Abstract
Introduction
Most current assessments of cerebral hemodynamics at altitude rely on transcranial dopplers (TCDs). This technology does not monitor cerebral blood flow (CBF), per se, but rather blood flow velocity. Equating CBF to blood flow velocity is predicated on multiple assumptions that remain unproven at altitude, such as constant diameter of medium-sized arteries. An increase in middle cerebral artery velocity as measured by TCD is interpreted as evidence of vasospasm within the context of subarachnoid hemorrhage, for example, but vasodilation within the context of ascent to altitude. Current techniques to directly assess CBF, such as Xenon CT (XeCT), are either too cumbersome or too expensive to allow serial measurements to assess the temporal dynamics of changes in CBF during ascent to high altitude.
We present a human feasibility study of a technique, previously validated in animal models, that allows measurement of CBF that is uniquely suited to investigation of blood flow changes at altitude.
Methods
Near infrared spectroscopy coupled with intravenous injection of indocyanin green dye (ICG-NIRS) was used to measure a CBF index in 3 patients undergoing laparoscopic surgery to assess feasibility of this technique in humans.
Results
Two of the 3 patients exhibited an increase in CBF index of 40% and 59%, during insufflation of CO2. The third patient exhibited a paradoxical decrease in CBF index of 39%.
Conclusions
The combination of ICG-NIRS and determination of CBF index is a promising, minimally invasive, portable and straightforward technique to provide repeated quantitative information regarding changes in CBF. This method is well suited to use in the alpine environment. Further refinements in technique and equipment, as well as formal validation studies in humans, are needed to increase the utility of ICG-NIRS.