Shock index: how to calculate it from heart rate and systolic blood pressure
The shock index is heart rate divided by systolic blood pressure. For a heart rate of 80 beats per minute and a systolic pressure of 120 mm Hg, the shock index is 80 ÷ 120 = 0.67.
Not a medical device. Not a diagnosis or dosing recommendation.
What is the shock index?
The shock index (SI) is a ratio of two vital signs: heart rate (HR) over systolic blood pressure (SBP).[1] Both inputs are standard vital signs, the ratio has no units, and published work describes it as the simplest and fastest of the bedside scores it was compared with.[1]
It was first described in 1967 by Allgöwer and Burri. Published work describes it as giving an approximation of haemodynamic status in addition to the traditional vital signs.[1]
This page explains how the ratio is calculated. It doesn't apply a shock or no-shock cutoff.
The shock index formula
shock_index = heart_rate_bpm / systolic_mm_hgIn words: divide heart rate (beats per minute) by systolic blood pressure (mm Hg). Magent's calculator reports the result to 2 decimal places.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
heart_rate_bpm | Heart rate | beats per minute (whole number) | 30–220 |
systolic_mm_hg | Cuff systolic blood pressure | mm Hg | 50–250 |
shock_index | Heart rate ÷ systolic pressure | no unit | 2 decimal places |
How the formula works
- Take the heart rate in beats per minute.
- Take the systolic blood pressure from a cuff reading, in mm Hg.
- Divide heart rate by systolic pressure.
- Round to 2 decimal places.
When heart rate and systolic pressure are equal, the ratio is exactly 1.00. A higher heart rate or a lower systolic pressure raises the ratio; a lower heart rate or a higher systolic pressure lowers it.
Units
There's nothing to convert. Heart rate is in beats per minute and systolic pressure in mm Hg, and the ratio is unitless.[1] Diastolic pressure and mean arterial pressure aren't used.
Worked examples
Example 1: heart rate 80, systolic 120
- 80 ÷ 120 = 0.666…
- Rounded to 2 decimals: 0.67
Example 2: heart rate 120, systolic 80
- 120 ÷ 80 = 1.5
- Rounded to 2 decimals: 1.50
Example 3: heart rate 60, systolic 120
- 60 ÷ 120 = 0.5
- Rounded to 2 decimals: 0.50
Examples 1 and 2 use the same two numbers swapped. Swapping heart rate and systolic pressure changes the ratio from 0.67 to 1.50, so the order matters.
Shock index reference table
These values are computed from the formula, heart rate ÷ systolic pressure, and rounded to 2 decimals as Magent's calculator reports. They show how the ratio moves; they aren't targets, ranges or risk categories.
| Heart rate \ Systolic | 80 mm Hg | 90 mm Hg | 100 mm Hg | 120 mm Hg | 140 mm Hg |
|---|---|---|---|---|---|
| 60 bpm | 0.75 | 0.67 | 0.60 | 0.50 | 0.43 |
| 80 bpm | 1.00 | 0.89 | 0.80 | 0.67 | 0.57 |
| 100 bpm | 1.25 | 1.11 | 1.00 | 0.83 | 0.71 |
| 120 bpm | 1.50 | 1.33 | 1.20 | 1.00 | 0.86 |
| 140 bpm | 1.75 | 1.56 | 1.40 | 1.17 | 1.00 |
To read it, find the heart-rate row and the systolic-pressure column. The diagonal where the two are equal is 1.00.
History of the shock index
The shock index comes from M. Allgöwer and C. Burri, Shock index, published in the Deutsche Medizinische Wochenschrift in 1967 (volume 92, issue 43, pages 1947–1950). That's why it's also called the Allgöwer shock index or the Allgöwer–Burri index.
Later researchers proposed variations, including a modified shock index and an age shock index.[1] Those variations are covered below.
When it's used
The shock index has been studied in patients at risk of or experiencing shock from many causes, including trauma, haemorrhage, myocardial infarction, pulmonary embolism and sepsis.[1] Recent research has looked at using it as soon as patients arrive in the emergency department.[1]
Where it's used
It's commonly discussed in emergency department triage. Because it uses values available on arrival, it can be calculated automatically in the electronic medical record and shown with the vital signs during triage.[1] Its inputs are available as soon as a patient arrives, so working it out doesn't need a blood sample or any extra test.[1]
Who uses it
Published work on the shock index focuses on emergency department teams. It describes the index as part of the triage and early assessment of critically ill patients.[1]
Limits of the calculation
- It's a ratio of two numbers, not a target for any intervention.
- It doesn't apply a shock or no-shock cutoff.
- It uses cuff systolic pressure. It doesn't use mean arterial pressure or diastolic pressure.
- Published work notes that heart rate and blood pressure can be affected by medications such as antihypertensives and beta-agonists, which also affects any ratio built from them.[1]
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- MAP estimates mean arterial pressure from cuff systolic and diastolic pressures. It gives a pressure in mm Hg, not a ratio.
- The modified shock index is a published variation that divides heart rate by mean arterial pressure instead of systolic pressure.[1] Magent's shock index calculator uses systolic pressure, so it doesn't compute the modified index.
- The age shock index multiplies the shock index by the patient's age.[1] This calculator doesn't apply an age term.
Glossary
- Shock index (SI): heart rate ÷ systolic blood pressure.[1]
- Heart rate (HR): beats per minute.
- Systolic blood pressure (SBP): the upper number of a cuff blood pressure reading, in mm Hg.
- Modified shock index (MSI): heart rate ÷ mean arterial pressure.[1]
- Mean arterial pressure (MAP): an estimated average arterial pressure from systolic and diastolic readings.
Common mistakes
- Dividing the wrong way round. It's heart rate ÷ systolic, not systolic ÷ heart rate (compare Examples 1 and 2).
- Using MAP or diastolic pressure. That gives a different index.
- Rounding too early. Divide first, then round once to 2 decimals.
- Entering values outside the accepted range. Heart rates outside 30–220 (whole numbers) and systolic pressures outside 50–250 mm Hg are rejected. They aren't estimated.
Source
Allgöwer M, Burri C. Shock index. Dtsch Med Wochenschr. 1967;92(43):1947-1950. DOI: 10.1055/s-0028-1106070
Further reading
- Koch E, Lovett S, Nghiem T, Riggs RA, Rech MA. Shock index in the emergency department: utility and limitations. Open Access Emerg Med. 2019;11:179-199.
Frequently asked questions
What is the shock index formula?
Shock index = heart rate ÷ systolic blood pressure. For 80 bpm and 120 mm Hg, that's 0.67.
How do you calculate the shock index?
Divide heart rate (beats per minute) by cuff systolic pressure (mm Hg), then round to 2 decimals. For example, 120 ÷ 80 = 1.50.
Can you give a shock index example?
With a heart rate of 60 and a systolic pressure of 120, the shock index is 60 ÷ 120 = 0.50. The reference table above gives more combinations.
What is the normal range for the shock index?
This page doesn't give a normal range or a cutoff. It calculates the ratio; what a value means depends on clinical context.
What is the modified shock index?
It's a published variation that uses mean arterial pressure instead of systolic pressure: heart rate ÷ MAP.[1] This calculator doesn't compute it.
Does the shock index have units?
No. It's a ratio of beats per minute to mm Hg, reported as a plain number.[1]