A system is described for the in vivo noninvasive measurement of intravascular PO2 at the microscopic level. Under special circumstances the method can also be used to measure interstitial PO2. The PO2 determination is based on the O2-dependent quenching of phosphorescence of palladium-porphyrins bound to albumin. This compound was injected intravenously in the dosage of 30 mg/kg body wt and dissolved in saline to a concentration of 15 mg/ml. The phosphorescence emission was excited by epi-illumination with a strobe xenon arc and measured by a photomultiplier in a well-defined tissue area as small as 15 x 30 microns. A selected portion of the phosphorescence decay was fitted by a single exponential, and the Stern-Volmer equation was used to calculate PO2. Calibration was performed in vitro using saline and blood and was in agreement with previous reports. In vivo observations were made in normal tissue regions from the unanesthetized hamster transparent skin fold chamber preparation. The method allows PO2 determinations, in the range of 0-80 mmHg, in microvessels with diameters of 15-100 microns. Simultaneous transillumination of the tissue also allows measurement of vessel diameter and red blood cell velocity in the same vessels.

Simultaneous measurements of intravascular and interstitial oxygen partial pressure (Po2) in any tissue have not previously been reported, despite the importance of oxygen in health and in disease. This is due to the limitations of current techniques, both invasive and noninvasive. We have optically measured microscopic profiles of Po2 with high spatial resolution in subcutaneous tissue and transplanted tumors in mice by combining an oxygen-dependent phosphorescence quenching method and a transparent tissue preparation. The strengths of our approach include the ability to follow Po2 in the same location for several weeks and to relate these measurements to local blood flow and vascular architecture. Our results show that (i) Po2 values in blood vessels in well-vascularized regions of a human colon adenocarcinoma xenograft are comparable to those in surrounding arterioles and venules, (i) carbogen (95% 02/5% CO2) breathing increases microvascular P02 in tumors, and (iii) in unanesthetized and anesthetized mice P02 drops to hypoxic values at <200 Am from isolated vessels but drops by <5 mmHg (1 mmlg = 133 Pa) in highiy vascularized tumor regions. Our method should permit noninvasive evaluations of oxygen-modifying agents and offer further mechanistic information about tumor pathophysiology in tissue preparations where the surface of the tissue can be observed.

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