Shock Index

Explaining shock index (SI)

Shock index (SI) is the ratio of heart rate (HR) to systolic blood pressure (SBP), expressed in beats per minute over mmHg. A normal resting SI is about 0.5 to 0.7 in healthy adults. When there's hypovolemia or shock, SI increases, providing an earlier warning than HR or SBP alone. It helps in risk stratification for conditions like trauma, sepsis, or MI. Thresholds above 0.7 signal abnormality, and values over 1.3 suggest high risk or need for interventions. However, it's limited in certain conditions like beta-blockade or hypertension.

Shock Index (SI) = heart rate ÷ systolic blood pressure
 e.g. 110 beats min⁻¹ ÷ 100 mm Hg = 1.10 (unit‑less)

---

Why it matters physiologically

When circulating volume falls or systemic vascular tone drops, the body tries to maintain cardiac output by raising heart rate while SBP drifts downward. The ratio changes earlier than either vital sign alone, so SI flags occult shock before frank hypotension appears. 

---

What the number tells you

Range (adult)	Typical interpretation	Clinical evidence/examples
0.5 – 0.7	Normal resting SI in healthy adults	—
> 0.7	Early hemodynamic stress; prompts closer monitoring	Predicts need for intervention in ED sepsis cohort.
≥ 0.9	Abnormal—high likelihood of compensated shock	Used as escalation trigger in obstetric hemorrhage guidelines.
≥ 1.0	Decompensation imminent; higher ICU admission & mortality	Linked to massive‑transfusion requirement after trauma.
≥ 1.4–1.7	Severe shock; urgent resuscitation needed	Strong predictor of adverse outcome in postpartum hemorrhage and septic shock meta‑analyses.

---

Key clinical uses

• Trauma & hemorrhage – SI > 0.9 at arrival identifies patients who will need massive transfusion or operative control even when vital signs look “normal.” 

• Sepsis – Persistently elevated SI after fluids correlates with progression to septic shock, vasopressor requirement, and increased 28‑day mortality. 

• Post‑partum hemorrhage – Obstetric protocols use SI ≥ 0.9 for transfer/activation and ≥ 1.3–1.7 for calling massive‑bleed teams. 

• Myocardial infarction, pulmonary embolism, stroke – SI adds prognostic discrimination over SBP or HR alone. 

---

Practical points & limitations

• Trend it. A rising SI is often more informative than any single value.

• Age or pediatric adjustment. In children SI normally runs higher; the SIPA score uses age‑specific cut‑offs. 

• Confounders. β‑blockers, pacemakers, atrial fibrillation, high spinal cord injury, or severe pain/anxiety can mask or exaggerate SI.

• Derived variants. Modified SI (HR ÷ MAP), Delta‑SI (arrival vs. prehospital), and Age‑SI (SI × age) may improve accuracy in specific settings but are not yet widely adopted.

Take‑home: Shock index is a quick bedside metric that integrates heart rate and systolic blood pressure into a single early‑warning number—values ≥ 0.9 signal compensated or overt shock and should trigger an immediate search for bleeding, sepsis, cardiogenic failure, or other causes, plus aggressive resuscitation and monitoring.