Shock Index Calculator

Q: Who came up with the shock index?

Two Swiss surgeons, Max Allgöwer and Cäsar Burri, described it in 1967. They wanted a fast way to grade blood loss using only a pulse and a blood pressure reading. The formula has not changed since: heart rate divided by systolic blood pressure.

Q: How much blood loss does a shock index of 1.0 suggest?

The original work linked shock index levels to rough blood loss: Around 0.5: normal blood volume Around 1.0: about 20% to 30% of blood volume lost 1.5 or higher: severe loss, often over 30% to 40% These are estimates, not exact amounts. A person's age, medicines, and baseline blood pressure all shift the picture.

Q: Is the shock index better than heart rate or blood pressure alone?

For spotting early shock, yes. A pulse of 105 alone may look mild. A systolic pressure of 100 alone may look fine. Together they give a shock index of 1.05, which is a red flag. In trauma and sepsis, the shock index predicts transfusion need, ICU admission, and death better than either vital sign by itself.

Q: Can vital signs look normal while the shock index is high?

Yes, and that is the main reason it is used. A patient with a heart rate of 118 and a systolic pressure of 118 mmHg has a shock index of 1.00. Both numbers sit near the edge of "normal," but the ratio says the body is working hard to keep pressure up. This is called compensated shock, and pressure can crash suddenly once the body can no longer keep up.

Q: What does a low shock index mean?

A shock index well under 0.5 usually just means a slow pulse paired with a strong blood pressure. That is common in athletes and in people taking beta blockers. But a low number can be false comfort. Beta blockers, calcium channel blockers, and pacemakers all hold the heart rate down, so a bleeding patient can still show a low shock index. Trust the full exam over one ratio.

Q: What is a normal modified shock index?

The modified shock index (MSI) divides heart rate by mean arterial pressure instead of systolic pressure. A normal MSI runs from about 0.7 to 1.3. Below 0.7: suggests a hypodynamic state, with low cardiac output Above 1.3: suggests a hyperdynamic state with low vascular tone, and is tied to higher death risk

Q: What is the pediatric shock index (SIPA)?

Children normally have faster pulses and lower blood pressure, so adult cutoffs flag too many healthy kids. The Shock Index, Pediatric Age-adjusted (SIPA) uses higher cutoffs by age: Ages 4 to 6: above 1.22 Ages 7 to 12: above 1.00 Ages 13 to 16: above 0.90 A SIPA over the cutoff in injured children is linked to more transfusions and longer hospital stays.

Q: What is a normal shock index in pregnancy?

Pregnancy raises heart rate and lowers blood pressure, so the normal range shifts to roughly 0.7 to 0.9. In postpartum bleeding, a shock index of 0.9 or higher is a warning sign, and 1.7 or higher points to severe blood loss that usually needs transfusion and a higher level of care. Young, healthy mothers can hide large blood loss behind normal-looking vitals, which is why this ratio is watched closely.

Q: What is the age-adjusted shock index?

It multiplies age in years by the shock index. Older adults often take heart rate medicines and have high baseline blood pressure, so a regular shock index can look fine while they are in shock. In older trauma patients, an age × shock index of 50 or more marks high risk for transfusion and death. Example: a 70-year-old with a shock index of 0.8 scores 56, which is a warning even though 0.8 alone is only borderline.

Q: What is the delta shock index?

It is the change in shock index between two points in time, such as in the ambulance versus in the emergency department. A rise of 0.1 or more is linked to greater need for blood products and higher death risk, even if both single readings look acceptable. The direction of change often matters more than one snapshot.

Introduction

The Shock Index (SI) is a quick bedside number that helps spot hidden shock. You find it by dividing heart rate by systolic blood pressure. This Shock Index Calculator runs that division and explains what the result means.

A normal Shock Index is below 0.70. As the number climbs, it often means the heart is beating fast to make up for low blood pressure. A high SI (1.00 or more) is linked to higher risk of death, need for blood transfusion, and ICU admission, even when the blood pressure alone still looks "normal."

Just enter the heart rate in beats per minute and the systolic blood pressure in mmHg or kPa. You can also pick a clinical context like trauma, sepsis, STEMI, or pulmonary embolism to get threshold tips for that group. The calculator shows your score, a severity tier, a step-by-step solution, charts, and the vitals needed to reach each cutoff.

This tool is for learning and quick reference. It does not replace a full exam, lab work, or the judgment of a trained clinician.

