pacemaker Failure

Fig. 10. Cutaway view of an implantable pulse generator (IPG, or pacemaker).

4.4. North American Society of Pacing and Electrophysiology and British Pacing and Electrophysiology Group Codes

To describe the function of a pacing system in a standardized manner, the North American Society of Pacing and Electrophysiology and British Pacing and Electrophysiology Group have developed a standard coding system (NASPE/BPEG code) (12). This code describes the pacing system's functionality using a multiletter designation. The first four letters are typically used, although this practice is evolving as new pacing features and indications are under development. In the four-letter code, the first letter indicates the pacing activity (A, atrial pacing; V, ventricular pacing; D, dual-chamber pacing; O, no pacing); the second indicates sensing (A, atrial sensing; V, ventricular sensing; D, dual-chamber sensing; O, no sensing); the third indicates the reaction to a sensed event (I, inhibit pacing; T, trigger pacing; D, inhibit and trigger; O, no reaction to sensing); and the fourth is used to describe unique device functionality (R, rate responsive, for example). Thus, a VVIR system would pace the ventricles (Vā€”), sense ventricular activity (-Vā€”), inhibit (i.e., withhold pacing) on detection of a sensed event in the ventricle (ā€”1-), and provide rate response to manage chronotropic incompetence (ā€”R). See Table 3 for a more complete explanation of the coding system.

Fig. 10. Cutaway view of an implantable pulse generator (IPG, or pacemaker).

4.5. Implantable Pulse Generators

The IPG is an implantable computer with an integral pulse generator and battery. The componentry is typically encased within a hermetically sealed stamped titanium housing with the battery taking up approximately half of the device volume. The most common battery chemistry used in modern pacemakers is lithium iodide. Device longevity is typically 8 to 10 years, but may vary significantly depending on system utilization (Fig. 9).

Electrically insulated feedthroughs connect the internal circuitry to an external connector block, which acts as the interface between the internal circuitry of the IPG and the leads. Typically, the connector block consists of a molded polyurethane superstructure housing metallic contacts. The contacts may be simple machined blocks or "spring-type" metallic beams. The connector block often has set screws to ensure permanent retention of the leads, and these may also enhance electrical contact. A cutaway view of an IPG can be found in Fig. 10, and the

Fig. 11. Schematic of the implantable pulse generator-to-lead interface. The IS-1 connector is the standard configuration for pacing. The DF-1 connector is the standard configuration for high-voltage defibrillation. (See connectorPlugln.mpg on the Companion CD.)
Electrogram
Fig. 12. The electrogram amplification and rectification scheme that is used in most modern implantable pacing and defibrillation systems. EGM, electrogram.
Amplification Plot

Fig. 13. Plot of electrical signals (amplitude and frequency) frequently encountered by pacing and defibrillation sensing algorithms. A bandpass filter for preferential detection of P waves and R waves is shown (parabolic line). This filter is designed to "reject" myopotentials and T waves.

Fig. 13. Plot of electrical signals (amplitude and frequency) frequently encountered by pacing and defibrillation sensing algorithms. A bandpass filter for preferential detection of P waves and R waves is shown (parabolic line). This filter is designed to "reject" myopotentials and T waves.

scheme for connection between the IPG and the leads is shown in Fig. 11.

4.6. Sensing Algorithms

To assess the need for therapeutic intervention, the pacing system must be able to detect and interpret the electrical activity of the heart accurately. The electrical activity of the heart, or electrogram (EGM), is recorded as a differential voltage measured between the bipolar electrode pair on the lead (bipolar lead) or between the cathode on the lead and the housing of the IPG (unipolar lead). This signal is then processed within the IPG and analyzed by the sensing algorithms. Typically, such signals are amplified, filtered, and rectified prior to undergoing analysis by the device (Figs. 12 and 13).

The resulting signal is then passed through a level detector to determine if it exceeds the minimum threshold for detection that was preprogrammed into the device by the clinician. The sensitivity setting, in millivolts, determines what is discarded as noise by the algorithm and which signals will be detected.

An ideal sensitivity setting is one that will reliably detect the event of interest (P wave in the atrium, R wave in the ventricle) and ignore physiological and nonphysiological signals.

Most rhythm management decisions are based on the HR detected. The modern IPG continuously measures the time from one sensed event to the next and compares the interval to the rates and intervals programmed by the clinician. For example, if two atrial events occur with a separation of 1500 ms (1.5 s), the HR is 40 beats/min (HR = 60/measured beat-to-beat interval; 60/1.5 = 40 beats/min).

To understand the logic behind sensing algorithms and pacing timing diagrams, specific terminology needs to be introduced. Table 4 includes the most commonly used terms and abbreviations. These terms are freely used in further discussions of the logic behind pacing and defibrillation sensing and therapies without further explanation. This table also provides the reader with the vocabulary required for interpreting and understanding current literature and publications on the topic.

