Cerebral Artery Dilatation Maintains Cerebral Oxygenation at Extreme Altitude and in Acute Hypoxia—An Ultrasound and MRI Study

Abstract

Transcranial Doppler is a widely used noninvasive technique for assessing cerebral artery blood flow. All previous high altitude studies assessing cerebral blood flow (CBF) in the field that have used Doppler to measure arterial blood velocity have assumed vessel diameter to not alter. Here, we report two studies that demonstrate this is not the case. First, we report the highest recorded study of CBF (7,950 m on Everest) and demonstrate that above 5,300 m, middle cerebral artery (MCA) diameter increases (n=24 at 5,300 m, 14 at 6,400 m, and 5 at 7,950 m). Mean MCA diameter at sea level was 5.30 mm, at 5,300 m was 5.23 mm, at 6,400 m was 6.66 mm, and at 7,950 m was 9.34 mm (P<0.001 for change between 5,300 and 7,950 m). The dilatation at 7,950 m reversed with oxygen. Second, we confirm this dilatation by demonstrating the same effect (and correlating it with ultrasound) during hypoxia (FiO2=12% for 3 hours) in a 3-T magnetic resonance imaging study at sea level (n=7). From these results, we conclude that it cannot be assumed that cerebral artery diameter is constant, especially during alterations of inspired oxygen partial pressure, and that transcranial 2D ultrasound is a technique that can be used at the bedside or in the remote setting to assess MCA caliber.

Cerebral Pressure–Flow Relationship in Lowlanders and Natives at High Altitude

“…Further, given the known limitations of this technique and the possibility of dilation in the MCA, 33,34 we did not place emphasis on absolute MCAv/PCAv values but only the relative changes. Importantly, the high temporal resolution provided by the transcranial Doppler ultrasound makes it an ideal tool to study the pressure-flow relationship.…”

Cerebrovascular Reactivity is Increased with Acclimatization to 3,454 M Altitude

“…In the present study, PetCO 2 was elevated 10 mm Hg above resting values, whereas changes in MCA diameter have been reported only with increases in 15 mm Hg or more. Moreover, dilation of the MCA in hypoxic conditions has been shown in altitudes 45,000 m, 3,18,39 and vessel diameter changes at 3,454 m are less likely to have occurred. 40 Nonetheless, if hypercapnia and/or hypoxia facilitated vessel diameter changes in the present study, this would have resulted in an underestimation of CBF.…”

“…2 In addition, the majority of studies have assessed CBF by means of transcranial Doppler ultrasound (TCD), [8][9][10][11] which for an accurate determination of flow, relies on unchanged vessel diameter. In conditions above 45,000 m (or the equivalent degree of hypoxia) 3,18 or changes in PaCO 2 more than 15 mm Hg 19 above resting values, changes in middle cerebral artery (MCA) diameter have been reported. Therefore, when interpreting TCD-derived data, these shortcomings need to be acknowledged.…”

Acute high-altitude sickness

“…CBF measured by transcranial Doppler in AMS yielded controversial results, possibly because of changes in the diameter of the middle cerebral artery that was assumed to remain unchanged [33]. However, changes in the diameter of the internal carotid artery did not correlate with high-altitude headache [34].…”