How to use our Shock Index Calculator

Enter the patient's heart rate and systolic blood pressure. The calculator divides heart rate by systolic blood pressure and shows the shock index, the severity tier, a step-by-step solution, and charts.

Clinical Context: Pick the setting that fits the patient: general, trauma, sepsis, STEMI, or pulmonary embolism. This is optional, but it adds threshold notes for that group.

Heart Rate: Type the pulse in beats per minute. Use whole numbers from 10 to 300.

Systolic Blood Pressure: Type the top blood pressure number, then choose mmHg or kPa. The range is 10 to 300 mmHg (1.33 to 40 kPa). Changing the unit converts the value for you.

Click Calculate to see the result, or Reset to clear the form and start again.

What Is the Shock Index?

The shock index (SI) is a simple number that shows how hard the heart is working compared to how strong the blood pressure is. You get it by dividing the heart rate by the systolic blood pressure (the top number in a blood pressure reading).

Shock Index = Heart Rate ÷ Systolic Blood Pressure

Example: a heart rate of 96 beats per minute divided by a systolic pressure of 112 mmHg gives an SI of 0.86.

Why the Shock Index Matters

When a person loses blood or goes into shock, the body tries to hide it. Blood vessels tighten and the heart beats faster, so blood pressure can look normal for a while. The shock index catches this early because it looks at both numbers together. A fast heart with a falling pressure pushes the number up, even when each vital sign alone still seems "okay."

Doctors and nurses use the shock index in trauma bays, emergency rooms, ambulances, and intensive care units. A high SI is linked to a greater chance of needing a blood transfusion, needing an ICU bed, and dying in the hospital.

Shock Index Ranges

Normal (below 0.70). Heart rate and blood pressure are in balance.
Borderline (0.70 to 0.89). Watch the patient closely. This range is tied to higher lactate levels, a sign of hidden shock.
Elevated (0.90 to 0.99). Higher risk of death and of needing a large transfusion.
High (1.00 and above). The heart rate has passed the blood pressure. This is a red flag for real shock and usually calls for fast action.

How Context Changes the Cutoff

The "danger line" shifts a bit depending on why the patient is sick:

Trauma: SI of 0.9 or higher raises the chance a massive transfusion is needed. SI of 1.0 or more is a strong warning.
Sepsis: SI of 0.7 or higher can point to hidden shock early on. An SI staying above 0.8 often means more vasopressor drugs are needed.
STEMI (a major heart attack): SI of 0.8 or higher links to higher death risk in the hospital.
Pulmonary embolism: SI of 1.0 or higher suggests the clot is blocking blood flow badly. Pair it with a score like sPESI.

Things That Can Throw It Off

The shock index is fast and free, but it is not perfect. Several things can skew it:

Heart rate medicines such as beta blockers keep the pulse slow, so the SI can look normal even in real shock.
Pacemakers set a fixed heart rate and hide the body's response.
Older adults often have high baseline blood pressure, so a "normal" SI may still hide trouble.
Children and pregnant patients have different normal vital signs, so adult cutoffs do not fit them well.
Pain, fear, fever, or caffeine can raise the heart rate without any shock at all.

The shock index is one clue, not a diagnosis. Always read it with the full picture: skin color, mental state, urine output, lactate level, and how the patient looks over time. A single reading tells you less than a trend of several readings.

Related Measures

Some teams also use the modified shock index, which divides heart rate by the mean arterial pressure instead of the systolic pressure, and the age-adjusted shock index (age × SI) for older trauma patients. Both try to fix the blind spots listed above.

Formulas used

Shock Index

\text{SI}=\frac{\text{HR (beats/min)}}{\text{SBP (mmHg)}}

Systolic blood pressure unit conversion (kPa to mmHg)

\text{SBP}_{\text{mmHg}}=\text{SBP}_{\text{kPa}}\times 7.5006157585

Rounding the shock index to two decimals

\text{SI}_{\text{rounded}}=\frac{\operatorname{round}(100\cdot \text{SI})}{100}

Heart rate needed to reach a target shock index

\text{HR}_{\text{target}}=\text{SI}_{\text{target}}\times \text{SBP}_{\text{mmHg}}

Systolic blood pressure that would produce a target shock index

\text{SBP}_{\text{target}}=\frac{\text{HR}}{\text{SI}_{\text{target}}}

Position of the shock index on the 0-2.0 display scale

\text{Position}\,(\%)=\min\!\left(\frac{\text{SI}}{2}\times 100,\;100\right)

Severity tier classification

\text{Tier}=\begin{cases}\text{Normal}, & \text{SI} < 0.70\\ \text{Borderline}, & 0.70 \le \text{SI} < 0.90\\ \text{Elevated}, & 0.90 \le \text{SI} < 1.00\\ \text{High}, & \text{SI} \ge 1.00\end{cases}

Frequently asked questions

Who came up with the shock index?

