The shock index (SI), the ratio of heart rate to systolic blood pressure, is a clinical tool for assessing injury severity. Age-adjusted SI models may improve predictive value for injured children in the out-of-hospital setting. We sought to characterize the proportion of children in the prehospital setting with an abnormal SI using established criteria, describe the age-based distribution of SI among injured children, and determine prehospital interventions by SI.

We performed a multi-agency retrospective cross-sectional study of children (<18 years) in the prehospital setting with a scene encounter for suspected trauma and transported to the hospital between 2018 and 2022 using the National Emergency Medical Services (EMS) Information System datasets. Our exposure of interest was the first calculated SI. We identified the proportion of children with an abnormal SI when using the SI, pediatric age-adjusted (SIPA); and the pediatric SI (PSI) criteria. We developed and internally validated an age-based distributional model for the SI using generalized additive models for location, scale, and shape to describe the age-based distribution of the SI as a centile or Z-score. We evaluated EMS interventions (basic airway interventions, advanced airway interventions, cardiac interventions, vascular access, intravenous fluids, and vasopressor use) in relation to both the SIPA, PSI, and distributional SI values.

We analyzed 1,007,863 pediatric EMS trauma encounters (55.0% male, median age 13 years [IQR, 8–16 years]). The most common dispatch complaint was for traffic/transport related injury (32.9%). When using the PSI and SIPA, 13.1% and 16.3% were classified as having an abnormal SI, respectively. There were broad differences in the percentage of encounters classified as having an abnormal SI across the age range, varying from 5.1 to 22.8% for SIPA and 3.7–20.1% for PSI. The SIPA values ranged from the 75th to 95th centiles, while the PSI corresponded to an SI greater than the 90th centile, except in older children. The centile distribution for SI declined during early childhood and stabilized during adolescence and demonstrated a difference of <0.1% at cutoff values. An abnormal PSI, SIPA and higher SI centiles (>90th centile and >95th centiles) were associated with interventions related to basic and advanced airway management, cardiac procedures, vascular access, and provision of intravenous fluids occurred with greater frequency at higher SI centiles. Some procedures, including airway management and vascular access, had a smaller peak at lower (<10th) centiles.

We describe the empiric distribution of the pediatric SI across the age range, which may overcome limitations of extant criteria in identifying patients with shock in the prehospital setting. Both high and low SI values were associated with important, potentially lifesaving EMS interventions. Future work may allow for more precise identification of children with significant injury using cutpoint analysis paired to outcome-based criteria. These may additionally be combined with other physiologic and mechanistic criteria to assist in triage decisions.