The effects of nitrous oxide and oxygen on transient hyperemic response in human volunteers.

UNLABELLED: The aim of this study was to determine the effects of breathing 100% oxygen or 50% nitrous oxide in oxygen on the indices of cerebral autoregulation derived from the transient hyperemic response (THR) test in human volunteers. Data were analyzed from nine healthy subjects. Middle cerebral artery (MCA) blood flow velocity (FV) was measured by transcranial Doppler ultrasound, and the THR test was performed using 10-s compression of the common carotid artery. Continuous measurement of P(ETCO2) and expired fractions of oxygen (F(ETO2)) and nitrous oxide (F(ETN2O)) was established, and mean arterial pressure (MAP) was recorded at 2-min intervals. All measurements were performed while the volunteers were breathing room air and were repeated 10 min after achieving F(ETO2) >0.95 and 10 min after achieving F(ETN2O) 0.48-0.52. Two indices derived from the THR test, the transient hyperemic response ratio (THRR) and strength of autoregulation (SA), were used to assess cerebral autoregulation. P(ETCO2) and mean arterial pressure did not change significantly throughout the study period. Breathing 100% oxygen did not change MCA FV, THRR, or SA. Inhalation of nitrous oxide resulted in a marked and significant increase in the MCA FV (from 48+/-9 to 72+/-8 cm/s; mean +/- SD) and a significant decrease in the THRR (from 1.5+/-0.2 to 1.2+/-0.1) and the SA (from 1.0+/-0.1 to 0.8+/-0.1) (P<0.05 for all). We conclude that breathing 50% nitrous oxide in oxygen results in both a significant increase in MCA FV and impairment of transient hyperemic response. IMPLICATIONS: Our study suggests that nitrous oxide impairs cerebral autoregulation and may have implications for its use in neurosurgical anesthesia and for interpretation of the results from studies of anesthetics in which nitrous oxide is used in the background.