MICROVASCULAR STUDIES ON THE ORIGINS OF PERFUSION-LIMITED HYPOXIA

Two forms of hypoxia are thought to exist in tumours: (1) hypoxia caus ed by limitations of its diffusion (chronic hypoxia); and (2) hypoxia caused by changes in perfusion (acute hypoxia). Indirect information s uggests the existence of perfusion-limited hypoxia, but there is no di rect proof that fluctuations in blood dow can lead to hypoxia, nor is there any information regarding potential causes of fluctuant flow. In this study, we have begun to explore these questions using R3230AC tu mours transplanted into rat dorsal-flap window chambers. Two types of fluctuant flow have been observed. The first type, usually confined to single vessels, is typified by instability of flow magnitude and dire ction, and total vascular stasis occurs, but only for a few seconds at a time (4% incidence). The second type of fluctuation occurs in group s of vessels and is cyclic, with cycle times ranging from 20-60 min. T otal vascular stasis does not necessarily occur, but the fluctuations in red cell flux are accompanied by changes in vascular oxygen content , as measured by microelectrodes. Another source of chronic hypoxia ha s also been identified in these experiments. Nine per cent (9%) of ves sels examined had plasma how, but very low or absent red cell flux ove r periods of many minutes.