How this instrument works
Rapid shallow breathing index divides respiratory rate, in breaths per minute, by tidal volume, converted from milliliters to liters. A patient breathing slowly and deeply produces a low figure; one breathing quickly in small, shallow gasps — the pattern clinicians call rapid shallow breathing — produces a high one. The ratio packages both signs of respiratory distress that a clinician would otherwise judge separately by eye into a single bedside number taken during a brief trial of unassisted or minimally assisted breathing.
The index comes from a 1991 study by Karl Yang and Martin Tobin, who measured breathing pattern in patients being considered for extubation from mechanical ventilation and tracked which of several bedside measures best predicted whether the attempt would succeed. Rapid shallow breathing index outperformed simpler measures like vital capacity or maximal inspiratory pressure, and a threshold of 105 became the figure most widely quoted afterward: below it, the odds favor a successful trial; at or above it, the odds shift toward failure.
The original study was a single-center trial in around 100 patients, and later attempts to reproduce its predictive strength across other hospitals have generally found a weaker relationship than the original numbers suggested — still useful, but not as sharp a cutoff as 105 sounds. Professional guidelines treat it as one input a clinician weighs alongside oxygenation, secretions, mental status, and the reason ventilation was started in the first place, never as a stand-alone green light to extubate.
- Enter Respiratory rate in breaths per minute, counted during a brief spontaneous-breathing trial.
- Enter Tidal volume in mL, the average breath size measured over the same trial.
- Read Rapid Shallow Breathing Index: respiratory rate divided by tidal volume in liters, recalculated as either field changes.
- Weigh the result — below 105 versus 105 and above — alongside oxygenation, secretions, and mental status, not as a lone decision point.
Worked example — 20 breaths/min at 400 mL
A patient breathing 20 times a minute at an average tidal volume of 400 mL: convert to liters (0.4 L) and divide, 20 / 0.4 = 50 — comfortably under the 105 threshold linked to a good chance of a successful weaning trial.
Compare a patient breathing 35 times a minute at only 250 mL a breath: 35 / 0.25 = 140, above the 105 cutoff, the fast-shallow pattern associated with a higher risk of failing the same trial. A middle case — 25 breaths/min at 350 mL — gives 25 / 0.35 ≈ 71.4, still under the line but closer to it than the first patient.
Questions
What does an RSBI below 105 mean?
It means the patient is breathing at a rate and depth statistically associated with a good chance of tolerating removal from the ventilator, per the original 1991 study. It is a favorable sign, not a guarantee — clinicians still confirm oxygenation, cough strength, and mental status before proceeding.
How accurate is the 105 threshold in practice?
Less sharp than the original single-center study suggested. Later trials across multiple hospitals found the threshold's ability to predict success or failure was weaker when tested more broadly, which is why current practice treats RSBI as one supporting number among several rather than a stand-alone pass/fail line.
Why divide by tidal volume in liters instead of milliliters?
Because the index was defined that way in the original study, and liters keep the resulting numbers in a convenient range — typically double digits to low hundreds — rather than the much smaller values milliliters would produce. The unit choice is a matter of the formula's original definition, not physiology.
Can RSBI alone decide when to extubate a patient?
No. Professional guidelines treat it as one input alongside oxygenation, secretion burden, cough strength, mental status, and the reason mechanical ventilation was started. A low RSBI supports proceeding to a breathing trial; it does not override clinical judgment about the rest of the picture.
Does a high RSBI mean the ventilator can never be removed?
No — it flags a higher statistical risk of failing that particular attempt, often because of fatigue, secretions, or an unresolved reason for ventilation. Many patients with an elevated reading go on to wean successfully once the underlying problem improves; the figure describes risk at a moment, not a permanent verdict.
Who first proposed this index and when?
Karl Yang and Martin Tobin, in a 1991 New England Journal of Medicine study that compared several bedside breathing measures against actual weaning outcomes and found this ratio outperformed the others they tested.
References
- Yang & Tobin 1991, NEJM — original RSBI study (PubMed)
- MacIntyre et al. 2001 — evidence-based weaning guidelines (PubMed)
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.