Pacemaker hemodynamics: Clinical implications

Abstract

Our review of the current literature and experience in caring for pacemaker patients suggests that a consideration of hemodynamics is a logical way to approach pacemaker selection and programming. Multiple clinical factors enter into the selection of a pacemaker or pacemaker programming settings in each case. It appears that in patients with sinus node disease, atrial-inhibited or dual-chamber pacing provides the best chance for preventing the development of chronic atrial fibrillation with its attendant risks of embolism and stroke. It is clear that AV synchrony has beneficial hemodynamic effects at rest in most patients. The results of Labovitz would suggest that in patients with marked left atrial enlargement, this may be less so. 10 The results of Stewart et al would further suggest that in patients with retrograde VA conduction, dual-chamber pacing is preferable. 11 Retrograde VA conduction can be intermittent and this makes it difficult to use its absence on a single test to decide on the type of pacemaker to use. It appears that baseline left ventricular function does not determine the relative improvement in cardiac output observed with AV synchrony or rate-adaptive pacing. However, in patients with severe congestive heart failure even a small improvement in cardiac output may result in significant clinical improvement. Studies have shown that in any given patient, there may be an optimal AV interval at rest. In general, this ranges from 100 to 150 milliseconds. In normal individuals the optimal AV interval shortens with increased heart rate during exercise in a predictable and linear fashion. The hemodynamic benefits of a shortened AV interval with faster heart rates in pacemaker patients have not yet been shown. Intuitively, however, this would appear to be a desirable approach and will probably be added to the design of future generations of dual-chamber pacemakers. Studies of the effect of different pacing modes on secretion of atrial natriuretic factor are intriguing and may contribute more to our understanding of pacing hemodynamics in the future. During exercise, heart rate increase is more important than AV synchrony and this has been shown by several studies. Thus, in active patients with chronotropic incompetence due to sick sinus syndrome, the addition of rate-adaptive pacing is important. Because single-chamber rate-adaptive atrial pacing leaves the patient exposed to the risk of future development of AV block and DDD pacing does not provide chronotropic support, it is likely that the new rate-adaptive dual-chamber (DDDR) devices will be used in a significant number of these patients. Dual-chamber rate-adaptive pacemakers are now available to meet the clinical needs of such patients. Baseline left ventricular function as measured by ejection fraction does not appear to predict the relative benefit of rate-adaptive pacing. There are several studies on the effect of different pacing modes on coronary blood flow in patients with ischemic heart disease. The physiologic effects of different modes of pacing on coronary blood flow are complex. In general, dual-chamber pacing provides a higher cardiac output without disturbing myocardial oxygen balance if the upper-rate limit is appropriately controlled. The upper-rate limit can be arbitrarily set to less than 100 or to a value less than that associated with angina on a treadmil test. Future studies of differing pacing modes and settings in patients with other types of heart disease are needed. Studies have been performed on the chronic effects of differing pacing modes on exercise and hemodynamics and the results are very interesting. It appears that dual-chamber pacing may reduce heart size and has long-term beneficial effects on exercise ability. In the patient with preexisting congestive heart failure, dual-chamber pacing appears to improve survival compared with VVI pacing. Although the new features added to pacemakers in the past 10 years may improve physiology, they also increase the likelihood of malfunction, pseudomalfunction, and other complications. Studies show that most of the complications of dual-chamber pacing can be treated by reprogramming. However, this requires knowledgeable and careful pacemaker follow-up. In addition, a pacemaker patient's condition and underlying rhythm may change over time requiring adjustments in pacemaker program settings by the physician. Currently in the United States, less than a third of permanent pacemaker implants are dual-chamber devices whereas the newly introduced single-chamber rate-adaptive devices are implanted in 20% of patients. The reason for the lack of enthusiasm for dual-chamber pacing is that many physicians are uncomfortable with atrial lead positioning. The use of single-chamber atrial pacing in the United States is extremely low, although this approach has enjoyed popularity in Europe. 75 A committee from the North American Society of Pacing and Electrophysiology suggested that DDD pacemakers are indicated in 60% to 80% of all implants. 76 The data reviewed here support an increased use of dual-chamber pacing.