The shock index (SI) is heart rate divided by systolic blood pressure — a triage aid first described by Allgöwer and Burri in 1967 that can reveal compensated shock before blood pressure alone looks alarming. This tool also computes mean arterial pressure (MAP), the modified shock index (HR ÷ MAP), and the age-adjusted shock index (age × SI), with literature-anchored interpretation bands. For education only — not a diagnostic tool. An elevated result warrants evaluation by a qualified health professional.
| Shock index (SI) | Interpretation |
|---|---|
| < 0.7 | Normal (classic 0.5–0.7 range, Allgöwer & Burri) |
| 0.7 – 0.9 | Borderline — upper end of the range; re-check and monitor |
| 0.9 – 1.0 | Elevated — marker of hemodynamic instability (Rady et al.) |
| 1.0 – 1.3 | High — associated with higher mortality in trauma studies |
| > 1.3 | Critical — markedly increased mortality risk (Vandromme et al.) |
| Modified shock index (MSI) | Interpretation |
|---|---|
| < 0.7 | Low — studied in association with higher mortality risk |
| 0.7 – 1.3 | Reference range |
| > 1.3 | Elevated — predicts in-hospital mortality (Liu et al., n=22,161) |
| Age × SI | Interpretation |
|---|---|
| < 50 | Below the value studied in older trauma patients |
| ≥ 50 | At/above the studied threshold — associated with instability in older patients (Zarzaur et al.) |
Systolic pressure alone is a late marker of shock: the body defends it until compensation fails. The modified shock index uses mean arterial pressure instead, bringing diastolic pressure — the phase that drives coronary perfusion — into the picture, and it has outperformed the classic index for mortality prediction in emergency-department cohorts. The age-adjusted shock index corrects for the fact that older patients carry less physiologic reserve: a “normal-looking” index in a 75-year-old can still signal danger.
The shock index (SI) is the ratio of heart rate to systolic blood pressure: SI = heart rate (beats per minute) ÷ systolic blood pressure (mmHg). It was described in 1967 by Allgöwer and Burri as a quick, noninvasive measure of hypovolemic shock, and it is now used as an early-warning and triage aid because tachycardia often precedes a drop in blood pressure during compensated shock.
Divide heart rate by systolic blood pressure, using the same measurement moment for both. Example: a heart rate of 120 bpm with a systolic pressure of 80 mmHg gives 120 ÷ 80 = 1.500. This calculator performs the division for you and interprets the result against literature-anchored bands.
The classic reference range reported by Allgöwer and Burri is 0.5–0.7. Values above 0.9 are treated as a marker of hemodynamic instability (Rady et al.), values of 1.0 or more have been associated with higher mortality in trauma studies, and values above 1.3 carry a markedly increased mortality risk (Vandromme et al.). Example: a heart rate of 70 bpm with a systolic pressure of 120 mmHg gives 70 ÷ 120 ≈ 0.583, which falls in the normal range.
The modified shock index replaces systolic pressure with mean arterial pressure (MAP): MSI = heart rate ÷ MAP, where MAP = diastolic pressure + (systolic − diastolic) ÷ 3. Because MAP incorporates diastolic pressure, the MSI reflects the diastolic phase that drives coronary perfusion. Example: heart rate 120 bpm, systolic 80 mmHg, diastolic 50 mmHg → MAP = 50 + (80 − 50) ÷ 3 = 60.0 mmHg, and MSI = 120 ÷ 60 = 2.000.
In a large emergency-department study of 22,161 patients (Liu et al.), the mean MSI was about 1.01, and an MSI of 1.3 or more predicted in-hospital mortality. In trauma research, MSI values below 0.7 or above 1.3 have been associated with a greater risk of death (Singer et al.). This calculator flags MSI above 1.3 as elevated and below 0.7 as low.
The age shock index is simply age in years multiplied by the shock index (Age SI = age × SI). Older patients can be in serious trouble with a deceptively reassuring shock index, so multiplying by age adjusts for reduced physiologic reserve. In a study of 16,077 blunt-trauma patients (Zarzaur et al.), an age × SI of 50 or more predicted early mortality and transfusion need in patients over 55; values above 50 were also linked to instability in patients over 65 (Kim et al.). Example: age 68, heart rate 130, systolic 85 → SI ≈ 1.5294 and Age SI ≈ 104.0.
An elevated shock index means the heart is beating fast relative to the pressure it is generating — the classic pattern of compensated shock, seen in hemorrhage, sepsis, severe dehydration, pulmonary embolism, and other states where cardiac output is threatened. It is a prompt for clinical evaluation, never a diagnosis by itself: blood pressure, perfusion signs, and the full clinical picture decide what is happening.
Several factors blunt or distort the index. Heart-rate-regulating drugs such as beta-blockers and calcium-channel blockers suppress the tachycardic response, and studies validating the thresholds excluded patients with arrhythmias, atrioventricular block, spinal cord injury, pregnancy, and severe hypertension. Older adults and people with chronic hypertension often run higher baseline pressures, so a “normal” index can be misleading — that is why the age-adjusted variant exists. The index is a population-level research association, not a personal diagnostic cutoff.
No. This tool is for education only and must not drive clinical decisions. The shock index is a triage aid, not a diagnosis. If a result is elevated, or if someone is unwell, seek evaluation by a qualified health professional promptly rather than relying on any online calculator.
Related calculators: ECG Heart Rate and Maximum Heart Rate.
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