Shock Index Calculator

Calculate Shock Index (SI), Modified Shock Index (MSI), and Age Shock Index for early identification of haemodynamic instability.

Important Clinical Disclaimer: Shock Index Calculator is intended for use by qualified healthcare professionals only. It is for educational and informational purposes and should not replace clinical judgement. Always interpret results alongside the full clinical picture and follow your local guidelines and protocols.
Vital Signs
Normal range: 60-100 bpm
Normal range: 90-140 mmHg
Optional - for Modified Shock Index (MSI)
Optional - for Age Shock Index
Clinical Context (Optional)
About Shock Index
What is Shock Index?

Shock Index (SI) was first described by Allgower and Burri in 1967 as a simple bedside calculation to assess haemodynamic status. It is calculated as:

SI = Heart Rate / Systolic Blood Pressure

A normal SI ranges from 0.5 to 0.7. Values above 0.9-1.0 suggest significant haemodynamic compromise and have been associated with increased mortality across multiple clinical settings.

Why Use Shock Index?

Traditional vital signs may remain "normal" in early shock due to physiological compensation. Shock Index can identify patients at risk of deterioration before overt hypotension develops - this concept is known as occult shock or compensated shock.

Modified Shock Index (MSI)

MSI uses Mean Arterial Pressure (MAP) instead of systolic BP:

MSI = Heart Rate / MAP
MAP = (SBP + 2 x DBP) / 3

MSI >1.3 has been associated with increased mortality. Some studies suggest MSI may be more sensitive than standard SI.

Age Shock Index

Age Shock Index accounts for the fact that older patients may have worse outcomes at the same SI:

Age-SI = SI x Age

Age-SI >50 has been associated with increased mortality in trauma patients.

Shock Index Interpretation
Shock Index Interpretation Clinical Significance
0.5 - 0.7 Normal Normal haemodynamic status
0.7 - 0.9 Borderline elevated Consider close monitoring; may indicate early compensation
0.9 - 1.0 Elevated Possible occult shock; increased mortality risk
1.0 - 1.4 High Significant haemodynamic compromise; consider fluid resuscitation
>1.4 Very high Severe shock; urgent intervention required; high mortality risk
Clinical Applications
Trauma
  • SI >1.0 associated with need for massive transfusion
  • Predicts mortality in blunt and penetrating trauma
  • Useful for triage decisions
  • Age-SI particularly useful in elderly trauma
Sepsis
  • SI >1.0 associated with increased mortality
  • May identify sepsis before lactate elevation
  • Consider alongside qSOFA criteria
GI Bleeding
  • SI >1.0 predicts need for intervention
  • Component of Glasgow-Blatchford Score
  • Helps identify high-risk patients
Obstetric Emergencies
  • Normal pregnancy: SI typically 0.7-0.9
  • SI >0.9 in pregnancy warrants investigation
  • Useful in ectopic pregnancy assessment
  • Postpartum haemorrhage monitoring
Limitations
Clinical judgement is essential. Shock Index is a screening tool and should not replace comprehensive clinical assessment.
  • Beta-blockers: May blunt heart rate response, falsely lowering SI
  • Pacemakers: Fixed heart rate prevents physiological tachycardia response
  • Pregnancy: Physiological changes mean higher baseline SI (typically 0.7-0.9)
  • Elderly patients: May have blunted tachycardia response; Age-SI may be more appropriate
  • Baseline hypertension: Relative hypotension may not be reflected in SI
  • Chronic conditions: Patients with chronic conditions may have altered baseline vital signs
  • Paediatric patients: Different normal ranges apply; not validated in children
Frequently Asked Questions

Shock Index (SI) is the ratio of heart rate to systolic blood pressure (HR/SBP). A normal SI is 0.5-0.7. Values above 0.9-1.0 suggest haemodynamic compromise and may indicate occult shock even when vital signs appear normal.

Modified Shock Index (MSI) is calculated as HR/MAP (Mean Arterial Pressure). MSI >1.3 has been associated with increased mortality in trauma and critical illness. Some studies suggest MSI is more sensitive than standard SI.

Shock Index is useful in trauma, sepsis, GI bleeding, ectopic pregnancy, and other conditions where early identification of shock is important. It can identify patients at risk before traditional vital sign abnormalities become apparent.

SI may be less reliable in patients on beta-blockers (blunted heart rate response), those with pacemakers, pregnant patients, elderly patients, and those with baseline bradycardia or hypertension. Clinical context is essential.
References
  1. Allgöwer M, Burri C. Shock index. Dtsch Med Wochenschr. 1967;92(43):1947-1950. PubMed
  2. Cannon CM, Braxton CC, Kling-Smith M, et al. Utility of the shock index in predicting mortality in traumatically injured patients. J Trauma. 2009;67(6):1426-1430. PubMed
  3. Singh A, Ali S, Agarwal A, Srivastava RN. Correlation of shock index and modified shock index with the outcome of adult trauma patients. J Emerg Trauma Shock. 2014;7(4):246-251. PMC
  4. Zarzaur BL, Croce MA, Fischer PE, et al. New vitals after injury: shock index for the young and age x shock index for the old. J Surg Res. 2008;147(2):229-236. PubMed
  5. Nathan HL, El Ayadi A, Hezelgrave NL, et al. Shock index: an effective predictor of outcome in postpartum haemorrhage? BJOG. 2015;122(2):268-275. PubMed
  6. NICE guideline [NG37]: Fractures (non-complex): assessment and management. NICE
Disclaimer

This calculator is provided for educational and informational purposes only. It is not intended to be a substitute for professional medical advice, diagnosis, or treatment.

  • Always seek the advice of a qualified healthcare provider with any questions regarding a medical condition.
  • Never disregard professional medical advice or delay seeking it because of information from this tool.
  • Clinical decision-making should always incorporate the full clinical context, patient preferences, and local protocols.
  • The creators of this tool accept no liability for decisions made based on its output.

If you are a patient: please discuss any results with your healthcare provider. This tool is designed for use by medical professionals and may not be appropriate for self-assessment.

Quick Reference
Shock Index (SI)

Formula: HR / SBP

  • Normal: 0.5-0.7
  • Elevated: >0.9
  • High risk: >1.0
Modified SI (MSI)

Formula: HR / MAP

  • Normal: <0.7
  • Elevated: >1.3
Age Shock Index

Formula: SI x Age

  • Elevated: >50
Related Tools
Clinical Pearl

A patient with HR 110 and SBP 110 has a "normal" blood pressure and a mildly elevated heart rate - easily dismissed as anxiety. But their Shock Index is 1.0, suggesting possible occult shock requiring further evaluation.