Shock Index - SI Ratio, MSI, and Triage Bands

A shock index calculator turns two vital signs - heart rate and systolic blood pressure - into a single bedside ratio that flags hemodynamic instability earlier than either number alone.

• • Reading a triage ratio at the bedside: Take an HR and SBP from a cuff and pulse oximeter, type them in, and get the shock index plus the modified shock index on one screen.
• • Checking the obstetric hemorrhage trigger: Match the result against the 0.9 to 1.0 obstetric band the ACOG postpartum hemorrhage protocol uses to escalate care.
• • Studying for a clinical or nursing exam: Work through the same SI = HR / SBP arithmetic used in emergency medicine, trauma, and sepsis teaching.
• • Screening a child in the ED: Enter age under 12 to switch to pediatric age-adjusted shock index bands and compare a child's reading to the right reference range.

The shock index was introduced by Allgower and Burri in 1967 to flag occult shock in trauma patients whose blood pressure had not yet dropped. Heart rate rises as the body tries to compensate for falling perfusion, so the HR / SBP ratio moves sooner than either vital sign crosses its own alarm threshold.

A reading around 0.5 to 0.7 is normal in a resting adult, 0.9 to 1.0 flags compensated instability, and 1.0+ is the textbook marker of overt shock.

When the same cuff pair needs the underlying MAP that drives the modified shock index, Mean Arterial Pressure Calculator does that arithmetic and shows the AHA-aligned bands alongside it.

The shock index calculator uses two ratios from the same vital-sign pair. The headline ratio divides heart rate by systolic blood pressure. When a diastolic reading is also available, the calculator divides heart rate by mean arterial pressure for the modified shock index, which is more sensitive to early hypoperfusion.

• HR: Heart rate in beats per minute, from a pulse count, pulse oximeter, or monitor.
• SBP: Systolic blood pressure in mmHg, the top number of a cuff reading.
• DBP: Diastolic blood pressure in mmHg, the bottom number of a cuff reading. Optional but recommended for MSI.
• Age: Optional age in years. Under 12 switches to pediatric age-adjusted shock index bands.

The result panel shows the shock index next to the modified shock index and MAP so the arithmetic stays visible. The band label names the clinical bucket.

If a cuff reading is questionable, the calculator returns a labeled Check measurement result, not a misleading ratio.

According to Allgower and Burri, 1967 (PubMed), the shock index is heart rate divided by systolic blood pressure and was introduced as a bedside ratio to flag occult shock in trauma patients

When the same systolic and diastolic pair needs an AHA stage label rather than a shock index, Blood Pressure Calculator interprets the reading.

Four concepts make the shock index easier to read at the bedside and explain why a single ratio can flag shock earlier than either vital sign on its own.

The lever most readers miss is that the shock index changes faster than either input, so a stable-looking vitals pair can hide a rising ratio.

When the same heart rate and systolic blood pressure pair is being used to estimate myocardial oxygen demand instead of hypoperfusion, Rate Pressure Product Calculator multiplies them into the rate-pressure product.

The form follows the order the numbers appear in a typical vital-signs set, so each field is read straight off the cuff and pulse oximeter.

1. 1 Take a pulse and a cuff reading: Count the radial pulse for 30 seconds and double, or read the heart rate from a pulse oximeter. Then take a seated blood pressure with a properly sized cuff.
2. 2 Enter the heart rate: Type the heart rate in bpm into the Heart Rate field. The calculator clamps the value to 30-220 bpm.
3. 3 Enter the systolic blood pressure: Type the top cuff number in mmHg into the Systolic Blood Pressure field. The calculator clamps it to 50-250 mmHg.
4. 4 Add the diastolic reading for MSI: Type the bottom cuff number into the Diastolic Blood Pressure field. With a diastolic reading on file, the calculator reports MAP and the modified shock index on the same screen.
5. 5 Add the patient age if pediatric: Type the patient's age in years. Leaving it at 30 uses adult bands; under 12 switches to pediatric age-adjusted bands.
6. 6 Read SI, MSI, and the band together: Use the shock index as the headline number, then check MSI (if MAP is available) and the severity band before deciding next steps.

An ER nurse reads HR 110 and BP 100 over 60 on a postpartum patient. They enter the four fields and the calculator returns SI 1.10, MAP 73.3 mmHg, MSI 1.50, and the Severe (>= 1.0) obstetric hemorrhage trigger band, the cue to call the protocol.

When the heart rate comes from a 6-second or 10-second rhythm strip rather than a wrist count, ECG Heart Rate Calculator derives the same bpm number from the strip.

The shock index arithmetic is two lines, but the calculator packages the ratio, modified shock index, MAP, and the triage band on one screen, so the bedside number and the next clinical action sit together.

• • Two numbers in, four results out: Heart rate and systolic blood pressure in, shock index, modified shock index, MAP, and triage band out, no manual arithmetic.
• • Bands aligned to the bedside: The bands use the 0.7 borderline, 0.9 mild, 1.0 severe, and 1.4 very severe thresholds cited in modern emergency-medicine reviews.
• • Obstetric hemorrhage trigger visible: The 0.9 to 1.0 obstetric hemorrhage band from ACOG Practice Bulletin 183 is labeled on the result, so it can drive a postpartum hemorrhage escalation at a glance.
• • Optional pediatric mode: Entering age under 12 switches the bands to pediatric age-adjusted thresholds, covering an adult trauma bay and a pediatric ED with the same form.
• • Invalid inputs are flagged: Systolic at or below diastolic, missing MAP, or out-of-range HR and SBP return a labeled Check measurement result rather than a misleading ratio.
• • Modified shock index included by default: When a diastolic reading is provided, MSI is reported next to the standard ratio, so the reader can compare both ratios on the same chart.

The headline benefit is that the vitals pair becomes a triage decision on the same screen, because the ratio moves faster than either vital sign.

When the shock index is being read alongside a flow number, Cardiac Output Calculator estimates cardiac output from stroke volume and heart rate.

Five factors can move a shock index result by enough to cross a triage band, and they explain why the same ratio can look different between patients.

• • The shock index is a screening tool, not a diagnosis. A normal ratio does not rule out early sepsis, occult bleeding, or cardiogenic shock, especially in patients on rate-control medications or with pacemakers.
• • Cuff-derived SBP and DBP can disagree with an intra-arterial line by a few mmHg in shock or with arrhythmias, so the ratio can shift slightly between cuff and invasive monitoring.
• • Pediatric age-adjusted bands are age-bin approximations, not patient-specific percentiles, and do not replace a full PALS assessment.

Treat the result as one number inside a larger clinical picture that includes symptoms, the trend across repeated readings, and the patient's medication and fluid context.

According to Koch et al., Anaesthesist 2019, a shock index around 0.7 already signals hemodynamic compromise, and the modified shock index is more sensitive than the standard ratio for early sepsis and trauma triage

According to ACOG Practice Bulletin 183, a shock index at or above 0.9 is a postpartum hemorrhage trigger because it catches compensated blood loss earlier than heart rate or blood pressure alone

When the same perfusion question is being read in body-size-adjusted terms, Cardiac Index Calculator divides cardiac output by body surface area.