Frequency‐Force Behavior of in Situ Ventricular Myocardium in the Dog

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Abstract
The effect of rate of beat on contraction of ventricular muscle was determined in trabeculae excised from five cat and five dog hearts, in the isovolumic canine left ventricle preparation, and in a recently developed isometric, in situ, papillary muscle preparation in the canine right ventricle perfused with oxygenated blood through the coronary vessels. Driven rates of excitation, experimental temperature (27°C), technique for recording contractile force development, and the measured maximal capacity for force development were all closely comparable for the excised and the in situ preparations of ventricular muscle. Contraction of in situ ventricular myocardium was affected only slightly, if at all, by alterations in rate of beat, in marked contrast to the profound dependence on rate of the contraction of excised ventricular muscle. Peak rate of force development by in situ myocardium was somewhat more sensitive to rate of beat, but still much less so than that of the excised ventricular tissues. This depression, in situ, of the myocardial frequency-force relationship is not attributable to effects of sympathetic outflow to the heart or of coronary flow on myocardial contraction or to negative inotropic substances in the perfusing blood which can be easily eluted from the tissue.

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