Nitrous oxide alters oxidative metabolic activities of rat neocortex in situ

Transcranial Doppler sonography: nitrous oxide and cerebral blood flow velocity in children

To determine the effect of nitrous oxide (N2O) on cerebral blood flow velocity (CBFV) and cerebrovascular resistance index (RI+) in children, ten ASA physical status I or II patients aged one to eight years old, scheduled for urological procedures, were studied. Anaesthesia was induced with thiopentone 2 mg.kg-1, fentanyl 5 micrograms.kg-1 and diazepam 0.3 mg.kg-1. Muscular relaxation was ensured by using vecuronium 0.1 mg.kg-1. After tracheal intubation, anaesthesia was randomly assigned to either a mixture of air in oxygen (N2/O2) or 70% N2O in oxygen (N2O/O2) producing an FIO2 of 30%. Three sets of measurements of CBFV and RI+ were made with both gas mixtures. The CBFV and RI+ were measured in the middle cerebral artery (MCA) with a transcranial Doppler monitor. Measurements were made while using the initial gas mixture, then the second gas mixture was administered, and finally, the patient again was given the initial gas mixture. A continuous caudal epidural or lumbar epidural block was performed before skin incision. Neuromuscular blockade was maintained with vecuronium 0.05 mg.kg-1. Temperature, heart rate, end-tidal CO2, arterial oxygen saturation, haematocrit and arterial blood pressure were maintained constant. Ventilation was adjusted to achieve normocapnia. The CBFV increased when 70% N2/O2 was replaced by 70% N2O/O2 (P less than 0.05) while the CBFV decreased when 70% N2/O2 was readministered (P less than 0.05). Likewise, the CBFV decreased when 70% N2O/O2 was replaced by 70% N2/O2 (P less than 0.05) while the CBFV increased when 70% N2O/O2 was readministered (P less than 0.05).

Detection of an oxidizable fraction of cytochrome oxidase in intact rat brain

Rapid-scanning reflectance spectrophotometry was used to evaluate the reduction-oxidation state of cytochrome oxidase in normoxic rat brain. Reflectance spectra were recorded from intact blood-perfused cerebral cortices after increased oxidative metabolic activity induced by direct cortical stimulation. Reflectance spectra taken from blood-free rat brain and from the rat ear vascular bed were used to identify cytochrome and hemoglobin components of spectra taken from intact brain. Cortical stimulation provoked shifts toward increased oxidation of cytochrome oxidase that were detectable in reflectance spectra. Observations of an oxidizable fraction of cytochrome oxidase demonstrate that a fraction of the cytochrome oxidase pool exists in a reduced state in normoxic brain. The presence of reduced cytochrome oxidase suggests that oxygen delivery to the brain is restricted by microvascular control mechanisms either as a function of brain metabolic physiology or as protection against oxygen toxicity.

Effect of nitrous oxide on local cerebral glucose utilization in rats

The influence of 70-80% N2O on local local cerebral glucose utilization (CMRg1) in the rat brain was studied with the [14C]deoxyglucose method in minimally restrained, spontaneously breathing animals 75 min following discontinuation of halothane anaesthesia. Nitrous oxide was found to have only small effects on local CMRg1 in the majority of the 25 structures analyzed. When corrections were made for a small difference in body temperature between nitrous oxide--breathing animals and those breathing air nitrous oxide was found to significantly increase local CMRg1 in some subcortical structures by 15-25% (red nucleus, thalamus, geniculate bodies, and superior colliculus), and to decrease local CMRg1 in nucleus accumbens and sensorimotor cortex by comparable amounts. Thus, although nitrous oxide does not alter overall glucose utilization in the brain, it differentially affects CMRg1 in some brain structures.