Ann Thorac Surg 2001;71:1945-1948
© 2001 The Society of Thoracic Surgeons
Original article: cardiovascular
Optimal position of atrial epicardial leads for temporary pacing in infants after cardiac surgery
Ichiro Kashima, MDa,
Ryo Aeba, MDa,
Toshiyuki Katogi, MDa,
Shiaki Kawada, MDa
a Division of Cardiovascular Surgery, Keio University, Tokyo, Japan
Accepted for publication March 13, 2001.
Address reprint requests to Dr Kashima, Division of Cardiovascular Surgery, Keio University, 35 Shinanomachi, Shinjuku, Tokyo 160-8582, Japan
e-mail: kashima{at}chp.kiyose.tokyo.jp
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Abstract
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Background. The atrial epicardial wall of pediatric patients was topographically assessed to identify the optimal position for temporary atrial pacing.
Methods. Unipolar electrodes were fixed at the cephalic wall between the right and left atrial appendages, the interatrial groove, and the right atrial appendage of fifteen pediatric patients who underwent definitive surgical repair. The performance of the three electrodes in terms of pacing patterns and six combinations of bipolar pacing were evaluated in terms of the pacing threshold, P-wave amplitude, slew rate, and lead impedance.
Results. Unipolar pacing of the right atrial appendage showed a significantly higher threshold than the other groups. A bipolar configuration of the cephalic atrial wall and interatrial groove had a significantly higher P-wave amplitude than groups without the electrode at the cephalic atrial wall, and a significantly higher slew rate than a unipolar configuration of the atrial appendage.
Conclusions. Bipolar pacing with the negative electrode at the cephalic atrial wall and the indifferent electrode at the interatrial groove is the most efficient method for pediatric patients.
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Introduction
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Atrial pacing is routinely applied to the therapeutic management of postoperative arrhythmia or for treating low cardiac output due to bradycardia [1]. Preservation of an appropriate heart rate by electrical pacing is an important adjunctive method with which to stabilize the postoperative hemodynamics of infants [2]. In as much as sequential atrioventicular contraction preserves physiologic filling of ventricles during diastole, atrial pacing is preferable for cardiac performance rather than ventricular pacing, particularly when atrioventricular conduction remains intact [3]. Moreover, to render atrial pacing reliable and effective, a high sensing performance with a low pacing threshold is required. Although an earlier study of adults has assessed pacing performances of electrodes fixed at the right atrial free wall or right atrial appendage [4–8], a comparable study of infants has not been published. The drawbacks to pediatric patients include a limited epicardial space that hardly admits two electrodes in the identical position for bipolar pacing and an extensive incision of the right atrial wall in most of the patients. With these considerations in mind, we made a topographic assessment of the atrial wall to elucidate the most appropriate position for the unipolar and bipolar atrial pacing.
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Patients and methods
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From September 1999 through January 2000, 15 consecutive patients aged 1 to 32 months (mean 13.2 ± 10.3) and weighting from 3.8 to 14.4 kg (mean 7.8 ± 3.4 kg), who underwent definitive surgical repairs for biventricular congenital heart conditions were enrolled in the study. These conditions included ventricular septal defects (n = 9), tetralogy of Fallot (n = 2), aortic stenosis (n = 2), atrioventricular canal defect (n = 1), and pulmonary stenosis (n = 1). Patients who underwent emergency surgery, who had repeated pericardotomy, univentricular repair, or who received high dose of inotropic agents, and patients with impaired atrioventricular conduction and heterotaxy were excluded from this series.
Intracardiac surgery was performed using standard cardiopulmonary bypass techniques with bicaval cannulation through the right atrium and aortic return. The myocardium was protected using anterograde cold crystalloid cardioplegia and topical hypothermia. After completion of the intracardidac repair, three unipolar electrodes (Unipolar Pediatric Temporary Pacing Lead, 6491; Medtronic France S.A, Fourmes, France) were fixed at three positions of the atrial wall (Fig 1): cephalic wall between the right and left atrial appendages where the Bachmann’s bundle is considered to indwell (site A), the interatrial groove (site B), and the right atrial appendage (site C). Additionally, an indifferent electrode was embedded subcutaneously within the chest wall. Provisional unipolar pacing indicated that the electrode was appropriate for the study provided that the pacing threshold was below 10 mA.
