How Much Does the ETT Add to Breathing Work?
ABSTRACT & COMMENTARY
Synopsis: In successfully extubated patients, work of breathing at the beginning and end of a spontaneous breathing T-piece trial is equal to the work of breathing immediately after extubation.
Source: Straus C, et al. Am J Respir Crit Care Med 1998;157:23-30.
Total work of breathing, inspiratory pressure-time-product, transpulmonary elastic, and resistive work of breathing were evaluated in 14 successfully extubated patients (5 with COPD), before and after a two hour spontaneous breathing trial using a T-piece, and again immediately after extubation. Endotracheal tube (ETT) size varied from 7 to 9 and the average length of mechanical ventilation was seven and a half days.
There were no significant differences in any evaluated variable except tidal volume, which increased from 464 ± 168 (beginning T-piece) to 492 ± 239 mL (post-extubation, p < 0.04). Total work of breathing was 1.57 ± 0.52 (beginning T-piece), 1.72 ± 0.59 (end T-piece), and 1.63 ± 0.45 joules/L (post-extubation). Transpulmonary elastic work of breathing tended to non-significantly increase post extubation: 0.85 ± 0.27 (beginning) vs. 0.92 ± 0.33 (end) vs. 1.00 ± 0.31 J/L (post extubation), while transpulmonary resistive work tended to non-significantly decrease (0.65 ± 0.31 beginning vs 0.70 ± 0.29 end vs 0.59 ± 0.26 post extubation). The work of breathing dissipated against the ETT was 11.0 ± 3.9% of the total work, and the work of breathing dissipated in the supraglottic area was 7.0 ± 4.3% of the total work of breathing. Straus and colleagues speculate that if glottic work could have been determined post extubation, the sum of supraglottic and glottic work would have equaled the work of the ETT.
COMMENT BY ROBERT M. KACMAREK, PhD, RRT
This study adds more support to the use of simple T-piece trials as an effective method for identifying patients' ability to breathe spontaneously following extubation. Specifically, Straus et al clearly show that in successfully extubated patients, removal of the endotracheal tube does not alter the patient's work of breathing. It should be noted that this study was performed by the group of investigators headed by Laurent Brochard, a long-time advocate of pressure support weaning. Straus et al go on to say that, "the use of pressure support ventilation at the end of the weaning period is probably needed mainly to compensate for the resistance and dead space of the circuity of the ventilator and not the endotracheal tube," as proposed by many (Brochard ARRD 1989;139:513). However, the work imposed by the newer ventilators, when set to flow, have minimal triggering.
This study only included patients who were successfully extubated, and did not address patients failing extubation. Straus et al comment on two patients who failed extubation in whom measurements were made. In both of these patients, work of breathing nearly doubled after extubation. However, none of the increase was due to increased supraglottic work.
These findings are similar to those of Nathan et al (Chest 1993;103:1215) and Ishaaya et al (Chest 1995; 107:204), in which work of breathing increased post extubation. Regardless of whether work of breathing stays the same or increases, these findings argue against the routine use of even low levels of pressure support during T-piece trials, since the use of pressure support would underestimate a patient's ability to breathe spontaneously. However, based on recent data by Esteban et al (Am J Respir Crit Care Med 1997;156:459-465) from a prospective randomized multicenter trial, the use of 7 cm H2O pressure support during a two-hour spontaneous breathing trial may be indicated in some patients.
The final word on what is "the best weaning or spontaneous breathing trial" is not yet available. However, I would bet my money on spontaneous breathing trials using T-pieces for the vast majority of patients. What we are not sure of is which patients will benefit from the use of low-level pressure support (for example, 5-7 cm H2O) and/or CPAP (e.g., 5 cm H2O) during these trials, and how short a trial (my bet is about 30 min) will successfully identify patients who are ready for extubation.