Efficacy

Factors that affect the Autocapture’s detection of an ER are listed in Box 31-3. One major challenge to this approach is the difficulty in distinguishing the ER signal and the pace-induced after-potential. A large electrode polarization artifact relative to the size of the ER can affect ER detection. This can be reduced with the use of low-polarization electrodes (made possible by increasing the microscopic electrode-tissue interface area).⁵ An alternative is to use a biphasic waveform that comprises a fast precharge followed by a negative postcharge to minimize the polarization effect.⁶ In one study, the effect of a modified fast prepulse on the Autocapture was tested in 45 patients with leads from two manufacturers (Medtronic 4024 Cap Sure, and Pacesetter 1450 K/T and 1470 T leads).⁷ Whereas the ER was independent of the type of pacing pulse, the polarization artifact was significantly less during the modified pulse compared with the conventional pacing pulse, resulting in an improved efficacy of the Autocapture algorithm (94% vs. 71% success rate in ER detection). An adequate ER amplitude of greater than 2.5 mV is recommended before activation of the Autocapture algorithm; this was present in 93% of 60 patients in one study.⁸ Neither the clinical data nor the conventional electrical parameters were effective in predicting the size of the ER signal. Body posture and exercise had relatively little effect on the ER.⁹ Recently, a new ER algorithm measuring the depolarization integral (area) instead of ER signal amplitudes (voltage) to determine ER has enhanced the accuracy of capture verification; in fact, the algorithm allows ER determination even with old high-polarization bipolar leads. Because of enhanced sensitivity, it has become possible to distinguish a small atrial ER from a pace-induced after-potential.

In a multicenter study, 113 patients received the Pacesetter Microny SR+ and were followed up for 1 year. ER was satisfactory for Autocapture in 102 of 113 patients.¹⁰ Even though ER was stable over time it correlated poorly with the R wave at the time of implantation. Acute and chronic pacing thresholds measured at the clinic using a standard surface ECG VARIO threshold test significantly correlated with that derived from the Autocapture, although the Autocapture threshold was higher (0.11 ± 0.22 V) because of the way in which threshold was derived. During Holter recordings, no failure of ventricular capture occurred, and backup pulses were used in 1.1% of all paced beats. Most of these were caused by fusion or pseudo-fusion beats (87%), undersensing of either the R wave or the ER (4.6%), and was the true cause of loss of capture in only 7%. Although these did not affect pacing performance, the need for backup pulses may negate the energy saving by the Autocapture itself. Similar positive results from the Autocapture algorithm in the medium term for safety and efficacy have been published.^11,12

Projected increases in battery longevity afforded by the Autocapture have been reported.^13,14 Compared with the factory-set pacemaker setting of 5 V, the Autocapture reduced the energy drain in the Microny SR+ (with 0.35 Ah) and increased device longevity by 53%. For the Regency SR+ with a larger battery (0.79 Ah), the increase in device longevity was even more significant (245%). However, when the conventional output was reduced to 2.5 V, the benefit of the Autocapture on battery life was much less impressive.^13,14

Clinical Implications

The main benefits of any automatic capture management algorithm are patient safety and effective capture during threshold changes. Programming of threshold can be simplified as the Autocapture threshold has been significantly correlated with bedside threshold assessment. The energy saving would be more important in patients with chronic high thresholds. Conversely, fusion and pseudo-fusion beats appear to be the main limitation, not only because they reduce battery energy but also because they may lead to erroneous threshold determination.¹⁵

Atrial Tachycardia Detection

A device can detect AT in four main ways (Table 31-1): (1) Most devices use a rate cutoff criterion, in which a sensed atrial rate exceeding a programmable value will result in AMS. Some systems are designed to avoid mode switch during atrial ectopic beats or short runs of AT. For example, four short cycles of seven consecutive beats are required before AMS occurs in the Medtronic Kappa 700. Interval number summation is used in the incremental and decremental counter of the Meta DDDR for short cycles and long cycles, respectively. (2) Some devices use a mean atrial rate, or matched atrial rate, based on a moving value related to the duration of the prevailing sensed atrial cycle as a criterion to move toward AMS. AMS will occur when the matched atrial interval has shortened to a predetermined duration. This algorithm is used in the Medtronic Thera DR, the Kappa 400 (GEM DR ICD), and the St. Jude Trilogy DR+/Affinity family. Because the process is gradual, the rapidity of AMS will depend not only on the AT detection rate or interval but also on the pre-existing sinus rate. It is easier for the matched atrial interval to reach the tachycardia detection interval when AT occurs in the setting of a higher resting sinus rate than from a sinus bradycardia. This is because the matching atrial interval starts from a shorter baseline duration on its gradual way to reach the tachycardia detection interval. (3) Sensors can be used to determine the physiological rate (e.g., Diamond and SmarTracking in Marathon). To take into account the fluctuation in sinus rate, a physiological heart rate range based on the sensor-indicated rate is used to define sinus rhythm, and rates beyond the upper end of the physiological range will activate AMS. (4) Complex algorithms are a combination of algorithms, or they use additional criteria (often from implantable cardioverter-defibrillators [ICDs]) to distinguish AF and other rhythms. For example, a PR logic and a rate criterion are instrumented in the AT500 (Medtronic, Inc.) to detect AF and AT.³³

Table 31-1 Classification of Different Methods of AT Detection in Current AMS Algorithms

AT, Atrial tachycardia; AMS, automatic mode switching; PVARP, postventricular atrial refractory period.