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Fig 1. Positions of three pacing leads. Site A is the cephalic atrial wall between right and left atrial appendage, namely, dome of left atrium adjacent to the aortic root. The Bachmann bundle is considered to indwell at site A, to originate at the anterior wall of the right atrium slightly medial to the superior vena cava, extend posteriorly over the roof of left atrium, then bifurcate. Site B is the interatrial groove. Site C is the right atrial free wall about 1 cm lateral from the right atrial appendage summit. (Ao = aorta; SVC = superior vena cava; SN = sinus node.)
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Pacing performance was evaluated 3 to 9 hours after termination of the cardiopulmonary bypass. All patients showed a regular sinus rhythm at the time of the study. Postoperative hemodynamics were stabilized in each patient without the need for inotropic agents, except for one patient to whom a small dose of dobutamine (3 µg · kg-1 · min-1) had already been administered before the study. Sedation of patients was achieved with intravenous administration of chlorpromazine (3 µg · kg-1 · min-1) with or without morphine hydrochloride (0.5mg/kg) coupled with vecuronuim bromide (0.2 mg/kg). We examined unipolar pacing performance of each atrial electrode with subsequent evaluation of six combinations of bipolar pacing, where one electrode functioned as the negative lead and the other functioned as an indifferent lead. Each combination of bipolar pacing was expressed by the position of the negative electrode in relation to that of the indifferent electrode (eg, A-B or C-A).
P-wave amplitude (mV) and P-wave slew rate (V/s) were used as indicators of the sensing performance. Slew rate is defined as the initial slope of electrical waveform. Given that the initial slope of the ventricular signal is generally low in the atrial electrodes, a higher slew rate indicates a more accurate discrimination of the P-wave from the ventricular signal [9]. Pacing variables included pacing threshold (V) at 0.5 milliseconds (msec) pulse width, and impedance (ohms) at 5.0 V with 0.5 msec pulse width. Pacing thresholds were obtained by increasing the stimulus until capture was consistent. When the pacing threshold exceeded 5.0 V, the lead impedance was measured at the threshold voltage. All measurements were made in triplicate using a Medtronic Pacing System Analyzer (5311B, Medtronic Inc, Minneapolis, MN) [7, 8]. The mean values were further analyzed.
All values were expressed as mean ± standard deviation of the mean. The analysis of variance (ANOVA) statistical analysis was performed using SPSS Ver. 9.0 for Windows (SPSS Inc, Chicago, IL). A multiple comparison was performed using Scheffe’s test as the posthoc test. A p value less than 0.05 was considered statistically significant.
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Results
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None of the patients developed sinus node dysfunction or bleeding subsequent to removal of the electrodes. Data from pacing and sensing variables are shown in Table 1. The threshold of the unipolar pacing at site C was significantly higher than that of the other pacing patterns (p < 0.0001). Lead impedance did not significantly differ among the pacing profiles. In terms of sensing variables, lead combinations of A-B (5.53 ± 2.27 mV) and B-A (5.48 ± 1.91 mV) showed a significantly higher P-wave amplitude than groups without the electrode at site A. The slew rate of the bipolar pacing pattern of A-B was significantly higher than that of the unipolar pacing with the electrode at site C (p = 0.046).
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Comment
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Infant cardiac output is improved by bringing the heart to a rate that is appropriate for the age coupled with regular cardiac rhythm, rather than by increments of the contractile force of the ventricle. Therefore, one essential means of managing postoperative patients with pathologic arrhythmia or bradycardia would be to preserve regular and appropriate heart rate by electrical pacing [2, 3]. Furthermore, Till and colleagues [10] reported that in the setting of defective atrioventricular conduction, atrial pacing, synchronous to the spontaneous ventricular rhythm, elevates the mean arterial pressure. Therefore, sequential atrioventricular contraction is preferable, if not indispensable, for a favorable postsurgical outcome.
