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Ann Thorac Surg 1995;59:300
© 1995 The Society of Thoracic Surgeons
DR RICHARD E. CLARK (Pittsburgh, PA): Doctor Foster, we know that approximately 70% to 80% of the patients who undergo just coronary artery bypass grafting have an abnormal motion of their septum. Have you used transesophageal echocardiography in such patients, and, if so, have you found that, with left ventricular and biventricular pacing, there is earlier activation of the septum, thus putting the septum in a little different position and making it contract earlier and more synchronously with total systole, to explain the increase in cardiac output? Or do you consider that a reasonable hypothesis?
DR NORMAN J. SNOW (Cleveland, OH):In most situations, we assume that physiologic pacing is the best, in that it produces ``normal conduction.'' To what do you attribute your finding that biventricular pacing is superior to atrial pacing, which you would expect to produce normal ventricular activation?
DR ARTHUR J. CRUMBLEY III (Charleston, SC): What you have demonstrated is less than a 10% improvement in cardiac index, and I would maintain that, although you have shown a statistically significant improvement, you have failed to demonstrate a significant improvement in practical terms.
DR DENTON A. COOLEY (Houston, TX): I would like to ask if any of these observations reflect on the physiologic characteristics of obstructive cardiomyopathy, with the recent trend toward using pacemakers for that condition?
DR FOSTER: It is well known that, depending on the site of stimulation, pacing can alter septal activation and contraction. As Dr Cooley points out, this fact has been exploited in the application of pacing for the relief of obstruction in patients with idiopathic hypertrophic subaortic stenosis.
Doctor Clark, epicardial ventricular pacing has clearly shown a reproducible pattern of early activation, early contraction, late relaxation, and systolic bulging in animal laboratory experiments using crystals and using angiography. The pacing sites that were chosen in these patients were close to the inferior septum of both ventricles, and undoubtedly caused an alteration in septal contraction. I have examined several patients using transesophageal echocardiography during atriobiventricular pacing. Analysis of the septum can be problematic, and, at this point, I cannot comment on it.
Doctor Snow, additional nonmechanical factors may explain why cardiac output was greater with biventricular pacing than with atrial pacing. It is known that ventricular pacing by itself causes changes in myocardial and systemic catecholamine release and causes reflex autonomic changes that affect venous return. In addition, neurohumoral substances such as atrial natriuretic peptides are released acutely during sequential atrioventricular pacing, which may cause vasodilatation. There may be fundamental differences in the release of these substances, depending on how and which chambers are paced and in what order. It should also be remembered that atrial pacing in these patients is not strictly ``natural'' because the intrinsic conduction system, like the septum, has been subjected to reperfusion injury, cardioplegia, and other insults during revascularization.
Doctor Crumbley, the data for the mean increase in cardiac index do not describe the individual changes for each patient. In 8 of the 18 patients, the output increased by more than 10%, with 50% the greatest increase, compared with the output seen for standard atrioventricular pacing of the right ventricle. The results yielded by the statistical tests confirmed (p = 0.012 to 0.002; see Fig 4) with a high level of certainty that the hemodynamic change is not due to chance but is due to atrio-biventricular pacing. Each patient is different in ways that might affect the acute pacing response. The mix of these patients with respect to diastolic versus systolic ventricular dysfunction, the presence of chronic hypertension, the use of pharmacologic beta-blockade, and so on, may be important. None of the patients have normal conduction systems or ventricular function, as each patient underwent recent coronary artery revascularization. The myocardium, autonomic nervous system, baroreceptor system, and neuroendocrine system are all affected by reperfusion, cardioplegia, moderate hypothermia, and cardiopulmonary bypass.
The acute hemodynamic effects of atrio-biventricular pacing have been identified by this study. The postoperative use of paraseptal biventricular pacing is supported by the findings and may be most beneficial in certain patient subsets. Most importantly, I believe these data suggest the potential for further clinical applications of atrio-biventricular pacing. As Dr Cooley reminds us in the case of monoventricular pacing and idiopathic hypertrophic subaortic stenosis, the therapeutic implications of biventricular pacing schemes will become more apparent only with further application and careful study.
Related Article
Ann. Thorac. Surg. 1995 59: 294-300.
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