Ischemia in a hollow internal organ can be detected in its incipient stages by obtaining a CO.sub.2 sample from within the organ of interest, measuring the partial pressure of CO.sub.2 sample, measuring the bicarbonate concentration of an arterial blood sample, and on the basis of these two measurements calculating the pH of the wall of the organ. The value of the pH is an indicator of the onset of ischemia in the organ. The CO.sub.2 sample is obtained by a novel catheter, multiple embodiments of which are disclosed. Also disclosed is a method for determining the vitality or adequacy of oxygenation of the whole body, or a solid internal organ, by the measurement of the pH of venous blood.