Influence of temperature on the mechanical properties of cardiac muscle.

The influence of temperature on contractile performance, total myocardial stiffness, and the elastic and viscous components of stiffness was studied in intact hearts (dog) and isolated cardiac muscle (cat). In 12 dogs on whole-heart bypass, stiffness (AP/AV) was determined by inducing sinusoidal 0.5-ml volume changes in paced, isovolumically contracting left ventricles and measuring the resultant pressure changes (AP). In eight cat papillary muscles contracting isometrically at Lmax, stiffness (AT/AL) was similarly determined from the changes in tension and length during sinusoidal length changes of 0.25% Lmax. Total stiffness was linearly related to ventricular pressure or muscular tension during both contraction and rest. As the temperature rose from 33° to 40°C in vivo and from 22° to 40°C in vitro, the slopes of the stiffness-pressure and stiffness-tension relationships and the intercept of the latter declined; therefore, stiffness is inversely related to temperature. Since both preparations responded as linear second-order systems, stiffness could be separated into its elastic and viscous components. The moduli of both components varied in the same direction as total stiffness during temperature changes; however, the viscous component was more sensitive to the influence of temperature.