ESTIMATION BY THE FOREIGN-GAS METHOD OF THE NET (SYSTEMIC) CARDIAC OUTPUT IN CONDITIONS WHERE THERE IS RE-CIRCULATION THROUGH THE LUNGS

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Abstract
Some fundamental kinetics were analyzed for the gas absorption by the blood in the foreign-gas methods for estimation of the cardiac output in man. The discussion applies specifically to the acetylene method but the conclusions apply to the other foreign gases. The absorption proceeds according to an equation of the first order and the mathematical analysis is developed accordingly. The condition where re-circulation occurs in the lungs is analyzed in detail and equations are derived for the proper computation of the true systemic circulation in such cases. The variables involved are the concs. of the foreign gas in the gas samples from the lung-bag system, the fraction of blood re-circulated, the short-circuit time, and the total time between samples. Graphs for computations with these variables are presented. Re-circulation through the lungs in conditions such as in patent ductus arteriosus or in inter-ventricular septal defects does not necessarily introduce a serious error and this error may be estimated. Re-circulation of blood from the coronary system cannot introduce an appreciable error. The error resulting from the common assumption that the absorption of the foreign-gas is linear with time is shown to be ordinarily small. Means are provided to estimate this error by equations and a graph. Timing of the gas sampling is discussed. The 2d sample may be taken later than is frequently believed possible. When there is re-circulation the first sample should be delayed 4 or 5 secs.; 2 or 3 secs. are not critical. Results of 41 studies on 22 patients with patent ductus arteriosus are presented. The avg. cardiac index is normal or close to it in this group. Many criticisms of the foreign-gas methods are invalid because they are based on quantitative misconceptions. The importance and difficulty of physiological standardization in cardiac output measurements are frequently underestimated.