Dominance--one hundred and fifteen years after Mendel's paper

Eight conscious chronically instrumented sheep were exposed to 1% inspired carbon monoxide (CO) for 35 min. In all sheep, carboxyhaemoglobin (COHb) levels at the end of the exposure were approximately 65%. Mean arterial blood pressure was unchanged with the exception of 2 sheep in which administration was stopped at 25 min following the sudden onset of hypotension. Oxygen delivery to the brain was sustained throughout the administration of CO due to a significant increase in cerebral blood flow (CBF). There was no evidence of either a metabolic acidosis or of lactate production by the brain suggesting the brain did not become hypoxic during the time-course of this study. Despite the apparent lack of hypoxia, oxygen consumption by the brain fell progressively and the sheep showed behavioural changes which varied from agitation to sedation and narcosis. The mechanism of these changes was therefore probably unrelated to hypoxia, but may have been due to raised intracranial pressure or a direct effect of CO on brain function. It is proposed that the time-course of progressive CO poisoning includes a phase in which CBF is elevated, blood pressure is unchanged and the brain is normoxic despite high COHb levels, but that this situation can rapidly evolve into a phase of haemodynamic collapse and severe hypoxia.