Physiologic mechanisms underlying the failure of the "shock index" as a tool for accurate assessment of patient status during progressive simulated hemorrhage

J Trauma Acute Care Surg. 2013 Aug;75(2 Suppl 2):S197-202. doi: 10.1097/TA.0b013e31829b73aa.

Abstract

Background: Shock index (SI), the ratio of heart rate (HR) to systolic arterial pressure (SAP), is a metric often used to diagnose patients at risk of impending cardiovascular instability and hemorrhagic shock. We hypothesized that if SI reflected impending cardiovascular instability and shock in an individual, then: (1) elevations in SI and HR would be greater in individuals with low tolerance (LT) to progressive lower-body negative-pressure (LBNP) compared with individuals with high tolerance (HT), and (2) LT would be associated with greater vagal withdrawal of the baroreflex sensitivity (BRS) compared with HT.

Methods: A total of 187 healthy subjects (HT, 125; LT, 62) underwent exposure to LBNP until a SAP of less than 80 mm Hg (instability) was achieved. HR and SAP were used to calculate SI, and BRS was determined from spontaneous fluctuations in R-R interval and diastolic arterial pressure. Maximal cardiac vagal withdrawal was calculated as the difference between BRS at baseline and BRS at 100% LBNP tolerance.

Results: Contrary to our hypothesis, SI at 60%, 80%, and 100% LBNP tolerance in LT (0.59 ± 0.03, 0.73 ± 0.04, and 0.97 ± 0.06, respectively) was lower (p ≤ 0.002) than SI in HT subjects at the same levels (0.66 ± 0.03, 0.84 ± 0.04, and 1.24 ± 0.06, respectively). Maximal cardiac vagal withdrawal was less (p = 0.045) in LT subjects (11.3 ± 2.2 ms/mm Hg) compared with HT subjects (14.9 ± 2.5 ms/mm Hg). The sensitivity of SI in identifying impending instability (SI, 0.9) at 80% and 100% LBNP tolerance was 13% and 63% in LT subjects and 34% and 91% in HT subjects, respectively.

Conclusion: The low sensitivity of the SI observed in LT individuals is associated with a lower capacity to withdraw cardiac vagal activity and can lead to an undertriage of those patients most likely to develop early hemorrhagic shock.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Adult
  • Electrocardiography
  • Female
  • Heart / physiology
  • Hemodynamics / physiology
  • Hemorrhage / diagnosis
  • Hemorrhage / physiopathology*
  • Humans
  • Lower Body Negative Pressure
  • Male
  • Models, Cardiovascular
  • Sensitivity and Specificity
  • Severity of Illness Index*
  • Shock / diagnosis*
  • Shock / physiopathology