Measurements of the perivascular PO2 in the vicinity of the pial vessels of the cat.

PO2's in the environment of the pial micro-vessels of the cat were measured using recessed tip oxygen microelectrodes. Measurements were made on the surface of vessels with internal diameters ranging from 200 micrometers to 22 micrometers. Blood oxygen partial pressures were also measured inside these vessels by penetrating the vessels with sharpened electrodes. Both intravascular and extravascular PO2 values decreased progressively from the large arterial vessels down to the small arterioles. The observed values of intravascular PO2 showed a systematic longitudinal decrease from 98.5 +/- 10.7 (SEM) mm Hg in the largest vessels down to 72.6 +/- 3.6 mm Hg in the smallest vessels. In addition to the longitudinal gradient, a transmural gradient was observed across the walls of the microvessels. The difference between blood PO2 and vessel surface PO2 was 27.0 +/- 2.5 mm Hg in the largest vessels and 6.0 +/- 2.2 in the smallest. The mean wall thickness in these groups of vessels were 27.0 +/- 1.5 and 7.5 +/- 0.8 micrometers respectively. Measurements of the minimum tissue PO2 on the exposed surface of the cortex yielded a value of 25.4 +/- 6.6 mm Hg. Systemic arterial partial pressure of oxygen averaged 94.7 +/- 4.7 mm Hg. The data indicate that significant gradients for oxygen exist both longitudinally and radially in association with the pial vessels. The longitudinal gradients represent losses of oxygen from the precapillary vessels. The transmural gradients are apparently the result of both consumption by the microvessel wall and diffusional gradients due to oxygen flux into the extravascular space.