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 (FiO(2)=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.

Figure 1

Ultrasound images demonstrating (A) velocity/ratio and (B) vessel diameter measurement and (C) composite of four magnetic resonance imaging (MRI) images, demonstrating middle cerebral artery (MCA) multiplanar reconstruction and analysis.

Figure 2

Composite of seven graphs, demonstrating changes in blood pressure, arterial oxygen saturation (SaO₂), regional cerebral oxygenation (rSO₂), end tidal CO₂ (ETCO₂), peak systolic, end diastolic and mean velocities, middle cerebral artery (MCA) diameter (MCA_Diam), calculated MCA flow (MCA_Flow), and oxygen delivery (note: blood pressure and EtCO₂ were not reassessed after oxygen administration at 7,950 m).

Figure 3

A graph demonstrating the correlation between transcranial Doppler (TCD) and magnetic resonance imaging (MRI) measurements of middle cerebral artery (MCA) diameter.