Document Type


Date of Award

Spring 5-31-1999

Degree Name

Master of Science in Biomedical Engineering - (M.S.)


Biomedical Engineering Committee

First Advisor

Peter Engler

Second Advisor

Joaquim J. Correia

Third Advisor

Stanley S. Reisman


One of the common and challenging problems confronting the physician in clinical practice is recurrent syncope. Despite extensive evaluations, the causes of these cases Of syncope are not found in more than 40% of the patients.

It is thought that most patients who experience unexplained syncope do so because of transient, unpredictable episodes of vasovagally mediated hypotension and bradycardia. It is hypothesized that impaired inhibition of parasympathetic tone during stress accompanies the development of syncope. Head-up tilt table testing reportedly provokes vasovagal episodes effectively in susceptible persons.

This study attempts to explain the role played by the sympathetic and parasympathetic systems in the syncope condition. Heart Rate Variability, which has been demonstrated to be a reflection of the relative activities of sympathetic and parasympathetic systems, is utilized to compare the two groups of patients that took part. in the study. The comparisons were performed between a set of normal subjects and patients with previous history of syncope. The syncope group was again categorized on the basis of their response to the tilt, i.e. positive or negative to tilt. Patients who experienced symptoms of syncope during the test were grouped positive and those whodid not as negative. Analysis was performed in the time domain as well as frequency domain. In the time domain the HF (parasympathetic) and LF (sympathetic + parasympathic) activity was analyzed before tilt and when in the tilted position, as a function of time. In the frequency domain, the LF and HF areas were compared between the two groups. The LF/HF ratio was a major parameter of interest in understanding the sympathovagal balance during the test.

It was found that in syncope patients the parasympathetic activity did not decrease when the patient was in the tilted position and also that their sympathetic activity did not pick up in response to the tilt as it does in normal subjects. This was clearly evident from the LF/HF analysis.



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