Effect of inner cannula removal on the work of breathing imposed by tracheostomy tubes: a bench study

Tracheotomy has been used to assist in weaning patients from mechanical ventilation. Some patients fail to be weaned from the ventilator despite tracheostomy. We hypothesized that removing the inner cannula from the tracheostomy tube would decrease the tube's imposed work of breathing (WOB(IMP)).

The hypothesis was tested using a lung model, by measuring the change in WOB(IMP) when the inner cannula was removed. A mechanical lung model was developed using a test lung to simulate a spontaneously breathing patient. WOB(IMP) was measured with a commercially available lung mechanics monitor. Shiley size 6, 8, and 10 nonfenestrated tracheostomy tubes were tested with the inner cannula in and out. Breathing conditions were simulated using tidal volumes (V(T)) of 300 and 500 mL matched with breathing frequencies of 12, 24, and 32 breaths per minute, by using a ventilator to simulate spontaneous breathing through one side of the test lung.

Under all the tested breathing conditions, WOB(IMP) for each of the 3 tracheostomy tubes was significantly reduced (p < 0.05) when the inner cannula was removed. Also, as simulated spontaneous inspiratory flow demand increased (ie, as V(T) and/or frequency were increased), WOB(IMP) also increased, and vice versa. With the cannula removed, WOB(IMP) was not significantly different between the size 6 and 8 tubes nor between the size 8 and 10 tubes when V(T) was 300 mL and frequency was 12 breaths per minute.

There was a significant decrease in WOB(IMP) with each tube when the inner cannula was removed. WOB(IMP) increased with an increase in inspiratory flow demand (ie, increase in V(T) and/or frequency), as well as when tube size decreased. In weaning a tracheostomized patient from mechanical ventilation, increasing the internal diameter of the tube by removing the inner cannula may be beneficial. Further study is needed to determine if these findings are clinically important.