Date of Award

2010

Document Type

Open Access Thesis

Degree Name

Medical Doctor (MD)

First Advisor

David Silverman

Abstract

The pulse transit time (PTT) has been investigated as an early noninvasive marker of hypovolemia, reflecting a combination of changes in the pre-ejection period (PEP) and vascular transit time (VTT). The use of photoplethysmography (PPG) has limited the analysis to the macrovascular peripheral circulation by nature of the detection mechanism of the PPG itself. Excluded is the richly innervated microcirculation that may have a significant influence on the vasomotor response to a hypovolemic challenge. Therefore the time required for the blood to travel from the PPG to the microvasculature (VTTm) under a laser Doppler flowmeter (LDF) would provide a more complete understanding of the physiologic response to hypovolemia. The present study sought to assess changes in the components of PTT, including VTTm, in a head-up tilt (HUT) model and in a post hoc analysis of data recorded from a two-unit blood withdrawal (BW) experiment performed by members of this research team. With IRB approval, 10 healthy volunteers were recruited for a 60o HUT test to simulate mild-to-moderate hypovolemia. Monitoring included a 3-lead EKG, Finapres and a PPG and LDF applied to both the finger and ear. Measurements were taken during the pre-tilt phase while the subject was supine and again upon tilting. The data from the BW study were retrospectively analyzed (see Appendix for methodology). Paired t-tests were performed and p-values are given where p < 0.05. During HUT and BW, PEP increased significantly (p < 0.001). While PTT also significantly lengthened with tilting (p = 0.02), no such change was observed with BW. There was no change in heart rate in either experiment. VTT also remained essentially the same after tilting and BW. As expected, VTTm was a significant addition to VTT, however it did not exhibit any significant changes in any region with either hypovolemic challenge. VTT and VTTm of the forehead were significantly different than the finger at baseline and after blood withdrawal. The component values of the ear and finger did not vary significantly. Results confirm previous reports of an increase in PEP in response to a mild-to-moderate hypovolemic insult. Flow through the microcirculation is a significant component of PTT. However, the VTTm did not exhibit a significant response to mild-to-moderate hypovolemia. VTT remained essentially the same in both conditions and PTT was only found to change significantly with tilting. It is interesting to note the lack of change in VTTm with tilting in the finger as well as the ear, even though the arterial network of the finger is more densely innervated by α-adrenergic receptors than other parts of the body. In a comparison with the existing literature, timing of the measurements and testing conditions are believed to significantly influence the results. Though our early timing of the measurements after the hypovolemic challenge might veil the presence of a sympathetic hemodynamic response, it is believed that baseline hydration status of the subject and time spent in the supine position before tilting are the root causes of this discrepancy with other studies.

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