Validation of echocardiography for measuring cardiac output by the multiple gas elimination technique

The multiple inert gas elimination technique (MIGET) is being increasingly used in respiratory physiology and pathophysiology. Six inert gases are giv en as an intravenous infusion then measured in samples of expired air and m ixed arterial and venous blood. This requires right-sided catheterization, a procedure that is sometimes ethically inappropriate. The present article reports a method in which inert gas levels in mixed venous blood were calcu lated, rather than measured, using Fick's law. Echocardiography was used to measure arterial inert gas levels and cardiac output. The method was valid ated in 11 men scheduled to undergo coronary bypass surgery. Cardiac output was either calculated based on biometrical (C) data or measured using four different methods in random order, namely Fick's law with oxygen (FiO(2)) or the inert gases (FiIG) as the tracers, thermodilution (TH), and echocard iography (E). Cardiac output values in L.min(-1) ((X) over bar +/- SD) were as follows : C, 4.99 +/- 0.39; FiO(2), 5.44 +/- 0.86; FiIG, 5.55 +/- 0.92; TH, 5.77 +/- 0.88; and E, 5.53 +/- 0.64. No significant differences were f ound among the four measured cardiac output values, of which the mean was 5 .57 +/- 0.70 L/min, a value that was significantly higher than the calculat ed value. This difference is probably ascribable to the use of dopamine, do butamine, or epinephrine in six of the 11 patients. A 1 L/min(-1) cardiac o utput error, in either direction, was found to have a merited influence on the distribution of alveolar perfusion at various (V) over dot A/(Q) over d ot ratios. Conversely, as expected, ventilation distribution was not influe nced by cardiac output. In conclusion, echocardiography provides satisfacto ry cardiac output estimations using the MIGET except in patients with septa l hypertrophy, subaortic membranes, a mitral valve prosthesis, or a mitral valve ring.