Shock Index
A calculated value (heart rate divided by systolic blood pressure) that helps identify patients in compensated shock who appear stable on individual vital signs but are at significant risk of decompensation.
In the Field
Shock index is a useful tool for the cases that fool you. A patient with a heart rate of 110 and blood pressure of 110 looks stable on each measurement individually, but their shock index is 1.0, which is the threshold suggesting compensated shock. The calculation catches patients whose body is compensating well enough to maintain blood pressure but who are heading for decompensation. Aid bag protocols at higher provider levels increasingly include shock index as a triage and disposition tool. For field providers without blood pressure capability, the radial pulse approach captures most of the same information and is more practical.
Common Mistake
Treating a patient as stable based on individual normal vital signs when the shock index calculation reveals significant risk of decompensation.
Technical Detail
The Shock Index (SI) is a clinical calculation defined as:
SI = Heart Rate / Systolic Blood Pressure
Both values use the standard units (beats per minute and millimeters of mercury), with the result expressed as a decimal value.
Normal range. The shock index in healthy adults is approximately 0.5 to 0.7. A patient with a heart rate of 70 and a systolic blood pressure of 120 has a shock index of 0.58, well within the normal range.
Threshold values. Several threshold values have clinical significance:
SI greater than 0.9. Suggests early shock or compensated shock. The patient may have normal-appearing individual vital signs but is at increased risk of decompensation.
SI greater than 1.0. Strongly suggests significant shock. Many trauma protocols use this threshold for triage and resuscitation decisions.
SI greater than 1.4. Suggests severe shock with high mortality risk in trauma populations.
The index has been validated in multiple trauma populations including military, civilian trauma, and hemorrhagic shock specifically. Higher SI values correlate with increased need for blood transfusion, increased mortality, and more aggressive intervention requirements.
Why the index works. Individual vital signs can be misleading in compensated shock:
Heart rate increases as the body compensates for blood loss, but a heart rate of 110 alone may not be alarming.
Blood pressure is maintained by compensatory mechanisms during early shock and may be normal or only slightly reduced.
Each individual value may fall within "normal" ranges or only mild abnormality, while the combination represents significant compensation.
The shock index captures the relationship between these two variables. A patient maintaining blood pressure only through marked tachycardia has a high shock index even if neither value individually is dramatically abnormal.
Variants. Several variants of the shock index have been proposed:
Modified Shock Index (MSI). Uses mean arterial pressure (MAP) in place of systolic blood pressure. MSI = HR / MAP. Some research suggests improved sensitivity.
Age Shock Index. Multiplies the shock index by age, addressing the relative significance of vital sign abnormalities in older patients who may have less compensatory reserve.
Pediatric Shock Index. Uses age-adjusted thresholds reflecting normal pediatric heart rates and blood pressures.
These variants have specific use cases but the standard shock index is the most widely used.
Field application. The shock index is most useful in advanced provider settings where blood pressure measurement is feasible:
EMS and tactical paramedic patient assessment.
Air medical and ground critical care transport.
Hospital triage.
Trauma center initial evaluation.
In the tactical environment at the point of injury, blood pressure measurement is often not feasible. Field providers rely on the more accessible perfusion indicators (mental status, radial pulse strength, skin signs) that capture similar information without requiring equipment. See the Radial Pulse and Perfusion entries.
For agencies and program planners considering shock index protocols, the practical considerations include:
Equipment. A blood pressure cuff (manual or automated) is required.
Training. Providers must be trained in blood pressure measurement and shock index calculation.
Protocol integration. Shock index thresholds should be integrated into established triage, resuscitation, and disposition protocols.
Documentation. Shock index values should be documented in patient care reports.
Time considerations. Blood pressure measurement adds time to assessment, which may not be feasible in all tactical scenarios.
The shock index is a useful adjunct rather than a replacement for clinical judgment. A patient with a low shock index can still have significant injury, and a patient with a high shock index may have non-trauma causes (pain, anxiety, dehydration). The calculation supports decision-making rather than replacing it.
