Limitation of the use of inert gases in the measurement of small gut mucosal blood flow.

The role of blood flow in absorption from the small gut would be best studied by a method able to measure exclusively blood flow in the small gut mucosa because this might vary independently of total small gut blood flow. Such a method should be repeatable to give values in control and experimental periods, and one would hope that it would be adaptable for use in man. We had hoped to meet these criteria by measuring the rate of removal of 133Xenon from fluid in the lumen of the small gut. Kety (1951) has described the factors involved in the use of inert gases to measure tissue blood flow. In muscle (e.g., Lassen, H0edt-Rasmussen, Lindjberg, Pedersen, and Munck, 1963), myocardium (e.g., Ross, Ueda, Lichtlen, and Rees, 1964), and brain (e.g., Lassen and Klee, 1956) blood flow has been measured from the rate of disappearance of radioactive inert gases out of the tissue. The validity of these methods depends upon the assumption that diffusion of gas through the tissue is so fast that it does not delay its removal, which is therefore dependent only upon blood flow. In the gut, if the same assumption holds true, then when the gas is dissolved in fluid placed in the gut lumen, the rate of its removal will be proportional to blood flow in the mucosa, and will be unaffected by blood flow in any other part of the gut wall.