Published in 1971, this study examined how beat frequency affects ventricular contraction in three preparations: trabeculae excised from cat and dog hearts, an isovolumic canine left ventricle setup, and a newly developed in situ isometric papillary muscle preparation perfused with oxygenated blood through the coronary vasculature.
Experimental conditions—temperature (27°C), stimulation rates, and force-recording techniques—were standardized across preparations to allow direct comparison. In excised ventricular muscle, researchers observed the expected robust frequency–force relationship, in which increasing heart rate substantially boosts contractile force.
By contrast, in situ ventricular myocardium showed only minimal, if any, enhancement of contraction with increased pacing rate. Peak force development was far less sensitive to frequency changes than in isolated tissue. Even the peak rate of force development—somewhat more responsive—remained markedly blunted relative to excised myocardium.
The investigators systematically ruled out potential confounders: sympathetic outflow, coronary perfusion effects, and easily eluted negative inotropic substances within perfusing blood. None accounted for the pronounced depression of the frequency–force relationship observed in vivo.
The study’s findings highlight fundamental physiological differences between intact and isolated cardiac muscle. Whereas excised tissue exhibits strong rate-dependent force modulation, the intact canine heart operates under conditions that significantly dampen this relationship, underscoring the importance of in situ context when interpreting cardiac contractility.
Source: Kavaler, F., Harris, R., Lee, R. J., & Fisher, V. J. (1971). Frequency-Force Behavior of in Situ Ventricular Myocardium in the Dog. Published May 1, 1971.
