Comparison of inspiratory work of breathing between flow-triggered and pressure-triggered demand flow systems in rabbits.

OBJECTIVES

Flow-triggered continuous positive airway pressure decreases the inspiratory work of breathing in adults when compared with pressure-triggered continuous positive airway pressure. However, the effect of flow-triggered continuous positive airway pressure on work of breathing in neonates is not known. Our objective was to determine if flow-triggering was superior to pressure triggering in the presence of narrow endotracheal tubes, such as those tubes used in neonates.

DESIGN

Prospective evaluation using within-animal comparison of flow-triggering and pressure-triggering demand flow systems.

SETTING

The animal laboratory in a university hospital.

SUBJECTS

Six spontaneously breathing white rabbits, tracheostomized and intubated with 3- and 4-mm inner diameter endotracheal tubes.

INTERVENTIONS

The animals were connected to a ventilator through a standard respiratory circuit. The ventilator was randomly operated in the following modes: flow-triggered continuous positive airway pressure, pressure-triggered continuous positive airway pressure, flow-triggered with 5 cm H2O pressure support ventilation, and pressure-triggered with 5 cm H2O pressure support ventilation.

MEASUREMENTS AND MAIN RESULTS

Esophageal pressure, airway pressure, and flow signals were monitored. Control data were obtained while the rabbits were breathing room air through the endotracheal tube. With 3-mm inner diameter endotracheal tubes, the negative deflection of esophageal pressure during flow-triggered continuous positive airway pressure was significantly less than control; however, negative deflection of esophageal pressure during pressure-triggered continuous positive airway pressure did not significantly differ from control. The application of 5 cm H2O pressure support ventilation with flow-triggering decreased negative deflection of esophageal pressure significantly compared with flow-triggered continuous positive airway pressure, pressure-triggered continuous positive airway pressure, and control. With endotracheal tube inner diameter of 4 mm, flow-triggered continuous positive airway pressure and pressure-triggered continuous positive airway pressure did not show any differences compared to control. Negative deflection of esophageal pressure differed under all conditions except control when results with the 3-mm inner diameter endotracheal tube were compared with the 4-mm inner diameter endotracheal tube.

CONCLUSIONS

Flow-triggering is superior to pressure-triggering in the presence of a 3-mm inner diameter endotracheal tube. This difference was not clear with a 4-mm inner diameter endotracheal tube. The size of the endotracheal tube may be the most important variable in evaluating the approach used to ventilate small neonates.