ROX Index in Mechanical Ventilation Weaning
Critical Clarification: ROX Index is NOT a Weaning Predictor
The ROX index is not validated or recommended for predicting successful weaning from mechanical ventilation. The ROX index (SpO₂/FiO₂ divided by respiratory rate) was developed and validated exclusively for predicting the need for intubation in patients receiving high-flow nasal cannula (HFNC) therapy, not for weaning mechanically ventilated patients 1, 2, 3.
What the ROX Index Actually Predicts
The ROX index serves a completely different clinical purpose:
Predicts HFNC success/failure: A ROX index >3.0 at 2,6, and 12 hours after HFNC initiation is 85.3% sensitive for identifying patients who will successfully avoid intubation 1
Identifies high-risk patients needing intubation: ROX <3.85 at 12 hours predicts need for invasive mechanical ventilation with 59.4% positive predictive value in COVID-19 patients 3
Different thresholds for different populations: ROX <14.8 predicts need for mechanical ventilation in community-acquired pneumonia patients 2, while ROX <6.9 predicts intubation in hypercapnic respiratory failure 4
The Correct Weaning Predictor: Rapid Shallow Breathing Index (RSBI)
For weaning from mechanical ventilation, use the frequency-to-tidal volume ratio (fR/VT), also called the Rapid Shallow Breathing Index (RSBI), which is the most validated and accurate predictor of weaning success 5, 6:
RSBI Thresholds and Performance
RSBI <105 breaths/min/L: Most reliable cutoff for predicting weaning success, with area under ROC curve of 0.89 7, 5
RSBI <80 breaths/min/L: Strongly predicts weaning success with likelihood ratio of 7.53 7, 5
RSBI >100 breaths/min/L: Strongly predicts weaning failure with likelihood ratio of 0.04 7, 5
Critical Measurement Technique
Measure after 30-60 minutes of spontaneous breathing, not during the first minute when respiratory drive may be suppressed—this improves ROC area from 0.74 to 0.92 5, 6
Use a handheld spirometer attached to the endotracheal tube during spontaneous breathing 7
Algorithmic Approach to Weaning Assessment
Step 1: Screen for Weaning Readiness
Before calculating any index, verify these criteria 5, 8:
- PaO₂/FiO₂ ratio ≥200
- PEEP ≤5 cm H₂O
- Intact cough on suctioning
- No sedative or vasopressor infusions
Step 2: Perform Spontaneous Breathing Trial (SBT)
Use pressure support 5-8 cm H₂O (not T-piece) for initial SBT—this increases success rate to 84.6% vs 76.7% 8, 6
Duration: 30 minutes for standard-risk patients, 60-120 minutes for high-risk patients 8, 6
Step 3: Calculate RSBI During SBT
If RSBI <80: Proceed with extubation (high likelihood of success) 5
If RSBI 80-105: Consider extubation with close monitoring 5
If RSBI >105: High risk of weaning failure, continue mechanical ventilation 5
Common Pitfalls to Avoid
Do not use ROX index for weaning decisions—it has no validation in this context and will lead to inappropriate clinical decisions 9, 1, 2
Do not measure RSBI in the first minute of spontaneous breathing—falsely low values occur due to suppressed respiratory drive 5, 6
Do not rely solely on maximum inspiratory pressure (PI,max)—it has poor specificity (only 25%) despite 80% sensitivity 7
Do not use minute ventilation alone—it performs no better than chance (ROC area 0.40) 7, 5
Special Considerations
Women have higher RSBI values than men independent of body size, and narrow endotracheal tubes (≤7 mm) further increase RSBI, leading to higher false-negative rates 5
For high-risk patients (prolonged ventilation, COPD, cardiac dysfunction, previous extubation failure), consider prophylactic noninvasive ventilation after extubation 8, 6
Systematic use of weaning protocols with RSBI results in better outcomes than clinical judgment alone, including shorter ventilation duration, fewer complications, and lower costs 7, 5