What is the shock index?
The shock index is the heart rate divided by the systolic blood pressure. Allgöwer and Burri proposed it in 1967 as a simple bedside sign of hypovolaemia.
It needs no extra equipment: every set of observations already has both numbers. That makes it a useful second look at the vital signs, especially when the heart rate is creeping up and the blood pressure has not yet fallen.
How do you calculate the shock index?
Shock index = heart rate (beats/min) ÷ systolic blood pressure (mmHg)
The result has no unit. The calculator shows up to two decimals and drops trailing zeros, so 1.10 appears as 1.1.
How to use the calculator
- Enter the Heart rate in beats per minute.
- Enter the Systolic BP in mmHg.
- Read the Shock index and the Interpretation.
- Check the working line, which shows the division.
Both values must be greater than zero. It’s best to use a heart rate and a blood pressure taken at the same time.
Interpretation
| Shock index | Label shown |
|---|---|
| 0.7 or less | Usual range (≤ 0.7) |
| Above 0.7 and below 1.0 | Borderline (0.7–1.0) |
| 1.0 or more | Elevated (≥ 1.0) |
The band is set on the value shown, rounded to two decimals, so the label always matches the number on screen.
A shock index of 1.0 or more has been linked to a higher chance of serious illness or injury, transfusion and admission to critical care (see the review by Koch and colleagues). It is best treated as a prompt to look closer and follow the trend, not as a diagnosis. Escalation criteria come from local protocols and early warning scores such as NEWS.
Worked examples
Example 1 – elevated. Heart rate 120/min, systolic 100 mmHg. 120 ÷ 100 = 1.2, Elevated (≥ 1.0).
Example 2 – usual range. Heart rate 80/min, systolic 130 mmHg. 80 ÷ 130 = 0.615…, shown as 0.62, Usual range (≤ 0.7).
Example 3 – borderline. Heart rate 98/min, systolic 112 mmHg. 98 ÷ 112 = 0.875, shown as 0.88, Borderline (0.7–1.0). Neither number would stand out on its own; the ratio does.
Limitations
- Heart rate is not only about volume. Pain, fever, anxiety, arrhythmias and drugs such as salbutamol raise it; beta-blockers, calcium channel blockers and pacemakers blunt it.
- Baseline blood pressure varies. In someone with long-standing hypertension a “normal” systolic may already be low for them, which hides a rise in the index.
- Adults only. Children and pregnant women need specific reference values.
- A snapshot. The direction over repeated observations matters more than one value.
Common mistakes
- Dividing the wrong way round. It is heart rate ÷ systolic, not systolic ÷ heart rate.
- Using mean or diastolic pressure. The classic index uses systolic pressure; the version with MAP is a different score.
- Mixing readings from different times. A heart rate from now and a blood pressure from an hour ago give a meaningless ratio.
- Taking false comfort from a normal value. A shock index in the usual range does not rule out bleeding or sepsis, especially in older people or those on rate-limiting drugs.
For the same patient, the mean arterial pressure calculator shows whether organ perfusion pressure is holding up, and in suspected infection the qSOFA score adds a quick sepsis screen.
Frequently asked questions
What is a normal shock index?
In healthy adults the shock index usually sits between about 0.5 and 0.7. The calculator labels values up to 0.7 as the usual range, values above 0.7 and below 1.0 as borderline, and 1.0 or more as elevated. The label is set on the number you see, rounded to two decimals.
Why use the shock index instead of heart rate and blood pressure separately?
Because the ratio moves before either number looks alarming. Early in blood loss or sepsis the heart rate rises while the systolic pressure is still held up. Each value can seem acceptable on its own, yet a heart rate of 105 with a systolic of 105 already gives a shock index of 1.0.
Who described the shock index?
Martin Allgöwer and Carl Burri, in a German paper published in 1967, which is why it is sometimes called the Allgöwer index. They proposed it as a simple bedside sign of blood volume loss. It has since been studied in trauma, bleeding, sepsis and general emergency patients.
Does a normal shock index rule out shock?
No. Beta-blockers, pacemakers, pain, anxiety, long-standing hypertension and age all change heart rate and blood pressure independently of blood volume. A value in the usual range is reassuring only alongside the rest of the assessment, and the trend over repeated observations matters more than one reading.
Is the shock index the same in children and pregnancy?
No. Children have higher normal heart rates and lower blood pressures, and pregnancy changes both, so different reference values are used. The bands in this calculator are the usual adult ones. For a child or a pregnant woman, it’s best to use age- or pregnancy-specific references and local protocols instead.
What are the modified and age shock index?
They are variants of the same idea. The modified shock index divides heart rate by mean arterial pressure instead of systolic pressure. The age shock index multiplies the classic shock index by the patient’s age in years. This calculator shows the classic version only.