Preoperative systemic immune-inflammation index as an incremental predictor of postoperative refractory hypoxemia in acute type a aortic dissection

BACKGROUND: Postoperative refractory hypoxemia (PRH) remains a clinically important pulmonary complication after surgery for acute type A aortic dissection (ATAAD). Because inflammation, coagulation activation, pulmonary vulnerability, and operative stress converge during ATAAD repair, a composite biomarker may provide more useful risk information than a single leukocyte-derived index. We evaluated whether the preoperative systemic immune-inflammation index (SII) was independently associated with PRH and whether it improved risk prediction beyond conventional clinical, pulmonary, and biomarker variables. METHODS: In this single-center retrospective cohort study, 490 patients undergoing surgery for ATAAD between January 2018 and December 2024 were analyzed. SII was calculated from absolute platelet, neutrophil, and lymphocyte counts and modeled primarily as a log2-transformed continuous variable, so that odds ratios represented the association per doubling of SII. The primary preoperative model included admission/preoperative variables, whereas an expanded perioperative model was used as a sensitivity analysis. Model discrimination, reclassification, decision-curve performance, and calibration were evaluated, and internal validation was performed using 1000 bootstrap resamples. RESULTS: PRH occurred in 158 of 490 patients (32.2%). Preoperative SII was higher in patients who developed PRH than in those who did not [1899.5 (1140.1-3043.6) vs. 667.5 (408.7-1173.7), P < 0.001]. In the primary preoperative prediction model, each doubling of SII was independently associated with PRH [adjusted odds ratio (OR), 2.80; 95% CI, 2.20-3.56; P < 0.001]. The base preoperative model had an AUC of 0.750 (95% CI, 0.705-0.794), whereas the SII-augmented model had an AUC of 0.846 (95% CI, 0.810-0.880). SII yielded the numerically largest incremental discrimination among SII, NLR, PLR, and MLR. Bootstrap validation of the SII-augmented model produced an optimism-corrected AUC of 0.834, calibration slope of 1.00, calibration intercept of 0.00, and Brier score of 0.153. CONCLUSIONS: Preoperative SII was independently associated with PRH after ATAAD surgery and provided incremental predictive information beyond conventional preoperative clinical, pulmonary, and biomarker variables. These findings support SII as a clinically accessible adjunctive risk-stratification marker rather than a stand-alone decision tool; external or temporal validation is required before routine implementation.