Do changes in cardiac output affect the inspiratory to end-trial oxygen difference?

The inspiratory to end-tidal oxygen difference during exercise

OBJECTIVE

Fast paramagnetic oxygen analyzers have made it possible to measure inspiratory to end-tidal oxygen concentration difference (P(I-ET)O2) breath-by-breath. It is now frequently displayed on monitors during routine anesthesia. We wanted to study the effects of major changes in metabolism, ventilation and circulation on P(I-ET)O2.

METHODS

Ten healthy male volunteers were studied under exercise. P(I-ET)O2 was measured with a fast-response paramagnetic differential oxygen sensor. Cardiac output was measured with non-invasive transthoracic electrical bioimpedance. Metabolism was measured with indirect calorimetry and ventilation with a side stream spirometer. After a rest period, the subjects cycled at 30 W and 60 W, 6 minutes on each work load and were then observed during 10 minutes of rest.

RESULTS

P(I-ET)O2 corresponded well to VO2/VA (the oxygen uptake to alveolar ventilation quotient) correlation showed r = 0.79. P(I-ET)O2 was influenced by changes in cardiac output which occurred primarily at the start and at the end of exercise. Expired minute ventilation (VE) multiplied by P(I-ET)O2 was related to cardiac output with a high intrapersonal correlation.

CONCLUSION

P(I-ET)O2 is a good measure of adequate ventilation in relation to the oxygen consumption level and multiplied by VE it might offer a non-invasive bedside parameter indicating changes in cardiac output.