Rapid Shallow Breathing Index Calculator

Background.

When a patient on a mechanical ventilator is ready to try breathing alone, one of the earliest signs of trouble is a change in the pattern of breathing rather than its total amount. Respiratory muscles that cannot cope with the load take small breaths and take them quickly. The rapid shallow breathing index captures that pattern in one number: breaths per minute divided by tidal volume in litres.

The ratio works because two patients can move the same air per minute in very different ways. Twenty-four breaths of 0.4 L and thirty-two breaths of 0.3 L both add up to 9.6 L a minute, but the first gives an index of 60 and the second 106.67. The shallower pattern is also less efficient, because every breath spends its first portion filling the airways that take no part in gas exchange.

Karl Yang and Martin Tobin introduced the index in the New England Journal of Medicine in 1991, measuring it while patients breathed without ventilator assistance and comparing it with whether the weaning trial succeeded. Low values went with success, high values with failure, and the index became one of the most widely used bedside weaning measures. Today it is usually read alongside a spontaneous breathing trial rather than on its own; the 2017 CHEST and American Thoracic Society guideline on liberation from mechanical ventilation, cited below, includes recommendations on how those trials should be run.

The arithmetic here is a clinical aid only. Whether to extubate depends on the trial itself, airway protection, secretions, gas exchange, circulation and the trend over time, judged by the treating team.

What is rapid shallow breathing index calculator?

The rapid shallow breathing index, also written f/VT, is the respiratory frequency in breaths per minute divided by the tidal volume in litres, giving a result in breaths per minute per litre. It rises when breathing becomes faster, shallower or both, the pattern seen when the respiratory muscles are being overloaded. It describes the pattern of spontaneous breathing at the moment it is measured and is not a measure of lung function or oxygenation.

How to use this calculator.

1. 01Measure breathing rate and tidal volume while the patient breathes spontaneously, using the conditions your unit specifies for weaning assessment; ventilator support during the measurement makes the index look better.
2. 02Enter the respiratory frequency in breaths per minute, averaged over a full minute rather than counted over a few seconds.
3. 03Enter tidal volume in litres. Ventilators usually display it in millilitres, so 400 mL becomes 0.4; entering 400 gives an answer a thousand times too small.
4. 04Read the index alongside the scope note and the rest of the clinical picture, and repeat it over time rather than relying on one reading.

The formula.

RSBI = f ÷ VT, where f is the respiratory rate in breaths per minute and VT is the average tidal volume in litres. Because VT is in the denominator, the index is very sensitive to shallow breathing: a fall in tidal volume from 0.4 L to 0.3 L at the same rate raises the index by a third, and a patient who compensates by breathing faster raises it further. Both changes push in the same direction, which is why the index separates a fast, shallow pattern from a slow, deep one even when minute ventilation (f × VT) is identical. The litre is a convention of the original study; the same ratio with VT in millilitres is a thousand times smaller.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Yang KL, Tobin MJ. A Prospective Study of Indexes Predicting the Outcome of Trials of Weaning from Mechanical Ventilation. NEJM. 1991.
2. [2]Ouellette DR et al. Liberation From Mechanical Ventilation in Critically Ill Adults. CHEST/ATS guideline. 2017.
3. [3]BIPM, The International System of Units (SI Brochure), 9th ed., version 3.01, coherent derived units and quantity equations.