Shock Index Calculator

The shock index (SI) is heart rate divided by systolic blood pressure. Described by Allgöwer and Burri in 1967, it captures a pattern that individual vital signs can miss: in early blood loss or sepsis, the heart rate climbs while systolic pressure is still held up, so the ratio rises before frank hypotension appears. A normal adult shock index is about 0.5–0.7, and in trauma studies values above roughly 0.9 have been associated with more transfusion, intensive care admission and death.

The modified shock index (MSI) divides heart rate by mean arterial pressure instead, bringing diastolic pressure into the picture. In a large emergency department cohort, an MSI above 1.3 or below 0.7 was associated with higher mortality, while the plain shock index was not significantly associated with mortality in that general emergency population. Enter heart rate, systolic and diastolic pressure to get both indices, the calculated MAP and an interpretation.

These ranges apply to adults; children have higher normal heart rates, so pediatric age-adjusted cut-offs are needed. Beta-blockers, pacemakers, pain, anxiety and pregnancy all affect the ratio, so treat it as a quick triage signal alongside the full clinical assessment rather than a diagnosis.

The shock index divides heart rate (beats/min) by systolic blood pressure (mmHg). Because compensatory tachycardia usually appears before systolic pressure falls, the ratio can flag hidden hypoperfusion. In healthy adults it sits around 0.5–0.7; above about 0.9 it is associated with worse outcomes.

The modified shock index uses mean arterial pressure as the denominator. MAP is estimated as one-third systolic plus two-thirds diastolic, reflecting the longer time the heart spends in diastole. An MSI between 0.7 and 1.3 is considered normal; higher values suggest reduced stroke volume and lower values may reflect a hyperdynamic state.