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

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. 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 < 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. 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% confidence interval 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. 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.