The rapid shallow breathing index (RSBI) is calculated by dividing a patient’s respiratory rate (breaths per minute) by their tidal volume (in liters). An RSBI below 105 breaths/min/L suggests the patient can likely breathe independently, while a value above 105 predicts a higher chance of weaning failure. The math itself is simple, but getting an accurate measurement depends on how and when you collect the numbers.
The RSBI Formula
RSBI = Respiratory Rate (breaths/min) ÷ Tidal Volume (liters)
The result is expressed in breaths/min/L. The key detail most people trip on is units: tidal volume must be in liters, not milliliters. Ventilators and spirometers often display tidal volume in milliliters, so you need to divide that number by 1,000 before plugging it into the formula.
A Quick Example
A patient is breathing 28 times per minute with an average tidal volume of 350 mL. First, convert the tidal volume: 350 mL ÷ 1,000 = 0.35 L. Then divide: 28 ÷ 0.35 = 80 breaths/min/L. That RSBI of 80 falls below the 105 threshold, suggesting the patient has a reasonable chance of tolerating independent breathing.
Now take a patient breathing 32 times per minute with a tidal volume of 250 mL (0.25 L). The calculation: 32 ÷ 0.25 = 128 breaths/min/L. That value exceeds 105 and indicates the patient is breathing rapidly and shallowly, a pattern associated with weaning failure.
How the Measurement Is Taken
RSBI was introduced by Yang and Tobin in 1991 as a bedside tool for predicting whether a mechanically ventilated patient could be successfully removed from the ventilator. The original method involves disconnecting the patient from the ventilator and letting them breathe on their own for a brief period, typically less than 5 minutes. During that window, a clinician records the breathing frequency and average tidal volume using a handheld spirometer (a Wright spirometer is the traditional tool).
The tidal volume used in the calculation is the average volume per breath over that one-minute observation, not a single breath. Breathing patterns can vary from one cycle to the next, so averaging over a full minute smooths out those fluctuations.
Some ICUs measure RSBI while the patient remains on low-level ventilator support (CPAP or minimal pressure support) rather than fully disconnected. Subgroup analyses in clinical practice guidelines have found no significant difference in predictive accuracy between these approaches, so the method your facility uses will depend on local protocol.
What the 105 Threshold Means
In Yang and Tobin’s original study of 100 mechanically ventilated patients, an RSBI above 105 breaths/min/L was highly predictive of weaning failure. The threshold had a sensitivity of 97% and a negative predictive value of 95%, meaning it caught nearly all patients who would fail if extubated. The specificity was 64%, meaning some patients with an RSBI above 105 could still have been successfully weaned but were flagged as high risk.
In practical terms, the index is better at ruling out failure than ruling it in. A low RSBI gives you confidence to proceed with a spontaneous breathing trial. A high RSBI is a caution signal, but it doesn’t guarantee the patient will fail.
When the Standard Cutoff May Not Apply
The 105 threshold was derived from a general medical ICU population. Different patient groups can shift the ideal cutoff. Research on patients with chronic obstructive pulmonary disease, for example, found that a lower cutoff of 62 breaths/min/L was more appropriate, because these patients tend to breathe with smaller tidal volumes at baseline.
Newer research has also explored tracking how RSBI changes over the course of a spontaneous breathing trial rather than relying on a single measurement. One study found that the rate of change in RSBI between 5 and 120 minutes into a trial predicted extubation failure with an overall accuracy of 88%, a sensitivity of 91%, and a specificity of 86%. The idea is that a patient whose RSBI climbs substantially during the trial is fatiguing, even if the initial number looked acceptable.
Common Pitfalls in Calculation
- Forgetting to convert milliliters to liters. This is the most frequent math error. A tidal volume of 400 mL entered as 400 instead of 0.4 produces an absurdly small RSBI that will mislead the clinical picture.
- Measuring too early. If you assess RSBI in the first few seconds after disconnecting from the ventilator, the patient’s breathing pattern may not have stabilized. The standard approach allows up to a few minutes of spontaneous breathing before recording values.
- Using ventilator-displayed values without verification. Some modern ventilators calculate and display RSBI automatically. Studies have validated these readings against the traditional spirometer method, but the accuracy depends on the ventilator model and whether it accounts for circuit compliance. When precision matters, a handheld spirometer remains the reference standard.
- Ignoring the clinical context. RSBI is one data point. Factors like the patient’s level of alertness, secretion burden, and underlying lung disease all influence whether a favorable number translates to successful extubation.
Putting It All Together
The calculation itself takes seconds: count the respiratory rate, measure the average tidal volume in liters, divide. The value you get is most useful as a screening tool. Below 105, the patient is a reasonable candidate for a longer spontaneous breathing trial. Above 105, the rapid, shallow pattern suggests the respiratory muscles are struggling to keep up with demand. For patients with chronic lung disease, a lower cutoff near 60 to 65 may be more accurate. And tracking changes in RSBI over the course of a breathing trial can reveal fatigue that a single snapshot would miss.