Rapid Shallow Breathing Index (f/VT) Calculator
Rapid Shallow Breathing Index (f/VT) Calculator
Respiratory rate divided by tidal volume in litres, during a spontaneous breathing trial. Yang and Tobin’s threshold of 105 came from 100 medical ICU patients in 1991, and its discrimination in later work is considerably worse.
Rapid shallow breathing index
f over VT, breaths/min/LRespiratory rate 24 breaths/min, average tidal volume 330 mL
Formula
- litres, not millilitres
- the index is published in breaths per minute per LITRE, so 330 mL enters as 0.33 L. Forgetting the division by 1,000 gives an answer a thousandfold too small, which is the one arithmetic error this calculation invites
- measured off support
- Yang and Tobin measured rate and tidal volume during unassisted spontaneous breathing in patients judged ready to wean. Pressure support raises the tidal volume and so lowers the index, so a value measured on support is systematically more favourable than the quantity that was validated
- 105
- the published threshold, from 100 medical ICU patients — 36 in derivation and 64 in validation. In validation, an f/VT under 105 had a sensitivity of 97% and a specificity of 64% for weaning success, and the authors noted that a value above 105 identified failure better than one below identified success
- the later performance
- Trivedi and colleagues pooled 10,946 patients for a sensitivity of 83% and a specificity of 58%. The COBRE-US secondary analysis found no association between f/VT and success of the trial or of extubation at any cut-off tested, with areas under the curve of 0.53 and 0.48
Worked example
Respiratory rate 24 breaths/min, average tidal volume 330 mL
Tidal volume in litres = 330 ÷ 1,000 = 0.33 L
f/VT = 24 ÷ 0.33 = 72.7 breaths/min/L
72.7 is below Yang and Tobin's 105 and below the 80 that the COBRE-US analysis used as its lowest stratum, whose median was 56 breaths/min/L
Halve the tidal volume to 165 mL at the same rate and the index doubles to 145.5 — the index is far more sensitive to tidal volume, because volume is in the denominator
Raise the rate to 35 at the original 330 mL and the index is 106.1, just across the threshold: two very different patients can sit on the same side of the line
Forget to convert and divide 24 by 330 and the answer is 0.07 — the unit error this page's division by 1,000 exists to prevent
What the index predicted, and in whom
| Study | Population | Performance of f/VT under 105 |
|---|---|---|
| Yang and Tobin, NEJM 1991 | 100 medical ICU patients (36 derivation, 64 validation), clinically stable and judged ready to wean | Sensitivity 97%, specificity 64% for weaning success. The paper noted that a value ABOVE 105 identified failure better than one below it identified success |
| Trivedi and colleagues, Chest 2022 | Systematic review and meta-analysis, 10,946 patients | Pooled sensitivity 83% (0.78 to 0.87), specificity 58% (0.49 to 0.66) |
| Varón-Vega and colleagues, Med Intensiva 2025 | Secondary analysis of the COBRE-US trial; median f/VT 56 (IQR 43 to 75) | No association with success of the trial (area under the curve 0.53) or of extubation (0.48), at any of the under-80, 80 to 105 and over-105 strata |
What changes the number without changing the patient
| Factor | Effect on f/VT | Why |
|---|---|---|
| Pressure support left on during the trial | Lowers it | Support increases tidal volume, which is the denominator |
| A small endotracheal tube | Raises it | Resistive work rises, tidal volumes fall and rate rises to compensate |
| Measuring in the first minute of the trial | Lowers it | Rate and depth drift over several minutes as the patient takes on the work |
| Pain, anxiety, fever | Raises it | Rate rises for reasons unrelated to respiratory muscle capacity |
Rate over depth, and what happened to the threshold
A patient whose respiratory muscles cannot sustain the work of breathing takes small breaths quickly rather than large breaths slowly. The rapid shallow breathing index is that observation reduced to one number: respiratory rate divided by tidal volume in litres. Yang and Tobin published it in 1991 from 100 medical ICU patients, 36 in a derivation set and 64 in a validation set, all clinically stable and all judged by their own physicians ready for a weaning trial. In the validation set an f/VT under 105 breaths per minute per litre had a sensitivity of 97% and a specificity of 64% for weaning success, and 105 has been quoted ever since.
Two things in that paper are usually dropped. The first is the asymmetry: the authors observed that a value above 105 identified weaning failure better than a value below 105 identified success, and said explicitly that a low index should not override a clinical judgement that extubation would be unsafe. The second is the population — one medical ICU, patients already selected as ready.
What has happened since is worth knowing before quoting the threshold. Trivedi and colleagues pooled 10,946 patients and found the pooled sensitivity of an f/VT under 105 to be 83% and the specificity 58%, both well below the original figures. A 2025 secondary analysis of the COBRE-US trial went further: across the under-80, 80-to-105 and over-105 strata the index showed no association with success of either the spontaneous breathing trial or extubation, with areas under the curve of 0.53 and 0.48. Its median value in that trial was 56 breaths per minute per litre, so most patients being assessed sat comfortably below the famous line.
The index is also easy to measure in a way that flatters it. Pressure support left on raises the tidal volume and so lowers the index; a value read in the first minute of a trial is more favourable than one read after several, because rate and depth drift as the patient takes on the work. Rapid shallow breathing remains a real sign; this arithmetic is one summary of it, with the discrimination its own literature reports. This supports a clinician’s judgement rather than replacing it.
Frequently asked questions
How is the rapid shallow breathing index calculated?
Respiratory rate in breaths per minute divided by tidal volume in LITRES. A rate of 24 with a tidal volume of 330 mL is 24 ÷ 0.33 = 72.7 breaths per minute per litre. Forgetting to convert millilitres to litres is the one arithmetic error the calculation invites.
What is a normal or favourable f/VT?
Yang and Tobin’s threshold was 105 breaths per minute per litre, with a value below it associated with weaning success in their 100 medical ICU patients. In a 2025 secondary analysis of the COBRE-US trial the median among patients undergoing spontaneous breathing trials was 56, interquartile range 43 to 75.
How well does the f/VT predict extubation success?
Much less well than the original figures suggest. Yang and Tobin reported a sensitivity of 97% and specificity of 64%; a meta-analysis of 10,946 patients found 83% and 58%; and a secondary analysis of the COBRE-US trial found no association with success of the trial or of extubation at any cut-off, with areas under the curve of 0.53 and 0.48.
Should the f/VT be measured on pressure support?
Not if it is being compared with the published threshold. Yang and Tobin measured rate and tidal volume during unassisted spontaneous breathing. Pressure support increases the tidal volume, which is the denominator, so a value measured on support is systematically more favourable than the quantity that was validated.
Does a low f/VT mean a patient can be extubated?
The index does not answer that, and its original authors said so: a value above 105 identified weaning failure better than a value below it identified success, and they advised against letting a low index override a clinical judgement that extubation would be unsafe.
Related calculators
References
- Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991;324(21):1445–50.
- WikiJournalClub. Yang-Tobin Study: a structured reproduction of Yang and Tobin (1991). wikijournalclub.org
- Trivedi V, Chaudhuri D, Jinah R, et al. The usefulness of the rapid shallow breathing index in predicting successful extubation: a systematic review and meta-analysis. Chest. 2022;161:97–111.
- Varón-Vega F, Tuta-Quintero E, Robayo-Amortegui H, et al. Clinical utility of rapid shallow breathing index in predicting successful weaning: secondary analysis of the COBRE-US trial. Med Intensiva. 2025;49(2):72–7.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/