Both high sensing performance and low pacing threshold are of importance for efficient cardiac pacing. The present study found that a negative electrode at site C was inappropriate for unipolar pacing. This may partly be attributable to the unusual arrangement of myocardial fibers at the appendages and mechanical stretch due to the purse-string sutures used for venous cannulation. With regard to the accurate sensing of the P wave, Brandt and associates [11] described that an amplitude exceeding that of the ventricular signal, and a sufficient slew rate were mandatory. Similarly, Scott and coworkers [12] described that a P-wave amplitude exceeding 1.0 mV was the minimal requirement for adequate detection of atrial activity. Furthermore, the reported optimal slew rate for the endocardiac electrode at the undamaged right atrial appendage was 0.6 to 1.2 V/s [9, 13]. Our observations concurred with these earlier findings and revealed that the bipolar pacing configuration using a combination of the negative electrode at site A and the indifferent electrode at site B achieved the most optimal sensing performance. This evidence was consistent with the reports of Ferguson and associates [6], who described that interelectrode distance was an important factor for optimal atrial sensing and that more closely spaced electrodes could theoretically yield better sensing performance.
Although the morphologic presence of the internodal tracts has been a subject of ongoing debate, the atrium is thought to be activated between the sinus and atrioventricular nodes by three preferential routes. These are the anterior internodal tract (Bachmann bundle), the middle internodal tract, and the posterior internodal tract [14, 15]. For atrial pacing with the electrodes fixed at the atrial wall, the site of pacing should be close to the internodal tracts, have muscle fibers oriented in the same direction for the preferential spread of excitation, be free of danger due to hemorrhage or unfavorable sequelae, and be separate from the surgical incision [6]. From a morphologic perspective, the interatrial groove and right atrial appendage are separated widely from the preferential pathways. Moreover, the muscular arrangement of the atrial appendages, where the muscle fibers cross in varying directions instead of being oriented in one, are contradictory for electrical pacing [16]. Therefore, the disadvantage at site C would be immutable even if the atrial appendage was intact due to direct bicaval cannulations. Some earlier studies on adults have reported on the inferiority of the electrode at the atrial appendage [7, 8]. The present study confirmed that the cephalic atrial wall between appendages fulfills all prerequisites for atrial pacing. In addition, as this region is supplied with blood by a branch of the sinus node artery that is a proximal branch of the left or right coronary artery [14], this segment of the atrium is rarely affected by ischemic injury. The sinus node artery courses through the area of the left atrium between the left atrial appendage and the left superior pulmonary vein when it arises from the left circumflex artery, or passes across the anteriomedial wall of the right atrium when it arises from the right coronary artery. Then the artery runs to the base of the superior vena cava and encircles the junction of the superior vena cava with the right atrium (atriocaval junction) in a clockwise or counterclockwise manner to penetrate the sinus node [16, 17]. Therefore, avoiding placing the electrode near the atriocaval junction can prevent injury to the sinus node artery in almost every case. Furthermore, a previous study on adults did not report impairment of the sinus node function itself by the procedure of placement of the electrode at site A [5], and the present study also showed no complications related to such placement.
The characteristics of electrodes over a longer postoperative time interval were not investigated in this study. However, we believe that site A will prove advantageous over longer time intervals given that it produced a superior performance even in the early postoperative period when there was thought to be an influence of the postperfusion effect, and given that this site was associated with a high tolerance to ischemic change. Although the number of enrolled patients was small and transitional evaluations with respect to time are lacking in the present study, site A is considered the optimal site for fixing the atrial pacing electrode.
In view of favorable pacing and sensing characteristics, we recommend a bipolar configuration for atrial pacing in children, with the negative electrode being fixed at the cephalic atrial wall and the indifferent electrode at the interatrial groove.
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Acknowledgments
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We thank T. Fukuda, MD, Director, Department of Cardiovascular Surgery, Tokyo Metropolitan Kiyose Children’s Hospital, for his helpful discussions.
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References
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Abstract
Introduction
