Contraction–excitation coupling?

Abstract

We are accustomed to thinking of excitation as preceding contraction. The timing, site of origin, and path of conduction of electrical impulses through the heart to activate cardiac myocytes are all important variables in determining the nature of the subsequent heart contraction and relaxation. But in the continuum of the cardiac cycle, contraction also precedes the next excitation. In this issue of the journal, Thomsen and colleagues make the provocative observation that just as variation in electrical activation can affect the subsequent contraction, periodic changes in mechanical contraction may be associated with variation in the subsequent electrical excitation. High frame rate ultrasound was used to perform tissue Doppler echocardiography (TDI) in 23 consecutive patients with spontaneous ventricular premature beats (VPBs). Eleven of these patients had no evidence of structural heart disease. TDI showed a highly significant decrease in left ventricular ejection time, peak myocardial velocity, and longitudinal systolic displacement in the normally conducted sinus beat immediately preceding the spontaneous ventricular premature systole, as compared with the TDI of earlier normally conducted complexes that were not followed immediately by a VPB. The authors interpret their data as evidence of a localized conduction block confined to an area immediately surrounding the ectopic pacemaker and speculate that this mechanism could be important in the development of cardiomyopathy and may be an underlying factor in some patients who are nonresponsive to cardiac resynchronization therapy. These preliminary results are very interesting and warrant further investigation in a much larger cohort of patients. TDI is a useful noninvasive technique, but it yields very indirect measures of ventricular performance. It is somewhat surprising that TDI is sensitive enough to detect areas of conduction block that are presumably relatively small.