RSBI Calculator — Rapid Shallow Breathing Index (Tobin Index)
Calculate the rapid shallow breathing index (RSBI, f/VT) from respiratory rate and spontaneous tidal volume to predict ventilator weaning success, with the classic cut-off of 105.
How it works
RSBI = respiratory rate (breaths/min) ÷ tidal volume (L). Below 105: weaning success more likely; 105 or above: failure more likely (Yang & Tobin 1991).
Patients who cannot sustain unassisted breathing tend to breathe fast and shallow. Yang and Tobin (NEJM 1991) measured the frequency-to-tidal-volume ratio during one minute of spontaneous breathing and found that a value under 105 breaths/min/L predicted successful weaning better than other bedside indices (sensitivity 0.97, specificity 0.64 in their cohort). Today the RSBI is used as a readiness screen before a spontaneous breathing trial rather than as a stand-alone decision rule; values measured on pressure support or CPAP are lower than on a T-piece.
Worked example
During a short off-support check a patient breathes 24 times per minute with an average tidal volume of 400 mL (0.40 L): RSBI = 24 ÷ 0.40 = 60 breaths/min/L — below 105, so weaning success is more likely; minute ventilation is 9.6 L/min.
Frequently asked questions
What is a good RSBI?
Below 105 breaths/min/L favors successful weaning; many clinicians are more confident when it is well below 80. Values of 105 or more predict failure of extubation.
How is RSBI measured?
Disconnect the patient from support (or set minimal pressure support and PEEP), let them breathe spontaneously for about a minute, and divide the respiratory rate by the average tidal volume in liters.
Does a low RSBI mean the patient can be extubated?
Not on its own. It indicates readiness for a spontaneous breathing trial. Extubation also depends on airway protection, secretions, mental status, oxygenation and the cause of respiratory failure.
Why is the Tobin index divided by liters, not milliliters?
The original index uses tidal volume in liters, giving values such as 60 or 105. Dividing by milliliters would give numbers 1,000 times smaller.
Related calculators
Clinical decision tool for education and for use by trained clinicians alongside clinical judgement — not a diagnosis and not a substitute for medical assessment. Scores and cut-offs come from the cited validation studies and may not apply to every patient; follow local protocols.