Correspondence

Duration of Mechanical Ventilation

N Engl J Med 1997; 336:1610-1611May 29, 1997

Article

To the Editor:

Ely et al. (Dec. 19 issue)1 advanced our understanding of the process of weaning patients from mechanical ventilation. How did the daily screening protocol influence decisions about weaning and extubation in the control group? The extent of the attending physicians' understanding of the study design and their knowledge of the random assignments should have been defined. Some awareness of the study would inevitably occur, given the oral and written communication between study personnel and attending physicians and the finite number (or number of groups) of intensivists, pulmonologists, and cardiologists. The lack of a positive communication (e.g., a statement that the patient had passed the daily screening and weaning trial) may have conditioned some attending physicians to delay weaning or forgo extubation. Because the majority of patients for whom communication did not take place would have been controls (the remainder being intervention-group patients who failed the screening), the outcome for controls might have been more adversely affected.

Scott K. Epstein, M.D.
Tufts University School of Medicine, Boston, MA 02111

1 References

1. 1

   Ely EW, Baker AM, Dunagan DP, et al. Effect on the duration of mechanical ventilation of identifying patients capable of breathing spontaneously. N Engl J Med 1996;335:1864-1869
   Full Text | Web of Science | Medline

To the Editor:

What were the existing practices of the intensive care units during the trial period, and how were the units organized? Who was managing the ventilators? What was the structure of the units (open vs. closed)? What approaches to ventilation were in use? The answers to these questions would make the implications of this interesting paper clearer. For example, some approaches meant to accelerate weaning from the ventilator (e.g., intermittent mandatory ventilation) do not do so.1 If the control patients were managed by some system of progressive reduction of support (intermittent mandatory ventilation or pressure-support ventilation with orders for progressive reduction in the amount of support provided) that was followed by rote and did not correspond to the actual ability of the patients to breathe spontaneously, this approach could artificially extend the period of ventilation.

William W. Merrill, M.D.
Tulane University School of Medicine, New Orleans, LA 70112

1 References

1. 1

   Schachter EN, Tucker D, Beck GJ. Does intermittent mandatory ventilation accelerate weaning? JAMA 1981;246:1210-1214
   CrossRef | Web of Science | Medline

To the Editor:

The respiratory status of the patients in the intervention and control groups may have been different. The difference may not have been detected because inspiratory force was not measured and endurance was not tested by trials of T-tube circuits in the control group. Although the ratio of the respiratory frequency to the tidal volume (RR:V_t), a good predictor of weaning failure,1 was measured, it is a poor indicator of endurance. A good rapid shallow-breathing index (RR:V_t) obtained in a rested patient may markedly deteriorate within minutes of spontaneous breathing. Furthermore, although the number of patients who did not exceed a shallow-breathing index of 105 was similar in the two groups, the mean values of this index may have been significantly different in the two groups, though still remaining below 105. It is possible that patients in the control group were markedly weaker than those in the intervention group and that their weakness caused the delay in extubation.

In many instances, delays in “liberating” patients from the ventilator may be due not to the lack of weaning protocols but to the fact that the care of critically ill patients and the decision to remove them from ventilator support do not rest with primary intensive care teams, either because of their lack of power or because they do not exist at many institutions. Weaning protocols may yield good results, but they are not superior to the bedside clinical judgment of experienced intensivists.2

Luis Teba, M.D.
West Virginia University School of Medicine, Morgantown, WV 26506-9166

2 References

1. 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;324:1445-1450
   Full Text | Web of Science | Medline

2. 2

   Leitch EA, Moran JL, Grealy B. Weaning and extubation in the intensive care unit: clinical or index-driven approach? Intensive Care Med 1996;22:752-759
   CrossRef | Web of Science | Medline

Author/Editor Response

The authors reply:

To the Editor: Dr. Teba suggests that patients in the control group may have been weaker than those in the intervention group, delaying extubation in the former. The prospective randomized design, the similar proportions of patients who passed daily screening tests in both groups, and the similar distributions of causes of respiratory failure suggest that such differences would be unlikely. Moreover, scores on the Acute Physiology and Chronic Health Evaluation II and acute-lung-injury scores were significantly higher for the patients in the intervention group.

Drs. Teba and Merrill question the potential effect attending physicians may have on outcomes. The decision to discontinue ventilatory support rested with the physicians in the intensive care units. One of eight pulmonologists or five cardiologists attended each of these patients; thus, our simple intervention was associated with better outcomes than the bedside clinical judgments of experienced intensivists. Their approaches to mechanical ventilation were remarkably similar in the intervention and control groups and were used in a nonstandardized manner. There was no predetermined schedule according to which ventilatory support was reduced; rather, physicians used their judgment to determine the best approach for each patient. The objective protocol enhanced this subjective, individualized approach.

Dr. Epstein asks about the level of awareness of the study and its potential influences on treatment. Because managing physicians were aware of the study design, such bias cannot be excluded absolutely. The random assignments were not communicated to attending physicians or other members of the managing team. The number of attending physicians and the frequencies of their rotations (every two to four weeks) might lessen the likelihood of systematic bias and possible conditioning effects of prompts (and their absence). We believe that “spillover” of study practice (i.e., the use of daily screening and spontaneous-breathing trials) would have reduced the magnitude of the differences observed. We share Dr. Epstein's concern regarding the effects of failed weaning trials on future weaning attempts and the likelihood that a subgroup of patients with tachypnea can be extubated successfully. Better identification of such patients is essential.

E. Wesley Ely, M.D., M.P.H.
Edward F. Haponik, M.D.
Bowman Gray School of Medicine, Winston-Salem, NC 27157-4649