Chapter 13: Cardiac Muscle Mechanics & the Cardiac Pump

Cardiac Muscle Mechanics & the Cardiac Pump explores the cellular mechanisms of excitation-contraction coupling, describing how calcium entry through dihydropyridine channels triggers massive calcium release from the sarcoplasmic reticulum via ryanodine receptors, a process modulated by sympathetic stimulation and cyclic AMP to regulate the inotropic state. The text details the hemodynamic events of the cardiac cycle, sequencing the phases of isovolumic contraction, rapid and reduced ejection, isovolumic relaxation, and ventricular filling, while correlating these mechanical events with electrocardiogram readings and the generation of heart sounds. Significant attention is given to the determinants of myocardial performance, specifically preload, which follows Starling's law of the heart, afterload, which is functionally defined by ventricular wall stress and aortic pressure, and contractility, which shifts the active length-tension relationship. The chapter utilizes pressure-volume loops to visualize stroke volume, work, and the end-systolic pressure-volume relationship (ESPVR), illustrating how positive inotropes shift the ESPVR to increase stroke volume independent of loading conditions. It further discusses the physiological regulation of cardiac output as the product of heart rate and stroke volume, explaining the reciprocal relationship between rate and filling time. Clinical measurement techniques are reviewed, including the Fick principle, indicator dilution, thermodilution, and noninvasive imaging modalities like echocardiography for assessing ejection fraction and wall motion. Finally, the text analyzes myocardial energetics, identifying wall stress as a primary determinant of oxygen demand, and addresses pathophysiological states such as heart failure, where compensatory mechanisms like fluid retention and hypertrophy attempt to maintain cardiac output despite depressed contractility.