Shock Index Calculator

What Is the Shock Index (SI)?

The Shock Index (SI) is a rapid, non-invasive bedside hemodynamic metric defined as the ratio of heart rate (HR) to systolic blood pressure (SBP). First introduced by Allgöwer and Buri in 1967, it provides early warning of occult hypoperfusion and circulatory failure in trauma, sepsis, gastrointestinal hemorrhage, ruptured ectopic pregnancy, and cardiogenic shock.

Formulas and Calculation Methods

This calculator evaluates three clinically validated indices:

• Standard Shock Index (SI): $$\text{SI} = \frac{\text{Heart Rate (bpm)}}{\text{Systolic Blood Pressure (mmHg)}}$$ Normal range: 0.5 to 0.7.
• Modified Shock Index (MSI): $$\text{MAP} = \frac{\text{SBP} + 2 \times \text{DBP}}{3}$$ $$\text{MSI} = \frac{\text{Heart Rate}}{\text{Mean Arterial Pressure (MAP)}}$$ Normal range: 0.7 to 1.3. Provides better vascular resistance sensitivity by incorporating diastolic pressure.
• Age-Adjusted Shock Index (SIA): $$\text{SIA} = \text{Age (years)} \times \text{Shock Index}$$ In geriatric trauma and adult critical care, an $\text{SIA} \ge 50$ is a strong independent predictor of in-hospital mortality.

Clinical Significance & Occult Shock Detection

During early stages of hypovolemia or vasodilation, compensatory sympathetic activation increases heart rate and peripheral vascular resistance to preserve systolic blood pressure. Conventional vital signs alone may appear normal while tissue perfusion deteriorates. An elevated Shock Index ($\ge 0.7$ to $0.9$) unmasks this compensatory phase, serving as a trigger for:

• Early activation of massive transfusion protocols (MTP) in trauma.
• Intensive fluid resuscitation and vasopressor readiness in sepsis.
• Rapid triage to level-1 trauma centers or intensive care units.
• Multidisciplinary assessment alongside other critical care tools such as the SAPS II Score Calculator, Revised Trauma Score Calculator, and qSOFA Score Calculator.

Shock Index Risk Tiers