Shock Index Calculator

Created by Aleksandra Zając, MD
Reviewed by Dominik Czernia, PhD candidate and Jack Bowater
Last updated: Mar 14, 2022

The shock index calculator quickly assesses just two parameters - heart rate and systolic blood pressure - to give you a valuable predictive measure. Don't forget to hit the advanced mode button and read on to find out:

  • How the prehospital shock index formula was introduced;
  • How to perform a shock index interpretation after you calculate it; and
  • Much more!

How to calculate shock index

The shock index formula is:

shock index = heart rate [beats/minute] / systolic blood pressure [mmHg]

The normal shock index should be smaller than 0.7. Analyzing the formula, you can clearly see the bigger the shock index is, the worse the patient's condition. It means that the disproportion between heart rate and systolic blood pressure is growing, which indicates increasing circulatory failure.

It is an old formula, first presented over 50 years ago - in 1967. Its utility still remains, however, and the index is widely used, mainly in emergency medicine.

Shock index score interpretation

The normal shock index value is more than 0.5 and less than 0.7. The general rule is that the greater the shock index, the worse state the patient is in. It means either the heart is beating too fast and/or the blood pressure is dropping too much, both of which are signs of uncompensated shock.

One piece of research showed an almost three times higher mortality rate in patients with a shock index over > 0.9, compared to patients within the normal shock index. In the same paper, the absolute change of the shock index during observation in the hospital was also a prognostic sign - a 0.3 or greater difference in the shock index correlated with an almost five times higher risk of death than a change of less than 0.3.

But mortality is not all there is. A shock index over 0.9 is linked to an increased risk of massive transfusion as well. Another study also found a correlation between a shock index greater than one, hyperlactatemia, and 28-day mortality.

Prehospital shock index formula

Shock index was first proposed by two Swiss doctors, Allgöwer and Burri, in 1967. Therefore, you might also find the formula under the name of Allgöwer's index, or Allgöwer's shock index, as well.

It has been used mainly in emergency medicine to assess the severity of shock. The index is quick to find and can help you estimate the risk of fatal outcomes, especially:

The normal shock index ranges from 0.5 to 0.7. The bigger the index, the higher is the risk of all the outcomes mentioned above.

The shock index calculator - an example

How do you calculate the shock index for this patient?

A male patient, aged 63, was admitted to the emergency room at 6 AM. He presented with chest pain, shortness of breath; he was pale and sweaty. He has a history of hypertension, prediabetes, and he underwent a left knee replacement two years ago. His body temperature was 36.9°C (98.4°F), his blood pressure was 165/109 mmHg, heart rate was 104 beats/minute. ECG showed manifestations of myocardial ischemia.

The formula for shock index is:

shock index = heart rate [beats/minute]/systolic blood pressure [mmHg]

From the history, we have two important parameters:

  • heart rate equal to 104 beats/minute; and
  • systolic blood pressure equal to 165 mmHg.

Therefore:

shock index = 104 / 165 ≈ 0.63

The patient's shock index is 0.63, which is within the normal range.

Modified shock index formula

Some clinicians prefer to use the modified shock index formula (MSI formula) instead of the 'regular' one. The difference lies in the pressure we're taking into account - in the modified formula, we're using mean arterial pressure (MAP) instead of systolic blood pressure.

modified shock index = heart rate [beats/minute] / MAP [mmHg]

You can calculate MSI in our shock index calculator - hit the advanced mode to expand the calculator panel.

To count MAP on your own, use the formula:

MAP=2×DBP+SBP3MAP = \frac{2 × DBP + SBP}{3}

where:
MAP - Mean arterial pressure;
DBP - Diastolic blood pressure; and
SBP - Systolic blood pressure.

Work on standardizing the modified shock index is still being done; however, a value of 1 is considered too high. You interpret this measure the same as the standard shock index - the bigger the value, the worse the prognosis.

Elevated MSI increases the risk of:

  • ICU (Intensive Care Unit) admission;
  • Sepsis and hyperlactatemia;
  • Short- and long-term mortality; and
  • Need for mechanical ventilation.

What is age shock index?

Age shock index is the shock index (SI) multiplied by the patient's age in years. It is a modified shock index and can identify patients at high risk of death due to acute myocardial infarction (AMI). It gives similar results to the GRACE score; however, it's much quicker to calculate bedside.

shock index = age [years] × heart rate [beats/minute] / systolic blood pressure [mmHg]

You can also calculate the age SI using our shock index calculator - again, hit the advanced mode to expand the calculator panel.

In one papers, there's a suggestion to consider any value above 50 as an elevated age shock index.

FAQ

What is compensated shock?

A compensated shock is a state where the body is still able to compensate shock (meaning relative or total fluid loss) via different mechanisms (circulatory centralization, release of specific hormones). Patients in a compensated shock won't be visibly hypotensive.

What is volemia?

Hypovolemia is a state of pathologically low fluids in your circulatory system. It can be due to bleeding or extreme dehydration. On the other hand, hypervolemia means too much fluid - e.g., due to renal or heart failure.

When we talking about volemia, we never talk about the term itself - we always refer to either hypovolemia or hypervolemia.

Does shock lower heart rate?

Generally, no. Shock will most probably increase your heart rate. This is because your body tries to compensate for relatively too little fluids, blood, and oxygen in the body - so it tries to speed up the blood circulation.

But this doesn't always mean that shock is always present with tachycardia (fast heart beating). If a patient is in shock because of conduction abnormalities, and their heartbeat is abnormally slow or not at all (asystole), you won't observe such a relationship.

What are the most common causes of shock?

The most common causes of shock include:

  1. Severe allergic reaction (or anaphylaxis);
  2. Major blood loss (due to an injury);
  3. Heart failure (especially acute heart failure, e.g., due to myocardial infarction);
  4. Sepsis;
  5. Pneumothorax;
  6. Severe, significant burns; and
  7. Dehydration.
Aleksandra Zając, MD
Heart rate (HR)
beats/minute
Systolic blood pressure (SBP)
mmHg
Shock Index (SI)
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