To the Editor.— It has been estimated that between 1 and 2 million pulmonary artery catheters were inserted for hemodynamic monitoring in the last decade. 1 The prevalence of the procedure has increased substantially over the past few years. It is therefore of interest that the methodology of a clinical hemodynamic measurement, cardiac output by thermodilution, which is made less frequently than blood pressure and heart rate only, be the subject of continued investigation. Some of the errors associated with the early use of this technique have been eliminated by improved technology. For example, those due to alterations in injectate temperature 2 are now minimized by the use of both a proximal and distal thermistor. The question of the importance of the respiratory effect on the measurement of cardiac output appears to have now been settled by a report inThe Journal. 3 The data presented in this article demonstrate