Pulmonary blood flow determined by continuous analysis of pulmonary N2O exchange

Measurement of mean pulmonary blood flow (Qp) as a function of pulmonary inert gas (N2O) uptake was studied with the aid of a mathematical model, fast response measurement of gas flow and gas concentrations at the mouth, and digital computer analysis of the data. The model treats the total pulmonary inert gas uptake as the sum of dead space, alveolar, lung tissue, and pulmonary blood flow uptakes. Analysis of any two breaths during breathing of a gas mixture (39 percent N2O, 21 percent O2, 40 percent N2 or He) in terms of the soluble (N2O) and the insoluble (N2 or He) inert gas yields two simultaneous equations with two unknowns which can be solved for Qp. No assumptions are required about the magnitude of the alveloar, dead space, or lung tissue volumes and constant FRC is not a requirement. The validity of the mathematical model and its sensitivity to known measurement errors was studied by computer simulation of respiratory gas exchange for N2O and N2. Comparison of Qp (N2O) with the direct Fick method (O2) in five anesthetized dogs showed agreement within plus or minus 20 percent. The proposed method has promise as a clinical method for determination of cardiac output on a breath-to-breath basis during regular breathing at rest or during exercise.