Why Does Tracheostomy Facilitate Weaning? Abstract & Commentary
February 1, 2002 4 minutes read
Why Does Tracheostomy Facilitate Weaning? Abstract & Commentary
Mohr and associates prospectively studied all patients in their surgical ICU who had difficulty weaning from ventilatory support and who underwent bedside percutaneous dilational tracheostomy during an 8-month period. Within 24 hours of performance of tracheostomy, before and after the procedure, they determined physiologic dead space (VdVt) from single-breath capnograms generated by the CO2SMO Plus bedside ventilation monitor (Novametrix Medical Systems, Inc), and they also recorded respiratory rate and tidal volume, minute ventilation, peak inspiratory and end-expiratory airway pressures, and arterial blood gases. Mohr et al also recorded demographic, diagnostic, and weaning outcome data on the patients.
During the study period, 42 patients underwent tracheostomy and survived at least 3 days. They had a variety of surgical illnesses, including isolated closed head injury (16 patients), vehicular and other trauma (13), general surgical procedures (10), abdominal aortic aneurysm repair (3), and coronary artery bypass grafting (3). Twenty-four patients met international consensus criteria for acute lung injury, and 6 had acute respiratory distress syndrome. Twenty-one patients were removed from ventilatory support within 3 days of tracheostomy, and in 21 weaning took 5 or more days.
There were no differences in any physiologic study variable before and after tracheostomy. Mean VdVt was 0.51 ± 0.10 before the procedure and 0.52 ± 0.11 afterward. Minute ventilation (10.2 L/min), respiratory rate (21 breaths/min), and tidal volume (445 mL) remained the same, as did peak inspiratory pressure, arterial pH, PCO2, PO2, and the ratio PO2/FIO2. When patients who weaned within 3 days of tracheostomy were compared with those who required more than 5 days, there were also no differences with respect to these variables. Mohr et al conclude that tracheostomy must facilitate weaning from ventilatory support by some mechanism other than improvement in ventilatory mechanics and gas exchange. (Mohr AM, et al. The role of dead space ventilation in predicting outcome of successful weaning from mechanical ventilation. J Trauma. 2001;51:843-848.)
Comment by David J. Pierson, MD
Compared with the normal upper airway, a tracheostomy tube has a substantially smaller volume and thus a reduced dead space. However, when patients fail to wean from ventilatory support, the issue is not one of the dead space of the normal unintubated airway vs. the tracheostomy tube, but rather the latter vs. an endotracheal tube that is already in place. In a bench study, Davis and colleagues1 demonstrated that the actual difference in dead space with endotracheal tubes vs. tracheostomy tubes was only about 10 mL. Thus, it would have been most surprising if the present study had discovered any differences in their in vivo measurements before and after tracheostomy.
So why does tracheostomy facilitate weaning? Actually, it may not help if management decisions are made according to protocol rather than left up to the individual physician. In a multicenter, randomized, controlled trial of early vs. delayed tracheostomy2 in which attempts were made to standardize the criteria for weaning and extubation, there were no differences in weaning time with respect to the timing of tracheostomy. Yet, Mohr et al acknowledge "the frequent observation that patients in whom extubation repeatedly fails are quickly weaned from mechanical ventilation after tracheostomy." After many years of making ventilator rounds in a surgical intensive care unit not unlike that of Mohr et al, I can wholeheartedly agree with that observation.
If tracheostomy does not change dead space or other aspects of ventilatory mechanics, and a controlled trial with rigid criteria for weaning and extubation showed no difference in weaning time with early vs. late tracheostomy, what accounts for the fact that the latter seems to hasten weaning in everyday practice? I believe the answer is contained in the quotation in the preceding paragraph: patients in whom extubation repeatedly fails are quickly weaned after tracheostomy. Weaning deals with 1 question ("Can the patient breathe without assistance?"), whereas extubation actually deals with 4 questions ("Can the patient breathe, and also maintain a patent upper airway, and avoid aspiration, and adequately clear airway secretions?"). Clinicians cannot seem to think of these issues separately. Neither, in fact, can investigators and the authors of review articles about weaning, who often mix up weaning and extubation even in the same paper.3
When dealing with an intubated patient, we tend to define weaning success operationally as "successful extubation." The patient must demonstrate the ability to ventilate spontaneously and also to protect the airway and clear secretions. Clinicians often dispense with prolonged trials of spontaneous, unassisted breathing, and extubate the patient directly from at least partial ventilatory support. If things do not go well, not only does mechanical ventilation have to be reinstituted, but an artificial airway has to be put back in as well. The situation is very different with a tracheostomy. If the patient develops respiratory distress when positive-pressure ventilation is taken away, it can simply be resumed. Ventilatory support can be taken off and put back multiple times with no problem. Whether and when to remove the tracheostomy tube can be dealt with later. For patients who fail "weaning" that actually consists of both weaning and extubation at the same time, what may be needed is an airway, not a ventilator.
I am convinced that in most cases patients wean promptly following tracheostomy because of a change in clinician behavior rather than because of any change in the patient. The results of this study are certainly consistent with this statement.
Dr. Pierson, MD, Professor of Medicine, University of Washington, Medical Director, Respiratory Care, Harborview Medical Center, Seattle, is Editor of Critical Care Alert.
References
- Davis K Jr, Branson RD, Porembka D. A comparison of the imposed work of breathing with endotracheal and tracheostomy tubes in a lung model. Respir Care. 1994;39:611-616.
- Sugarman HJ, et al. Multicenter, randomized, prospective trial of early tracheostomy. J Trauma. 1997; 43:741-747.
- Pierson DJ. Weaning from mechanical ventilation: Why all the confusion? Respir Care. 1995;40:228-232.
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