The Most Reliable Indicator of Successful Ventilatory Weaning

The frequency-to-tidal volume ratio (f/Vt) <100 is the most reliable indicator of successful ventilatory weaning. 1, 2

Evidence Supporting f/Vt Ratio as Superior Predictor

The f/Vt ratio (also known as Rapid Shallow Breathing Index or RSBI) has consistently demonstrated the highest accuracy among weaning predictors, with values <100 breaths/minute/L strongly predicting weaning success 1, 2
Analysis using receiver-operating-characteristic (ROC) curves shows the area under the curve for f/Vt ratio (0.89) is significantly higher than other traditional weaning indices 1, 3
The American Thoracic Society recommends f/Vt ratio as the most accurate predictor of weaning success, with values <105 breaths/minute/L indicating patients likely to be successfully weaned 2
Values <80 breaths/minute/L generate a likelihood ratio of 7.53, indicating a strong prediction of weaning success 1, 2

Oxygenation parameters such as PaO₂/FiO₂ ratio are unreliable predictors of weaning outcome with an area under ROC curve of only 0.48 2
While adequate oxygenation is necessary for weaning, it is not sufficient to predict successful weaning 1

PCO₂ levels alone are not reliable predictors of weaning success 1
Ventilatory parameters must be considered in conjunction with respiratory mechanics and muscle strength 2

Maximum Inspiratory Pressure (PI,max or NIF) has poor specificity despite reasonable sensitivity 1, 2
At a threshold of -30 cm H₂O, PI,max has sensitivity of 0.67-0.86 but specificity as low as 0.21 1
The area under the ROC curve for PI,max (0.61) is significantly lower than that for f/Vt ratio (0.89) 1, 3

The f/Vt ratio should be measured after at least 1-2 minutes of spontaneous breathing, not during the first minute when respiratory drive may still be suppressed 1, 2
Measurements taken after 30-60 minutes of spontaneous breathing have even better predictive value (ROC area 0.92) than when measured during the first minute (ROC area 0.74) 1, 2
Women have higher f/Vt ratios than men, and narrow endotracheal tubes (≤7 mm internal diameter) in women further increase f/Vt, leading to higher false-negative rates 1, 2

The f/Vt ratio is most useful when there is genuine doubt about a patient's ability to wean; it is less helpful when clinicians already strongly suspect successful weaning 1
A systematic screening approach using multiple parameters (including f/Vt ≤105, PaO₂/FiO₂ ratio ≥200, PEEP ≤5 cm H₂O) followed by a spontaneous breathing trial provides the most comprehensive assessment 2, 4
While newer indices like the Weaning Index (WI) have been proposed, the f/Vt ratio remains the most widely validated and accessible predictor 5