Eccentricities of activitrax pulse generators

BRIEF

REPORTS

Eheniricities

of Activitrax

Pulse Generators

Agustin Castellanos, MD, Pedro Fernandez, and Robert J. Myerburg, MD

MD,

Richard J. Thurer,

t present, and on a worldwide basis, the Activitrax (VVI plus activity or VVIR) pulse generator is the A most frequently implanted unit that uses signals other

Phyllis Rhymes,

MD,

RN,

than intracardiac electrograms to control the rate of stim-

ulation. Because all pulse generators have eccentricities,’ it is of clinical importance to describe some of the unexpetted varieties of normal function observed during the evaluation of these devices.

From the Division of Cardiology, Department of Medicine, University of Miami School of Medicine, P.O. Box 016960, Miami, Florida 33101. Manuscript received November 17, 1988; revised manuscript received and accepted January 3,1989.

For this purpose, 13 Holter recordings were obtained in 8 patients with implanted Activitrax pulse generators (models 8400 and 8403). Because their normal modes of operation have been previously reported,2,3 only those

El 630 ML2

A

MLi

FlioURE 1. Normal recycling (A) and partial ms (6). ML2 = monitoring l&ad 2.

recycling

with

escape

interval

(El) equalling

programmed

upper

cycle

length

of 480

El

ML2

FIGURE 2. Partial maker cycle length

874

recycling with escape interval equalling was 780 ms. Abbreviation as in Figure

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upper

cycle

length

(480

ms) at a time when

the pace-

relating to the phenomenon to be described will be discussed here. All pacing systems were functioning well (except for temporary, correctable myopotential inhibition in Zpatients) as determinedfrom repeat evaluations in our pacemaker clinic. The eccentricities noted occurred with pacing and sensing safety margins >2:1 and with the following programmed parameters4: basic rate-60 or 70 beatslmin (cycle lengths of 1,000 or 860 ms); programmed upper rate-l 25 beatslmin (cycle lengths of 480 ms); activity threshold-medium; and rate response-5 or 7. Four patients showed a specific type of partial recycling characterized by the sudden appearance of an escape interval with a duration shorter than the pacemaker cycle length present at that particular moment. In 3 of these patients, the escape intervals had durations invariably equal to the programmed upper cycle length. Normal and partial recycling followed ventricular extrasystales appearing during long runs of otherwise uninterrupted pacemaker rhythm with a cycle length of 620 to 640 ms (Figure I). In Figure IA, the moment of normal sensing within the premature QRS complex (indicated by an arrow) was obtained by subtracting the duration of the pacemaker cycle length from the moment of occurrence of the pacemaker spike following the ventricular extrasystole. Thus calculated, the normal escape interval measured 630 ms. Figure IB, obtained 1 minute later, showed partial recycling because the escape interval following a similar extrasystole did not measure 630 ms, but 480 ms (a value equalling the programmed upper cycle length). The latter corresponded to a rate of 12.5 beatslmin. Similarly, in Figure 2, obtained from another patient, the escape interval following the fifth (natural) beat also measured 480 ms. On the other hand, the interval between this unexpectedly early paced beat and the immediatelyfollowingpaced beat measured 780 ms (rate of 77 beatslmin). In 1 patient (Figure 3),partial recycling after some natural ventricular complexes was

El

FIGURE 3. Normal recycling the pacemaker cycte length

manifested as an escape interval, which was not only shorter than the programmed upper cycle length but always had a value of 600 ms (corresponding to a rate of 100 beatslmin). The latter is shown in Figure 3B, while normal recycling is depicted in Figure 3A.

Holter recordings obtained in these patients, after simple reprogramming to the classic VVI mode (VVI without activity), did not reveal any evidence of partial recycling. The eccentricity depicted in Figures 1 and 2, also noted by Botella Solana et a1,3was described in the 1986 Medtronic Technical Manual provided with Activitrax model numbers 8400,8402 and 8403.4 According to this manual, an occasional pacemaker escape will be emitted at the programmed upper cycle length when a sensed ventricular event occurs within an 8-ms window after the pacemaker activity detection circuit has been activated. In addition, escape intervals of 600 ms (as in Figure 3) were described as “. . . being clinically observed when a basic rate pacing output preceded the intrinsic rhythm.“4 However, this statement does not apply to Figure 3 where a paced beat followed a run of the patient’s intrinsic rhythm. We have been unable to obtain more extensive explanations for these differences in response (Anderson K, personal communication). Many pulse generators of nominally similar operation may have circuits that are different in design and, more important, in actual function.’ The latter can produce electrocardiographic changes capable of being puzzling and difficult to interpret correctly. Our findings tend to corroborate these general statements, especially since the manufacturers have removed the 8-ms window from the most current Activitrax pacemakers, and also omitted the description of the eccentricities from the 1987 Technical Manual provided with these units. Therefore, a physician or specialized pacemaker nurse detecting partial recycling during evaluation of one of the many earlier units still in use may not identify the shorter-than-expected

El

(A) and partial recycling was 1,020 ms. Abbreviation

(s) With escape as in Figure

interval

with

a consistent

value

of 505

ms tit a time when

1.

