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Modified Shock Index Calculator

Enter heart rate and systolic and diastolic blood pressure.

How it is calculated

The tool first finds the mean arterial pressure from the cuff reading: MAP = (SBP + 2 × DBP) / 3, in mmHg, where SBP is systolic and DBP is diastolic pressure. Then MSI = HR / MAP, with heart rate HR in beats per minute. MSI has no unit and is shown with two decimals. The category is taken from that rounded value: below 0.7 is labelled "Low (< 0.7)", 0.7 to 1.3 inclusive is "Normal" and above 1.3 is "High (> 1.3)". All three inputs must be above zero, otherwise no result appears. The label is a screening aid only; it ignores age, medication, pregnancy and the clinical picture, and it is not medical advice.

Worked example

A patient has a heart rate of 88 bpm and a blood pressure of 120/80 mmHg. The mean arterial pressure is (120 + 2 × 80) / 3 = 280 / 3 = 93.3 mmHg. The modified shock index is 88 / 93.33 = 0.94. That lies between 0.7 and 1.3, so the tool labels it "Normal". For comparison, a heart rate of 130 with 90/50 gives a MAP of 63.3 mmHg and an MSI of 2.05, which is labelled "High (> 1.3)".

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Common mistakes

  • Entering the MAP in the diastolic box, or pulse pressure (systolic minus diastolic) as the diastolic value. The tool calculates MAP itself from SBP and DBP.
  • Swapping systolic and diastolic. If the diastolic value is not below the systolic one, the tool shows no result; check the order before reading anything into a missing result.
  • Confusing MSI with the classic shock index (HR divided by systolic pressure). The two use different denominators, so their values cannot be compared.
  • Reading "Normal" as reassurance. The label covers everything from 0.7 to 1.3, and a single reading says nothing about a trend; repeated measurements matter more.
  • Entering a heart rate from a pulse oximeter or a moving patient without checking it. A wrong HR changes the index in direct proportion.

Sources

About this calculator

The Modified Shock Index (MSI) is heart rate divided by mean arterial pressure (MAP), where MAP itself is calculated as diastolic pressure plus one-third of the pulse pressure (systolic minus diastolic). This tool uses the bands from the emergency-medicine literature: 0.7 to 1.3 is shown as normal, above 1.3 as high (a marker of shock and higher mortality) and below 0.7 as low, which can also be abnormal.

Because MSI reacts to blood loss and shock sooner than standard vital-sign thresholds, it's used by emergency physicians, trauma teams, and paramedics for rapid triage — for instance in trauma, sepsis, or postpartum hemorrhage — to flag patients who look stable on paper but are compensating for significant blood loss. This tool is for reference and education only, not medical advice.

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Comments (1)

  • Editorial team

    This index divides heart rate by mean arterial pressure, so it is not the same number as the older heart rate over systolic pressure index. Compare values with published cut-offs only if they use the same definition. One reading is a snapshot and cuff error moves it, so a clinician would look at the trend and the person, not the number alone. Do you usually work with this version or the systolic one?

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