Current issues of ACP Journal Club are published in Annals of Internal Medicine


Diagnosis

The absence of shallow breathing was the best predictor of successful weaning from mechanical ventilation

ACP J Club. 1991 Sept-Oct;115:53. doi:10.7326/ACPJC-1991-115-2-053


Source Citation

Yang KL, Tobin MJ. A prospective study of indexes predicting the outcome of trials of weaning from mechanical ventilation. N Engl J Med. 1991 May 23;324:1445-50.


Abstract

Objective

To assess predictors of success and failure in weaning patients from mechanical ventilation, including 2 new indexes.

Design

Blinded comparison of predictors for weaning from mechanical ventilation.

Setting

3 university-affiliated intensive care units.

Patients

100 medical patients who were ventilated for a mean of 8.2 days and judged to be clinically stable enough for a weaning trial.

Description of the tests

Sensitivity and specificity were calculated for the ratio of respiratory frequency to tidal volume (f/VT and for CROP, an integrated measure (thoracic compliance, respiratory rate, arterial oxygenation, and maximal inspiratory pressure [ PImax]), as well as for traditional predictors, in 36 intubated patients for whom independent decisions on weaning were made. The derived prediction values were applied to a validation sample of 64 patients. Respiratory frequency (f), VE, PImax, thoracic compliance, and oxygenation index were the traditional weaning tests done. 2 new indexes were evaluated: rapid shallow breathing (f/VT) and CROP. Successful weaning was defined as the ability to sustain spontaneous breathing for ≥ 24 hours. Criteria for weaning failure were either objective (based on arterial blood gases) or subjective (based on clinical status).

Main outcome measures

Sensitivity and specificity.

Main results

In 60 patients weaning was a success and in 40 patients it failed. Successful wearing was predicted with sensitivity, specificity, and the likelihood ratio of a positive and negative test result for PI max of 100%, 11%, {1.12, and 0}*; for f/VT of 97%, 64%, {2.69, and 0.05}*; for VT of 97%, 54%, {2.11, and 0.06}*, for CROP of 81%, 57%, {1.88, and 0.33}*; and for VE of 78%, 18%, {0.95, and 1.22}*, respectively. Areas under the receiver operating characteristic curve, an integrated measure of sensitivity and specificity, were greatest for f/VT (0.89), VT (0.87), CROP (0.78), and f (0.76). The area under the receiver operating characteristic curve for the f/VT ratio (0.89) was larger than that under the curves for CROP (0.78, P < 0.05), PI max (0.61, P < 0.001), and VE (0.40, P < 0.001).

Conclusion

Although PI max was the most sensitive predictor of success in weaning patients from mechanical ventilation; f/VT ratio and VT were more accurate in predicting success or failure.

Sources of funding: In part, the American Lung Association and the Veterans Affairs Research Service.

Address for article reprint: Dr. K.L. Yang, Division of Pulmonary and Critical Care Medicine, the University of Texas Health Science Center at Houston, 6431 Fannin, Suite 1.274, Houston, TX 77030, USA.


Commentary

The value of an index for "weaning" from mechanical ventilation is its ability to predict respiratory endurance (1). Endurance reflects the ability of the respiratory capacity to meet the respiratory demands of resistive load from airways or endotracheal tube resistance, and elastic load from conditions that decrease compliance (e.g., fibrosis). Of the new indices that the authors present, the average VT and the ratio of respiratory frequency to tidal volume (f/VT) determined during a 1-minute bedside trial of spontaneous ventilation, are simple and warrant widespread consideration. 24 hours is a reasonable period of time to define successful liberation from mechanical ventilation but failures that occur shortly thereafter, often in less closely monitored environments, can have serious consequences.

The f/VT index was the best predictor of successful weaning in this study. The usefulness of f/VT, however, should not eliminate careful systematic attention to other measurements that correlate closely with reversible physiologic abnormalities, such as bronchospasm and weakness associated with electrolyte abnormalities (2). The decreased predictive accuracy of indices in this study for patients requiring longer periods of ventilation, underscores the need for continued attention to these other clinical and laboratory parameters, particularly in patients who require a more gradual process of "weaning" (1).

The other index introduced (CROP) integrates measurement of respiratory load, strength, and gas exchange. The predictive value of this index is not as great as that of f/VT and its complexity makes it less attractive for bedside use in the intensive care unit.

Kenneth W. Presberg, MD
Medical College of WisconsinMilwaukee, Wisconsin, USA


References

1. Marini JJ. Weaning from mechanical ventilation. N Engl J Med. 1991;324:1496-8.

2. Hall JB, Wood LDH. Liberation of the patient from mechanical ventilation. JAMA. 1987;257:1621-8.


Update

Other studies of f/VT have restricted themselves to patients who, after a weaning process were considered ready for extubation (3-5). Some patients required reintubation. The studies did not find f/VTuseful for discriminating between those who fail extubation and those who do not.

Deborah J. Cook, MD MSC
McMaster UniversityHamilton, Ontario, Canada

3. Lee KH, Hul KP, Chan, TB, Tan WC, Lim TK. Rapid shallow breathing (frequency-tidal volume ratio) did not predict extubation outcome. Chest. 1994;105:540-3.

4. Eqstein SK. Etiology of extubation failure and the predictive value of the rapid shallow breathing index. Am J Resp Crit Care Med. 1995;152:545-9.

5. Capdevila XJ, Perrigault PF, Perey PJ, Roustan JP, d'Athis F. Occlusion pressure and its ratio to maximum inspiratory pressure are useful predictors for successful extubation following t-piece weaning trial. Chest. 1995;108:482-9.