Shock Index Calculator
Shock index (heart rate ÷ systolic blood pressure), modified shock index and age-adjusted shock index — early warning of haemorrhage, sepsis and occult shock before blood pressure falls.
How it works
SI = HR ÷ SBP (normal 0.5–0.7); MSI = HR ÷ MAP; Age SI = SI × age
Blood pressure is a late sign of shock — young patients in particular compensate with tachycardia and vasoconstriction until they have lost a third of their blood volume. The shock index captures that compensation by dividing heart rate by systolic pressure: as the heart speeds up and pressure slips, the ratio rises above the normal 0.5–0.7 long before hypotension appears. Values above 0.9 predict the need for massive transfusion in trauma, ICU admission in sepsis and mortality in pulmonary embolism and acute coronary syndrome. The modified index uses mean arterial pressure to account for diastolic pressure, and multiplying by age recognises that the same index carries more risk in older patients. Beta-blockers, pacemakers and pain can all distort it.
Worked example
Heart rate 112, blood pressure 104/66, age 68: SI = 1.08 (elevated), MAP 79, MSI 1.42, age SI 73 — significant risk despite a “normal” systolic pressure.
Frequently asked questions
What shock index is normal?
0.5 to 0.7 in adults. Children run higher: paediatric age-adjusted (SIPA) cut-offs are about 1.2 for ages 4–6, 1.0 for 7–12 and 0.9 for 13–16.
Is it useful in obstetrics?
Yes — an obstetric shock index above 0.9–1.0 after delivery flags significant postpartum haemorrhage earlier than blood pressure alone.
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Clinical decision tool for education and for use by trained clinicians alongside judgement — not a diagnosis and not a substitute for medical assessment. Scores validated in specific populations may not apply to every patient.