Simple and noninvasive indicator of pulmonary gas exchange impairment using pulse oximetry

We postulated that the fractional inspired oxygen concentration (FIO2) required to achieve a certain value of arterial oxygen saturation (SaO2) can be used as an indicator of pulmonary gas exchange impairment in patients during mechanical ventilation. We tested this hypothesis in 20 patients. By reducing FIO2 in increments of 10 vol% of capacity while monitoring SaO2 with pulse oximetry, we could determine FI98, FI97, FI96, and FI95; that is, the FIO2 that yields 98, 97, 96, and 95% SaO2, respectively. On the basis of our data, we chose FI98 as the most appropriate index, as an SaO2 of 97% or below could not be achieved even with a low FIO2 in some of the patients. To test the significance of the newly proposed index, we compared FI98 with the alveolar-arterial oxygen tension difference, P(A - a)O2, and with the respiratory index, which are routinely used elsewhere. The correlation between FI98 and P(A - a)O2 was excellent: P(A - a)O2 = 490.5 * FI98 + 117.2 with a correlation coefficient of 0.906 (P less than 0.01). FI98 also correlated significantly with the respiratory index: respiratory index = 4.354 * FI98 - 0.776 (r = 0.889, P less than 0.01). We conclude that FI98 may be used as a simple index for the rough estimation of pulmonary gas exchange impairment without the need for invasive procedures. However, further studies are needed to confirm the validity of our method in hemodynamically unstable patients or when other brands of pulse oximeters are used.