A Patient-Anchored, Oxygen Uptake Rate-Informed Gradient-Boosting Virtual Sensor for Arterial Oxygen Saturation Across a Nine-Hour Monitoring Gap

In intensive care, the arterial oxygen saturation of a ventilated patient is followed through the bedside monitor’s pulse oximetry reading, which describes the present situation only; whether it will hold over the coming hours is left to the attention of the staff. Soft sensors exist for intermittently measured variables, but none can estimate the saturation hours ahead without a recent reading of it. We ask whether the continuously recorded ventilator and expired-gas signals can estimate saturation nine hours after the last available reading. Using 4017 hourly records of 31 ventilated children, we build a virtual sensor as three nested implementation scenarios of one code base: X0 (XGBoost 2.1.4 through our pipeline, no physics term active), XP (X0 plus a blend toward the last reading and the cumulative oxygen uptake rate over the gap per kilogram of body mass, derived from an oxygen balance of the arterial blood), and v4 (XP plus a patient-level anchor that recalibrates the nine-hour-old reading toward the patient’s own history). Out-of-fold predictions under patient-grouped five-fold cross-validation are scored, with paired patient bootstrap intervals, on the 3730 h of 30 patients whose inputs contain no saturation value younger than nine hours. From X0 to XP, the mean per-patient MAPE decreases from 3.313% to 3.282%, resolved for this summary only; from XP to v4, it decreases to 3.115%, the pooled MAPE decreases from 3.885% to 3.729% (both resolved), the pooled correlation rises from 0.754 to 0.793, hypoxaemia below 90% at the end of the gap is detected with an AUC of 0.939 against 0.900 for the last reading, and the ensemble shrinks from 55 to 19 inputs and from 9637 to 474 leaves. An exact decomposition of every prediction shows that the two-coefficient anchor earns 82% of the improvement over persistence; most of the nine-hour information is the patient’s own level, the gas-derived terms add a small but resolved contribution, and the sensor is a trend indicator rather than an alarm.