Doppler study of middle cerebral artery blood flow velocity and cerebral autoregulation during a simulated ascent of Mount Everest

Abstract

Objective. To explore cerebral hemodynamics in 8 healthy volunteers in a hypobaric chamber up to the altitude of Mount Everest after a progressive stepwise decompression to 8848 m. Methods. Physiological, clinical, and transcranial Doppler data were collected after at least 3 days at 5000, 6000, and 7000 m and within 4 hours of reaching 8000 m and returning to sea level. Results. Three subjects were excluded at 8000 and 8848 m because of acute neurological deficits. Heart rate increased; mean arterial pressure remained stable; PaO2 and PaCO2 decreased with altitude; hemoglobin (Hb) and hematocrit (Ht) increased; arterial O2 content decreased over 6000 m; middle cerebral artery blood flow velocity (MCAv) increased only during acute exposure to 8000 m; and the corresponding pulsatility (PI) and resistivity indices (RI) decreased over 5000 m. PI and RI correlated with heart rate. The transient hyperemic response (THR) of MCAv to common carotid compression was depressed at 8000 m. Conclusions. At 8000 m, the increase in MCAv seemed to reflect the normal hemodynamic response to acute hypoxia. The decrease of THR at this altitude could be an indication of impaired cerebral autoregulation. The role of impaired cerebral autoregulation in the genesis of acute neurologic deficits, observed at 8000 m and above in 3 subjects, remains speculative.