About the RSBI Calculator
The rapid shallow breathing index (RSBI), also written f/VT, is the respiratory rate divided by the tidal volume in litres. Patients who are not ready to come off a ventilator tend to breathe fast and shallow, which pushes the ratio up; patients who are ready breathe more slowly and deeply. Yang and Tobin introduced the index in 1991 and found that a value below 105 breaths/min/L during a short period of unassisted breathing predicted successful weaning.
Enter the respiratory rate and the average spontaneous tidal volume measured while the patient breathes without pressure support — classically over one minute on a T-piece or with minimal support. The calculator returns the RSBI, the minute ventilation and where the value sits against the 105 threshold.
The RSBI was validated in adult intensive care patients on mechanical ventilation. It is one input to a spontaneous breathing trial and an extubation decision, not a substitute for it: pressure support, PEEP, sedation, the timing of the measurement and the patient’s size all change the value, and pediatric thresholds differ.
With the default inputs, the rsbi (f/vt) is 71 breaths/min/L. Change any value above to recalculate instantly.
How to use the rsbi calculator
- 1Put the patient on minimal or no ventilator support (T-piece or CPAP 0–5) as per local protocol.
- 2Measure the respiratory rate over one minute.
- 3Record the average exhaled tidal volume in mL.
- 4Enter both values and read the RSBI.
- 5Use the result together with the full spontaneous breathing trial assessment.
Formula and method
The respiratory frequency in breaths per minute is divided by the spontaneous tidal volume expressed in litres (mL ÷ 1000). The result has units of breaths per minute per litre. Rapid, shallow breathing — a high rate with a small tidal volume — is a sign of respiratory muscle load exceeding capacity, so the ratio rises when weaning is likely to fail.
Yang and Tobin derived the 105 threshold in 36 adult ventilated patients and tested it prospectively in 64 more, where it had a sensitivity of 0.97 and specificity of 0.64 for successful weaning. Later studies show the predictive value depends heavily on how and when the index is measured.
- f
- Respiratory rate (breaths/min)
- VT
- Spontaneous tidal volume (litres)
Worked examples
Ready-to-wean pattern
A rate of 25 breaths/min with a 350 mL (0.35 L) tidal volume gives 25 ÷ 0.35 = 71 breaths/min/L, comfortably below 105. Minute ventilation is 25 × 0.35 = 8.75 L/min.
Rapid shallow breathing
At 34 breaths/min and only 250 mL per breath, the RSBI is 34 ÷ 0.25 = 136, above the 105 cut-off and consistent with weaning failure, even though minute ventilation is similar to the first example.
Slow, deep breathing
Fourteen breaths of 500 mL give an RSBI of 14 ÷ 0.5 = 28 breaths/min/L, 77 below the threshold.
Frequently asked questions
What is a good RSBI?+
An RSBI below 105 breaths/min/L, measured during unassisted breathing, was the threshold Yang and Tobin found to predict successful weaning. Lower values are more favourable; values above 105 suggest the patient is likely to fail.
Should tidal volume be in mL or litres?+
Litres. The ventilator usually shows tidal volume in mL, so divide by 1000 first — 400 mL is 0.4 L. Using mL by mistake gives an RSBI 1000 times too small.
Can RSBI be measured on pressure support?+
It can, but pressure support and PEEP increase tidal volume and lower the RSBI, making the patient look better than they would unassisted. The original threshold was derived without support, so record the settings used.
Does a low RSBI guarantee successful extubation?+
No. The RSBI predicts the ability to breathe spontaneously, but extubation also depends on airway protection, cough strength, secretions, mental status and the cause of respiratory failure. Specificity in the original study was only 0.64.
For healthcare professionals and education only. This calculator implements a published formula or score but is not a diagnosis and does not replace clinical judgment. Verify inputs and results against the original source before using them in patient care.