Table 4

Pacing and Timing Abbreviations

AP Atrial pace

AS Atrial sense

AR Atrial refractory event

AEI Atrial escape interval: longest allowable interval between ventricular and atrial event (also called VA interval)

ARP Atrial refractory period

AV Atrioventricular

AV interval Longest allowable interval between atrial and ventricular event

LR Lower rate: slowest pacing rate allowed

LR interval Longest period of time allowed before delivery of a pacing stimulus

MS Mode switch

PAV Paced atrioventricular interval: longest allowable interval between paced atrial beat and paced or sensed ventricular beat

PMT Pacemaker-mediated tachycardia

PVAB Postventricular atrial blanking period

PVARP Postventricular atrial refractory period

SAV Sensed atrioventricular interval: longest allowable interval between sensed atrial beat and paced or sensed ventricular beat

TARP Total atrial refractory period (AV + PVARP)

UAR Upper activity rate (also called maximum sensor-indicated rate)

UR Upper rate: fastest pacing rate allowed

UR interval Shortest allowable interval between paced beats or a sensed and paced beat

UTR Upper tracking rate: fastest rate the ventricles may be paced in 1:1 synchrony with the sensed atrial rate (also called maximum tracking rate)

VA interval Time between ventricular and atrial event

VP Ventricular pace

VR Ventricular refractory event

VRP Ventricular refractory period

VS Ventricular sense

VSP Ventricular safety pacing

Post-Alrial Ven tri ciliar Blanking

Post-Alrial Ven tri ciliar Blanking

Fig. 14. A typical dual-chamber timing diagram, including subdiagrams for the atrial and ventricular channels. The sequence of events begins with a paced atrial beat P. This paced beat occurs when the maximum allowable interval between sensed atrial events is exceeded. For example, if the minimum rate is programmed to 60 beats/min, an atrial pace will occur when a 1000-ms interval between sensed events is exceeded. Immediately following this pacing pulse, both the atrial and ventricular sensing algorithms are blanked. This means that the threshold detector ignores all sensed activity within that period. The system is blanked to avoid sensing the resultant atrial depolarization on the atrial channel and the atrial pacing spike and the atrial depolarization on the ventricular channel. Concurrently, in the atrium a paced atrioventricular (PAV) interval occurs. This is the longest interval that will be allowed by the device without a paced ventricular beat. The PAV is commonly programmed to 150 ms and is set to optimize filling of the ventricle caused by the atrial contraction. During a cardiac cycle, if the PAV value is reached (meaning an intrinsic ventricular beat does not occur within the programmed interval following the paced atrial beat), a ventricular pacing pulse is then delivered. This pacing pulse is again accompanied by blanking in both channels to avoid oversensing of the pacing pulse and the resultant ventricular depolarization. This interval is referred to as the postventricular atrial blanking (PVAB) period on the atrial channel. Concurrently, the postventricular atrial refractory period (PVARP) occurs on the atrial channel in which the device attempts to avoid sensing of retrograde P waves (i.e., atrial contractions conducted through the atrioventricular node in a retrograde manner), and far-field R waves. The ventricular refractory period (VRP) occurs on the ventricular channel to avoid oversensing of T waves. Following these intervals, the timing is repeated. If the atrial rate stays above the minimum programmed rate (the lower rate) and the SAV is never reached, the device will never pace unless inappropriate sensing occurs.

The decision processes and behaviors of the typical pacing algorithm are usually described using a timing diagram (Fig. 14). An understanding of this diagram will provide the basis for the analysis of the behavior of pacing systems and will communicate the various parameters of concern to the clinician and device manufacturer. The concepts associated with pacemaker timing are shown in Fig. 14; the information is presented in an alternate form in Fig. 15.

The actual behaviors of pacing systems can deviate from the ideal for a number of reasons. For example, the pacing pulse can be of an inadequate energy to pace the chamber, losing capture on one or more beats (Fig. 16). Another situation that commonly arises is oversensing. In this case, the device inappropriately identifies electrical activity as an atrial or ventricular event (Fig. 17). Clinically, this is resolved by reprogramming the device to a lower sensitivity. Conversely, if a system is under-sensing, the sensitivity is increased. Assessment of the behavior of the pacing system is vastly simplified through the use of marker channels. These are shown below the electrocardiograms of Figs. 16 and 17. The marker channel reports the behavior of the pacing system to allow a quick assessment of the performance of the algorithms and device output levels.

Pacing Blanking Period

Fig. 15. Blanking and refractory periods. The top trace represents the electrocardiogram. The portion of the diagram on the left is a situation in which both the atrial and ventricular leads are pacing. The portion on the right is a situation in which the system is sensing intrinsic atrial and ventricular activity (i.e., no pacing is occurring). AP, atrial pace; AS, atrial sense; AV, atrioventricular; PVAB, postventricular atrial blanking period; PVARP, postventricular atrial refractory period; VP, ventricular pace; VS, ventricular sense.

Fig. 15. Blanking and refractory periods. The top trace represents the electrocardiogram. The portion of the diagram on the left is a situation in which both the atrial and ventricular leads are pacing. The portion on the right is a situation in which the system is sensing intrinsic atrial and ventricular activity (i.e., no pacing is occurring). AP, atrial pace; AS, atrial sense; AV, atrioventricular; PVAB, postventricular atrial blanking period; PVARP, postventricular atrial refractory period; VP, ventricular pace; VS, ventricular sense.

Loss Capture Pacemaker
Fig. 16. A surface EKG (above) and pacemaker marker channel (below) printed from a programmer. Note the loss of capture on the atrial channel (indicated by the arrow); notice that no P wave follows the pacing pulse.

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