Comparison of non-invasive to invasive oxygenation ratios for diagnosing acute respiratory distress syndrome following coronary artery bypass graft surgery: a prospective derivation-validation cohort study

Abstract

Objective: To determine if non-invasive oxygenation indices, namely peripheral capillary oxygen saturation (S_pO₂)/fraction of inspired oxygen (F_iO₂) and partial pressure of alveolar oxygen (P_AO₂)/F_iO₂ may be used as effective surrogates for the partial pressure of arterial oxygen (P_aO₂)/F_iO₂. Also, to determine the S_pO₂/F_iO₂ and P_AO₂/F_iO₂ values that correspond to P_aO₂/F_iO₂ thresholds for identifying acute respiratory distress syndrome (ARDS) in patients following coronary artery bypass graft (CABG) surgery.

Methods: A prospective derivation-validation cohort study in the Open-Heart ICU of an academic teaching hospital. Recorded variables included patient demographics, ventilator settings, chest radiograph results, and S_PO₂, P_aO₂, P_AO_2, S_aO₂, and F_iO₂. Linear regression modeling was used to quantify the relationship between indices. Receiver operating characteristic (ROC) curves were used to determine the sensitivity and specificity of the threshold values.

Results: One-hundred seventy-five patients were enrolled in the derivation cohort, and 358 in the validation cohort. The S_PO₂/F_iO₂ and P_AO₂/F_iO₂ ratios could be predicted well from P_aO₂/F_iO₂, described by the linear regression models S_PO₂/F_iO₂ = 71.149 + 0.8PF and P_AO₂/F_iO₂ = 38.098 + 2.312PF, respectively. According to the linear regression equation, a P_aO₂/F_iO₂ ratio of 300 equaled an S_PO₂/F_iO₂ ratio of 311 (R² 0.857, F 1035.742, < 0.0001) and a P_AO₂/F_iO₂ ratio of 732 (R² 0.576, F 234.887, < 0.0001). The S_PO₂/F_iO₂ threshold of 311 had 90% sensitivity, 80% specificity, LR+ 4.50, LR- 0.13, PPV 98, and NPV 42.1 for the diagnosis of mild ARDS. The P_AO₂/F_iO₂ threshold of 732 had 86% sensitivity, 90% specificity, LR+ 8.45, LR- 0.16, PPV 98.9, and NPV 36 for the diagnosis of mild ARDS. S_PO₂/F_iO₂ had excellent discrimination ability for mild ARDS (AUC ± SE = 0.92 ± 0.017; 95% CI 0.889 to 0.947) as did P_AO₂/F_iO₂ (AUC ± SE = 0.915 ± 0.018; 95% CI 0.881 to0.942).

Conclusions: P_aO₂ and S_aO₂ correlated in the diagnosis of ARDS, with a P_aO₂/F_iO₂ of 300 correlating to an S_PO₂/ F_iO₂ of 311 (Sensitivity 90%, Specificity 80%). The S_PO₂/ F_iO₂ ratio may allow for early real-time rapid identification of ARDS, while decreasing the cost, phlebotomy, blood loss, pain, skin breaks, and vascular punctures associated with serial arterial blood gas measurements.

Keywords: Acute respiratory distress syndrome; Coronary artery bypass graft surgery; Oxygenation indices; Ratio of arterial oxygen partial pressure to fractional inspired oxygen; Ratio of partial pressure of alveolar oxygen to fractional inspired oxygen; Ratio of peripheral capillary oxygen saturation to fractional inspired oxygen.

Fig. 1

Patient flow diagram

Fig. 2

Scatter plots for S/F vs. P/F ratios (top) and P_A/F vs. P/F ratios (bottom) for the derivation data set. The S/F vs. P/F line of best fit equals (S/F = 71.149 + 0.8PF); (P < 0.0001; r = 0.926); whereas the P_A/F vs. P/F line of best fit equals (P_A/F = 38.098 + 2.312PF); (P < 0.0001; r = 0.759)

Fig. 3

Scatter plots for S/F vs. P/F ratios (top) and P_A/F vs. P/F ratios (bottom) for the validation data set. The S/F vs. P/F line of best fit equals (S/F = 43.309 + 0.869PF); (R² 0.909; F 3565.427; < 0.0001); (< 0.0001; r = 0.954); whereas the P_A/F vs. P/F line of best fit equals (PAFR = − 1.537 + 2.435PFR); (R² 0.493; F 345.762; < 0.0001); (< 0.0001; r = 0.702)