Effects of Tracheotomy on Work of Breathing
ABSTRACT & COMMENTARY
Synopsis: Both work of breathing and intrinsic PEEP in patients on pressure support were significantly reduced when their endotracheal tubes were replaced by tracheostomy tubes of the same inner diameter. Studies of resistive work of breathing through the endotracheal tubes removed from the patients showed that it was measurably higher than through unused tubes of the same size, suggesting a subtle buildup of secretions on the tubes’ inner walls.
Source: Diehl J-L, et al. Am J Respir Crit Care Med 1999;159:383-388.
Diehl and colleagues at the university of Paris measured work of breathing (WOB) in eight patients before and after they underwent tracheotomy because of the anticipated need for prolonged mechanical ventilation. The patients had a variety of medical illnesses; all were unable to be weaned completely from ventilatory support after a mean of 31 days but all were able to breathe spontaneously on pressure support (PS) ventilation. The inner diameter of the tracheostomy tube was matched in each instance to that of the previous endotracheal tube. Both on the day before and six hours after tracheotomy, Diehl et al measured the patients’ WOB on their baseline PS settings (which ranged from 8-20 cm H2O) and again at both 5 cm H2O less (PS-5) and 5 cm H2O more (PS+5) than this initial inspiratory pressure level. WOB was calculated from esophageal pressure-tidal volume loops as both WOB per liter and the power of breathing (WOB/min).
At the patients’ baseline PS levels, WOB fell from 0.9 ± 0.4 to 0.4 ± 0.2 J/L after tracheotomy, a statistically significant difference. This was associated with a drop in intrinsic positive end-inspiratory pressure (auto-PEEP) from 4.0 ± 1.6 to 0.8 ± 1.0 cm H2O (P < 0.05). Both the initial and post-tracheotomy values for WOB and auto-PEEP were greater while the patients breathed at PS-5, and less while they were on PS+5, but the changes (all significant) were all in the same direction as in the baseline measurements. The pressure-time index of the respiratory muscles at the patients’ baseline PS levels also decreased after tracheotomy, from 181 ± 92 to 80 ± 56 cm H2O.s/min (P < 0.05). Three of the patients had ineffective breathing efforts prior to tracheotomy and all had improved ventilator synchrony after the procedure.
Diehl et al also made in vitro measurements using the endotracheal tubes removed from the patients in comparison with new endotracheal tubes of the same size and with the tracheostomy tubes used in the patients. At the baseline PS levels for the patients, the mean resistive WOB was 0.46 J/L with the patients’ endotracheal tubes, 0.33 J/L with the new endotracheal tubes, and 0.26 J/L with the tracheostomy tubes. Mean percentage reduction in resistive WOB with tracheostomy tubes was 21% vs. new endotracheal tubes, and 43% vs. the removed endotracheal tubes at the same PS level. The differences in WOB between the new and removed endotracheal tubes of the same manufactured inner diameter were accounted for by deposition of small but significant amounts of secretions on the walls of the latter.
COMMENT BY DAVID J. PIERSON, MD
This study provides physiologic documentation of the benefit of tracheotomy with respect to WOB, and also shows that the latter can be affected measurably by insidious obstruction of endotracheal tubes during their clinical use. Tracheotomy thus reduces WOB in three ways. The effective radius of a tube exerts an effect on resistance to airflow that is inversely proportional to its fourth power, so that small changes in this effective radius can have a great effect on resistance and WOB. Because resistance is also directly proportional to the length of a tube, the shorter tracheostomy tube has less total resistance even if its diameter is unchanged. And, because auto-PEEP is reduced by tracheotomy, the WOB related to overcoming this pressure in initiating breaths from the ventilator (or breathing spontaneously during a T-piece trial) is also reduced accordingly.
Tracheotomy clearly tends to facilitate weaning in difficult-to-wean patients. I have always thought that this was mainly because of its effects on the clinician’s thinking rather than on the patient’s physiology. Because for many of us it tends to be hard to think of discontinuation of positive-pressure ventilation (that is, weaning) and removal of the endotracheal tube (extubation) as separate clinical steps, uncertainty about whether a patient can breathe without assistance tends to prolong the duration of mechanical ventilation, since, if the answer is "no," the patient may have to be reintubated. In contrast, with a tracheostomy it is a simple matter to wean off the positive-pressure breaths and then to put them back again if the patient does not tolerate spontaneous breathing.
That there are physiologic as well as psychologic reasons for easier weaning with a tracheostomy is nicely shown by the findings of Diehl et al in this study. The differences in WOB and auto-PEEP before and after tracheotomy are not great in magnitude, but they may be sufficient to make a difference clinically in the frail, hard-to-wean patient.
The decrease in effective diameter of an endotracheal tube after a period of clinical use (also shown by other investigators: Wright et al. Am Rev Respir Dis 1989;140:10-16; Villafane MC, et al. Anesthesiology 1996;85:1341-1349) is troubling. There has been a tendency in many centers to extend the period of endotracheal intubation and to avoid tracheotomy for as long as possible if there was a reasonable hope that the patient could eventually be weaned and extubated. Seldom do we think of changing the endotracheal tube in such cases, and it is often left in place, as in this study, for many weeks. However, if deposits on the tube’s inner walls gradually increase resistance to flow and thus WOB for the patient, this practice may need to be reconsidered.