Tracheostomy in the critically ill: the myth of dead space

Benefits and advantages of tracheostomy have been vigorously debated. There is a lack of consensus as to whether perceived clinical improvement is attributable to fundamental changes in respiratory dynamics. We compare the effect of tracheostomy versus endotracheal tube on dead space, airway resistance and other lung parameters in critically ill ventilated patients. Data collected included patients who were admitted to surgical, burn and neurosurgical intensive care units at the University of North Carolina. Twenty-four intubated patients were included in our analysis with various aetiologies of respiratory failure. Tracheostomy was deemed necessary either for severe neurological devastation or failure to wean from the ventilator. The diameter of the endotracheal tubes ranged from 6-8 mm and the tracheostomy tube diameters were from 6.4-8.9 mm. Internal diameters between endotracheal tube and tracheostomy tubes, ventilator settings and sedation were kept consistent throughout the study. Respiratory parameters were measured using the Respironics' non-invasive cardiac output 2 device (Phillips, Andover, MA) immediately prior to tracheostomy and repeated within 24 hours of tracheostomy. Only two (8%) of the patients had slight improvement (>6% decrease in dead space). The average dead space of endotracheal versus tracheostomy tubes was 41±12.6% and 40±14.6%, respectively (P=0.75). The remaining 22 patients (92%) had no significant change in dead space, compliance or other respiratory parameters. This study shows that there is no significant difference in respiratory mechanics and dead space with a tracheostomy versus endotracheal tube.