Abstract & Commentary
Endotracheal Tube Size Affects Weaning Predictors
By David J. Pierson, MD, Editor, Professor, Pulmonary and Critical Care Medicine, Harborview Medical Center, University of Washington, Seattle, is Editor for Critical Care Alert.
Synopsis: In this study of physiologic variables during spontaneous breathing in patients who had been weaned and were about to be extubated, those with smaller-diameter endotracheal tubes breathed more rapidly and shallowly than those with larger tubes. If these "weaning parameters" had been used to decide whether to extubate the patients, most of those with smaller tubes would not have qualified.
Source: Mehta S, et al. Impact of endotracheal tube size on preextubation respiratory variables. J Crit Care 2010; Epub ahead of print; PMID: 20079602.
In this secondary analysis of data from a study on the pre-extubation prediction of post-extubation work of breathing in patients recovering from acute respiratory failure,1 Mehta and colleagues sought to determine whether the size of a patient's endotracheal tube (ETT) affected respiratory rate, tidal volume, and other "weaning parameters" commonly measured during spontaneous breathing and used to predict successful extubation. They studied 22 mainly elderly medical ICU patients admitted with a variety of causes for acute respiratory failure. The patients, all of whom had been ventilated for more than 24 hours, had been successfully weaned from ventilatory support, were awake with adequate spontaneous secretion clearance, and were judged by their physicians to be ready for extubation.
The investigators used an esophageal balloon, and a miniature pneumotachograph/pressure transducer placed just proximal to the ETT, to measure the pressure and flow variables of interest. Separately and in random order, for 15 minutes each, the patients breathed spontaneously on pressure support (PS; 5 cm H2O), continuous positive airway pressure (CPAP; 5 cm H2O), and a T-piece circuit without positive inspiratory or expiratory pressure. After 15 minutes, the following were measured and/or calculated: respiratory rate, tidal volume, f/VT (rapid shallow breathing index), airway occlusion pressure, pressure-time product, and work of breathing. Following these measurements, the patients were extubated, and the same variables were assessed 15 and 60 minutes later by means of a tight-fitting mask.
Twenty patients completed the study, of whom 8 were intubated with 7.0 or 7.5 mm inner diameter ETTs and 12 had 8.0 mm tubes. When patients with the two smaller ETT sizes, considered together, were compared to the others, the spontaneous respiratory rate was significantly higher with the smaller tubes under all three breathing conditions while intubated and also 15 minutes after extubation. Tidal volumes were lower and pressure-time products higher on PS and CPAP in the patients with smaller ETTs. Although the differences were nonsignificant, the negative change in esophageal pressure, airway occlusion pressure, and work of breathing all tended to be higher with the smaller ETTs. However, during T-piece breathing, differences in f/VT according to ETT size were both statistically and clinically significant: mean f/VT was 69 ± 35 breaths/L in the patients with 8.0 mm ETTs, as compared to 122 ± 57 breaths/L in those with the smaller tubes (P < 0.05).
Commentary
Patients in this study who were intubated with smaller ETTs breathed more rapidly and shallowly than those with larger tubes. The f/VT, for which a value of 105 breaths/L is recommended as a cutoff for a higher likelihood of extubation failure, was well below this threshold in patients with an 8.0 mm ETT but above it in those with 7.0 and 7.5 mm tubes. Thus, in this group of patients who had already shown readiness for weaning and who were successfully extubated during the study, the mean value for f/VT in those with the smaller tubes would have led to interruption of the process if used as an indicator of extubation readiness.
A major defect in this study is the uneven distribution of ETT size with respect to gender. All the patients with 7.0 or 7.5 mm tubes were women, and all those with 8.0 mm tubes except two were men. Women tend to be smaller than men. In the absence of factors predicting a difficult intubation, clinicians may select smaller ETTs for smaller patients and larger ones for larger patients. No information is given as to the body sizes of the patients in this study, and thus whether ETT size was scaled to body size cannot be determined.
As pointed out by the authors, healthy non-intubated women breathe about 4 breaths/min faster than do their male counterparts, with tidal volumes averaging 200 mL less, even when corrected for body surface area. The more rapid respiratory rates, smaller tidal volumes, and higher f/VT values found in the (female) patients with 7.0 and 7.5 mm ETTs, as compared with the (nearly all male) patients with 8.0 mm ETTs, are consistent with differences between the sexes among intubated patients as documented by Epstein et al to be independent of ETT size.2 However, I agree with the authors' contention that, because none of the measured variables were significantly different between the patient groups 60 minutes following extubation, gender differences probably do not completely explain the findings.
It is probably safe to conclude that smaller ETTs do tend to make patients breathe faster during brief trials without ventilatory support and also that if an arbitrary f/VT threshold is applied some patients with smaller tubes may "fail" whereas the same patients would "pass" if their tubes were larger. This conclusion illustrates the fallacy inherent in using "weaning parameters" to decide when a patient is ready to attempt weaning from ventilatory support. In fact, publication of this study in 2010 is an interesting reminder of the sea change that has occurred in weaning during the dozen years since its data were collected.
A comprehensive review of all the published studies on predicting readiness for weaning3 concluded that no single "parameter" or combination of variables could reliably predict when a patient could safely be liberated from mechanical ventilation. This is because, once they have clinically improved and are generally stable, many patients can be weaned and extubated despite being considered "unready" by their physicians. Documentation of this fact by numerous studies was a main reason that the 2001 international weaning guidelines3 called for "checking" to see whether a patient is ready by means of a spontaneous breathing trial (SBT) instead of trying to predict when weaning should start using the f/VT or some other assessment prior to an SBT. At the institution in which I practice, routine assessment of f/VT and other "weaning parameters" prior to an SBT was discontinued a decade ago; now the respiratory therapists use these measurements primarily to sort out the physiologic reasons for a "failed" SBT.
As with other health care interventions, implementation of evidence-based practice weaning guidelines has been incomplete in many settings. "Weaning parameters" are still widely used in deciding when an SBT should be done. This study should serve as a reminder that this approach is likely to relegate some patients to continued invasive mechanical ventilation who could be immediately weaned and extubated, thus reducing their risk for ventilator-associated pneumonia and other complications, and also shortening their ICU stay.
References
- Mehta S, et al. Prediction of post-extubation work of breathing. Crit Care Med 2000;28: 1341-1346.
- Epstein SK, Ciubotaru RL. Influence of gender and endotracheal tube size on preextubation breathing pattern. Am J Respir Crit Care Med 1996;154(6 pt 1):1647-1652.
- MacIntyre NR, et al; American College of Chest Physicians; American Association for Respiratory Care; American College of Critical Care Medicine. Evidence-based guidelines for weaning and discontinuing ventilatory support: A collective task force facilitated by the American College of Chest Physicians; the American Association for Respiratory Care; and the American College of Critical Care Medicine. Chest 2001;120(6 Suppl):375S-395S.
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