Cerebral Hemodynamics at Altitude Using Diffuse Correlation Spectroscopy and Transcranial Doppler

Abstract

Studies of cerebral hemodynamics at altitude have been hampered by the lack of a portable, real-time, noninvasive monitor of intracranial blood flow. Most studies to date have relied on cerebral blood flow velocities (CBF-V) measured by transcranial Doppler, as a surrogate for cerebral blood flow. The use of this measurement to accurately reflect cerebral blood flow is predicated on the recently disproven notion that middle cerebral artery (MCA) diameter remains constant with changes in altitude. We used a novel technology, diffuse-correlation spectroscopy (DCS), which is not dependent on MCA diameter changes and has been previously validated against Xenon CT, to assess the cerebrovascular response to hyperventilation during acclimatization and compared this to traditional transcranial Doppler (TCD). Eleven subjects from the Xtreme Alps medical research expedition were studied at sea level and 2 and 7 days after ascent to 4559 m before and after hyperventilation to reduce end-tidal CO2 by 50%. Transcranial Doppler was used to measure peak systolic velocity (PSV) in the MCA. Diffuse-correlation spectroscopy was used to measure changes in relative CBF (rCBF). Diffuse-correlation spectroscopy revealed a decrease in rCBF of 19.6% ± 18.9% following hyperventilation at sea level, 28.9% ± 13.6% following hyperventilation after 2 days at altitude and 33.0% ± 29.0% following 7 days at altitude. PSV as measured by TCD demonstrated a decrease of 32% ± 12.7%, 28.9% ± 32.3% and –29.4% ± 7.6% respectively. Diffuse-correlation spectroscopy provides a safe, portable, and noninvasive measure of cerebral blood flow at altitude, reflecting cerebral perfusion at the parenchymal level. By contrast TCD provides a measure of velocity of flow through large resistance vessels, with no direct information about end-organ perfusion. While DCS correlates poorly with TCD, further studies are necessary to elucidate which test is a more accurate reflection of cerebral blood flow.