Tumour oxygenation under normobaric and hyperbaric conditions

Abstract

Tumor oxygenation during exposure to normobaric and hyperbaric O2 was assessed by means of a cryophotometric micromethod and a specially constructed pressure chamber. The tumors investigated were DS-carcinosarcomas implanted s.c. into the thigh of Sprague-Dawley rats. The animals were anesthetised and put on a heating pad. Mean arterial blood pressure was monitored throughout the entire time of pressurization. Cryobiopsies were removed from the tumors after exposure to O2 for 30 min at 1 bar (atmospheric pressure), 2, 3 or 4 bar. Oxyhemoglobin saturation (HbO2) values of single red blood cells in tumor microvessels were determined photometrically in the frozen tissue samples as a quantitative measure for tumor oxygenation. Frequency distribution curves of HbO2 showed that there was a marked improvement of the O2 supply to tumor tissue in O2 atmospheres at 1 bar compared with exposure to air at 1 bar. Pressurization in O2 atmospheres up to 2 and 3 bar did not substantially alter the distribution curve of the HbO2 values in comparison with the data sampled at 1 bar O2 exposure, yet led to a significant drop in the occurrence of HbO2 values below 5% saturation. Thus, pressurization particularly raises those tissue O2 partial pressures (pO2) that are in a range where the radiosensitivity is critically influenced by the pO2. Pressurization up to 4 bar caused a shift of the HbO2 distribution curve to values significantly higher than those from tumors exposed to O2 at 1 bar, with no values below 35% saturation. Using these data and a previously published model for computing tissue pO2 values it can be shown that radiobiological hypoxia is unlikely to occur in DS-carcinosarcomas exposed to an O2 atmosphere at 4 bar under the conditions chosen. Hyperbaric oxygenation is recommended as an efficient adjuvant of X irradiation, particularly in superficial tumors.