OXYGEN AVAILABILITY TO THE BRAIN DURING INFLOW OCCLUSION OF THE HEART IN NORMOTHERMIA AND HYPOTHERMIA

Oxygen tension of tissues in vivo

It seems fair to say that prior to the use of the oxygen electrode there was a static concept of the oxygen tension of tissues. This was true in spite of the fact that Bazett and Sribyatta, with the gas bubble technic, had demonstrated one example of a labile oxygen tension, in the subcutaneous space of man. 17 They of course could not show the rapidity with which the changes take place. Experiments with the polarographic technic have shown that profound changes in oxygen tension can take place very rapidly, all the way from values approaching that of arterial blood down to zero.

In vasodilated skin oxygen tension approaches that of arterial blood. When different concentrations of oxygen are breathed, the changes in oxygen tension in such skin closely follow those of the arterial blood. When the circulation to skin is limited by peripheral arterial occlusion, oxygen is reduced and does not increase on vasodilatation as much as does that of normal skin. When a normal person breathes oxygen, the oxygen in the tissues that use little oxygen quickly reaches a pressure 4 or 5 times that produced when he breathes air. In the case of tissues that use greater amounts of oxygen this relationship is not so close. The higher tensions have not been found in the tissues that use the most oxygen. The transfer of oxygen from the capillaries to the tissue is rapid but not instantaneous.

The usual ranges of oxygen tension of various tissues such as skeletal muscle, heart muscle, brain, etc., are not yet known in terms of millimeters of mercury, but this should now be possible. It is hoped that local metabolisms will be measured reasonably accurately in intact tissues within the body. Perhaps the storage of oxygen by myoglobin and cytochromes can be derived, again in situ, from the metabolism and the rate of change of oxygen tension when the circulation is arrested.

The potentialities of the oxygen electrode are great, but limitations of the method should be respected. Further development of the method may free it of some of its defects. Other methods may supplant it, but if so they will need to be equally rapid and more capable of exact calibration.

In closing this presentation I want to thank our chairman, my friend Dr. Arthur Merrill, for allowing me to speak to you. I want also to thank Dr. George Brown, in memoriam, for his pioneering in the paths of study of the interrelations of circulatory and tissue physiology and medicine.