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Title: Variability of biological signals in critical illness
Author: Willard, Terence B.
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2006
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Inhealthy physiological systems all organs act to maintain relative constancy or homeostasis through a highly complex and integrated communication control network of feedback loops. This constant interaction results in a fluctuation in the beat to beat value of the nominal heart rate. Heart rate variability (HRV) is used to describe the variation in the intervals between consecutive heartbeats or the intervals between consecutive R peaks of the QRS complex obtained from an electrocardiogram (ECG). The hypothesis that illness and injury is associated with a reduction in heart rate variability has been tested in various clinical settings and continues to be evaluated. However standard measures of heart rate variability using power spectral analysis have not always been conclusive and the interpretation of results is still being debated. Newer methods based on ideas from nonlinear mathematics are controversial. Inparticular, the method of approximate entropy (ApEn) may give misleading results. In this study a novel normalized entropy measure L was developed for assessing HRV, using theoretical methods and data from an acute hypovolaemic shock model for validation. Electrocardiogram data from the shock model experiments was processed to . obtain the R-R intervals for analysis and used to validate the measure L. The results were statistically significant in showing the differences between the baseline state and the post shock state and between the baseline and the post resuscitation state after allowance was made for the effects of changes in heart rate. From theoretical analysis and from experimental data the method was shown to be valid in both the time and frequency domain. Theoretical predictions of the inconsistencies of approximate entropy were confirmed by experimental results and in particular the method did not give statistically significant results for the experimental data using the accepted range of tolerance values and number of elements being compared. The standard measures of power spectral analysis were not statistically significant. However, by modifying the analysis to that of amplitude rather than power a statistically significant reduction was shown in the total amplitude and high frequency amplitude, after allowance was made for changes in heart rate, at the same states as the measure L.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available