Destination Mode

Either the VVI(R) mode or the DDI(R) mode is used. No studies have been performed on the relative merits of the VVI(R) mode versus the DDI(R) mode. Obviously, during AMS, no AV synchrony occurs, and the DDI(R) is functionally equivalent to the VVI(R) mode. Theoretically, the DDI(R) mode may avoid AV dissociation when a sinus pause occurs at AT termination if the AMS algorithm has not yet resynchronized to sinus rhythm. The VVI mode during AMS has been described as VDI because the maintenance of As controls the perpetuation or termination of AMS, but this designation does not strictly conform to the standard pacemaker code. However, when AF is undersensed during AMS, atrial pacing in DDI(R) destination mode may paradoxically perpetuate paroxysmal AF. Apart from passive handling of AT in terms of the AMS algorithm, some devices deliver an active pacing intervention on the detection of frequent atrial ectopic beats, which are thought to herald the onset of AF, or an active pacing to terminate AT.

Resynchronization

Some AMS algorithms use the same onset criteria to resynchronize after AT termination, and some others, for example, Thera DR, use slower criteria of resynchronization to avoid intermittent AMS during short runs of AT.

Automatic Mode Switching Sensitivity

As most AMS algorithms detect AT by a rate cutoff criterion, a slow atrial rate (e.g., atrial rate slowing after antiarrhythmic medications) may fall below the tachycardia detection rate, and AMS will not occur. Conversion of AF to atrial flutter is a special situation in which alternate flutter waves coincide with the postvertricular atrial blanking period (PVAB), and the effectively detected atrial rate falls below the tachycardia detection rate and prevents AMS (Figure 31-5).

FIGURE 31-5 Sensing of atrial flutter (2:1) by a DDD pacemaker. The atrial flutter cycle length is shorter than the sum of the A-V interval plus the PVAB. The blanking periods are shown in purple. Every alternate flutter wave falls in the PVAB, resulting in 2:1 sensing. AEGM, Atrial electrocardiogram; ECG, surface electrocardiogram; MC, marker channel; PVAB, postventricular atrial blanking period.

(From Lau CP, Leung SK, Tse HF, Barold SS: Automatic mode switching of implantable pacemakers. I: Principles of instrumentation, clinical and hemodynamic considerations, Pacing Clin Electrophysiol 25:967–983, 2002.)

In the currently available devices, the postventricular atrial refractory period (PVARP) is opened (completely or on a conditional high rate) to enhance AMS. In other words, sensing occurs in the second part of the PVARP beyond the PVAB. The latter is designed to prevent far-field R-wave sensing. Atrial undersensing may occur because of an inappropriately long PVAB. Undersensing can often be avoided by appropriate adjustment of the PVAB, provided the atrial channel exhibits no far-field sensing. The widely varying amplitude of atrial ECG in AF, both temporally in a patient and between patients, can result in AMS failure when the atrial sensitivity is programmed incorrectly. During electrophysiological study, acutely recorded atrial ECGs (ECGs) during AF and sinus rhythm show similar mean amplitude, but the variability in amplitude is substantially wider and the minimum amplitude considerably smaller in AF compared with sinus rhythm (minimum atrial ECG: 1.4 ± 1.1 and 2.0 ± 0.8 mV, respectively).³⁴ A high-programmed atrial sensitivity may cause As of far-field signals or noise, whereas a low atrial sensitivity can lead to undersensing during AF (Figure 31-6).³⁵ Optimal programming of atrial sensitivity for AMS requires three times the safety margin compared with two times for sinus P-wave sensing (Figure 31-7).³⁵

FIGURE 31-6 Effect of programmed atrial sensitivity in three mode switching pacemakers during persistent atrial fibrillation (AF). A, Significant undersensing of AF began to occur when the sensitivity was above 1 mV. B, Minimum oversensing of noise occurred when the sensitivity level was above 2 mV.

(From Leung SK, Lau CP, Lam CT, et al: Programmed atrial sensitivity: A critical determinant in atrial fibrillation detection and optimal automatic mode switching, Pacing Clin Electrophysiol 21:2214–2219, 1998.)

FIGURE 31-7

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