Cardiovascular Dynamics in Complete Heart Block at Various Heart Rates

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Abstract
Cardiovascular dynamics were deter-mined with the indicator-dilution technique in 13 patients with complete heart block resulting from coronary disease. An electrode catheter placed in the right ventricle was connected to an external pacemaker and used to control the heart rate. Cardiac output, stroke volume, central blood volume, brachial artery pressure, peripheral resistance and left ventricular work were measured at various heart rates from a minimal rate of 25 to a maximum of 125 beats/min. The cardiac index at a slow heart rate averaged 1. 84 L/min M.2 As the heart rate increased to 76 beats/min. the cardiac output increased to 3.09[plus or minus] 0. 77 L/min/M.2 The left ventricular work and mean pressure also increased while the stroke volume and peripheral resistance decreased. Further increase in the heart rate to 125 beats/min resulted in a fall in the cardiac output and stroke volume with a further increase in the left ventricular work. Exercise at a fixed heart rate at the "optimum rate" of pacing (76 beats/min) resulted in an increase in the cardiac output, stroke volume and mean pressure with a decrease in the peripheral resistance. It is, therefore, suggested that variations in stroke volume and peripheral resistance are important regulatory mechanisms of the cardiac dynamics and that these mechanisms become of great significance when the heart rate cannot be increased.