Imposed work of breathing and methods of triggering a demand-flow, continuous positive airway pressure system.

OBJECTIVES

To compare the inspiratory imposed work of breathing during spontaneous ventilation with continuous positive airway pressure using three methods of triggering "ON" the demand-flow system of a ventilator: a) conventional pressure triggering with the pressure measuring/triggering site inside the ventilator on the exhalation limb of the breathing circuit; b) tracheal pressure triggering from the tracheal or carinal end of the endotracheal tube; and c) flow-by (flow triggered) triggering.

DESIGN

Multitrial tests under simulated clinical conditions using a mechanical lung model.

SETTING

A research laboratory at a university medical center.

INTERVENTIONS

Spontaneous breathing with continuous positive airway pressure, at peak sinusoidal inspiratory flow rate demands of 30, 60, and 90 L/min with sizes 6, 7, 8, and 9 mm internal diameter endotracheal tubes at each flow rate during conventional pressure triggering, tracheal pressure triggering, and flow-by.

MEASUREMENTS AND MAIN RESULTS

Pressures were measured at the tracheal end of the endotracheal tube, "Y" piece of the breathing circuit, and inside the ventilator on the exhalation limb of the breathing circuit. Volume measured between the endotracheal tube and lung model and pressure measured at the tracheal end of the endotracheal tube were integrated to generate pressure-volume (work) loops to calculate the inspiratory imposed work of the total breathing apparatus (i.e., endotracheal tube, breathing circuit, and ventilator). Significantly (p < .05) greater decreases in pressure during spontaneous inhalation were measured for all methods of triggering at the tracheal end of the endotracheal tube than at the Y piece or inside the ventilator. Inspiratory-imposed work was significantly lower during tracheal pressure triggering compared with conventional pressure triggering and flow-by under most conditions. For example, with a 7-mm internal diameter endotracheal tube at a peak inspiratory flow rate demand of 60 L/min, imposed work was 382% and 315% lower, respectively, during tracheal pressure triggering compared with the conventional pressure triggering and flow-by triggering methods. Under all conditions, inspiratory imposed work was lower during flow-by triggering compared with conventional pressure triggering. The smaller the internal diameter of the endotracheal tube and the greater the peak inspiratory flow rate demand, the greater the inspiratory imposed work of breathing for all methods of triggering. Under all conditions, inspiratory-imposed work was significantly greater at a peak inspiratory flow rate demand of 90 L/min than at 60 L/min, and at a peak inspiratory flow rate demand of 60 L/min than at 30 L/min.

CONCLUSIONS

An endotracheal tube is a resistor in the breathing apparatus over which a pressure decrease must be developed by the patient in order to inhale spontaneously. An endotracheal tube, therefore, imposes substantial resistance and work. The results indicate that the pressure measuring/triggering site for a ventilator's demand-flow system should be at the tracheal or carinal end of an endotracheal tube so as to effectively decrease the resistance of the endotracheal tube, thus, decreasing the patient's work of breathing.