Changes in respiratory mechanics after tracheostomy.

OBJECTIVE

To determine the effects of tracheostomy on respiratory mechanics and work of breathing (WOB).

DESIGN

A before-and-after trial of 20 patients undergoing tracheostomy for repeated extubation failure.

SETTING

Surgical intensive care unit at a university teaching hospital and a level I trauma center.

PATIENTS

A consecutive sample of 20 patients who met extubation criteria (Pa(O2), >55 mm Hg; pH >7.30; and respiratory rate, <30/min on room air continuous positive airway pressure after 20 minutes) but failed extubation on 2 occasions were eligible for the study.

INTERVENTIONS

Respiratory mechanics, lung volumes, and WOB were measured before and after tracheostomy.

MAIN OUTCOME MEASURES

Patients in whom extubation fails often progress to unassisted ventilation after tracheostomy. The study hypothesis was that tracheostomy would result in improved pulmonary function through changes in respiratory mechanics.

RESULTS

Data are given as means +/- SDs. After tracheostomy, WOB per liter of ventilation (0.97+/-0.32 vs. 0.81+/-0.46 J/L; P<.09), WOB per minute (8.9+/-2.9 vs. 6.6+/-1.4 J/min; P<.04), and airway resistance (9.4+/-4.1 vs. 6.3+/-4.5 cm H20/L per second; P<.07) were reduced compared with breathing via an endotracheal tube. These findings, however, do not fully explain the ability of patients to be liberated from mechanical ventilation after tracheostomy. In 4 patients who were extubated before tracheostomy, WOB was significantly greater during extubation than when breathing through an endotracheal or tracheostomy tube (1.2+/-0.19 vs. 0.81+/-0.24 vs. 0.77+/-0.22 J/L).

CONCLUSIONS

We believe that the rigid nature of the tracheostomy tube represents reduced imposed WOB compared with the longer, thermoliable endotracheal tube. The clinical significance of this effect is small, although as respiratory rate increases, the effects are magnified. In patients in whom extubation failed, WOB may be elevated because of incomplete control of the upper airway. Future studies should evaluate the cause of increased WOB after extubation.