What is the significance of the Rox index in predicting weaning from mechanical ventilation (MV)?

ROX Index in Predicting Weaning from Mechanical Ventilation

The ROX index is NOT a validated predictor for weaning from mechanical ventilation—it was developed specifically for high-flow nasal cannula (HFNC) therapy, not for patients on invasive mechanical ventilation. The established weaning predictor you should use is the rapid shallow breathing index (RSBI), also called the frequency-to-tidal volume ratio (fR/VT).

Critical Distinction: ROX Index vs. RSBI

The evidence provided addresses two completely different clinical scenarios:

ROX Index (Not for MV Weaning)

• ROX index = (SpO₂/FiO₂)/Respiratory Rate 1, 2, 3
• Developed and validated for predicting success of high-flow nasal cannula therapy in patients with hypoxemic respiratory failure 2, 3
• In COVID-19 patients on HFNC, a ROX index >3.0 at 2,6, and 12 hours was 85.3% sensitive for identifying HFNC success 2
• In pneumonia patients with hypoxemic ARF on HFNC, a ROX index ≥4.88 after 12 hours predicted lower risk of requiring mechanical ventilation 3
• One recent study attempted to apply ROX to mechanically ventilated patients, but this is not the standard or validated use 1

RSBI (The Actual MV Weaning Predictor)

• RSBI = Respiratory Rate/Tidal Volume (breaths/min/L) 4, 5
• The American Thoracic Society confirms RSBI is the most accurate predictor of weaning success from mechanical ventilation with an area under the ROC curve of 0.89 4, 5
• RSBI <105 breaths/min/L indicates likely weaning success 4, 5
• RSBI <80 breaths/min/L strongly predicts weaning success (likelihood ratio 7.53) 4, 5
• RSBI >100 breaths/min/L strongly predicts weaning failure (likelihood ratio 0.04) 4, 5

How to Properly Use RSBI for Weaning Assessment

Measurement Technique

• Measure RSBI after 1-2 minutes of spontaneous breathing, NOT during the first minute when respiratory drive may still be suppressed 4, 5
• The area under the ROC curve improves from 0.74 when measured in the first minute to 0.92 when measured at 30-60 minutes 4
• Can be measured with a handheld spirometer or directly from ventilator parameters with satisfactory concordance (0.94) 6

Systematic Weaning Protocol

The American Thoracic Society recommends screening patients daily for five readiness criteria 4, 5:

• PaO₂/FiO₂ ratio ≥200 4, 5
• PEEP ≤5 cm H₂O 4, 5
• fR/VT ≤105 breaths/min/L 4, 5
• Intact cough on suctioning 4, 5
• Absence of sedative or vasopressor infusions 4, 5

Critical Caveats

• Women have higher RSBI values than men (not explained by body size), leading to higher false-negative rates 4, 5
• Narrow endotracheal tubes (≤7 mm) in women further increase RSBI, causing especially high false-negative rates 4, 5
• RSBI is less helpful when pretest probability of successful weaning is already very high (>0.84), and most useful when outcome is uncertain 4
• Traditional indices like maximal inspiratory pressure (PI,max), minute ventilation, and PaO₂/PAO₂ ratio perform poorly compared to RSBI 4

Additional Weaning Assessment Tools

Airway Occlusion Pressure (P0.1)

• P0.1 ≤4 cm H₂O indicates likely extubation success 7
• P0.1 >6 cm H₂O predicts weaning failure 7
• Combining P0.1 with RSBI increases specificity from 0.40 to 0.60, though overall accuracy doesn't improve 4, 5

Spontaneous Breathing Trial (SBT)

• After meeting screening criteria including RSBI ≤105, perform a 2-hour SBT 4, 7
• Monitor for respiratory distress: respiratory rate 10-30/min, SpO₂ >92%, absence of exhaustion, agitation, hypertension, tachycardia 7
• Most SBT failures occur within the first 30 minutes 7

Bottom Line Algorithm

1. Screen daily for the five readiness criteria, including RSBI ≤105 4, 5
2. Measure RSBI after at least 1-2 minutes of spontaneous breathing 4, 5
3. If RSBI <80: Strong predictor of success—proceed with SBT 4, 5
4. If RSBI 80-105: Reasonable chance of success—proceed with SBT and monitor closely 5
5. If RSBI >100: High likelihood of failure—delay weaning attempt 4, 5
6. Adjust thresholds for women and those with narrow ETTs (≤7 mm) 4, 5

Do not confuse the ROX index (for HFNC therapy) with validated mechanical ventilation weaning predictors like RSBI.