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Canadian Journal of Anesthesia 49:761-762 (2002)
© Canadian Anesthesiologists' Society, 2002

Correspondence

Bedside indices to predict weaning from mechanical ventilation

Seetharaman Hariharan, MD, Areti Y. Kumar, MD and Anitha Shenoy, MD

Barbados, West Indies

To the Editor:

An experienced intensivist may be able to predict whether a patient can be weaned successfully from mechanical ventilatory support or not. However, it is always helpful to have criteria on the basis of which the outcome may be predicted. After the introduction of the rapid shallow breathing index - the frequency to tidal volume ratio (breaths•min-1•L-1) by Yang and Tobin,1 many studies have found it to be a very effective and simple bedside index.2–4 In an attempt to further improve the accuracy of this index, we modified it by incorporating the weight of the patient as the ratio of frequency to the tidal volume corrected for patient’s weight (breaths•min-1•mL-1•kg-1). We hypothesized that the tidal volume corrected for the patient’s weight would be more accurate than the tidal volume per se as an absolute value since it eliminates the factors contributed by the anthropometric differences in individuals.

After approval by the Ethical Committee, adult patients admitted to a Multidisciplinary Intensive Care Unit for mechanical ventilation were studied. The diagnoses included adult respiratory distress syndrome, chronic obstructive pulmonary disease, pulmonary edema of various origins, pneumonia and lung abscess. The number of days of mechanical ventilation ranged from four to 16 days. The minute ventilation (VE) and the frequency of respiration (f) were measured using a Wright respirometer. The spontaneous tidal volume (VT) was calculated by dividing ‘VE’ by ‘f’. This VT was then corrected for patient’s weight. The primary clinicians were blinded to the measurements of the study and decision regarding extubation or re-institution of mechanical ventilation was left to their discretion. None of them used the rapid shallow breathing index for their decision-making. Weaning was considered successful if the patient could sustain spontaneous breathing without any form of ventilatory support for 24 hr and longer.5 Among the 30 patients studied, 23 were weaned successfully while weaning failed in seven patients due to several reasons. The threshold value of "7" for the modified index was fixed by a post hoc analysis. The accuracy of each index is shown in the Table.Go


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  		TABLE Accuracy of indices
 
The rapid shallow breathing index - the f/VT ratio is a very useful bedside criterion to predict weaning outcome. The modification of this index as the ratio of f to the VT corrected for patient’s weight was more accurate in predicting the outcome of weaning in our patients. This has to be further validated by a larger prospective study.

References

1 Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med 1991; 324: 1445–50.[Abstract]

2 Epstein SK. Etiology of extubation failure and the predictive value of the rapid shallow breathing index. Am J Respir Crit Care Med 1995; 152: 545–9.[Abstract]

3 Chatila W, Jacob B, Guaglionone D, Manthous CA. The unassisted respiratory rate-tidal volume ratio accurately predicts weaning outcome. Am J Med 1996; 101: 61–7.[Medline]

4 Vassilakopoulos T, Zakynthinos S, Roussos C. The tension-time index and the frequency/tidal volume ratio are the major pathophysiologic determinants of weaning failure and success. Am J Respir Crit Care Med 1998; 158: 378–85.[Abstract/Free Full Text]

5 Lessard MR, Brochard LJ. Weaning from ventilatory support. Clin Chest Med 1996; 17: 475–89.[Medline]





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