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(Circulation. 1995;92:1849-1859.)
© 1995 American Heart Association, Inc.
Articles |
Accelerometer Systolic Time Intervals as Fast-Response Sensors of Upright Posture in the Young
From the Departments of Pediatrics (M.O.) and Medicine (K.G., F.I.M.), University Heart Center, University of Arizona Health Sciences Center, Tucson, Ariz; and the Division of Pediatric Cardiology (D.T.), University of Illinois College of Medicine (Chicago).
Correspondence to Marc Ovadia, MD, Assistant Professor of Pediatrics/Cornell University Medical College, North Shore University Hospital, 300 Community Dr, Manhasset, NY 11030.
Background Sensors of posture may improve rate-adaptive
pacing in syndromes where syncope occurs in the upright posture,
particularly in the young. No sensor of posture has been described to
date. Previous studies suggest that two sensors currently under
investigation (preejection period [PEP] and left
ventricular ejection time [LVET] systolic time
intervals [STIs] and accelerometers) may be affected by posture. A
PEP-sensing pacemaker is available commercially in which heart rate
(HR) decreases with an increase in PEP
(
[HR]/[PEP]<0). In
patients with upright syncope, it is not known how such algorithms
respond to posture. Also, it is not known whether STIs correlate with
posture independent of autonomic tone.
Methods and Results We studied accelerometer-derived STIs in head-upright tilt-testing with ß-blockade and catecholamine stimulation in patients with syncope 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 to four tilts (60°) at baseline, during esmolol infusion (500 µg/kg load, 50 to 140 µg/kg per minute), after esmolol withdrawal, and during isoproterenol infusion if not contraindicated. PEP, LVET, and 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-upright) 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 were observed during tilt with ß-blockade and esmolol withdrawal, and during isoproterenol infusion. STI changes occurred immediately on postural change and were stable. Postural change of PEP was greater than the ß-adrenergic effect by 6:1. Postural change of STIs was independent of vagal tone.
Conclusions First, accelerometer-derived STIs detect postural changes. Because these changes are independent of autonomic tone and are rapid and stable, they may be useful as fast-response sensors of upright posture in rate-adaptive pacemakers. Second, with postural change, HR increases when PEP increases. However, PEP-sensing pacemakers presently under investigation assume the opposite (inverse) mathematical relationship. Therefore, current PEP-sensing pacemakers use an incorrect algorithm for physiological postural responses in syncope patients. These data predict a paradoxical tachycardic response to the supine posture in patients implanted with these devices.
Key Words: pacing • arrhythmias • nervous system • autonomic • testing • pediatrics
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