DO CHANGES IN CARDIAC-OUTPUT AFFECT THE INSPIRATORY TO END-TIDAL OXYGEN DIFFERENCE

Background. The paramagnetic technique has made it possible to monitor the end-tidal oxygen concentration and P-(I-ET)O-2, i.e. inspiratory to end-tidal oxygen difference, breath-by-breath. Little is known about the implications of a changing P-(I-ET)O-2, but so far studies have shown it to be a quick and sensitive variable to detect hypoventilation. This study was designed to observe the circulatory effects on P-(I-ET)O-2 in an experimental setting but monitored as in a clinical situation. Methods. We assessed the oxygen difference during changes in cardiac output induced by intravenous ephedrine-hydrochloride in 12 healthy male volunteers. P-(I-ET)O-2 was measured with a fast-response paramagnetic differential oxygen sensor. Cardiac output was measured with non-invasive transthoracic electrical bioimpedance. As stimultaneous changes in metabolism and ventilation will also influence P-(I-ET)O-2, oxygen uptake and expired minute volume were monitored. After a rest period, the subjects had an intravenous injection of ephedrine-hydrochloride 0.1 mg . kg(-1) followed by a 30-min observation period. Results. Cardiac output increased significantly as did the oxygen uptake and the ventilation. We found no biologically significant correlation between cardiac output and P-(I-ET)O-2. The P-(I-ET)O-2 was influenced by ventilation and metabolism.