Shock Index Calculator (SI, MSI, Age-SI)
Enter heart rate, systolic blood pressure, diastolic blood pressure, and patient age to compute three hemodynamic screening scores at once: the standard Shock Index (SI), the Modified Shock Index (MSI, which uses mean arterial pressure instead of systolic), and the Age Shock Index (Age-SI). Each score is mapped to its clinical risk band so you can quickly judge hemodynamic stability at a glance. Results update in real time as you type.
Formula
Worked example
A 55-year-old patient with HR 110 bpm, SBP 100 mmHg, DBP 70 mmHg: SI = 110/100 = 1.10 (elevated). MAP = (2x70+100)/3 = 80 mmHg. MSI = 110/80 = 1.38 (elevated). Age-SI = 55x110/100 = 60.5 (above the 50 threshold for high risk).
What is the Shock Index?
The Shock Index (SI) is a bedside ratio defined as heart rate divided by systolic blood pressure. It was first described by Allgower and Burri in 1967 as a quick way to detect hemodynamic instability in trauma patients. A normal resting adult has a heart rate lower than the systolic blood pressure, so SI sits between 0.5 and 0.7. When a patient starts to decompensate, the heart rate rises and blood pressure falls, pushing the ratio above 0.9 before frank hypotension or tachycardia alone crosses the classical alert thresholds. This makes SI particularly useful for detecting occult or compensated shock, where individual vital signs can appear deceptively normal.
Standard SI, Modified SI, and Age Shock Index: which to use?
The standard SI uses only heart rate and systolic blood pressure, making it quick to compute without any equipment beyond a BP cuff. The Modified Shock Index (MSI) replaces systolic BP with mean arterial pressure, which accounts for diastolic tone as well and is thought to better reflect end-organ perfusion pressure. Studies in sepsis and trauma suggest MSI is more sensitive than SI for predicting ICU admission and 28-day mortality at the cost of needing one extra reading. The Age Shock Index multiplies SI by the patient age, because older adults compensate less well: an SI of 1.0 in a 70-year-old carries a different prognosis than the same value in a 25-year-old. An Age-SI above 50 has been associated with significantly higher in-hospital mortality in several retrospective cohorts.
Clinical applications and evidence base
SI is used across trauma, obstetrics, emergency medicine, and critical care. In trauma, an SI greater than 0.9 on arrival predicts the need for massive transfusion, early acute coagulopathy, and 30-day mortality with better sensitivity than either heart rate or blood pressure alone. In obstetric hemorrhage, an SI above 0.9 serves as a trigger for escalated intervention in many postpartum hemorrhage protocols. In sepsis, MSI above 1.0 at triage has been linked to increased need for vasopressors and mechanical ventilation. In acute myocardial infarction, an SI above 0.7 on presentation correlates with larger infarct size and worse left ventricular function. Despite its utility, SI is a single-variable ratio and must be interpreted alongside the patient history, examination, mental status, urine output, and lactate.
Limitations and special populations
Several factors alter the interpretation of SI without indicating true hemodynamic compromise. Beta-blockers and other rate-controlling agents can blunt the tachycardic response, keeping SI artificially low even in shock. Athletic individuals and some pregnant patients have baseline vital sign patterns that push SI outside the normal range without pathology. Elderly patients are often on antihypertensives that lower baseline SBP, potentially inflating SI at rest. Children have higher resting heart rates and lower baseline blood pressures, so the adult thresholds do not apply, and pediatric-specific indices should be used. Shock Index should never be used in isolation: a trend of three or more serial measurements, combined with lactate, urine output, skin perfusion, and mental status, gives a far more reliable picture of hemodynamic trajectory than any single value.
