1Shock Index Reference
| Shock Index | Category | Clinical Significance |
|---|---|---|
| <0.6 | Normal | Normal haemodynamics |
| 0.6–1.0 | Normal–Borderline | Mild compensated shock possible |
| 1.0–1.4 | Elevated | Significant haemodynamic compromise — may predict massive transfusion |
| ≥1.4 | Severe | Shock — high mortality, massive transfusion likely needed |
| ≥1.7–2.0 | Critical | Severe shock — immediate intervention required |
Clinical Applications
- Trauma: SI ≥1.0 predicts massive transfusion with ~70% sensitivity. Activate MTP (massive transfusion protocol) at SI ≥1.0–1.4 in context of trauma
- Obstetric haemorrhage: SI ≥1.0 in PPH indicates significant blood loss (>1000 mL), SI ≥1.7 predicts severe maternal morbidity. Valuable because BP may remain deceptively normal in young women
- Sepsis screening: SI ≥1.0 combined with clinical features suggests haemodynamic compromise in sepsis
- Pulmonary embolism: SI ≥1.0 in suspected PE predicts right heart strain and adverse outcomes — consider thrombolysis
- Upper GI bleeding: SI ≥1.0 at presentation predicts adverse outcomes and need for urgent intervention
Why Shock Index Can Be More Sensitive Than BP Alone
A young, healthy patient can maintain systolic BP in the normal range while developing significant hypovolaemia through compensatory tachycardia. A patient with HR 110 and SBP 110 has a normal blood pressure but a shock index of 1.0 — suggesting significant circulatory stress. SI captures the haemodynamic relationship that isolated BP or HR values miss.
2Frequently asked questions
What is the shock index?
Shock Index (SI) = Heart Rate / Systolic Blood Pressure. Normal SI: 0.5–0.7. SI >1.0 indicates haemodynamic compromise — the heart rate exceeds systolic BP, reflecting compensated or overt shock. SI >1.4 correlates with severe haemorrhage and massive transfusion requirement. It is a simple bedside tool requiring only pulse oximetry and BP measurement.
How is shock index used in trauma?
In trauma: SI >1.0 predicts the need for blood transfusion. SI >1.4–1.5: high likelihood of massive transfusion protocol (MTP) activation (≥10 units pRBCs in 24 hours). In obstetric haemorrhage: SI >0.9 predicts severe postpartum haemorrhage. In UGIB: SI >1.0 is a red flag for major bleed requiring urgent endoscopy. SI is particularly useful when exact blood loss is unknown.
What are the types of shock?
Hypovolaemic (most common): haemorrhage, burns, dehydration — high SI, cold peripheries, collapsed veins. Distributive (septic, anaphylactic, neurogenic): SI >1 early (vasodilation), warm peripheries in early sepsis. Cardiogenic: SI >1, cold peripheries, raised JVP, pulmonary oedema, S3 gallop. Obstructive (PE, tamponade, tension pneumothorax): SI >1, specific clinical signs. Each type requires different immediate management.
What are the initial steps in managing shock?
ABCDE approach: Airway — ensure patent. Breathing — high-flow O2 (15L via non-rebreather mask). Circulation — large-bore IV access ×2, blood cultures, urgent bloods (FBC, coag, crossmatch, lactate, ABG), IV fluid bolus 250–500 mL crystalloid and reassess (avoid fluid overload in cardiogenic shock). Disability — GCS, glucose. Exposure — identify source. Activate massive transfusion protocol if haemorrhagic shock with SI >1.4.
When does shock index fail?
Shock index is less reliable in: elderly patients (chronotropic incompetence — cannot mount appropriate tachycardia; beta-blockers blunt HR response), patients on vasopressors (BP maintained artificially), and neurogenic shock (paradoxical bradycardia with hypotension — SI may be normal or low despite severe haemodynamic compromise). Always interpret SI alongside clinical assessment, lactate, and urine output.
What is the modified shock index?
Modified Shock Index (MSI) = HR / Mean Arterial Pressure. Normal MSI: 0.7–1.1. MSI >1.4 predicts in-hospital mortality in sepsis and trauma better than SI alone. MSI incorporates diastolic BP which SI ignores, making it more sensitive for early shock detection. However, both SI and MSI are screening tools — neither replaces comprehensive haemodynamic assessment.
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.