A Comparison of the Imposed Work of Breathing with Endotracheal and Tracheostomy Tubes in a Lung Model

Abstract

Background

Artificial airways have been shown to increase flow resistance and the work of breathing (WOB) as inside diameters decrease and inspiratory flow increases. We designed a study to evaluate the imposed work of breathing (WOB_I) in a lung model, using both endotracheal (ETT) and tracheostomy tubes (TT). We hypothesized that the shorter, more rigid TT might result in a lower WOB_I than that imposed by a thermolabile ETT of the same size.

Materials & Methods

A patient model was created using an intubation mannequin, simulated trachea, and two-chamber test lung. One side of the test lung was connected to a ventilator to simulate spontaneous breathing. The ventilator was set to deliver a V_r of 500 mL at flows of 0.5, 1.0, and 1.5 L/s. ETT with inside diameters (ID) of 6.0-, 7.0-, 8.0-, and 8.5-mm ID and TT with ID of 5.0-, 7.0-, 8.5-, and 9.0-mm ID were placed in the model. WOB_I was measured by a VenTrak respiratory monitor. A pneumotachograph and pressure tap were placed at the proximal airway and a second pressure tap was located below the distal tip of the artificial airway. WOB and peak negative pressure (PNP) were recorded from the average of 10 breaths at each flow setting.

Results

WOB_I and PNP varied inversely with ID and directly with inspiratory flow. With each increase in flow, both WOB_I and PNP increased (p < 0.01) for each tube. For equivalent ID, 7.0- and 8.5-mm ID ETTs had significantly higher PNP and WOB_I at 1.0 L/s (p < 0.05) and 1.5 L/s (p < 0.01) than did 7.0- and 8.5-mm TTs.

Conclusion

The WOB_I of artificial airways increases with decreasing ID, increasing length, and increased patient demand. For identical ID, the shorter, more rigid TT results in a lower WOB_I and PNP than equivalently sized ETT. Clinicians should be aware of the contribution of artificial airways to WOB_I and select the appropriate airway for patients.