Shock Index: Formula, Normal Range & What It Tells You

The shock index is one of the fastest bedside numbers in medicine — heart rate divided by systolic blood pressure — and it often flags trouble before blood pressure ever drops.

What is the shock index?

The shock index (SI) is the heart rate divided by the systolic blood pressure. It is a simple, dimensionless marker of circulatory stress: as a patient deteriorates, the heart rate climbs and the systolic pressure falls, so the ratio rises — frequently earlier than either vital sign looks alarming on its own.

Shock index formula

Shock index = heart rate ÷ systolic blood pressure

For example, a heart rate of 120 with a systolic pressure of 80 gives a shock index of 1.5 — clearly abnormal. Get it instantly with our shock index calculator.

Normal shock index and how to interpret it

Shock index	Interpretation
0.5–0.7	Normal
0.7–0.9	Borderline — reassess
> 0.9–1.0	Elevated — likely haemodynamic instability
> 1.3	High risk of serious deterioration

The trend matters as much as the single value — a rising shock index over serial readings is a red flag.

Modified shock index

The modified shock index (MSI) = heart rate ÷ mean arterial pressure. Because it uses MAP instead of systolic pressure, some clinicians find it a more sensitive marker of perfusion. A normal MSI is roughly 0.7–1.3. Work out MAP with our MAP calculator.

Where the shock index is used

• Trauma — an elevated shock index predicts the need for transfusion and major haemorrhage.
• Sepsis — helps flag patients heading toward septic shock.
• Obstetrics — used to detect postpartum haemorrhage early.
• Triage and early-warning scores — because it needs only two routine vital signs.

Limitations

The shock index is a screening signal, not a diagnosis. It can be misleading in patients on beta-blockers (a blunted heart-rate response), those with chronic hypertension, pacemakers, or arrhythmias. Always read it alongside the full clinical picture rather than in isolation.

Key takeaways

• Shock index = heart rate ÷ systolic blood pressure.
• Normal is 0.5–0.7; above 0.9–1.0 suggests instability.
• It often rises before blood pressure falls, giving early warning.
• The modified shock index uses mean arterial pressure instead.
• Interpret the trend and the clinical context, not just one value.

Frequently asked questions

What is the shock index?

The heart rate divided by the systolic blood pressure — a quick, dimensionless marker of circulatory stress that often rises before blood pressure falls.

What is the shock index formula?

Shock index = heart rate ÷ systolic blood pressure.

What is a normal shock index?

About 0.5 to 0.7. Values above 0.9–1.0 suggest haemodynamic instability, and above 1.3 indicates high risk.

How do you calculate shock index?

Divide the heart rate by the systolic blood pressure. For example, a heart rate of 100 and a systolic pressure of 100 gives a shock index of 1.0.

What is the modified shock index?

Heart rate divided by mean arterial pressure rather than systolic pressure; a normal value is roughly 0.7–1.3.

Why is the shock index useful?

It uses two routine vital signs and often detects deterioration earlier than blood pressure alone, making it valuable in trauma, sepsis, and obstetrics.

What does a high shock index mean?

That the heart rate is high relative to the blood pressure, suggesting the circulation is stressed and the patient may be heading toward shock.

What are the limitations of the shock index?

It can mislead in patients on beta-blockers, with chronic hypertension, pacemakers, or arrhythmias, so it must be read with the full clinical picture.