Regional cerebral blood flow in humans at high altitude: gradual ascent and 2 wk at 5,050 m

Abstract

The interindividual variation in ventilatory acclimatization to high altitude is likely reflected in variability in the cerebrovascular responses to high altitude, particularly between brain regions displaying disparate hypoxic sensitivity. We assessed regional differences in cerebral blood flow (CBF) measured with Duplex ultrasound of the left internal carotid and vertebral arteries. End-tidal Pco2, oxyhemoglobin saturation (SpO2), blood pressure, and heart rate were measured during a trekking ascent to, and during the first 2 wk at, 5,050 m. Transcranial color-coded Duplex ultrasound (TCCD) was employed to measure flow and diameter of the middle cerebral artery (MCA). Measures were collected at 344 m (TCCD-baseline), 1,338 m (CBF-baseline), 3,440 m, and 4,371 m. Following arrival to 5,050 m, regional CBF was measured every 12 h during the first 3 days, once at 5-9 days, and once at 12-16 days. Total CBF was calculated as twice the sum of internal carotid and vertebral flow and increased steadily with ascent, reaching a maximum of 842 ± 110 ml/min (+53 ± 7.6% vs. 1,338 m; mean ± SE) at ∼ 60 h after arrival at 5,050 m. These changes returned to +15 ± 12% after 12-16 days at 5,050 m and were related to changes in SpO2 (R(2) = 0.36; P < 0.0001). TCCD-measured MCA flow paralleled the temporal changes in total CBF. Dilation of the MCA was sustained on days 2 (+12.6 ± 4.6%) and 8 (+12.9 ± 2.9%) after arrival at 5,050 m. We observed no significant differences in regional CBF at any time point. In conclusion, the variability in CBF during ascent and acclimatization is related to ventilatory acclimatization, as reflected in changes in SpO2.

The Contribution of Arterial Blood Gases in Cerebral Blood Flow Regulation and Fuel Utilization in Man at High Altitude

“…Acute hypo-and hypercapnia changed arterial and jugular venous pH at both altitudes. Arterial and internal jugular venous HCO 3 − was decreased at HA relative to SL. Arterial HCO 3 − was increased during hypercapnia, and decreased during hypocapnia at both altitudes.…”

“…3,17 Briefly, the left ICA was measured at least 2 cm from the bifurcation ensuring there was no turbulent or retrograde flow at the site of velocity measurement. The right VA was measured between C6 and C4.…”

“…Moreover, most of these data come from transcranial Doppler ultrasound estimations of CBF that are confounded by constriction or dilation of the insonated vessel in response to both changes in blood gases and ascent to HA. 3 In addition, ventilatory acclimatization elicits slightly different ramifications to arterial than cerebral extracellular fluid; internal jugular venous blood provides a better estimation of brain extracellular fluid parameters than does arterial blood 4 but has not been measured at HA. A number of studies have measured CBF over the course of acclimatization to HA and have found CBF to return to SL values, or remain slightly elevated, after 2 weeks (see ref.…”

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.…”

Regulation of Brain Blood Flow and Oxygen Delivery in Elite Breath-Hold Divers

“…During clamp the identical duration and changes in arterial blood gases were replicated while the subjects breathed. 5,260 m. 10,11,27 Conversely, sea level sensitivity to isocapnic or poikilocapnic hypoxia is greater in the VA. 7,12 Euoxic hypocapnia also elicits greater decreases in VA flow through a broad range of PaCO 2 (~15 to 40 mm Hg); 7 however, this is not a universal finding through smaller changes in PCO 2 . 28 It thus seems that during simultaneous changes in PaO 2 and PaCO 2 , ICA and VA reactivities are similar, as was the case in the present study where ICA and VA showed similar profiles during clamp and apnea.…”