Shock Index

About the Shock Index (SI)

The shock index is the heart rate (HR) divided by systolic blood pressure (SBP). It was first proposed by Allgöwer and Burri in 1967, who studied shock severity in patients either at risk of or experiencing shock from a variety of causes: trauma, myocardial infarction, hemorrhage, pulmonary embolism or sepsis (amongst others). The SI was intended as an approximation of hemodynamic status in addition to traditional vital signs.

Shock Index = Heart Rate /Systolic Blood Pressure

Vital signs are most commonly within normal ranges even in the compensatory phases of shock so prediction and risk stratification tools can become crucial in triggering early intervention. For example, patients with advanced age and chronic hypertension may not appear in hemodynamic compromise from the start.

Interpretation

• Normal shock index values range between 0.5 and 0.7. SI values >1.0 were widely found to predict increased risk of mortality and other markers of morbidity, such as need for massive transfusion and admission to intensive care units.
• A retrospective study by Cannon et al. found that SI values >0.9 were predictive of significantly higher mortality (15.9%) whilst another study by Vandromme et al. evidenced that SI>0.9 had a 1.6-fold higher risk for massive transfusion.
• Elevation in SI has also been correlated with reduced left ventricular end-diastolic pressure and circulatory volume even with HR and SBP within normal range.
• A study by Berger at el. showed that when the SI was >0.7, subjects had a 3 times higher likelihood of hyperlactatemia when compared to those with SI <0.7.

Further research is needed to compare SI with SOFA and qSOFA models as predictor for the development of septic shock. But pairing the higher sensitivity of SIRS criteria with the specificity of SI >1.0 may lead to a more accurate way to identify septic patients in need of critical intervention.

References

Original reference

Allgöwer M, Burri C. The “shock-index”. Dtsch med Wochenschr 1967; 92(43): 1947-1950. DOI: 10.1055/s-0028-1106070

Validation

Cannon CM, Braxton CC, Kling-Smith M, Mahnken JD, Carlton E, Moncure M. Utility of the Shock Index in Predicting Mortality in Traumatically Injured Patients. J Trauma. 2009;67(6):1426–1430.

Vandromme MJ, Griffin RL, Kerby JD, McGwin G Jr., Rue LW III, Weinberg JA. Identifying Risk for Massive Transfusion in the Relatively Normotensive Patient: Utility of the Prehospital Shock Index. J Trauma. 2011;70(2):384–390.

Other references

Berger T et al. Shock Index and Early Recognition of Sepsis in the Emergency Department: Pilot Study. WestJEM. 2013;14(2):168–174.

Zarzaur BL, Croce MA, Fischer PE, Magnotti LJ, Fabian TC. New vitals after injury: shock index for the young and age × shock index for the old. J Surg Res. 2008;147(2):229–236.