Shock Index
Heart rate / systolic BP ratio for hemodynamic instability detection
Calculator
Why use it
Detects occult shock earlier than HR or SBP alone. Simple, fast bedside calculation. Useful in trauma triage, postpartum hemorrhage, and GI bleeding. Better predictor of transfusion need than individual vital signs.
About
The Shock Index (SI) is the ratio of heart rate to systolic blood pressure (HR/SBP). A normal SI is 0.5-0.7 in healthy adults. An elevated SI (> 0.9-1.0) is an early indicator of hemodynamic compromise, often detecting occult shock before traditional vital signs (HR and SBP individually) become abnormal.
When to use
- Trauma triage and hemorrhage assessment
- Postpartum hemorrhage evaluation
- GI bleeding severity assessment
- Any suspected occult hypovolemia
Formula
Pearls & pitfalls
- Normal SI = 0.5-0.7. A "normal" HR of 90 with SBP of 90 gives SI = 1.0 — already concerning.
- SI detects shock before hypotension develops (compensatory mechanisms maintain BP initially).
- In pregnancy, normal SI may be slightly higher (up to 0.9) due to physiologic changes.
- Pediatric SI norms are different — use age-adjusted references (SIPA).
- Beta-blockers blunt tachycardia and can mask elevated SI — interpret with caution.
- Modified Shock Index (MSI) = HR/MAP may be more accurate in some settings.
Next steps
SI > 0.9: Consider occult hemorrhage, early resuscitation. SI > 1.0: Likely significant hemodynamic compromise — aggressive fluid/blood resuscitation. SI > 1.4: Severe shock — massive transfusion protocol consideration.
Evidence
Described by Allgöwer and Burri (1967). Multiple trauma studies validate SI > 0.9-1.0 as predictor of transfusion need, ICU admission, and mortality. Validated in postpartum hemorrhage (Nathan et al., Obstet Gynecol 2015).