Procurement implications. Shock index capability is reflected in:
Manual or automated blood pressure cuffs in advanced aid bags and EMS units.
Provider training in blood pressure measurement and shock index interpretation.
Protocols and documentation systems that include shock index thresholds.
Triage and resuscitation guidelines that incorporate shock index into decision points.
The shock index is one of several tools that have moved trauma assessment from purely empirical observation toward more systematic, calculation-based evaluation, particularly at advanced provider and trauma center levels.
SI = Heart Rate / Systolic Blood Pressure
Both values use the standard units (beats per minute and millimeters of mercury), with the result expressed as a decimal value.
Normal range. The shock index in healthy adults is approximately 0.5 to 0.7. A patient with a heart rate of 70 and a systolic blood pressure of 120 has a shock index of 0.58, well within the normal range.
Threshold values. Several threshold values have clinical significance:
SI greater than 0.9. Suggests early shock or compensated shock. The patient may have normal-appearing individual vital signs but is at increased risk of decompensation.
SI greater than 1.0. Strongly suggests significant shock. Many trauma protocols use this threshold for triage and resuscitation decisions.
SI greater than 1.4. Suggests severe shock with high mortality risk in trauma populations.
The index has been validated in multiple trauma populations including military, civilian trauma, and hemorrhagic shock specifically. Higher SI values correlate with increased need for blood transfusion, increased mortality, and more aggressive intervention requirements.
Why the index works. Individual vital signs can be misleading in compensated shock:
Heart rate increases as the body compensates for blood loss, but a heart rate of 110 alone may not be alarming.
Blood pressure is maintained by compensatory mechanisms during early shock and may be normal or only slightly reduced.
Each individual value may fall within "normal" ranges or only mild abnormality, while the combination represents significant compensation.
The shock index captures the relationship between these two variables. A patient maintaining blood pressure only through marked tachycardia has a high shock index even if neither value individually is dramatically abnormal.
Variants. Several variants of the shock index have been proposed:
Modified Shock Index (MSI). Uses mean arterial pressure (MAP) in place of systolic blood pressure. MSI = HR / MAP. Some research suggests improved sensitivity.
Age Shock Index. Multiplies the shock index by age, addressing the relative significance of vital sign abnormalities in older patients who may have less compensatory reserve.
Pediatric Shock Index. Uses age-adjusted thresholds reflecting normal pediatric heart rates and blood pressures.
These variants have specific use cases but the standard shock index is the most widely used.
Field application. The shock index is most useful in advanced provider settings where blood pressure measurement is feasible:
EMS and tactical paramedic patient assessment.
Air medical and ground critical care transport.
Hospital triage.
Trauma center initial evaluation.
In the tactical environment at the point of injury, blood pressure measurement is often not feasible. Field providers rely on the more accessible perfusion indicators (mental status, radial pulse strength, skin signs) that capture similar information without requiring equipment. See the Radial Pulse and Perfusion entries.
For agencies and program planners considering shock index protocols, the practical considerations include:
Equipment. A blood pressure cuff (manual or automated) is required.
Training. Providers must be trained in blood pressure measurement and shock index calculation.
Protocol integration. Shock index thresholds should be integrated into established triage, resuscitation, and disposition protocols.
Documentation. Shock index values should be documented in patient care reports.
Time considerations. Blood pressure measurement adds time to assessment, which may not be feasible in all tactical scenarios.
The shock index is a useful adjunct rather than a replacement for clinical judgment. A patient with a low shock index can still have significant injury, and a patient with a high shock index may have non-trauma causes (pain, anxiety, dehydration). The calculation supports decision-making rather than replacing it.
Procurement implications. Shock index capability is reflected in:
Manual or automated blood pressure cuffs in advanced aid bags and EMS units.
Provider training in blood pressure measurement and shock index interpretation.
Protocols and documentation systems that include shock index thresholds.
Triage and resuscitation guidelines that incorporate shock index into decision points.
The shock index is one of several tools that have moved trauma assessment from purely empirical observation toward more systematic, calculation-based evaluation, particularly at advanced provider and trauma center levels.