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Variation in contractile force of the isovolumic contracting left ventricle of the dog was studied in open-chested in situ hearts. The electrocardiogram and intraventricular pressures were recorded at various heart volumes. Spontaneous changes in heart rate and rhythm occurred at all volumes. Isovolumic systolic pressure development (contractile force) varied with rate and rhythm. Contractile force increased with heart rate (treppe) regardless of pacemaker origin. When a premature beat was followed by a compensatory pause, the premature beat showed a decrease and the next beat an increase in contractile force (postextrasystolic potentiation). The magnitude of the changes varied directly with the prematurity of the beat. Mechanical alternans was observed with electrical alternans, despite the absence of significant volume change. Rate-induced changes, postextrasystolic potentiation and mechanical alternans were additive when they occurred simultaneously. For practical purposes, ventricular volume (filling), hence muscle fiber length, remained constant during these rate and rhythm change, therefore could not affect the strength of contraction. Contractile force changes directly attributable to rate and rhythm changes do, therefore, occur in the intact mammalian heart.

Thin-walled vessels have been visualized histologically in the mitral valve of the dog; these vessels were increased in number and caliber in animals with chronic impairment of cardiac lymph flow. Injection of diluted India ink into the free edge of the anterior mitral leaflet of the beating heart revealed extensive networks of vessels, grossly visible on the atrial surface of the valve. Histological study showed the India ink to be within thin-walled channels. Thus, there is compelling evidence that the visualized vessels are lymphatics and that their size and number increase when the cardiac lymphatic drainage is chronically impaired. The possible significance of the presence of lymphatic capillaries in the mitral valve is discussed.

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