The Rapid Shallow Breathing Index (RSBI), also called the Tobin index, is one of the most widely used bedside predictors of ventilator weaning readiness in critical care. It condenses a simple but clinically important pattern — fast, shallow breathing — into a single number that helps clinicians decide whether a patient is likely to tolerate a trial of extubation.

How RSBI works

RSBI is calculated as respiratory rate (breaths per minute) divided by tidal volume (in liters), measured during the first few minutes of a spontaneous breathing trial. Patients who breathe with a rapid, shallow pattern — a sign of impending respiratory muscle fatigue — produce a high RSBI, while patients who breathe slowly and deeply produce a low RSBI.

The index was introduced by Karl Yang and Martin Tobin in a 1991 study published in the New England Journal of Medicine. In that study, an RSBI below 105 breaths/min/L was strongly associated with successful weaning, while values at or above 105 were associated with a much higher rate of weaning failure. This cutoff remains the standard reference point used in critical-care practice today.

Inputs and what they mean

Respiratory rate is the patient's own spontaneous breathing rate during the trial, not a rate set on the ventilator. Tidal volume is the spontaneous volume of air moved per breath, in liters (400 mL is entered as 0.4 L). Both values should be measured in the first 1-3 minutes of a spontaneous breathing trial, since RSBI can change as the trial progresses and fatigue develops or resolves.

A common misunderstanding is entering tidal volume in milliliters without converting to liters — since RSBI divides by tidal volume, using the wrong unit changes the result by a factor of 1,000.

Limits and edge cases

RSBI is a screening tool, not a standalone extubation decision. It has recognized false positives and false negatives, and its predictive accuracy varies across patient populations and clinical settings. RSBI should always be interpreted alongside the rest of the weaning assessment — oxygenation, hemodynamic stability, secretion burden, mental status, and the patient's overall trajectory.

The calculator guards against a tidal volume of zero (which would make the ratio undefined) by treating it as a missing input. This tool is for clinical education and decision support only — actual ventilator weaning and extubation decisions must be made by a qualified clinician at the bedside.