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| The University of Tennessee Health Science Center | The University of Memphis | LeBonheur Children's Medical Center | St. Jude Childens Research Hospital | The Regional Medical Center |
The Memphis Center For Biomedical Engineering in Pediatrics
Effects of Orthostatic Stress on Heart Rate and Heart Rate Variability in Near Term Infants Exposed to In Utero Cocaine.
Executive summary of thesis by Vijay John
Introduction:
This research was carried out to understand the effects of orthostatic stress on heart rate variability in near term and full term infants exposed to cocaine in utero. All the infants were healthy 5-days or less postnatal age at the time of the study.
Orthostatic stress is induced by head-up tilt using a tilt table at 25°, causing the lowering of blood pressure in the body parts above the level of the heart mainly the blood supply to the brain. The reflex heart rate response that follows regulates the cardiac output thus bringing the blood pressure back to normal.
Hypothesis:
This research was based on the hypothesis that in utero cocaine exposed near-term and full term infants have a significant difference between heart rate variability as determined by different measurements that include time and frequency domain measurements, with orthostatic stress compared to control infants of the same gestational age.
Background:
The heart is designed to match cardiac output with the needs of the body. Thus sudden changes in heart rate are common and expected in response to physical or mental stress and exercise. Even in the absence of external perturbations, the normal heartbeat is not characterized by clockwise regularity. This fluctuation around the mean heart rate is called heart rate variability (HRV). HRV therefore is a reflection of the cardiovascular control exerted by the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS) on the heart that helps regulate cardiac output.
Cocaine is a sympathomimetic compound with potent local anesthetic properties. A sympathomimetic agent produces effects similar to those of impulses conveyed by adrenergic postganglionic fibres of the sympathetic nervous system. The principal hypothesis accepted to date to explain the sympathomimetic effects of cocaine is that this drug inhibits neuronal monoamine neurotransmitter reuptake by binding to a transporter or uptake site thereby increasing the effective concentration of neurotransmitter at adrenergic receptor sites.
Methods:
The subjects used in this study are near-term infants with gestational ages ranging from 36-38 wks. We plan to collect data from 23 control infants and 21 cocaine infants with prenatal cocaine exposure respectively. A 30 min segment of the ECG was recorded with the neonate in supine position and following a 25° degree head-up tilt position. All recordings were done on infants in quiet sleep state. The artifact free RR intervals calculated using the R-wave peak detection algorithm is used for the data analysis. Heart rate variability was determined using time and frequency domain analysis techniques. The time domain analysis included descriptive statistics, instantaneous variability, Poincare plot, DRRn+1 versus DRRn plot, approximate entropy, Fractal analysis. The frequency domain technique used was power spectral analysis.
Future work:
Test the hypothesis using the results from the different time and frequency domain techniques mentioned. Expected completion of the research, data analysis and manuscript preparation will be December 2001.