Shock Index Calculator

Background.

Heart rate and blood pressure can both look acceptable while a patient is quietly losing blood. Early in a hemorrhage or in sepsis, the body defends its blood pressure by speeding the heart and tightening the blood vessels, so the systolic reading may hold steady while the pulse climbs. Each number can stay inside its usual range; it is the relationship between them that shifts. In 1967 Allgöwer and Burri proposed dividing the heart rate by the systolic pressure to expose that shift, and the shock index has been studied in trauma, sepsis and obstetric bleeding ever since.

The ratio is as simple as it sounds: beats per minute over millimeters of mercury, with the units set aside. A pulse of 100 and a systolic pressure of 110 give 0.91. The index rises when the heart speeds up, when the pressure falls, and fastest when both happen together.

Bleeding after childbirth is a setting where watching the pressure alone is especially misleading. Blood volume expands substantially during pregnancy, so a woman can lose a large amount of blood before her pressure falls, and blood loss judged by eye is often underestimated. A ratio that climbs while the pressure still looks reassuring gives the team a second prompt alongside the measured loss.

What it does not do is diagnose. No cutoff is applied here, because no single threshold holds across settings; studies use different values for different populations and outcomes. Plenty of things move one input without any loss of circulating volume. Pain, fear and fever push the heart rate up; beta-blockers and pacemakers hold it down. Someone with long-standing hypertension may be in trouble at a systolic pressure that would be normal for someone else, pregnancy and childhood shift the baseline for both numbers, and an irregular rhythm such as atrial fibrillation makes a single heart-rate figure unreliable.

The index earns its place as a trend. A ratio climbing across successive sets of observations says more than any one value, and it sits alongside the examination, the mechanism of injury and the clinician's judgment rather than in place of them.

What is shock index calculator?

The shock index is the heart rate in beats per minute divided by the systolic blood pressure in millimeters of mercury, both taken at the same assessment. By convention it is reported as a plain number, without units. Because it combines two vital signs that move in opposite directions as circulating volume falls, it is used as an early bedside indicator of circulatory strain, particularly in trauma, sepsis and bleeding after childbirth.

Behind the ratio is the body's first defence of its circulation. Blood pressure depends on cardiac output, which is stroke volume times heart rate, and on how tightly the blood vessels are constricted. When circulating volume falls, each beat ejects less blood; pressure sensors in the carotid arteries and the arch of the aorta respond by speeding the heart and narrowing the vessels. Early on that response holds the systolic pressure up while the heart rate carries the extra load, and the shock index records that trade in a single number. That is why it can move before either vital sign leaves its usual range, and why it is attractive at triage and in ambulances: it needs only two observations that are recorded anyway.

How to use this calculator.

1. 01Enter the heart rate in beats per minute, measured at the same moment as the blood pressure. A monitor reading works; with an irregular rhythm, count the pulse for a full minute rather than estimating from a few seconds.
2. 02Enter the systolic blood pressure in mmHg, the upper number of the reading. Diastolic and mean pressure are not used.
3. 03Read the ratio together with the scope note beside it; no value is labelled normal or abnormal, and the trend across repeated readings matters more than any single figure.
4. 04Recalculate with each new set of observations and compare them: a ratio rising over time carries more information than a single figure.
5. 05Note anything that blunts or exaggerates the pulse response beside each result: a beta-blocker, a pacemaker, pregnancy, childhood or old age, pain, fever or an irregular rhythm.

The formula.

SI = HR ÷ SBP, where HR is the heart rate in beats per minute and SBP the systolic blood pressure in mmHg, both from the same set of observations. Both must be positive, and nothing else enters the calculation: no constants, age adjustments or thresholds. With heart rate on top and pressure underneath, the index responds to both halves of the compensation pattern: a faster pulse raises it, and a falling pressure raises it further.

The pressure effect is not linear. At a pulse of 100, a systolic drop from 120 to 100 mmHg moves the index from 0.83 to 1.0, while a further 20 mmHg fall to 80 lifts it to 1.25. Proportional changes multiply, so a 10 percent rise in pulse together with a 10 percent fall in systolic pressure raises the index by 1.1 ÷ 0.9, about 22 percent, more than either change alone. Because it is a ratio, it also hides the absolute values: a pulse of 90 over a pressure of 100 and a pulse of 135 over 150 both give 0.9. An index above 1 says only that the pulse number is larger than the systolic number.

The ratio leaves out information a clinician would use. Diastolic pressure plays no part, so the narrowing gap between systolic and diastolic pressure that can accompany early blood loss, as the vessels constrict, is invisible to it. It also assumes that the compensation it is meant to reveal is working. Where the heart cannot speed up, the index stays deceptively low: beta-blockers and fixed-rate pacemakers cap the pulse, a high spinal cord injury that cuts sympathetic outflow produces a low pressure with a slow pulse, and very late in shock the heart can slow as compensation fails. Finally, it assumes both readings are accurate and simultaneous; a pulse counted during a spike of pain and a pressure taken minutes later describe two different moments.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Allgöwer M, Burri C. Shock index. Deutsche Medizinische Wochenschrift. 1967 (BIBLIOGRAPHIC).
2. [2]Koch E et al. The value of the shock index for outcomes in emergency patients: systematic review. 2019.
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.
4. [4]NIST Special Publication 811, Guide for the Use of the International System of Units, 2008 edition.
5. [5]ACEP Clinical Policy: Critical Issues in Adult Patients With Severe Sepsis and Septic Shock.