ACCELEROMETER SYSTOLIC-TIME INTERVALS AS FAST-RESPONSE SENSORS OF UPRIGHT POSTURE IN THE YOUNG

Background Sensors of posture may improve rate-adaptive pacing in synd romes where syncope occurs in the upright posture, particularly in the young. No sensor of posture has been described to date. Previous stud ies suggest that two sensors currently under investigation (preejectio n period [PEP] and left Ventricular ejection time [LVET] systolic time intervals [STIs] and accelerometers) may be affected by posture. A PE P-sensing pacemaker is available commercially in which heart rate (HR) decreases with an increase in PEP (partial derivative[HR]/partial der ivative[PEP]<0). In patients with upright syncope, it is not known how such algorithms respond to posture. Also, it is not known whether STI s correlate with posture independent of autonomic tone. Methods and Re sults We studied accelerometer-derived STIs in head-upright tilt-testi ng with beta-blockade and catecholamine stimulation in patients with s yncope or presyncope using an ultra-low-frequency accelerometer placed on the chest. Thirty-two patients age 6 to 22 years with unexplained recurrent syncope or presyncope underwent tilt-testing involving two t o four tilts (60 degrees) at baseline, during esmolol infusion (500 mu g/kg load, 50 to 140 mu g/kg per minute), after esmolol withdrawal, a nd during isoproterenol infusion if not contraindicated. PEP, LVET, an d other indexes were quantified, and their relations to posture and to autonomic state were determined. With tilt, PEP increased from 98.9+/ -2.2 to 109.1+/-2.8 msec (P<.0001), and LVET decreased (supine-to-upri ght) from 295.5+/-4.5 to 247.2+/-4.7 msec (P<.0001). PEP/LVET changed from 0.337+/-0.01 to 0.45+/-0.02 (P<.0001). Similar postural changes w ere observed during tilt with beta-blockade and esmolol withdrawal, an d during isoproterenol infusion. STI changes occurred immediately on p ostural change and were stable. Postural change of PEP was greater tha n the beta-adrenergic effect by 6:1. Postural change of STIs was indep endent of vagal tone. Conclusions First, accelerometer-derived STIs de tect postural changes. Because these changes are independent of autono mic tone and are rapid and stable, they may be useful as fast-response sensors of upright posture in rate-adaptive pace makers. Second, with postural change, HR increases when PEP increases. However, PEP-sensin g pacemakers presently under investigation assume the opposite (invers e) mathematical relationship. Therefore, current PEP-sensing pacemaker s use an incorrect algorithm for physiological postural responses in s yncope patients. These data predict a paradoxical tachycardic response to the supine posture in patients implanted with these devices.