Techniques for investigating substrate metabolism in patients.

Studies of dynamic aspects of substrate metabolism in patients can be made with a variety of techniques. Methods include indirect calorimetry, substrate load tests, measurement of arteriovenous differences with use of the Fick principle, and isotope infusions. Recent developments in stable isotope methodology have made available safe techniques for accurate measurement of the turnover of plasma substrates in patients. The use of deuterated, 13C, and 15N isotopes enables the turnover of several substrates to be measured at the same time. It is hoped that the primed constant infusion of stable isotopes combined with indirect calorimetry will lead to a greater understanding of metabolism in ill patients.