Indirect assessment of autonomic nervous system balance for prediction of hemorrhage: A preclinical study in a porcine model

Hemorrhage is a common and significant complication of trauma or surgical procedures. Rapid and accurate assessment of a patient's condition during hemorrhage is crucial for management. The primary objective was to describe pupillometry parameters and parasympathetic tone activity (PTA) during hemorrhage. The secondary objective was to develop a model to predict blood losses. In this preclinical study, 15 pigs were used. After anesthesia, 35 mL/kg of blood was removed over 1 h. Hypovolemia was maintained for 90 min. Indirect assessment of autonomic nervous system balance was based on the monitoring of heart rate (HR), blood pressure (BP), shock index (SI), PTA and pupil diameter (PD). PD and the pupillary light reflex (PLR) were measured using a portable pupillometer. HR and SI significantly increased during hemorrhage, whereas BP decreased. PD increased significantly during hemorrhage (+0.03 mm/min, [0.01; 0.04]; then +0.01 mm/min, [0.01; 0.02]) and the hypovolemia period (+0.004 mm/min, [0.002; 0.005]). PLRs were absent in 251/254 measurements. Large variations in PTA parameters were observed over time, which prevented statistical analysis. The best models predicted blood losses with a mean absolute error ranging from 3.61 to 3.67 mL/kg. In conclusion, hemorrhage was associated with an increased PD potentially influenced by anesthesia. The selected models allowed reasonable prediction of blood losses with a slight tendency to overestimate. The study provides proof of concept for the use of this kind of models to predict blood losses. The results must be confirmed in other contexts: with different species, other anesthetic protocols, or different surgical procedures.