Influence of Digitalization on the Contribution of Atrial Systole to the Cardiac Dynamics at a Fixed Ventricular Rate

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Abstract
The cardiovascular hemodynamic effects of 1 mg of g-strophanthin were studied in 6 patients with complete heart block and fixed ventricular rate. Digitalization resulted in 21, 22, 13 and 28 per cent increases in cardiac index, stroke index, ventricular power and stroke power, respectively, in the absence of any change in ventricular or atrial rates. The arterial pressure did not change significantly after digitalization. Variations in the "P-R interval" of the electrocardiogram resulted in significant changes in ejection time, stroke power and tension-time index. For the "optimum P-R interval" between 100 and 300 mseconds there was an increase in ejection time. Very short "P-R intervals" and P waves occurring during ventricular systole resulted in little or no change in ejection time. Digitalization diminished the contribution of atrial systole to ventricular filling. Atrial systole occurring during the "optimum P-R interval" prolonged isometric contraction time. This increase was diminished or abolished after digitalization. It is concluded that digitalis exerts its effect by a primary inotropic action, thereby increasing cardiac output, stroke volume and stroke power while decreasing ejection time and isometric contraction time independently of changes in ventricular or atrial rates. Its possible effect on the peripheral circulation producing vasodilatation is also suggested on the basis of a decrease in the peripheral resistance after digitalization in the absence of any significant changes in the systemic pressure.

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