Calculate Shock Index (SI), Modified Shock Index, MAP, and pulse pressure for rapid hemodynamic assessment. Estimates hemorrhage severity and transfusion need.
The Shock Index (SI) is the ratio of heart rate to systolic blood pressure (HR/SBP), a simple yet powerful bedside indicator of hemodynamic compromise. Normal SI ranges from 0.5 to 0.7; values above 0.9-1.0 indicate significant hemodynamic instability and correlate strongly with the need for massive transfusion, ICU admission, and in-hospital mortality.
The power of SI lies in its ability to detect occult shock before traditional vital signs become overtly abnormal. A patient with HR 100 and SBP 115 has "normal" vital signs individually, but an SI of 0.87 — clearly elevated — suggests compensated shock that warrants closer monitoring and intervention. This is particularly valuable in trauma, where compensatory mechanisms can mask significant hemorrhage until sudden cardiovascular collapse.
This calculator also computes the Modified Shock Index (HR/MAP), mean arterial pressure, pulse pressure, and provides estimated hemorrhage class correlation, offering a comprehensive hemodynamic snapshot from basic vital signs. Check the example with realistic values before reporting.
Individual vital signs (HR, SBP) have poor sensitivity for detecting early shock. Heart rate can be normal in hemorrhage (vagal response to blood loss), and blood pressure is maintained until late in the shock cascade. The ratio of HR to SBP captures compensatory tachycardia relative to blood pressure decline, detecting physiological stress earlier.
In trauma settings, SI >0.9 at presentation independently predicts the need for massive transfusion with higher sensitivity and specificity than SBP or HR alone.
Shock Index (SI) = Heart Rate / Systolic Blood Pressure Normal: 0.5-0.7 Modified Shock Index (MSI) = Heart Rate / Mean Arterial Pressure MAP = DBP + (SBP − DBP) / 3 Pulse Pressure = SBP − DBP (Normal: 30-60 mmHg) Hemorrhage Classification: SI <0.6: Class I (<15% blood volume) SI 0.6-1.0: Class II (15-30%) SI 1.0-1.4: Class III (30-40%) SI >1.4: Class IV (>40%)
Result: Shock Index 1.22 — Shock Likely
HR 110 / SBP 90 = SI 1.22, indicating significant hemodynamic compromise consistent with Class III hemorrhage (30-40% blood volume loss). MAP is 70 mmHg, approaching the threshold for organ ischemia. This patient needs aggressive fluid resuscitation, type and crossmatch, and consideration of massive transfusion protocol. Identify and control the bleeding source.
Pregnancy: Normal pregnancy increases HR and decreases SBP, resulting in higher baseline SI (~0.7-0.9 in third trimester). Elderly: Higher baseline SBP masks hemorrhage; apply age-adjusted SI. Pediatric: Normal pediatric SI ranges are age-dependent (infants normally have SI >1.0). Athletes: Lower resting HR may delay SI elevation during hemorrhage.
When SI is elevated but the diagnosis is unclear, advanced monitoring aids differentiation: Point-of-care ultrasound (IVC collapsibility for volume status, cardiac contractility), Lactate (tissue hypoperfusion marker), Central venous pressure (preload assessment), Pulse pressure variation (fluid responsiveness in ventilated patients).
Common activation criteria incorporate SI: ABC Score (Assessment of Blood Consumption) uses SI ≥1.0 as one of four criteria. TASH Score (Trauma Associated Severe Hemorrhage) uses multiple variables. Most protocols deliver balanced ratios of RBCs:FFP:Platelets (1:1:1) until hemorrhage control is achieved.
Compensatory tachycardia occurs before blood pressure drops. A patient losing blood may have SBP 110 but HR 105, giving SI 0.95 — already elevated. SBP does not fall significantly until approximately 30% of blood volume is lost (Class III hemorrhage). SI captures the compensatory heart rate response earlier.
Yes. Elderly patients often have higher baseline SBP (from hypertension) and may not mount adequate tachycardia (beta-blockers, pacemakers, autonomic dysfunction). This means their SI may appear normal despite significant blood loss. For elderly patients, a lower threshold (SI >0.7-0.8) should raise concern. Age-Shock Index (Age × SI) attempts to account for this.
SI was primarily validated in traumatic hemorrhage, but it can be elevated in any form of shock: distributive (sepsis), cardiogenic (heart failure, MI), and obstructive (PE, tamponade). However, the hemorrhage class estimates are specific to hemorrhagic shock and should not be applied to other shock types.
Beta-blockers blunt the tachycardic response to hemorrhage, falsely lowering SI. In patients on beta-blockers, a "normal" SI should not be reassuring if clinical suspicion for hemorrhage exists. Other markers (lactate, base deficit, clinical appearance) should be used alongside SI in this population.
Massive transfusion protocols are typically activated for: estimated blood loss >1.5L, SI >1.0 with active bleeding, hemodynamic instability despite 2L crystalloid, or anticipated need for >10 units RBCs in 24 hours. Many trauma centers use SI >1.0 as a scoring component in their activation criteria (ABC score, TASH score).
In unstable patients: every 5-15 minutes or with each vitals measurement. Trending SI is important — an increasing SI suggests ongoing hemorrhage or worsening hemodynamic status. A falling SI in response to treatment indicates successful resuscitation. Document SI trends alongside vital signs.