Reliability of Shock Index as a Severity of Disease Marker
The shock index (heart rate divided by systolic blood pressure) is a reliable and valuable tool for assessing disease severity across multiple conditions, particularly in identifying patients at high risk for adverse outcomes including mortality. 1
What is Shock Index?
- Shock Index (SI) = Heart Rate / Systolic Blood Pressure
- Normal range: 0.5-0.7
- Modified Shock Index (MSI) = Heart Rate / Mean Arterial Pressure
- Age Shock Index (ASI) = SI × Age
Evidence Supporting Shock Index Reliability
Trauma and Hemorrhage
- The European guideline on management of major bleeding and coagulopathy following trauma (2023) specifically recommends shock index for assessing the degree of hypovolaemic shock and transfusion requirements 1
- Shock index can identify patients who require hospital-based intervention in cases of gastrointestinal bleeding 1
- SI ≥1.3 has a positive likelihood ratio of 4.9 for predicting mortality in emergency department patients 2
Sepsis
- In sepsis patients, MSI ≥1.59 (with comorbidities) and MSI ≥1.67 (without comorbidities) predict the need for mechanical ventilation with sensitivities of 68.75% and 83.33%, respectively 3
- MSI is useful for early assessment of COVID-19 patients 1
Stroke
- SI >0.7 is associated with significantly worse outcomes in stroke patients, with adjusted odds ratios of:
- 2.00 for in-hospital mortality
- 1.46 for longer hospital stays
- 1.50 for discharge to destinations other than home
- 1.41 for inability to ambulate independently at discharge 4
Heart Failure
- Modified Shock Index and Age-adjusted Shock Index are independent predictors of in-hospital mortality in decompensated heart failure patients 5
Emergency Department Triage
- MSI >1.7 is associated with increased likelihood of both hospital admission (positive likelihood ratio = 6.29) and in-hospital mortality (positive likelihood ratio = 5.12) in emergency department patients 6
Limitations of Shock Index
Variable Specificity: The specificity of SI for predicting mortality can be lower compared to other scoring systems like JAAM-DIC or ISTH overt DIC scores in certain conditions 1
Context-Dependent Utility: The 2013 European trauma guidelines noted that shock index "may be useful in drawing attention to abnormal values, but is too insensitive to rule out disease and should not lower the suspicion of major injury" 1
Requires Clinical Context: Guidelines recommend using SI as part of a comprehensive assessment that includes patient physiology, anatomical injury pattern, mechanism of injury, and response to initial resuscitation 1
Comparison to Other Tools: When added to existing stroke mortality prediction models that already include the National Institutes of Health Stroke Scale (NIHSS), SI shows little to no improvement in predictive ability 4
Optimal Cutoff Values by Condition
| Condition | Recommended SI Cutoff | Significance |
|---|---|---|
| Trauma/Hemorrhage | SI >1 | Indicates unstable patient [1] |
| Emergency Department | MSI >1.7 | Predicts admission and mortality [6] |
| Stroke | SI >0.7 | Associated with worse outcomes [4] |
| Heart Failure | IShock 0.9, IShockM 1.26 | Predicts in-hospital mortality [5] |
| Sepsis | MSI ≥1.59-1.67 | Predicts need for mechanical ventilation [3] |
Clinical Application
Initial Triage: Use SI to rapidly identify potentially unstable patients requiring immediate attention
Risk Stratification:
Resource Allocation: SI can help guide decisions about level of care, especially in resource-limited settings 2
Monitoring Response: Serial SI measurements can track response to resuscitation efforts
Conclusion
Shock index is a simple yet powerful tool for assessing disease severity across multiple conditions. Its greatest strength lies in its simplicity and accessibility, requiring only heart rate and blood pressure measurements. While it should not be used in isolation, it provides valuable information for risk stratification and clinical decision-making, particularly in emergency and critical care settings.