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.

Factors affecting assessment of cerebral autoregulation using the transient hyperaemic response test.

The transient hyperaemic response in the middle cerebral artery blood flow velocity on the release of brief compression of the ipsilateral common carotid artery has been validated as an indicator of cerebral autoregulation. We evaluated, in three stages, the effect of experimental factors such as duration of compression of the common carotid artery and magnitude of the decrease in blood flow velocity during common carotid artery compression on the transient hyperaemic response. In stage 1, 13 healthy volunteers underwent six transient hyperaemic response tests each; two tests each for either 3, 6 or 10 s duration of compression of the common carotid artery. In stage 2, 10 volunteers underwent four transient hyperaemic response tests each; two tests each for either 10 or 15 s duration of compression of the common carotid artery. In stage 3, data from the transient hyperaemic response tests using 10 s compression from the 23 volunteers who participated in stages 1 and 2 were analysed to evaluate the relationship between magnitude of decrease in blood flow velocity at the onset of compression and the transient hyperaemic response. The transient hyperaemic response ratio (blood flow velocity after the release of compression/baseline blood flow velocity) increased significantly when the duration of common carotid artery compression increased from 3 to 6 s, or from 6 to 10 s (stage 1); increase in the duration from 10 to 15 s did not have any significant effect (stage 2). The transient hyperaemic response ratio correlated significantly with the magnitude of decrease in blood flow velocity after compression, up to the values of the compression ratio (percent decrease in blood flow velocity at the onset of compression) of 40% but not more (stage 3). We conclude that experimental factors such as duration of common carotid artery compression and magnitude of the decrease in blood flow velocity during common carotid artery compression can significantly influence the transient hyperaemic response. These factors should be controlled if the transient hyperaemic response test is used for a comparison between repeated measurements. A compression time of 10 s and a compression ratio of 40% or more, allow maximum expression of the hyperaemic response in healthy volunteers.

Reliability of the transient hyperemic response test in detecting changes in cerebral autoregulation induced by the graded variations in end-tidal carbon dioxide.

UNLABELLED: The transient hyperemic response (THR) in the middle cerebral artery (MCA) after the release of brief compression of the ipsilateral common carotid artery has been used to study cerebral autoregulation. We conducted the present study to evaluate the reliability of THR to detect changes in cerebral autoregulation induced by graded variations in PETCO2. Seven healthy adult volunteers were recruited. Fifteen THR tests were performed on every volunteer: three at baseline PETCO2, three each at PETCO2 of 7.5 mm Hg and 15 mm Hg above the baseline, and then three each at PETCO2 of 7.5 mm Hg and 15 mm Hg below the baseline. Transient hyperemic response ratio (THRR) and strength of autoregulation (SA) were calculated using established formulae. Both THRR and SA were highly sensitive (96%) in detecting the changes in cerebral autoregulation induced by graded changes in PETCO2. The within-individual variability of SA was significantly smaller than that of THRR at all levels of PETCO2. IMPLICATIONS: This study demonstrates the reliability of the THR test, when used for repetitive measurements, in detecting changes in cerebral autoregulation induced by graded changes in PETCO2. This test may provide a simple and noninvasive method of evaluating changes in cerebral autoregulation within an individual.

Comparison between alfentanil, pethidine and placebo in the treatment of post-anaesthetic shivering.

We have compared the effects of pethidine, alfentanil and placebo in the treatment of post-anaesthetic shivering. Ninety patients who shivered after routine surgery were allocated randomly to receive normal saline (n = 30), alfentanil 250 micrograms (n = 30) or pethidine 25 mg (n = 30). After 10 min, 26 patients had stopped shivering in the pethidine group which was significantly more than the incidence in the two other groups (placebo = 7; alfentanil = 12) (P < 0.0002). Alfentanil was not significantly different from normal saline in affecting shivering. We conclude that alfentanil 250 micrograms was not effective in the treatment of post-anaesthetic shivering.