Shock Index Calculator
Shock Index Calculator
Divide heart rate by systolic blood pressure to catch compensated shock earlier than either number does alone.
Shock Index
HR ÷ SBPHeart rate 118 per minute, systolic BP 96 mmHg
Formula
- Normal range
- approximately 0.5 to 0.7 in a stable adult
- > 0.9
- predicts mortality and transfusion need in trauma; also used in sepsis and postpartum haemorrhage
- modified shock index (mSI)
- uses mean arterial pressure instead of systolic pressure as the denominator
- age shock index
- shock index × age; performs better than the plain index in older trauma patients
Worked example
Heart rate 118 per minute, systolic BP 96 mmHg
118 ÷ 96 = 1.23
1.23 is 0.9 or above → markedly raised
Shock index thresholds
| Shock index | Interpretation |
|---|---|
| < 0.5 | Below normal |
| 0.5 – 0.7 | Normal |
| 0.7 – 0.9 | Raised |
| ≥ 0.9 | Markedly raised — higher mortality and transfusion risk in trauma |
Why a ratio beats either number alone
Shock index is heart rate divided by systolic blood pressure, and its value lies in what it catches that either number alone does not: early, compensated shock. A young, previously fit patient can lose a significant volume of blood or fluid while maintaining a normal blood pressure through peripheral vasoconstriction, so blood pressure alone looks reassuring right up until it suddenly is not. Because heart rate tends to rise earlier in that compensatory response, the ratio moves before either component crosses its own individual threshold of concern.
A normal shock index is roughly 0.5 to 0.7. Above 0.9 it predicts both mortality and the likelihood of needing transfusion in trauma, and the same underlying idea has been used to identify higher-risk presentations of sepsis and postpartum haemorrhage, where early compensated shock is just as easy to miss on the individual numbers.
It has real limitations, and they are worth stating plainly. Beta-blockers and rate-limiting calcium channel blockers blunt the tachycardic response and can mask a rising index in a patient who is nonetheless deteriorating; a paced rhythm fixes the heart rate independent of the patient’s physiological state; and the index is unreliable in any significant arrhythmia, where the pulse rate does not track stroke volume in the usual way.
Two variants extend the idea. The modified shock index substitutes mean arterial pressure for systolic pressure as the denominator, which is less sensitive to measurement technique. The age shock index multiplies the ordinary shock index by the patient’s age and outperforms the plain index specifically in older trauma patients, in whom an apparently “normal” heart rate and blood pressure can already represent substantial physiological reserve lost.
Frequently asked questions
What is a normal shock index?
Roughly 0.5 to 0.7 in a healthy, haemodynamically stable adult. Values above 0.9 are markedly raised and are associated with worse outcomes in trauma, sepsis and postpartum haemorrhage.
Why is shock index better than blood pressure alone at detecting shock?
Vasoconstriction can keep blood pressure normal even as significant volume is lost, especially in younger patients. Heart rate tends to rise earlier in that compensatory response, so the ratio detects the change before blood pressure does.
What limits the reliability of shock index?
Beta-blockers and rate-limiting drugs blunt the tachycardic response, a paced rhythm fixes the heart rate, and significant arrhythmia disconnects pulse rate from true stroke volume — all of these can mask a rising index.
What is the difference between shock index and the age shock index?
The age shock index multiplies the ordinary shock index by the patient’s age. It performs better than the plain index in older trauma patients, where normal-looking vital signs can still represent a large loss of physiological reserve.
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References
- Rady MY, Nightingale P, Little RA, Edwards JD. Shock index: a re-evaluation in acute circulatory failure. Resuscitation. 1992;23(3):227–34.
- Vandromme MJ et al. Identifying risk for massive transfusion in the relatively normotensive patient: utility of the prehospital shock index. J Trauma. 2011;70(2):384–8.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.