Implanted Cardiac Pacemakers of Variable Frequency: Hemodynamic Studies of Two Patients

Studies were made to assess the advantages of variable frequency control in two patients with complete heart block in whom cardiac pacemakers had been inserted. Ventricular rate could be varied electromagnetically by use of an external induction coil. With the patient at rest, cardiac output was relatively independent of the ventricular rate of between 66 and 120. One patient, exercised at a fixed ventricular rate of 66, showed an increase in cardiac output from augmented stroke volume alone. A further increase in cardiac output, despite a decrease in stroke volume, occurred when the ventricular rate was set at the "physiological" level compatible with the exercise load. This effectively differentiates the role of heart rate and stroke volume as determinants of cardiac output.