Shock Index (HR/SBP Ratio) Calculator
- Shock Index (HR/SBP Ratio): Explanation and Clinical Context
The Shock Index (SI) is a simple, bedside physiological ratio calculated by dividing the heart rate (HR) by the systolic blood pressure (SBP). It serves as a rapid and sensitive indicator of hemodynamic compromise and circulatory shock.
A normal Shock Index typically ranges from 0.5 to 0.7. Values above 0.9 are considered abnormal and have been strongly correlated with an increased risk of morbidity and mortality across various clinical conditions, including trauma, sepsis, myocardial infarction, and gastrointestinal bleeding.
The index reflects the balance between cardiac output and peripheral vascular resistance. As perfusion deteriorates, compensatory tachycardia and hypotension lead to an elevated SI. This simple metric has been shown to outperform traditional vital signs in identifying patients at risk of shock or requiring intensive resuscitation.
Clinically, SI is especially valuable because it is independent of age and comorbidities, and can be calculated immediately on presentation. An SI >1.0 has been associated with a significantly higher likelihood of requiring critical care interventions and predicts poor outcomes even in normotensive patients.
Interpretation Summary:
- SI < 0.5: May indicate bradycardia or hypertension; usually hemodynamically stable.
- SI 0.5–0.9: Normal to mildly elevated; compensated state.
- SI 0.9–1.3: Suggests possible hypovolemia or early shock.
- SI > 1.3: Indicates decompensated or severe shock; immediate resuscitation needed.
Reference:
Allgöwer M, Burri C. “Shock Index.” Dtsch Med Wochenschr. 1967;92(43):1947–1950.
Rady MY, Smithline HA, Blake H, Nowak R, Rivers E. “A comparison of the Shock Index and conventional vital signs to identify acute, critical illness in the emergency department.” Ann Emerg Med. 1994;24(4):685–690.
Birkhahn RH, Gaeta TJ, Terry D, Bove JJ, Tloczkowski J. “Shock Index in diagnosing early acute hypovolemia.” Am J Emerg Med. 2005;23(3):323–326.
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