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Shock Index Calculator

Calculate Shock Index (SI) by dividing heart rate by systolic blood pressure. A rapid bedside tool for identifying occult shock and hemodynamic compromise.

By Christopher FloiedPublished Updated

Must be greater than 0

Results

Shock Index

1.11

Shock likely

How to Use This Calculator

1

Enter your input values

Fill in all required input fields for the Shock Index Calculator. Follow the units and input format printed next to each field. Where a unit selector is available, choose the units that match your data.

2

Review your inputs

Double-check that all values are correct and match the units or format shown for each field. Incorrect units are the most common source of calculation errors and can produce results that are off by factors of 2, 10, or more.

3

Read the results

The Shock Index Calculator computes the output in your browser and displays the results, with units clearly labeled, as soon as they are ready — no data sent to a server.

4

Explore parameter sensitivity

Try adjusting individual input values to see how the output changes. This is a quick and effective way to develop intuition about how different parameters influence the result and to identify which inputs have the largest effect.

When to Use This Calculator

  • •Use the Shock Index Calculator for personal health tracking and wellness monitoring, establishing a baseline and tracking changes over time.
  • •Use it when recording fitness metrics to track progress toward health or athletic goals.
  • •Use it to compare measurements before and after a lifestyle, diet, or training change to quantify the impact.
  • •Use it as a conversation starter before a doctor's appointment, bringing objective data to discuss with a healthcare professional.

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About Shock Index Calculator

The Shock Index Calculator divides heart rate by systolic blood pressure to produce a simple ratio identifying hemodynamic compromise. Normal SI is 0.5-0.7. Values above 0.9-1.0 suggest hypovolemia or shock, often before traditional vital signs become abnormal. Shock Index is particularly useful for detecting compensated shock — a dangerous state where the body maintains near-normal BP through tachycardia.

The Math Behind It

First described by Allgower and Burri in 1967, Shock Index exploits the physiological relationship between heart rate and blood pressure during hemorrhage. In early hemorrhage (up to 30% blood loss), the body compensates by increasing heart rate while maintaining BP through vasoconstriction. Traditional vital signs may appear normal, but SI rises because HR increases while SBP holds. **SI interpretation**: - 0.5-0.7: Normal - 0.7-0.9: Monitor closely - 0.9-1.1: Early shock - >1.1: Significant shock - >1.5: Severe shock, high mortality Trauma patients with SI > 0.9 have 2-3x higher mortality. In obstetric hemorrhage, SI > 0.9 predicts need for transfusion. **Modified SI** (MSI = HR/MAP) and Age-adjusted SI exist but the original SI remains most validated.

Formula Reference

Shock Index

SI = HR / SBP

Variables: HR = heart rate, SBP = systolic BP

Worked Examples

Example 1: Compensated Shock

HR 110, SBP 100 mmHg

Step 1:SI = 110/100 = 1.10

SI 1.10 — compensated shock despite 'normal' looking BP. Needs aggressive evaluation.

Example 2: Normal Patient

HR 72, SBP 120

Step 1:SI = 72/120 = 0.60

SI 0.60 — normal.

Common Mistakes & Tips

  • !Relying on a single reading. Trends matter — rising SI indicates worsening hemodynamics.
  • !Applying to patients on beta-blockers who cannot mount tachycardia.
  • !Ignoring context. Anxiety and pain can elevate HR without true shock.

Related Concepts

MAP Calculator

Mean Arterial Pressure for another hemodynamic view.

GCS Calculator

Glasgow Coma Scale often used alongside SI in trauma.

Used in These Calculators

Calculators that build on or apply the concepts from this page:

Glasgow Coma Scale Calculator

Frequently Asked Questions

Why is SI better than BP alone?

BP is maintained by compensatory mechanisms until ~30% blood loss. SI captures the tachycardia that precedes BP drop, detecting compensated shock.

What SI predicts transfusion need?

SI > 0.9 is associated with need for blood transfusion. SI > 1.0 correlates with massive transfusion protocol activation.

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