TEMPERATURE MAY BE AN APPROPRIATE SENSOR FOR CHRONOTROPICALLY INCOMPETENT PATIENTS WITH POSTURAL SYNCOPE

Chronotropically incompetent patients benefit most from sensor driven rate response during exercise. Postural syncope may occur despite the chronotropic response because of the failure of currently available se nsors to respond physiologically to postural changes. Seven chronotrop ically incompetent patients with postural syncope who had a dual chamb er rate adaptive pacemaker (Circadia(R)) that modulates heart rate in response to temperature change were studied with respect to: (1) respo nse to exercise; and (2) head-up tilt (HUT). During exercise, continuo us-wave Doppler of aortic velocities and two-dimensional echocardiogra phic derived measurements of left ventricular systolic function were u sed to assess cardiac function. Patients exercised longer (by an avera ge of 168 sec) in the DDDR compared to the DDI mode (P = 0.013). Incre ase in exercise duration wets due mostly to the sensor driven increase during DDDR pacing. During DDDR pacing, heart rate increased from 71 +/- 6 to 121 +/- 17 ppm compared to 70 +/- 1 to 103 +/- 21 ppm for the DDI pacing (P = 0.038). Stroke volume as assessed by Doppler derived stroke distance (SD) contributed more significantly to the cardiac out put increase during exercise in the DDI mode (SD increased from 13.4 /- 4 to 18 +/- 7 cm in DDI compared to 13 +/- 4 to 14 +/- 2 cm in DDDR mode), although these mechanisms were insufficient to fully compensat e for failure of appropriate chronotropic response. In response to the HUT, right ventricular temperature increased from 36.78 degrees C +/- 0.29 degrees C to 36.89 degrees +/- 0.28 degrees C (P = 0.0002), and heart rate increased from 54 +/- 3 to 71 +/- 8 ppm (P = 0.0003) in the DDDR mode. No significant change in heart rate occurred in the DDI mo de in response to the HUT. Strong positive correlation of temperature and heart rate was noted in all patients in response to HUT (P = 0.001 , R(2) = 0.755-0.976). We conclude that temperature sensor responds ph ysiologically to exercise and HUT, Therefore, temperature sensing rate adaptive dual chamber pacing may be appropriate for chronotropically incompetent patients with posture related syncope.