THE IMPORTANCE OF TIMING MUSCLE-CONTRACTION IN DYNAMIC CARDIOMYOPLASTY

This acute dynamic cardiomyoplasty (CMP) study used ten dogs (weight r ange 21-32 kg) and was designed to determine the importance of the tra in of stimuli initiation time when applied to the thoracodorsal nerve, which innervates the latissimus dorsi (LD) muscle that is wrapped aro und the ventricles. Using the P wave of the cardiac electrogram to tri gger a special delay circuit, the stimulus train could be initiated fr om the apex of the R wave to any time throughout and at the end of the isovolumic period, signaled by opening of the aortic valve. The cardi ac electrogram (which contained the R wave), left ventricular pressure (LVP), aortic flow velocity (AFV), beat-by-beat stroke volume (SV), f emoral artery pressure, and the envelope of the stimulus train were re corded as the onset of the stimulus train was varied from the R wave t o the end of the isovolumic period with a pumping ratio of one LD cont raction for every seven ventricular contractions. In four dogs there w as a pronounced increase in the augmentation in LVP, AFV, and SV when the stimulus train was initiated later than 40 msec after the first pe ak of the R wave. In five dogs the augmentation in LVP, AFV, and SV wa s not as clearly apparent, although all of these dogs exhibited an opt imal train delay. Data were not obtained on one dog due to an anomalou s LD muscle blood supply. For all of the dogs, the optimum train delay from the R wave averaged 58 msec (range 40-80 msec). The average augm entation in SV was 26% (range 13%-45%). The same muscle-wrap tightness was used in all dogs. In one dog, the muscle-wrap tightness was varie d, and by tightening the wrap the SV augmentation increased from 17% t o 27%. For all dogs the range of augmentation in SV (13%-45%) perhaps represents variations in muscle-wrap tightness, which may be a major u ncontrolled factor in dynamic CMP.