Rapid Shallow Breathing Index (RSBI) for Weaning from Mechanical Ventilation
Definition and Measurement
The rapid shallow breathing index (RSBI), also known as the frequency-to-tidal volume ratio (fR/VT), is calculated by dividing respiratory rate (breaths/minute) by tidal volume (liters) and represents the most validated single predictor of successful weaning from mechanical ventilation. 1
The measurement technique requires:
- A handheld spirometer attached to the endotracheal tube while the patient breathes spontaneously for 1 minute 1
- Measurement after 30-60 minutes of spontaneous breathing, NOT during the first minute when respiratory drive may still be suppressed 2, 3
- The area under the ROC curve improves from 0.74 to 0.92 when measured at 30-60 minutes versus the first minute 2, 3
Critical Cutoff Values
RSBI ≤ 105 breaths/min/L: This threshold provides the best separation between weaning success and failure, with positive predictive value of 0.78 and negative predictive value of 0.95 1, 2
RSBI < 80 breaths/min/L: This value strongly predicts weaning success with a likelihood ratio of 7.53, meaning patients with this value are 7.53 times more likely to wean successfully 1, 2
RSBI > 100 breaths/min/L: This value strongly predicts weaning failure with a likelihood ratio of 0.04, indicating only a 4% chance of successful weaning 1, 2
Comparative Predictive Accuracy
The RSBI demonstrates superior discriminative power compared to traditional weaning indices:
- RSBI area under ROC curve: 0.89 1
- Minute ventilation: 0.40 1
- PaO2/PAO2 ratio: 0.48 1
- Maximum inspiratory pressure (PI,max): 0.61 1
This makes RSBI the highest-performing single weaning predictor among conventional indices 1, 2.
Clinical Application Algorithm
Step 1: Ensure Prerequisites
- PaO2 ≥ 55 mmHg on FiO2 ≤ 0.40 (do not attempt weaning below this threshold) 1, 2
- Hemodynamic stability without vasopressors 2
- Resolution or significant improvement of primary condition 2
Step 2: Initiate Spontaneous Breathing Trial
- Use pressure support ventilation 5-8 cm H2O with PEEP 5 cm H2O 2, 4
- FiO2 ≤ 40% 2
- Duration: 30 minutes for standard-risk patients, 60-120 minutes for high-risk patients 2, 3
Step 3: Measure RSBI
- Measure after 30-60 minutes of spontaneous breathing 2, 3
- Calculate: respiratory rate ÷ tidal volume (in liters) 1
Step 4: Interpret Results
- RSBI < 80: Proceed with extubation (high likelihood of success) 2
- RSBI 80-105: Acceptable for weaning trial 2
- RSBI > 100: High risk of weaning failure; reconsider timing 2
Important Caveats and Pitfalls
Gender and Endotracheal Tube Size: Women have higher RSBI values than men independent of body size, and narrow endotracheal tubes (≤ 7 mm internal diameter) further increase RSBI in women, leading to higher false-negative rates 2
Timing of Measurement: Measuring RSBI during the first minute of spontaneous breathing yields falsely low values due to suppressed respiratory drive, reducing predictive accuracy 2, 3
Ventilator-Displayed Values: Ventilators significantly overestimate RSBI compared to Wright spirometer measurements; if using ventilator-displayed values, average the readings from 0,15,30,45, and 60 seconds for best correlation with standard technique 5
COPD Patients: RSBI values may behave differently in COPD patients, with less variation across different measurement strategies 6
Limitations: Approximately 10% of patients who pass RSBI criteria will still fail extubation due to factors not evaluated by this index, such as upper airway patency, secretion clearance, and bulbar function 3
Integration with Comprehensive Weaning Protocol
While RSBI is the most accurate single predictor, systematic use of a complete weaning protocol that includes RSBI ≤ 105, PaO2/FiO2 ≥ 200, PEEP ≤ 5 cm H2O, intact cough on suctioning, and absence of sedative or vasopressor infusions produces superior outcomes compared to RSBI alone 2. Protocol-driven weaning reduces mechanical ventilation duration by approximately 25 hours and ICU length of stay by 1 day 2, 4.