Trauma Patient Survival Times in a Resource-Limited South African Trauma System: Implications for Combat Casualty Evacuation Timelines

Abstract

Abstract Introduction Future United States military conflicts may involve near-peer adversaries and large-scale combat operations resulting in mass casualties with delayed evacuation. Health service support planning for such conflicts will require data informing patterns of survival among injured combatants. We determined the probability of survival to 24 hours post injury for trauma patients with blunt or penetrating non-compressible torso hemorrhage (NCTH) in a relatively resource-constrained environment. We hypothesized that the conditional probability of 24-hour survival would rise with survival to intermediary timepoints of progressively increasing duration. Materials and Methods This secondary analysis of trauma patients treated in South Africa’s Western Cape leveraged data collected during the Epidemiology and Outcomes of Prolonged Trauma Care (EpiC) study. EpiC captures data from point-of-injury through either hospital discharge or patient death. Patients must survive to reach ambulance or facility care to be included in EpiC. Inclusion criteria for this analysis included patients with NCTH, defined as an abbreviated injury scale (AIS) severity score of 2+ in the chest, abdomen, and/or pelvis body regions without AIS 3+ in any other body region. The primary outcome was survival at 24 hours post injury. We considered intermediary timepoints at: 1, 2, 3, 6, 12, and 18 hours post injury. We calculated unadjusted conditional Kaplan-Meier curves, visually describing crude overall 24-hour survival trajectories conditional on survival to the prespecified set of intermediary timepoints. To adjust for potential confounding, we generated an adjusted 24-hour survival curve using a Cox proportional hazards model adjusting for age, sex, new injury severity score (NISS), and triage early warning score (TEWS). Using the adjusted survival curve, we estimated conditional probabilities of 24-hour survival given survival to each of the intermediary timepoints individually. We plotted the linear trend line of conditional 24-hour survival probability versus intermediary timepoints, using segmented regression trendlines to account for changes in the relationship. Results We included 2,685 patients: 111 (4.1%) died within 24 hours. Survival probability estimates consistently demonstrated that survival to intermediary timepoints of increasing duration was associated with higher probability of 24-hour survival. An inflection point in 24-hour survival trajectory occurred at 3.6 (95% CI: 2.8-4.3) hours. For every hour that a patient survived up to the first 3.6 hours after injury, the probability of 24-hour survival increased by 0.53% (95% CI: 0.38%-0.69%). For every additional hour of survival after 3.6 hours and before 24 hours post injury, the probability of 24-hour survival increased by 0.07% (95% CI: 0.05%-0.10%). Conclusions Among civilian patients with blunt force injury or penetrating NCTH injuries who survived to ambulance retrieval (role 1 equivalent) or health facility (role 2 equivalent), the overall 24-hour mortality was 4.1%. Conditional survival probability estimates consistently demonstrated that survival to intermediary timepoints of increasing duration was associated with higher probability of 24-hour survival. We identified 3.6 hours from time of injury as a threshold beyond which patients derive relatively less additional 24-hour survival benefit progression to subsequent intermediate time points. Military medical doctrine should acknowledge increased risk of casualty deaths when evacuation timelines exceed three and a half hours.