Development and internal validation of the Low Cardiac Index Score: a simplified bedside tool for continuous intraoperative screening

<title>Abstract</title> <p> Purpose Advanced haemodynamic monitoring of cardiac index is restricted to selected high-risk cases, leaving most patients undergoing major surgery without real-time assessment. We aimed to develop and internally validate a simplified bedside score for continuous, minute-by-minute screening of low cardiac index using routinely available parameters. Methods Retrospective single-centre study of adults undergoing major abdominal surgery with advanced haemodynamic monitoring (January 2023 to February 2026). Low cardiac index was defined as pulse contour cardiac index &lt; 2.2 l.min ^− 1 .m ^− 2 . The dataset was split 80/20 at the patient level. Twelve candidate features were ranked using SHapley Additive exPlanations (SHAP) values; the top five were used to fit a generalised linear mixed model, yielding the integer-weighted Low Cardiac Index (LCI) score. A rule-out threshold (sensitivity ≥ 0.90) was selected, and test-set performance was evaluated with patient-level cluster bootstrap. Results The analysis cohort comprised 136 patients and 42,346 timepoints (15.5% low cardiac index prevalence). The derived formula was LCI score = 57 + (2 × DBP) + (0.5 × Age) + PPV – HR – SBP. At a threshold of ≥ 30, the score achieved an area under the receiver operating characteristic curve of 0.84 (95% CI 0.75, 0.91), sensitivity 0.92 (0.80, 0.99), and negative predictive value 0.97 (0.92, 1.00). Performance was stable across 20 random splits and outperformed individual haemodynamic parameters. Conclusion The LCI score is a simple bedside rule-out tool for low cardiac index, with a score below 30 yielding a negative predictive value of 0.97. External validation and prospective assessment of clinical impact are warranted. </p>