Shock Index Calculator

Shock Index Calculator for Critical Care. A normal SI is roughly 0.5–0.7. An SI ≥ 0.9 is the most widely cited threshold for occult hypoperfusion and predicts higher mortality, ICU admission, and transfusion need across trauma and sepsis cohorts. In the four-class hemorrhage scheme, Class I (SI < 0.6) is no shock, Class II (0.6 to <1.0) mild, Class III (1.0 to <1.4) moderate, and Class IV (≥ 1.4) severe shock with the highest transfusion and mortality rates. A value ≥ 1.0 should prompt aggressive resuscitation and source control; ≥ 1.4 warrants early massive transfusion consideration. The action is escalation, not diagnosis: SI tells you how sick, not why.

How this calculator works

The Shock Index (SI) is heart rate divided by systolic blood pressure (SI = HR ÷ SBP), both taken from the same vital-sign set. It is a unitless ratio that captures early hemodynamic compromise better than either value alone, because a rising pulse and falling pressure move the ratio in the same direction and often shift before hypotension appears. Two variants extend it: the Modified Shock Index (MSI = HR ÷ mean arterial pressure) and the Age-Adjusted Shock Index (SI × age), used in older trauma patients.

When to use this calculator

Use SI at the front end of triage for undifferentiated shock, hemorrhage, sepsis, and trauma, and to flag occult hypoperfusion in patients who are still normotensive but tachycardic. It is most useful in the ED, prehospital setting, and early trauma bay for deciding on escalation, transfusion activation, and closer monitoring. Do not rely on it in patients on beta-blockers or with pacemakers (blunted HR response), in atrial fibrillation or other rate-variable arrhythmias, in late/decompensated pregnancy where baseline physiology shifts, or as a standalone rule-out in children (age-specific pediatric SI, SIPA, is required).

Inputs used

  • Heart rate
  • Systolic blood pressure

Clinical interpretation

A normal SI is roughly 0.5–0.7. An SI ≥ 0.9 is the most widely cited threshold for occult hypoperfusion and predicts higher mortality, ICU admission, and transfusion need across trauma and sepsis cohorts. In the four-class hemorrhage scheme, Class I (SI < 0.6) is no shock, Class II (0.6 to <1.0) mild, Class III (1.0 to <1.4) moderate, and Class IV (≥ 1.4) severe shock with the highest transfusion and mortality rates. A value ≥ 1.0 should prompt aggressive resuscitation and source control; ≥ 1.4 warrants early massive transfusion consideration. The action is escalation, not diagnosis: SI tells you how sick, not why.

Worked example

A 34-year-old trauma patient arrives with HR 118 and SBP 92 mmHg. SI = 118 ÷ 92 = 1.28. This sits in the Class III range (1.0 to <1.4) of the Mutschler shock classification, which corresponds to overt hemorrhagic shock, a base deficit around 8, and a substantially elevated likelihood of needing blood products. Despite an SBP that is not yet frankly hypotensive, the SI signals the team to activate massive transfusion protocol readiness and secure hemorrhage control, whereas a raw SBP of 92 alone might have been dismissed as borderline.

Limitations and safety notes

SI is falsely reassuring in patients whose heart rate cannot rise appropriately (beta-blockade, calcium-channel blockade, high spinal cord injury, athletic bradycardia, pacemakers) and unreliable in atrial fibrillation or other irregular rhythms where HR is unstable. It performs worse in the elderly, where age-adjusted SI or SIPA-type adjustments outperform the raw ratio, and standard adult thresholds do not transfer to children. A single early SI can miss evolving hemorrhage, so serial measurement matters more than any one value, and compensated young patients can maintain a deceptively normal SI until sudden decompensation.

Frequently asked questions

What is a normal Shock Index?

Roughly 0.5 to 0.7 in healthy adults. Values at or above 0.9 are commonly used to flag occult hypoperfusion, and 1.0 or higher signals overt shock warranting aggressive resuscitation.

How is Shock Index different from just checking blood pressure?

SI combines heart rate and systolic pressure into one ratio, so it rises when a compensating tachycardia appears even while SBP is still normal. That lets it catch early or occult shock that a blood-pressure reading alone would miss.

Can I use the adult Shock Index in children?

No. Children have age-dependent normal heart rates and pressures, so adult thresholds misclassify them. Use the age-adjusted pediatric shock index (SIPA), which sets different cutoffs by age band.

When does the Shock Index become unreliable?

In patients on beta-blockers or other rate-limiting drugs, with pacemakers, spinal cord injury, or atrial fibrillation, the heart-rate response is blunted or erratic, so SI can look falsely normal despite significant hemodynamic compromise.

What is the Modified Shock Index?

MSI is heart rate divided by mean arterial pressure instead of systolic pressure. Because it incorporates diastolic pressure, some studies find it a slightly more sensitive marker of adverse outcomes than the standard SI.

References

  • 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. PMID: 8092595.
  • Mutschler M, Nienaber U, Brockamp T, et al. A critical reappraisal of the ATLS classification of hypovolaemic shock: does it really reflect clinical reality? Resuscitation. 2013. PMID: 22835498.
  • Moffat B, Vogt KN, Inaba K. The Shock Index: is it ready for primetime? Crit Care. 2013. PMID: 24090407.
  • Fröhlich M, Driessen A, Böhmer A, et al. Is the shock index based classification of hypovolemic shock applicable in multiple injured patients with severe traumatic brain injury? An analysis of the TraumaRegister DGU. Scand J Trauma Resusc Emerg Med. 2016. PMID: 27955692.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: March 31, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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