Effects of sensor selection on exercise stroke volume in pacemaker dependent patients.

The effects of sensor selection and sensor blending on the cardiovascular response to graded exercise was evaluated in 10 patients (age 74 +/- 2 yrs; 7 men and 3 women) implanted with a dual sensor rate adaptive VVIR pacemaker (Vitatron Topaz model 515). Patients underwent three graded exercise tests (GXT) with sensor programming randomly assigned. For a given graded exercise text the pacemaker was programmed into activity sensing (ACT), QT sensing, or dual sensing (ACT = QT). Data were recorded at rest and during each stage of the graded exercise text. Oxygen uptake (VO2) was measured continuously using a Q Plex I system. Heart rate (HR), stroke volume (SV), and cardiac output (Qc) were measured by impedance cardiography. Systolic time intervals were calculated from simultaneous recordings of the ECG, phonocardiogram, and the impedance cardiogram. In response to the GXT no differences in peak VO2 were observed across the three sensor settings. Regardless of the sensor setting Qc increased linearly with each increment in VO2. The HR response to ACT only pacing was significantly higher than in the other two pacing conditions. During ACT only pacing SV failed to rise in response to exercise. The increased exercise Qc during QT and ACT = QT pacing were mediated by significant increases in both HR and SV. The QT and dual pacing conditions were also associated with longer diastolic filling times. The data indicate that the mechanisms responsible for the increase Qc during exercise were different for ACT versus ACT = QT or QT sensor-driven pacing.