SYNCHRONICITY OF VENTRICULAR CONTRACTION: OBSERVATIONS COMPARING HAeMODYNAMIC EFFECTS OF ATRIAL AND VENTRICULAR PACING

Abstract

In the course of earlier investigations it was noted that when the heart was paced by ventricular stimulation, mean left atrial pressure was higher for any given cardiac output, aortic pressure, and stroke work than when the atrium was paced at the same rate. Although the changes observed in those initial observations were not always striking, it was felt desirable to clarify the mechanisms by which they took place and to find what influenced their magnitude. Accordingly, a more systematic investigation was undertaken.Two explanations were entertained that it was thought might account for the observed phenomena. The first was that, as suggested by Koch (1920) and Wiggers (1925), ventricular stimulation does not produce as well-organized a wave of ventricular depolarization as is the case when the impulse is normally propagated, and so results in a less synchronous ventricular contraction. Secondly, it was reasoned that if the ventricle had perchance been stimulated at such a rate and at such a time in the cardiac cycle that the paced ventricle and the spontaneously beating atrium were contracting simultaneously, the atrium would have been contracting against a closed atrio-ventricular valve, thus depriving the ventricle of that portion of its filling and the consequent influence on contraction which normally results from atrial systole (Sarnoffand Mitchell, 1961). The results of experiments designed to examine the validity of these two explanations over the whole range of ventricular function and to ascertain the effects of such changes on flow are the subject of this communication. A preliminary report on a portion of these data has been previously presented (Linden et al., 1959). METHODSMongrel dogs of both sexes weighing 17-5 to 31-5 kg. were anesthetized with morphine, chloralose, and urethane. The chest was opened under intermittent positive pressure breathing and the appropriate cannulations made so as to be able to record continuously aortic blood flow and heart rate, as well as right atrial, left atrial, aortic, and pulmonary arterial pressures and left ventricular diastolic pressure. The techniques used were essentially the same as those described (Samoff et al., 1960a) with the following additions. After pericardiotomy, pacing electrodes were placed on the right or left atrium or both, on the right or left ventricle or both, and sometimes on all four chambers. Each cardiac electrode, as well as the indifferent electrode, was connected to a multiplexing switch so that the impulse could be applied to the atrium and then instantaneously changed from atrium to ventricle and back again as desired. No special effort was made to apply the ventricular electrode to any particular place on the ventricular myocardium although generally it was applied to the lateral wall of the ventricle near the atrioventricular sulcus rather than in the vicinity of the ventricular septum. The electrodes for atrial pacing were applied on or near the atrial appendage.

Effects of Varying Pacemaker Sites on Left Ventricular Performance

“…It has been known that the conduction velocity in ventricular muscle tissue is slower than the velocity in the specialized conducting system (Wiggers 1925) . The abnormal direction of excitation and slower conduction velocity with ventricular stimulation produce a less synchronous ventricular contraction than that resulting from right atrial stimulation (Wiggers 1925;Gilmore et al . 1963).…”

Dynamic Geometry of the Left Ventricle during Ventricular Pacing: Correlation with Cardiac Pumping Action

“…In this study, heart rate was constant throughout the experiment. Although we did not investigate the hemodynamic response to ventricular pacing in comparison with RA pacing at equivalent end-diastolic volume, Gilmore et al (1963) have presented that the left ventricle produced less stroke work from any given left ventricular end-diastolic pressure during ventri cular stimulation. It is unlikely that temporal asynchrony could alter the contractility of individual muscle fibers to cause the changes in ventricular func tion.…”

Regional Contraction patterns of the Left Ventricle during Ventricular Pacing

“…The decrease in cardiac performance observed during ventricular pacing was related to the severity of asynchrony rather than the direction of the ventricular depolarization or change in regional myocardial tension. regional myocardial contraction; straingauge arch; ventricular pacing; ventricular asynchrony; cardiac performance Decreased cardiac performance during ventricular pacing has been ascribed to the lack of synchrony resulting from altered ventricular activation and contraction (Wiggers 1925; Gilmore et al 1963;Finney 1965;Miyazawa et al 1976). This is due to slow passage of the depolarization wave through ventricular muscle, as compared with the relatively rapid activation through the normal conduction system.…”