140 SI Shock index

Shock index (heart rate / systolic pressure): normal range, the 0.9 threshold, causes and limitations.

The shock index (SI) is the heart rate divided by the systolic blood pressure. In blood loss, dehydration or sepsis, the body first compensates by speeding up the heart while blood pressure is still maintained. The shock index captures this early phase: it may rise before blood pressure falls. It is used in emergency departments, trauma care and obstetrics for rapid triage.

HaemodynamicsShockEmergency triage

FormulaHR (beats/min) / SBP (mmHg)

Normal0.5–0.7

Increased risk≥ 0.9

ExampleHR 110, SBP 100 → SI 1.1

Figure 1. Shock index zones. In the study by Rady et al., SI of 0.9 or more identified emergency patients with higher acuity even when vital signs looked normal.

Clinical significance

A raised shock index suggests that the circulation is under strain: the heart beats faster to keep cardiac output up while each beat ejects less blood. Typical causes are bleeding (trauma, gastrointestinal or postpartum haemorrhage), dehydration, sepsis, pulmonary embolism and acute heart failure. In emergency patients, a shock index of 0.9 or higher was associated with more frequent admission to intensive care and more intensive treatment, even when heart rate and blood pressure alone looked acceptable.

Limitations

Beta-blockers, pacemakers and well-trained athletes can blunt the heart-rate response
Pain, anxiety, fever and stimulants raise heart rate without shock
Long-standing hypertension may mask relative hypotension
The index is a trigger for assessment, not a diagnosis

Seek emergency care

if a high shock index comes with bleeding, dizziness or fainting, confusion, cold clammy skin, fast breathing or chest pain.

References

Rady MY, Smithline HA, Blake H, Nowak R, Rivers E. A comparison of the shock index and conventional vital signs to identify acute, critical illness in the emergency department. Ann Emerg Med. 1994;24(4):685–690. PubMed 8092595

This article is for general information and does not replace a consultation with a doctor. Measurements must be interpreted by a physician together with symptoms and other tests.

Published on 9 October 2024