Why is the Shock Index more useful than systolic BP alone in trauma?

Systolic BP can remain apparently normal during compensated haemorrhagic shock because vasoconstriction maintains pressure while cardiac output falls. The Shock Index captures the relationship between heart rate (which rises early in haemorrhage) and BP simultaneously. An SI above 1.0 in a trauma patient indicates the compensatory tachycardia is no longer maintaining perfusion -- a condition that is not apparent from the SBP value alone. Illustrative example only -- verify with current agency trauma protocols.

What is permissive hypotension and when does it apply?

Permissive hypotension (also called hypotensive resuscitation) is the strategy of targeting a lower-than-normal SBP (typically 80-90 mmHg in penetrating trauma, 90-100 mmHg in blunt trauma without TBI) to avoid disrupting clot formation at the bleeding site while maintaining minimum perfusion. Aggressive fluid resuscitation that normalises BP in uncontrolled haemorrhage can worsen coagulopathy and increase bleeding. Permissive hypotension does NOT apply in traumatic brain injury (where MAP target is higher) -- confirm local protocol targets. Illustrative example only.

What is Tranexamic Acid (TXA) and when should it be given in the prehospital setting?

Tranexamic acid (TXA) is an antifibrinolytic agent that prevents clot breakdown, reducing blood loss in significant haemorrhage. The CRASH-2 and MATTERs studies demonstrated survival benefit when given within 3 hours of injury (greatest benefit within 1 hour). Many Australian and US trauma systems now include TXA in prehospital protocols for significant haemorrhage (SI >=1.0, major mechanism, expected blood loss). Timing of administration is critical -- beyond 3 hours, TXA may be harmful. Always follow your agency protocol and medical director authorisation for TXA administration. Illustrative example only.

Shock Index — Free Paramedics Calculator

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A trauma patient is on scene after a high-speed MVC. Vitals are HR 118 bpm and SBP 102 mmHg. Before calling a trauma alert and initiating transport, the crew needs the shock index calculated to quantify haemodynamic instability and support the decision.

Shock Index

Haemodynamics

Heart rate (bpm)

Systolic BP (mmHg)

Shock Index = Heart Rate ÷ Systolic BP Ranges: <0.6 Normal · 0.6–1.0 Monitor · 1.0–1.4 Haemorrhagic shock likely · ≥1.4 Critical
Normal SI in adults is 0.5–0.7. SI >1.0 is associated with significantly increased mortality.

💡 SI is a rapid bedside screening tool — always assess in conjunction with full clinical picture.

Reference: Mutschler M et al. — Critical Care 2013 · NCBI — Shock Index review

⚕️ Clinical safety: 🇦🇺 Verify with facility drug formulary and senior clinician · Meets AHPRA/ACSQHC standards

1 What this calculator does

Calculates the Shock Index (SI) from heart rate and systolic blood pressure. Classifies haemodynamic status from normal to critical haemorrhage and provides a recommended prehospital action for each threshold. Useful for early identification of occult shock when BP appears borderline normal.

2 Formula & professional reasoning

Shock Index = Heart rate (bpm) / Systolic BP (mmHg)
Thresholds:
  SI <0.6: Normal -- haemodynamically stable
  SI 0.6-0.9: Mildly elevated -- monitor, early haemorrhage or pain response
  SI 1.0-1.3: Moderately elevated -- haemorrhagic shock likely, IV access + fluid
  SI >=1.4: Severely elevated -- critical haemorrhage, immediate transport + massive haemorrhage protocol

The Shock Index inversely reflects cardiac output relative to compensatory tachycardia. Normally the heart rate is significantly lower than the systolic BP, giving an SI below 1.0. As haemorrhage develops, the heart rate rises and the BP falls -- the ratio crosses 1.0 when tachycardia no longer maintains adequate pressure, indicating significant blood loss (typically >750-1000 mL in an adult). The SI is particularly useful in the 'golden hour' because it can identify patients in compensated shock who have a normal-appearing SBP due to vasoconstriction. An SI of 1.0 or above in a trauma patient should trigger immediate IV access and expedited transport regardless of the absolute BP value.

3 Worked examples

⚠️ Illustrative example only — not clinical or professional instruction.

Basic

Normal shock index -- haemodynamically stable

Given: HR: 85 bpm | SBP: 120 mmHg

Working: SI = 85 / 120 = 0.71

Answer: Shock Index: 0.71 -- Mildly elevated. Monitor closely. May reflect pain response or early haemorrhage.

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. SI 0.71 is in the mildly elevated range. In a trauma patient, this warrants close monitoring and reassessment every 5 minutes. A rising SI on serial measurements is more concerning than a single value.

