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The Shock Index is a simple bedside tool for rapid assessment of hemodynamic status and shock severity. It can detect occult shock before overt hypotension develops. However, it should be used in conjunction with full clinical assessment. Single measurements have limitations - trending is more valuable. Not validated in all populations (pregnancy, athletes, elderly, beta-blockers).
Rapid assessment of hemodynamic status and shock severity
Normal: 60-100 bpm
Normal: 90-140 mmHg
| Shock Index | Interpretation | Blood Loss (Trauma) | Action |
|---|---|---|---|
| <0.5 | Very Low (Bradycardia/HTN) | None | Investigate cause |
| 0.5-0.7 | Normal | <15% (<750 mL) | Routine monitoring |
| 0.7-1.0 | Borderline / Class II | 15-30% (750-1500 mL) | Close monitoring, IV access |
| 1.0-1.4 | Elevated / Class III | 30-40% (1500-2000 mL) | Active resuscitation needed |
| >1.4 | Critical / Class IV | >40% (>2000 mL) | Aggressive resuscitation, blood products |
The Shock Index (SI) is calculated as the ratio of heart rate to systolic blood pressure. It provides a simple, objective measure of hemodynamic status that can detect physiologic derangement earlier than vital signs alone.
Shock Index = Heart Rate ÷ Systolic Blood Pressure
Normal value: 0.5-0.7. Values >1.0 indicate significant hemodynamic compromise.
In shock, the body compensates by increasing heart rate and initially maintaining blood pressure through vasoconstriction. As shock progresses:
SI correlates with hemorrhage severity and blood transfusion requirements:
Modified Shock Index (MSI = SI × Age/10):
MSI = Heart Rate ÷ Mean Arterial Pressure
Uses MAP instead of SBP. Normal <1.3. May be more sensitive as MAP reflects overall perfusion pressure better.
Age SI = (Heart Rate ÷ Systolic BP) × (Age ÷ 10)
Incorporates age to account for different physiologic reserves. Higher values indicate worse prognosis in elderly trauma patients.
RSI = Systolic BP ÷ Heart Rate
Inverse of SI. Normal 1.5-2.0. Values <1.0 indicate shock. Some find this more intuitive (higher is better rather than lower is better).
Change in shock index over time. More valuable than single measurement. Rising SI despite treatment indicates ongoing blood loss or inadequate resuscitation. Falling SI suggests adequate resuscitation.
SI 0.5-0.7 (Normal)
SI 0.7-1.0 (Borderline)
SI 1.0-1.4 (Elevated)
SI >1.4 (Critical)
Normal shock index is 0.5-0.7 in healthy adults. This corresponds to typical values like HR 70 and SBP 120 (70/120 = 0.58). Values persistently above 0.7 suggest physiologic stress or early shock. Values below 0.5 may indicate bradycardia or hypertension and warrant investigation.
Young, healthy patients can maintain normal blood pressure despite significant blood loss (up to 30%) through compensatory mechanisms (vasoconstriction, tachycardia). Shock index detects this "occult shock" earlier by capturing the compensation (elevated HR) before BP drops. By the time BP falls, the patient has lost significant compensatory reserve.
Pediatric shock index requires age-adjusted normative values because children have higher baseline heart rates and different blood pressures. General SI cutoffs don't apply directly to pediatrics. Pediatric-specific charts are available, or use other pediatric shock assessment tools (e.g., PEWS).
Beta-blockers blunt the heart rate response to stress, potentially masking shock. A patient on beta-blockers may have a "normal" SI despite significant hypovolemia because HR cannot increase appropriately. Rely more heavily on other signs of perfusion (lactate, base deficit, mental status, skin perfusion, urine output) in these patients.
SI roughly correlates with ATLS hemorrhage classes: Class I (<15% loss): SI <0.7.Class II (15-30% loss): SI 0.7-1.0. Class III (30-40% loss): SI 1.0-1.4.Class IV (>40% loss): SI >1.4. However, individual variation exists and SI is just one assessment tool.
SI can help predict transfusion needs but shouldn't be the sole determinant. SI >1.0 correlates with increased transfusion likelihood, and SI >1.4 strongly predicts massive transfusion needs. However, transfusion decisions should incorporate clinical context, hemoglobin, ongoing bleeding, and hemodynamic response to resuscitation.
No. Single measurements provide a snapshot but trending SI over time is more valuable. A rising SI despite treatment suggests ongoing blood loss or inadequate resuscitation. A falling SI indicates adequate resuscitation. Serial measurements (every 15-60 minutes in acute settings) provide better clinical information than isolated values.
Athletes often have resting bradycardia (HR 40-60) and may have lower baseline SI values. However, they should still mount appropriate tachycardia in response to shock. If an athlete has SI >1.0, take it seriously - it still indicates significant physiologic stress even if their baseline is lower.