What Is the Shock Index?

The shock index (SI) is the ratio of heart rate to systolic blood pressure:

Shock Index = Heart Rate ÷ Systolic BP

It was first described in the 1960s and has since been validated as a rapid, bedside indicator of circulatory shock—often before overt hypotension develops.

Normal and Abnormal Values

Shock Index	Interpretation
0.5 – 0.7	Normal
0.7 – 0.9	Elevated; monitor closely
≥ 0.9	High risk for occult shock and poor outcomes
≥ 1.1 – 1.4	Severe shock; aggressive resuscitation often needed

Why It Is Useful

In early shock, the body compensates by increasing heart rate to maintain blood pressure. A patient may therefore have a "normal" blood pressure but a shock index that reveals underlying instability. This is particularly valuable in:

• Trauma and hemorrhage
• Sepsis and septic shock
• Acute myocardial infarction
• Pulmonary embolism
• Obstetric emergencies

Modified Shock Index

Some clinicians use the modified shock index, which incorporates mean arterial pressure (MAP) or diastolic blood pressure. These variants may improve accuracy in specific populations like pregnant patients or those on beta-blockers.

When to Act

A shock index ≥ 0.9 in a patient with concerning symptoms (tachycardia, pallor, altered mental status, or oliguria) should trigger immediate evaluation for the cause of shock and preparation for fluid resuscitation or blood transfusion.

Calculate your patient's shock index with our shock index calculator.