Studies on Starling's Law of the Heart

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Abstract
This investigation was designed to study the mechanical properties of human myocardium in vivo, and to ascertain whether Starling's law applies to the human ventricle by determining whether the length of a segment of myocardium influences its force of contraction. A modified Walton-Brodie strain-gauge arch was sewn to the right ventricle of 10 patients and to the left ventricle of one patient at the time of operation. This instrument permitted the controlled alteration of the length of the myocardial segment to which it was attached, as well as the measurement of the tension exerted by this segment of myocardium throughout the cardiac cycle. In all 11 patients studied it was noted that as myocardial length was increased, the end-diastolic tension tended to rise slightly at first, but with further stretching the end-diastolic tension increased more strikingly. As the segment was stretched the maximal systolic tension, which it developed, increased more than did its resting tension. Thus, within limits, the active or developed tension rose as the myocardial segment increased in length. In five patients the active tension declined after the myocardial segment had been stretched to values ranging between 15 and 50 per cent above baseline length, i.e., a descending limb of the length-active tension relationship was evident. No change in duration of active tension occurred with progressive stretching of the myocardial segment, and changes in the rate of development of tension paralleled the changes in the active tension. In six patients the segment of myocardium was stretched progressively and was then permitted to shorten in the opposite direction. Hysteresis of the myocardium was demonstrated, since, at any given myocardial segment length, the resting and the maximal tensions were higher when the segment was being lengthened than when it was being shortened. These observations provide a description of the mechanical properties of functioning human myocardium and indicate that this tissue behaves in accordance with the Frank-Starling principle.
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