Abstract & Commentary
Synopsis: Chronic apical right ventricular pacing in patients with congenital heart block is associated with late findings consistent with deleterious left ventricular remodeling.
Source: Thambo JB, et al. Circulation. 2004;110: 3766-3772.
In this paper, Thambo et al report results of a study conducted on a small group of patients with congenital complete AV block to see whether or not they suffered any detrimental effects from chronic pacing from the right ventricular apex. The study population was composed of 23 patients with a mean age of 24 years. Each had received a dual chamber pacemaker with ventricular pacing from the RV apex at least 5 years previously. Only patients without other cardiac abnormalities were included in the study. These patients with congenital AV block were then compared to a control group of 30 healthy volunteers who were matched for age, gender, weight, and height. All of the congenital AV block patients had undergone clinically indicated echocardiography prior to initiation of pacing. Patients in both groups underwent echocardiography and exercise testing as part of the study. During the study echocardiogram, Thambo and colleagues evaluated the following parameters of ventricular dyssynchrony: 1) interventricular dyssynchrony, defined as the difference between the aortic and pulmonary preejection delays; and 2) septal to posterior wall motion delay, defined as the shortest interval between maximum displacement of the left ventricular septum and that of the posterior wall at the papillary muscle level. The following hemodynamic variables were also evaluated: left ventricular filling time, cardiac output, left ventricular stroke volume and ejection fraction, and the severity of mitral regurgitation. Tissue Doppler imaging was used to assess intra-left ventricular dyssynchrony and delayed longitudinal contraction.
In the group with congenital AV block, the echocardiographic ejection fraction was 69 ± 6%, and no patient had significant mitral regurgitation prior to receiving their pacemaker. The escape rhythm was junctional (QRS width less than 120 msec) in 19 patients and ventricular (QRS width greater than 120 msec) in 4 patients. Pacing was first instituted at an average age of 8 ± 4 years. Ten patients had initially received an epicardial pacing device before being later converted to a transvenous system. At the time of the long-term measurements made in this study, all patients were programmed in either VDD or DDD/DDDR modes with 100% ventricular pacing.
After long-term endocardial pacing, 3 patients manifest New York Heart Association functional class II or III heart failure. Their left ventricular ejection fractions were 39%, 41%, and 45% respectively. The remaining patients were asymptomatic and had left ventricular ejection fractions greater than 55%. In the congenital AV block group, echocardiographic examination after chronic right ventricular pacing showed LV remodeling with LV dilatation and asymmetrical hypertrophy. The ratio of posterior wall to septal wall thickness was 1 ± 0.1 before implantation vs 1.3 ± 0.2 after long-term RV pacing. Measures of both interventricular dyssynchrony (55 ± 18 vs 25 ± 8 m/sec) and of the septal to posterior wall motion delay (84 ± 26 vs 41 ± 16) were greater after long-term right ventricular pacing than before implantation. Similar findings consistent with dyssynchrony were not seen in the healthy control subjects.
Mean resting cardiac output was decreased in the congenital AV block patients during chronic right ventricular pacing, compared with that measured in the control patients (3.8 ± 0.6 vs 4.9 ± 0.8 L/min). Stroke volume was also lower in the paced group. The ratio of the area of mitral regurgitation to the area of the left atrium was higher in the paced group than in controls (16 ± 8 vs 5 ± 2). Estimates of interventricular dyssynchrony, intraventricular left ventricular dyssynchrony, the extent of left ventricular myocardium displaying delayed longitudinal contraction, and septal to posterior wall motion delay were all significantly higher after chronic RV pacing in the congenital AV block group than in controls. During exercise testing, the performance of patients with chronic RV pacing was significantly worse than that of matched controls (123 ± 24 vs 185 ± 39 watts). Heart rates during exercise were not significantly different between the groups.
Thambo and colleagues conclude that chronic apical right ventricular pacing in patients with congenital heart block is associated with late findings consistent with deleterious left ventricular remodeling. Although symptoms are only reported by a few individuals, patients with congenital complete heart block have lower peak exercise capacity than matched controls, presumably due to this electromechanical dyssynchrony.
Comment by John P. DiMarco, MD, PhD
This paper provides some intriguing new insights into the management of patients with congenital complete AV block. Often these patients present with few or only minor symptoms as children, teenagers, or as young adults. At first it was uncertain whether or not these patients would benefit from permanent pacing. However, even in the absence of reported symptoms, implantation of a dual chamber pacemaker significantly improves exercise performance in such patients. There are also numerous reports of unexpected sudden death among unpaced patients with congenital AV block, and this is another reason to recommend pacing. Longevity in congenital AV block patients after dual chamber pacing has been thought to be restored to normal. This paper by Thambo and colleagues, however, suggests that chronic right ventricular atrial pacing fails to restore these patients to their full functional potential due to dyssynchrony induced by pacing from the right ventricular apex.
Although it may seem attractive to proceed with biventricular pacing systems in patients with congenital complete heart block based on these data, we must be cautious. Many young individuals with pacemakers will require multiple procedures over the years, since problems with lead malfunction are more common in young, physically active individuals. Adding a third pacing lead in might produce a short-term benefit in many, but any advantage might be negated by the increased complexity of the devices, which entails an increased risk of future lead malfunction. If reliable and more durable biventricular systems become available, however, they may become the preferred pacing mode in the future for virtually all patients who require ventricular pacing, not just those with congenital AV block.
Dr. DiMarco, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville, is on the Editorial Board of Clinical Cardiology Alert.