Date of Award


Document Type

Open Access Thesis

Degree Name

Medical Doctor (MD)

First Advisor

Keith Ruskin, MD

Second Advisor

David Silverman, MD


The present study employed lower body negative pressure (LBNP), a rapidly titratable, safe and reversible means of inducing simulated hypovolemia, for a comparison of transcranial Doppler (TCD) ultrasound of the middle cerebral artery and laser Doppler (LD) flowmetry of the forehead microvasculature. With IRB approval, 9 healthy volunteers (26.3±2.7 years) were monitored continuously with EKG, noninvasive finger arterial blood pressure (BP), and TCD positioned at the transtemporal window. After a baseline (Base) period, subjects underwent rapid onset of LBNP to -70 mmHg over the course of 1 minute, followed by progressive declines of ~10 mmHg until lightheadedness or had a BP decline >20% of baseline BP. Changes in the peak (systolic) and trough (diastolic) values with each heart beat were analyzed at Base, at approx. 30 seconds prior to the onset of lightheadedness (Presympt) and at onset of symptoms (Sympt). In the 6 subjects who subsequently became lightheaded, forehead LD flow decreased by 10.9±11.7% at Presympt (p=NS for interphase difference). It then decreased by an additional 20.4±18.7% with the onset of lightheadedness (p=0.035 for Presympt vs. Sympt). Peak TCD readings decreased by 29.3±9.7% from Base to the time of the Presympt measurement (p=0.001); they then increased by 4.1±12.9% with the onset of Sympt (p=NS). In the 2 subjects who remained asymptomatic, LD did not change significantly in the Presympt and Sympt phases where Sympt was the time when the study was terminated because the BP cutoff was reached. In these asymptomatic subjects, the TCD flow velocity declined progressively. The present findings suggest that monitoring of the microvasculature in the distribution of the carotid arteries provides a better indication of changes in perfusion associated with lightheadedness than measurement of velocity at the middle cerebral artery. The discordance between LD and TCD is consistent with autoregulatory mechanisms at the level of the forehead microvasculature that have previously been reported in the context of systemic administration of phenylephrine.