Regional differences in the cerebral blood flow velocity response to hypobaric hypoxia at high altitudes

Abstract

Symptoms of acute mountain sickness (AMS) may appear above 2,500 m altitude, if the time allowed for acclimatization is insufficient. As the mechanisms underlying brain adaptation to the hypobaric hypoxic environment are not fully understood, a prospective study was performed investigating neurophysiological changes by means of near infrared spectroscopy, electroencephalograpy (EEG), and transcranial doppler sonography at 100, 3,440 and 5,050 m above sea level in the Khumbu Himal, Nepal. Fourteen of the 26 mountaineers reaching 5,050 m altitude developed symptoms of AMS between 3,440 and 5,050 m altitude (Lake-Louise Score ⩾3). Their EEG frontal beta activity and occipital alpha activity increased between 100 and 3,440 m altitude, i.e., before symptoms appeared. Cerebral blood flow velocity (CBFV) in the anterior and middle cerebral arteries (MCAs) increased in all mountaineers between 100 and 3,440 m altitude. During further ascent to 5,050 altitude, mountaineers with AMS developed a further increase in CBFV in the MCA, whereas in all mountaineers CBFV decreased continuously with increasing altitude in the posterior cerebral arteries. These results indicate that hypobaric hypoxia causes different regional changes in CBFV despite similar electrophysiological changes.

Figure 1

Changes in cerebral blood flow velocity (CBFV) for all mountaineers in the anterior cerebral artery (A), middle cerebral artery (B), and in the posterior cerebral artery (C). Differences in CBFV in the middle cerebral artery between mountaineers who developed acute mountain sickness (AMS) or did not are shown in panel (D). Different altitudes were compared using Wilcoxon test; differences between AMS and non-AMS mountaineers with the Mann–Whitney U-test.

Figure 2

Changes in frontal beta activity in non-acute mountain sickness (non-AMS) (A) and AMS (B) mountaineers. Panel (C) shows changes in occipital EEG alpha activity in non-AMS and panel (D) in AMS mountaineers. Different altitudes were compared using Wilcoxon test; differences between AMS and non-AMS mountaineers with the Mann–Whitney U-test. EEG, electroencephalography.