The shock index is a simple bedside ratio — heart rate divided by systolic blood pressure — that can reveal hemodynamic instability before it shows up in either vital sign alone. This calculator computes the ratio and classifies it against normal, borderline, and elevated ranges.

How the shock index works

The shock index is simply heart rate divided by systolic blood pressure: SI = HR ÷ SBP. Allgöwer M and Burri C first described it in 1967 as a fast way to gauge cardiovascular status using two vital signs already being measured at the bedside.

Because it combines two values into one ratio, the shock index can flag a mismatch — a rising heart rate paired with a falling systolic blood pressure — before either individual value crosses a typical alarm threshold. A normal range is roughly 0.5 to 0.7; values at or above 0.9 to 1.0 are associated with occult shock and increased mortality risk.

Inputs and what they mean

Heart rate (bpm) is the number of heartbeats per minute, from a monitor, ECG, or manual pulse count.

Systolic blood pressure (mmHg) is the pressure during the heart's contraction phase, read from a blood pressure cuff or arterial line. Both values should be measured at the same point in time — mixing a heart rate from one moment with a blood pressure from another can produce a misleading ratio.

Limits and edge cases

The shock index is a screening tool, not a diagnosis. Beta-blockers and other rate-controlling medications can blunt the heart-rate rise expected in shock, keeping the shock index falsely low despite real hemodynamic compromise. Pre-existing hypertension, advanced age, pain, and anxiety can also shift the ratio independent of true perfusion status. A single elevated reading should prompt closer evaluation and trending, not an automatic diagnosis of shock — and a normal reading does not rule shock out, especially early in compensated blood loss.