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Heart rate variability is commonly interpreted as a measure of vagal tone. Changes in heart rate variability and more specifically respiratory sinus arrhythmia (RSA) are however also commonly observed when breathing is altered without changes in heart. This study examined how decreasing vagal tone through a low level (6%MVC) handgrip (HG) contraction and changing breathing frequency, alone or in combination, affected heart rate variability and respiratory sinus arrhythmia. ECG and respiratory recordings were obtained in 16 university students. Results show that decreasing breathing rate did not affect mean heart rate but resulted in a significant increase (p < 0.05) from baseline values in RSA amplitude (%) (SP: 0.11+/-0.07; SP-4: 0.14+/-0.07). On the other hand, under spontaneous breathing conditions sustained handgrip contraction resulted in a small yet significant increase in mean heart rate (SP: 62+/-6 bpm; SP-4: 69+/-11 bpm; p < 0.05) but was not associated with significant changes in RSA amplitude. The slope of the relationship between RSA amplitude and respiratory cycle duration was taken to reflect the vagal responsiveness to respiratory stimulation. A slight downward parallel shift from baseline was observed under sustained HG but significant differences in slope or y-intercept were not observed. These results suggest that indices of heart rate variability and respiratory sinus arrhythmia may be more appropriate indices of cardiac vagal efferent activity modulations by respiration than tonic activity.

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Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Document formatted into pages; contains x, 86 p.; also includes graphics (some col.). Includes bibliographical references (p. 78-86). Available online via OhioLINK's ETD Center.

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Thesis (Ph. D.)--University of Wisconsin--Madison, 1958. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

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