The effect of CO₂- and O₂-gas mixtures on laser Doppler measured cochlear and skin blood flow in guinea pigs

The effects of carbogen (5% CO₂:95% O₂) 10% CO₂-in-air and 100% O₂ on cochlear blood flow (CBF), skin blood flow (SBP), blood pressure (BP) and arterial blood gases were investigated in the anesthetized, respired or self-respiring guinea pig. In respired animals, CBF and SBF were increased with carbogen and 10% CO₂-in-air and decreased with O₂. BP was elevated with each gas. In freely breathing animals, only 10% CO₂-in-air caused a small increase in CBF; both carbogen and O₂ caused CBF to decrease. SPF changes were similar in form, but larger than those seen in respirated subjects. No consistant change in BP was seen during breathing of these mixtures. Arterial PO₂ was increased by carbogen and 10% CO₂-in-air for both groups. PCO₂ increased for both CO₂ gas mixtures during forced respiration; but in free-breathing animals PCO₂ only increased for 10% CO₂-in-air (normal PCO₂ values were maintained with carbogen thorough increased breathing rate). The observed changes in CBF were consistant with a balance between a combined vasoconstrictive effect of PO₂ and vasodilation effect of PCO₂ on cochlear vessels. Analysis of cochlear vascular conductivity (CBF/BP) indicated that vasodilation was significant only with 10% CO₂-in-air in respirated animals. In all other conditions the increased CBF apparently reflects the increase profusion pressure associated with respiration of each gas. For clinical purposes, while carbogen does not appear to directly cause vasodilation of cochlear vessels it does lead to an increased oxygenation of the cochlea blood and would appear to avoid the cochlear vasoconstriction caused by 100% O₂.