Standard

Shock index at 1.0 threshold -- haemorrhagic shock

Given: HR: 118 bpm | SBP: 102 mmHg

Working: SI = 118 / 102 = 1.157

Answer: Shock Index: 1.16 -- Moderately elevated. Haemorrhagic shock likely. IV access, cautious fluid resuscitation, expedite transport.

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. An SBP of 102 mmHg appears borderline acceptable in isolation, but the SI of 1.16 reveals significant haemodynamic compromise. This patient is likely in compensated haemorrhagic shock. Initiate 2 large-bore IV lines, permissive hypotension strategy (target SBP 80-90 mmHg in penetrating trauma), and trauma team pre-notification.

Advanced

Critical haemorrhage -- SI above 1.4

Given: HR: 140 bpm | SBP: 90 mmHg

Working: SI = 140 / 90 = 1.556

Answer: Shock Index: 1.56 -- Severely elevated. Critical haemorrhage. Immediate transport. Massive haemorrhage protocol. Trauma team activation.

💡 Illustrative example only. Verify all clinical decisions with your agency protocol, medical director and partner clinician. Meets AHPRA/ACSQHC prehospital care standards. SI above 1.4 is associated with haemodynamic instability requiring surgical intervention. Minimise scene time, activate trauma team en route, consider TXA administration per protocol, and maintain target SBP per local permissive hypotension guidelines. Do not delay transport for fluid resuscitation beyond 2 attempts at IV access.

4 Sanity check

Shock Index thresholds

Normal <0.6 | Mildly elevated 0.6-0.9 | Haemorrhagic shock likely 1.0-1.3 | Critical >=1.4

Some sources use a threshold of 0.9 as the upper limit of normal for adults.

SI limitations in specific populations

Elderly patients: may not mount a tachycardia -- lower SI may still indicate significant haemorrhage | Beta-blockers: blunt tachycardia response -- SI may underestimate haemorrhage | Pregnancy: normal HR is higher, SBP lower -- adjusted thresholds apply

Serial measurements more useful than a single value

A rising SI on serial 5-minute measurements is more clinically significant than a single elevated value | Trend over time guides resuscitation response

SI does not replace clinical assessment

Skin colour, capillary refill, level of consciousness and mechanism of injury all inform the haemorrhage assessment alongside the SI

5 Common errors

Error	Cause	Consequence	Fix
Treating a normal-appearing SBP as reassuring when the HR is elevated	Focusing on the SBP in isolation without calculating the SI	Missing compensated shock -- the patient deteriorates rapidly once compensatory mechanisms fail	Calculate the SI for every significant trauma patient. An SBP of 100 mmHg with an HR of 110 bpm (SI 1.1) is a very different clinical picture from an SBP of 100 with an HR of 70 (SI 0.7). The SI reveals the relationship between the two values.
Not recalculating the SI after interventions or during transport	Calculating once and not repeating	Missing deterioration or improvement in haemodynamic status during transport	Recalculate the SI every 5 minutes in an unstable trauma patient. A rising SI despite fluid administration suggests ongoing haemorrhage. A falling SI suggests the patient is responding to resuscitation.
Applying standard SI thresholds to elderly patients on beta-blockers	Not adjusting for medications that blunt the tachycardia response	Underestimating haemorrhage -- beta-blockers prevent the heart rate from rising, maintaining a falsely reassuring SI	In patients taking beta-blockers, a 'normal' or mildly elevated SI may still represent significant haemorrhage. Combine SI with clinical assessment of skin perfusion, mental status, capillary refill and mechanism of injury.
Using the shock index as the sole criterion for a trauma team activation	Mechanistically applying a threshold without clinical synthesis	Under- or over-triaging based on a single calculated parameter	SI is a support tool for decision-making alongside mechanism of injury, anatomical injury patterns, physiological trends and clinical assessment. Pre-notification decisions should integrate all available information.

6 Reference & regulatory links

🇦🇺 Australia

🇺🇸 United States

7 Professional workflow

Common tools used alongside this one:

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Revised Trauma Score (RTS)

Revised Trauma Score combines GCS, SBP and respiratory rate for a more comprehensive trauma severity assessment alongside the Shock Index

→

❤️

Mean Arterial Pressure (MAP)

Mean Arterial Pressure provides a more complete perfusion pressure picture when the SI suggests haemodynamic compromise

→

⏱️

Scene Time Calculator

Scene Time Calculator helps enforce the platinum 10-minute guideline -- high SI scores should trigger immediate preparation for transport

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Input	Result
HR 118, SBP 102	SI 1.16 -- haemorrhagic shock likely
HR 140, SBP 90	SI 1.56 -- critical haemorrhage
HR 85, SBP 120	SI 0.71 -- mildly elevated, monitor
Serial measurement interval	Every 5 minutes in unstable trauma