Inspiratory end-tidal oxygen content difference: a sensitive indicator of hypoventilation.

The effect of progressive hypoventilation on end-tidal gas concentrations and corresponding partial pressures in arterial blood was studied in anesthetized pigs. Oxygen, CO2, and nitrous oxide concentrations were measured continuously with fast infrared and paramagnetic sensors as ventilation was decreased gradually in 12.5% increments at 5-min intervals. Samples for blood gas determinations were obtained at 3 min after each respirator adjustment. An increasing difference between inspiratory and end-tidal oxygen concentrations (FIO2 - PetO2) was the most sensitive indicator of hypoventilation and exceeded the sensitivity of end-tidal CO2. Decreasing PetO2 was followed by a decrease in PaO2, but no detectable change in arterial oxygen saturation until the ventilation was decreased to 37% of the initial value. The rapidly decreasing alveolar oxygen was replaced by nitrous oxide, and a sudden drop in ventilation was characterized by a change in the end-tidal values of all the three gases and peaked waveforms. Thus, PetO2 and FIO2 - PetO2 are sensitive and valuable indicators of adequate ventilation and appropriate oxygen supply.