Shock Index clinical reference ranges
| Shock Index (SI) | Category | Clinical Significance |
|---|---|---|
| < 0.5 | Low | Possible bradycardia or hypertension - not shock |
| 0.5-0.7 | Normal | Hemodynamically stable |
| 0.7-0.9 | Borderline | Monitor closely for deterioration |
| 0.9-1.2 | Elevated | Likely early/occult shock; 3x higher mortality in trauma |
| > 1.2 | High | Probable overt shock; high mortality and transfusion risk |
| MSI > 1.0 | Elevated MSI | Increased ICU admission, sepsis mortality risk |
| MSI > 1.4 | High MSI | High 28-day mortality in sepsis and trauma |
| Age-SI > 50 | Elevated Age-SI | Higher in-hospital mortality in older patients |
Thresholds used in trauma, emergency, and critical care literature. Values are guidelines only, always interpret in context.
Frequently asked questions
What is a normal Shock Index value?
A normal Shock Index in a resting healthy adult is between 0.5 and 0.7. This reflects a heart rate that is lower than the systolic blood pressure, which is the typical resting state. Values above 0.9 are considered abnormal in most clinical guidelines and have been associated with significantly worse outcomes in trauma and sepsis cohorts.
What does a Shock Index above 1.0 mean?
An SI above 1.0 means heart rate has exceeded systolic blood pressure, which strongly suggests hemodynamic compromise. In trauma settings, SI above 0.9 to 1.0 is linked to roughly a three-fold increase in 30-day mortality and a higher probability of needing massive transfusion. Above 1.2, the risk of overt shock and adverse outcome is substantial and warrants urgent intervention.
How is the Modified Shock Index (MSI) different from the standard Shock Index?
The Modified Shock Index replaces systolic blood pressure with mean arterial pressure (MAP) in the denominator. MAP captures both the systolic peak and the diastolic pressure, giving a better estimate of the average perfusion pressure across the cardiac cycle. Some studies suggest MSI is more predictive of ICU admission, mechanical ventilation, and 28-day mortality than SI alone, particularly in sepsis. An MSI above 1.0 is considered elevated, and above 1.4 is considered high.
What is the Age Shock Index and why does age matter?
The Age Shock Index multiplies the standard Shock Index by the patient age: Age-SI = Age x HR / SBP. Older patients have a reduced physiological reserve and compensate less effectively during hemodynamic stress, so the same SI value carries a higher absolute risk. An Age-SI above 50 has been associated with significantly higher in-hospital mortality in elderly trauma and emergency patients and acts as a more age-sensitive alert threshold.
Can Shock Index be used in children?
Not with the adult thresholds. Children have higher normal heart rates and lower baseline blood pressures than adults, so their resting SI is naturally higher. The adult cutoffs of 0.9 or 1.0 would flag many healthy children as abnormal. Pediatric emergency medicine uses age-specific reference ranges and purpose-built pediatric scoring tools. This calculator is designed for adults only.
Does Shock Index work if the patient is on beta-blockers?
With caution. Beta-blockers blunt the tachycardic response to shock, so an SI that looks normal in a beta-blocked patient may not reflect true hemodynamic stability. Similarly, patients on antihypertensives may have lower baseline blood pressures, making SI appear elevated at rest. Always factor in the medication history when interpreting SI, and rely more heavily on serial lactate, mental status, and urine output in these patients.
How often should Shock Index be recalculated?
Serial measurements are more informative than any single reading. In acute settings, recalculating SI every 15 to 30 minutes during active resuscitation gives a trend that reflects whether the patient is improving or deteriorating. A falling SI in response to fluid or blood product administration suggests a response; a rising or persistently high SI despite resuscitation indicates ongoing hemodynamic compromise.
Sources
- Allgower M, Burri C. "Shock index." Deutsche Medizinische Wochenschrift, 1967. Foundational paper defining the SI formula.
- Mutschler M et al. "The shock index revisited - a fast guide to transfusion requirement? A retrospective analysis on 21,853 patients derived from the TraumaRegister DGU." Critical Care, 2013.
- Rady MY et al. "Shock index: a re-evaluation in acute circulatory failure." Resuscitation, 1992. MSI and clinical risk evidence.