How to Calculate the Rapid Shallow Breathing Index (RSBI)
The RSBI is calculated by dividing the respiratory rate (breaths per minute) by the tidal volume (in liters): RSBI = Respiratory Rate (breaths/min) / Tidal Volume (L). 1
Measurement Technique
Standard Method
- Measure RSBI using a handheld spirometer (Wright spirometer) attached to the endotracheal tube while the patient breathes spontaneously for 1 minute. 1
- The patient should be breathing room air or on minimal support during measurement 1
- Record the respiratory frequency and average tidal volume over the 1-minute period 1
- Calculate: RSBI = fR (breaths/min) / VT (L) 1
Alternative Ventilator-Based Method
- RSBI can be calculated directly from ventilator display parameters, though this method systematically overestimates the value compared to spirometry 2, 3
- When using ventilator-displayed values, average the RSBI readings at 0,15,30,45, and 60 seconds for best correlation with the standard technique (r = 0.76, ICC = 0.79) 3
- The ventilator method overestimates RSBI by approximately 17 breaths/min/L on average 3
Critical Measurement Conditions
Ventilator Settings During Measurement
- Measure RSBI with the patient on spontaneous breathing mode with PSV = 0 and PEEP = 0 for optimal accuracy. 4
- RSBI measured on CPAP 5 cm H₂O is significantly lower (19.1% reduction) than measurements without CPAP 5, 6
- RSBI measured on CPAP 0 cm H₂O is still 9.4% lower than handheld spirometer measurements due to base flow delivered by ventilators 6
- The level of ventilator support significantly affects RSBI values (median 71 breaths/min/L on 5 cm H₂O CPAP vs. 90 breaths/min/L on T-piece, P<0.001) 5
Timing Considerations
- Do not measure RSBI during the first minute of spontaneous breathing, as respiratory drive may still be suppressed. 1
- Wait until steady-state breathing is achieved, though this does not necessarily require 30-60 minutes 1
- Time of day does not significantly affect RSBI values 5
Interpretation for Weaning
Threshold Values
- RSBI <105 breaths/min/L predicts successful weaning with positive predictive value of 0.78 and negative predictive value of 0.95. 1
- RSBI <80 breaths/min/L has a likelihood ratio of 7.53 for successful weaning 1
- RSBI >100 breaths/min/L has a likelihood ratio of 0.04 for successful weaning (highly predictive of failure) 1
- The area under the ROC curve for RSBI is 0.89, the highest among 10 weaning indices evaluated 1
Important Caveats
- RSBI should not be used as the sole criterion for weaning decisions; consider the patient's general status, comorbidities, and duration of mechanical ventilation. 4
- Women have higher RSBI values than men, which cannot be explained by body size alone 1
- Women with narrow endotracheal tubes (≤7 mm internal diameter) have especially high false-negative rates 1
- RSBI is less helpful when the pretest probability of successful weaning is very high (>84%) compared to uncertain cases 1
- Approximately 10% of patients who pass RSBI criteria will still fail extubation 7, 8
Common Pitfalls to Avoid
- Never measure RSBI through the ventilator with CPAP support, as this will artificially lower the value and may lead to premature extubation attempts. 5, 6
- Do not rely on ventilator-displayed RSBI values without understanding they overestimate by ~17 breaths/min/L 3
- Avoid measuring during the initial transition to spontaneous breathing when drive is suppressed 1
- Do not use RSBI in isolation—always assess upper airway patency, bulbar function, secretion management, and cough effectiveness before extubation 7, 9