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

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.