title: 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.
authors.affiliation: Centre for Altitude, Space and Extreme Environment Medicine, Institute of Human Health and Performance, Charterhouse Building, UCL Archway Campus, University College London, London, UK. mark@medicstravel.com
authors.forename: Mark H
authors.initials: MH
authors.lastname: Wilson
authors.forename: Mark E G
authors.initials: ME
authors.lastname: Edsell
authors.forename: Indran
authors.initials: I
authors.lastname: Davagnanam
authors.forename: Shashivadan P
authors.initials: SP
authors.lastname: Hirani
authors.forename: Dan S
authors.initials: DS
authors.lastname: Martin
authors.forename: Denny Z H
authors.initials: DZ
authors.lastname: Levett
authors.forename: John S
authors.initials: JS
authors.lastname: Thornton
authors.forename: Xavier
authors.initials: X
authors.lastname: Golay
authors.forename: Lisa
authors.initials: L
authors.lastname: Strycharczuk
authors.forename: Stanton P
authors.initials: SP
authors.lastname: Newman
authors.forename: Hugh E
authors.initials: HE
authors.lastname: Montgomery
authors.forename: Mike P W
authors.initials: MP
authors.lastname: Grocott
authors.forename: Christopher H E
authors.initials: CH
authors.lastname: Imray
mesh_terms: D000208:Acute Disease; D000328:Adult; D000532:Altitude Sickness / Q000000981:diagnostic imaging / Q000503:physiopathology; D001783:Blood Flow Velocity; D006801:Humans; D000860:Hypoxia / Q000000981:diagnostic imaging / Q000503:physiopathology; D018810:Magnetic Resonance Angiography; D008279:Magnetic Resonance Imaging; D008297:Male; D008875:Middle Aged; D020768:Middle Cerebral Artery / Q000000981:diagnostic imaging / Q000503:physiopathology; D010101:Oxygen Consumption; D010313:Partial Pressure; D011859:Radiography; D014463:Ultrasonography; D014664:Vasodilation; D055815:Young Adult