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BRIEF

REPORTS

escape intervals as an eccentricity after consulting the 1987 manual. Consequently, these unusual modes of operation can be construed to indicate pacing system malfunction. The initial Activitrax pulse generators had 2 unexpetted, yet normal variations in the duration of their escape intervals. It is possible for these eccentricities to be considered as reflecting malfunctions, especially if their evaluation is made with the help of the Technical Manuals provided with more recently manufactured models (as with second generations) of the same pacemaker.

Association of the Acquired Disease of Atrioventricular John F. Cardella,

MD,

Vladimir

I. Kanjuh,

Bicuspid Valves MD,

Addendum: While this manuscript was in press, a pertinent article by Den Dulk et al (Am J Cardiol 1988; 61:107-112) came to our attention. 1. Furman

S. Pacemaker eccentricity. PACE 1981;4;261. L, Smedgard P, Kruse I, Anderson K. Rate responsive pacing by means of activity sensing. Stimucoeur 1984:12:181~184. 3. Botella Solana S, More1 Cabedo S, Sanjuan Manez R, Garcia Civera R, Llacer Escorihuela A, Lindemans FW. Partial re-cycling in activitrax pacemaker ECG’s. Stimucoeur 1987;15:188-193. 4. ActivitraxTM Multiprogrammable Rate Response (Activity) Pulse Generator. Models 8400/8402/8403. Technical Manual. Medtronic Inc., Minneapolis, Minnesota. First Edition, 1986; Second Edition, 1987. 2. Ryden

Aortic

Valve

and Jesse E. Edwards,

he acquired bicuspid valve has been considered by T some to be a result of rheumatic endocarditis. The distinction between congenital and acquired bicuspid aortic valves has been reported.‘,* Even more has been reported on the causes of aortic stenosis3-6 and regurgitation.7,8 When aortic stenosis is associated with mitral (and tricuspid) valvular disease, the etiology of the aortic stenosis is usually attributed to rheumatic fever.3,gJ0 Although one of us3 has attributed the acquired bicuspid aortic valve to rheumatic fever, we could not find specific From the Departments of Pathology and Radiology, University of Minnesota, Minneapolis, and the Jesse E. Edwards Registry of Cardiovascular Disease, Suite 200, St. Paul Heart and Lung Center, 255 North Smith Avenue, St. Paul, Minnesota 55102. This study was supported by the United Hospital Foundation, St. Paul, Minnesota. Manuscript received October 3, 1988; revised manuscript received and accepted January 3, 1989.

1

with

Rheumatic

MD

confirmation or denial from others regarding this question. Accordingly, starting with specimenshaving an acquired bicuspid aortic valve, we examined the atrioventricular valves for the presenceof rheumatic stigmata. This was done with the idea that the findings might, as circumstantial evidence, provide an answer to the question regarding a causeof acquired bicuspid aortic valve. The acquired bicuspid aortic valve is characterized by commissural fusion at 1 aortic valve commissuresuch that 2 cuspsare converted into the conjoined cu~p.~J~The fusion of 2 adjacent hemicuspsresultsin a raphe that is as tall asthe free edgesof the cusps.This is in contrast to the classic raphe of congenital bicuspid aortic valve*J r in which the upper aspectof the raphe is usually distinctly below the level of the free edgesof the cusps.Also, in someexamplesof acquired bicuspid aortic valve one may observe elements of the 2 cuspsthat have been fused. Among specimensin the Cardiouascular Registry of United Hospital, St. Paul, there were 75 hearts with an acquired bicuspid aortic valve. In some specimensthe valves were stenotic through calcific deposits.In 70 cases all the cardiac valves were present, while in 3 the mitral valve had been replaced, and in 2 both the mitral and tricuspid valves had beenreplaced. No casejudged to be a congenital bicuspid aortic valve was included in this study. Gender was known in 66 cases(48 men and 18 women). Of the 56 casesin which the age was known, the mean age was 59 years (61 years for men and 56 for

women).

FIGURE 1. In 75 cases of acquired bicuspid aortic valve, the associated changes in the atrioventricular valve are given. Mitral and tricuspid disease, when present, refers to rheumatic type of disease. 876

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The 3 aortic commissuresmay be designatedaccording to the related cusps. Thus, the commissurebetween the posterior and right cusps may be termed the P-R commissure.The R-L commissurelies betweenthe right and left cusps,while the L-P commissurelies between the left andposterior cusps.Using thesedesignationsthe incidence of fused commissure among the 75 casesof acquired bicuspid aortic valve was P-R commissure in 43 cases,R-L in 26 and L-P in 6. In some casesabnormalities that were consideredto be of rheumatic nature were observedin the mitral valve,