Regulation of cardiac output during rapid volume loading

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Abstract
Interactions between heart rate (HR) and the Frank-Starling mechanism in augmenting cardiac output (CO) during acute volume loading were studied in dogs under varying conditions. In normal conscious dogs with a low HR of 70-90 beats/min, end-diastolic diameter (EDD) was maximal and CO was increased reflexly by cardioacceleration. Anesthetized open-chest dogs with high HR ranging from 140-160 beats/min, responded with bradycardia and CO was adjusted solely by increasing stroke volume (SV). In anesthetized open-chest dogs with low HR, EDD was less than maximal and increases in both HR and SV contributed to augment CO. The cardioacceleration reflex and the Frank-Starling mechanism are important determinants of the cardiac response to elevated preload. Although the relative contribution of each control mechanism varies with initial conditions, their interplay extends the range of cardioregulation beyond that attainable by either loop operating alone.