What the shock index measures
When a patient loses blood, the heart beats faster to keep blood pressure up. For a while, systolic pressure may look normal even though the body is compensating hard. The shock index (SI) combines the two numbers:
SI = heart rate ÷ systolic blood pressure
In healthy adults, it is usually about 0.5 to 0.7. A rising shock index can reveal hidden bleeding or sepsis earlier than the heart rate or blood pressure alone.
Variants
- Modified shock index (MSI) = heart rate ÷ mean arterial pressure, where MAP = (systolic + 2 × diastolic) ÷ 3. Values above about 1.3 or below 0.7 have been associated with higher mortality.
- Age shock index = age × SI. In trauma patients aged 55 or older, a value of 50 or more has been linked to higher mortality.
Worked example
A 60-year-old with a heart rate of 110, blood pressure 100/70 mmHg:
| Result | Value |
|---|---|
| Shock index | 1.10, abnormal |
| Mean arterial pressure | 80 mmHg |
| Modified shock index | 1.38 |
| Age shock index | 66 |
| Pulse pressure | 30 mmHg |
The blood pressure alone might not alarm anyone, but all three indices point toward significant blood loss or shock.
Interpreting the shock index
| Shock index | Usual interpretation |
|---|---|
| 0.5 to 0.7 | Usual range |
| 0.7 to 0.99 | Borderline or mildly elevated |
| 1.0 to 1.39 | Abnormal: associated with more transfusion, ICU admission and mortality |
| 1.4 or more | Severe |
Limits
- Medicines such as beta blockers blunt the rise in heart rate and can hide shock.
- Pain, anxiety and fever raise heart rate without blood loss.
- Chronic high blood pressure means a “normal” reading may already be low for that patient.
- Pregnancy raises the normal shock index; obstetric thresholds of about 0.9 to 1.0 are often used.
- In children, normal heart rate and blood pressure vary with age, so adult cut-offs do not apply.
The shock index is a quick screening tool. It complements, and never replaces, a full clinical assessment, lactate and the response to treatment.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.