Shock Index Calculator
Calculate shock index (SI) from heart rate and systolic blood pressure with interpretation. Includes modified shock index (MSI) and age shock index (ASI) for comprehensive hemodynamic assessment.
About This Calculator
The Shock Index (SI) is a rapid, bedside hemodynamic assessment tool used primarily in emergency medicine to evaluate the severity of shock and circulatory failure. First described by Allgöwer and Burri in 1967, the shock index is calculated by dividing heart rate by systolic blood pressure. It helps clinicians identify patients at risk of hemodynamic instability even when vital signs appear within normal ranges individually.
The formula is: SI = Heart Rate (bpm) / Systolic Blood Pressure (mmHg). A normal shock index ranges from 0.5 to 0.7. Values above 0.7 indicate progressively worsening hemodynamic status, with SI > 0.9 associated with significantly increased mortality risk, need for massive transfusion, and hyperlactatemia. The modified shock index (MSI) uses mean arterial pressure instead of systolic pressure, and the age shock index (ASI) adjusts for patient age to improve risk stratification in acute myocardial infarction.
This calculator also computes the Modified Shock Index (MSI = HR / MAP, where MAP = (2 × DBP + SBP) / 3) and the Age Shock Index (ASI = Age × HR / SBP) when optional inputs are provided. These variants offer complementary prognostic information — MSI correlates with ICU admission and sepsis risk, while ASI helps identify high-risk AMI patients similarly to the GRACE score but with faster bedside calculation.
Frequently Asked Questions
How is the shock index calculated?
The shock index (SI) is calculated by dividing the heart rate (beats per minute) by the systolic blood pressure (mmHg): SI = HR / SBP. The normal shock index ranges from 0.5 to 0.7. A higher value indicates greater hemodynamic instability.
What is a normal shock index range?
The normal shock index ranges from 0.5 to 0.7. Values below 0.5 may indicate bradycardia, while values between 0.7 and 0.9 suggest compensated shock. A shock index above 0.9 is associated with significantly increased mortality risk and need for massive transfusion.
What is the modified shock index (MSI)?
The modified shock index (MSI) uses mean arterial pressure (MAP) instead of systolic blood pressure: MSI = HR / MAP. MAP is calculated as (2 × DBP + SBP) / 3. An MSI above 1.0 is considered elevated and correlates with increased risk of ICU admission, sepsis, and mortality.
What is the age shock index (ASI)?
The age shock index multiplies the standard shock index by the patient's age: ASI = Age × HR / SBP. It helps identify patients at high risk of death from acute myocardial infarction (AMI). An ASI above 50 is generally considered elevated.
Can I rely on this calculator for diagnosis?
No. This calculator is for educational and reference purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional for medical assessment and decision-making.
What does a shock index over 0.9 mean?
A shock index over 0.9 is associated with significantly higher mortality rates — research shows nearly three times higher mortality compared to patients with a normal shock index. It is also linked to increased risk of massive transfusion, hyperlactatemia, and occult shock. An absolute change of 0.3 or more during observation correlates with nearly five times higher risk of death.
What is the formula for shock index?
The shock index formula was first proposed by Swiss doctors Allgöwer and Burri in 1967: shock index = heart rate (beats/minute) / systolic blood pressure (mmHg). The index is also known as Allgöwer's index and is widely used in emergency medicine for rapid hemodynamic assessment.
What are the most common causes of shock?
The most common causes of shock include severe allergic reaction (anaphylaxis), major blood loss from injury, heart failure (especially acute myocardial infarction), sepsis, pneumothorax, severe burns, and dehydration. The shock index helps rapidly assess severity in these emergency situations.