Clinical Performance of Spo 2/Fio 2 and Pao 2/Fio 2 Ratio in Mechanically Ventilated Acute Respiratory Distress Syndrome Patients: A Retrospective Study

Objectives: The present study aims to evaluate the severity classification of acute respiratory distress syndrome (ARDS) in mechanically ventilated patients according to peripheral oxygen saturation by pulse oximetry (Spo2)/Fio2 ratio compared with Pao2/Fio2 ratio and the relationship between Spo2/Fio2 ratio and venous admixture. Design: Retrospective observational study. Setting: Medical-surgical ICU. Patients: A cohort of 258 mechanically ventilated patients with ARDS already enrolled in previous studies. Interventions: None. Measurements and Main Results: Gas exchange, Spo2, and respiratory mechanics were measured on ICU admission and during the positive end-expiratory pressure (PEEP) trial. Radiological data from CTs were used to compute lung recruitability and to assess different lung compartments. A nonlinear association was found between Spo2/Fio2 and Pao2/Fio2. Considering the possible confounding factors of the pulse oximeter on the relationship between Spo2/Fio2 and Pao2/Fio2 ratio, arterial pH, and Paco2 had no effect. Spo2/Fio2 and Pao2/Fio2 ratio demonstrated a moderate agreement in classifying ARDS severity (intraclass correlation coefficient = 0.63). Between the correspondent classes according to Spo2/Fio2 vs. Pao2/Fio2 ratio-derived severity classifications, there was no difference in terms of respiratory mechanics, gas exchange, lung radiological characteristics and mortality in ICU, and within two levels of PEEP. A Spo2/Fio2 ratio less than 235 was able to detect 89% of patients with a venous admixture greater than 20%, similarly to a Pao2/Fio2 ratio less than 200. Conclusions: Spo2/Fio2 ratio can detect oxygenation impairment and classify ARDS severity similarly to Pao2/Fio2 ratio in a more rapid and handy way, even during a PEEP trial. However, our results may not be applicable to different patient populations; in fact, the pulse oximeter is merely a monitoring device and the information should be personalized by the physician on the patient's characteristics and conditions.