The effects of hyperoxic and hypercarbic gases on tumour blood flow

Carbogen (95% O₂ and 5% CO₂) has been used in preference to 100% oxygen (O₂) as a radiosensitizer, because it is believed that CO₂ blocks O₂-induced vasoconstriction. However, recent work suggests that both normal and tumour arterioles of dorsal flap window chambers exhibit the opposite: no vasoconstriction vs constriction for O₂ vs carbogen breathing respectively. We hypothesized that CO₂ content might cause vasoconstriction and investigated the effects of three O₂-CO₂ breathing mixtures on tumour arteriolar diameter (TAD) and blood flow (TBF). Fischer 344 rats with R3230Ac tumours transplanted into window chambers breathed either 1%, 5%, or 10% CO₂ + O₂. Intravital microscopy and laser Doppler flowmetry were used to measure TAD and TBF respectively. Animals breathing 1% CO₂ had increased mean arterial pressure (MAP), no change in heart rate (HR), transient reduction in TAD and no change in TBF. Rats breathing 5% CO₂ (carbogen) had transiently increased MAP, decreased HR, reduced TAD and a sustained 25% TBF decrease. Animals exposed to 10% CO₂ experienced a transient decrease in MAP, no HR change, reduced TAD and a 30-40% transient TBF decrease. The effects on MAP, HR, TAD and TBF were not CO₂ dose-dependent, suggesting that complex physiologic mechanisms are involved. Nevertheless, when ≥ 5% CO₂ was breathed, there was clear vasoconstriction and TBF reduction in this model. This suggests that the effects of hypercarbic gases on TBF are site-dependent and that use of carbogen as a radiosensitizer may be counterproductive in certain situations.