The utility of shock index in differentiating major from minor injury

The importance of early recognition of hemorrhagic shock and its effects on outcome have long been recognized. Traditional vital signs are relatively insensitive as early diagnostic markers of hemorrhage. The shock index (SI); heart rate (HR) divided by systolic blood pressure (SBP), has been suggested as such a marker. We tested the diagnostic utility of the SI in differentiating major from minor injury in trauma patients.

Retrospective study of a prospectively collected observational cohort at a level I trauma center. Demographics, injury mechanism, HR, SBP, base deficit and lactate were recorded and Injury Severity Score were calculated. Major injury was defined as either a change in hematocrit greater than 10 or blood transfusion requirement during initial 24 h, or Injury Severity Score greater than 15.

One thousand four hundred and thirty-five trauma patients were enrolled, average age 35.2±16.9 years. Two hundred and forty-two were classified as major injury. The area under the receiver operator characteristic curves for SI [0.63 95% confidence interval (CI) 0.59-0.67] was significantly less than that for base deficit (0.72, 95% CI: 0.69-0.76) or lactate (0.69, 95% CI: 0.65-0.73). The diagnostic performance of SI was slightly better than HR (0.58) but not SBP (0.61). To reach sensitivity of 90%, the SI must be 0.5, well in the range of a normal SBP and HR.

The SI can be a valuable tool, raising suspicion when it is abnormal even when other parameters are not, but is far too insensitive for use as a screening device to rule out disease. A normal SI should not lower the suspicion of major injury.