Two Swiss surgeons, Max Allgöwer and Cäsar Burri, described it in 1967. They wanted a fast way to grade blood loss using only a pulse and a blood pressure reading. The formula has not changed since: heart rate divided by systolic blood pressure.

How much blood loss does a shock index of 1.0 suggest?

The original work linked shock index levels to rough blood loss:

Around 0.5: normal blood volume
Around 1.0: about 20% to 30% of blood volume lost
1.5 or higher: severe loss, often over 30% to 40%

These are estimates, not exact amounts. A person's age, medicines, and baseline blood pressure all shift the picture.

Is the shock index better than heart rate or blood pressure alone?

For spotting early shock, yes. A pulse of 105 alone may look mild. A systolic pressure of 100 alone may look fine. Together they give a shock index of 1.05, which is a red flag. In trauma and sepsis, the shock index predicts transfusion need, ICU admission, and death better than either vital sign by itself.

Can vital signs look normal while the shock index is high?

Yes, and that is the main reason it is used. A patient with a heart rate of 118 and a systolic pressure of 118 mmHg has a shock index of 1.00. Both numbers sit near the edge of "normal," but the ratio says the body is working hard to keep pressure up. This is called compensated shock, and pressure can crash suddenly once the body can no longer keep up.

What does a low shock index mean?

A shock index well under 0.5 usually just means a slow pulse paired with a strong blood pressure. That is common in athletes and in people taking beta blockers.

But a low number can be false comfort. Beta blockers, calcium channel blockers, and pacemakers all hold the heart rate down, so a bleeding patient can still show a low shock index. Trust the full exam over one ratio.

What is a normal modified shock index?

The modified shock index (MSI) divides heart rate by mean arterial pressure instead of systolic pressure. A normal MSI runs from about 0.7 to 1.3.

Below 0.7: suggests a hypodynamic state, with low cardiac output
Above 1.3: suggests a hyperdynamic state with low vascular tone, and is tied to higher death risk

What is the pediatric shock index (SIPA)?

Children normally have faster pulses and lower blood pressure, so adult cutoffs flag too many healthy kids. The Shock Index, Pediatric Age-adjusted (SIPA) uses higher cutoffs by age:

Ages 4 to 6: above 1.22
Ages 7 to 12: above 1.00
Ages 13 to 16: above 0.90

A SIPA over the cutoff in injured children is linked to more transfusions and longer hospital stays.

What is a normal shock index in pregnancy?

Pregnancy raises heart rate and lowers blood pressure, so the normal range shifts to roughly 0.7 to 0.9. In postpartum bleeding, a shock index of 0.9 or higher is a warning sign, and 1.7 or higher points to severe blood loss that usually needs transfusion and a higher level of care. Young, healthy mothers can hide large blood loss behind normal-looking vitals, which is why this ratio is watched closely.

What is the age-adjusted shock index?

It multiplies age in years by the shock index. Older adults often take heart rate medicines and have high baseline blood pressure, so a regular shock index can look fine while they are in shock. In older trauma patients, an age × shock index of 50 or more marks high risk for transfusion and death. Example: a 70-year-old with a shock index of 0.8 scores 56, which is a warning even though 0.8 alone is only borderline.

What is the delta shock index?

It is the change in shock index between two points in time, such as in the ambulance versus in the emergency department. A rise of 0.1 or more is linked to greater need for blood products and higher death risk, even if both single readings look acceptable. The direction of change often matters more than one snapshot.

What is the reverse shock index multiplied by GCS (rSIG)?

The reverse shock index flips the formula: systolic blood pressure divided by heart rate. Multiply that by the Glasgow Coma Scale score and you get rSIG. Unlike the shock index, a low rSIG is bad. In trauma patients, values under about 16.5 predict higher death risk. Adding GCS brings brain injury into a number that otherwise only watches circulation.

Why does the shock index use systolic pressure instead of diastolic?

Systolic pressure reflects how much blood the heart pushes out with each beat. When blood volume drops, stroke volume falls and systolic pressure follows, while the pulse climbs. That pairing is what the ratio is built to catch. Diastolic pressure tracks vessel tone more than pumped volume, so it moves in a less useful way for this purpose.

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