Adaptation of pressure support ventilation to increasing ventilatory demand during experimental airway obstruction and acute lung injury.

OBJECTIVE

To estimate the changes in the relative amount of ventilatory assistance offered by inspiratory pressure support during changing ventilatory demand with external airway obstruction and with oleic acid-induced acute lung injury.

DESIGN

Prospective, controlled, crossover study.

SETTING

Experimental laboratory in a university anesthesiology department.

SUBJECTS

Eight dogs anesthetized with pentobarbital.

INTERVENTIONS

An external resistor was placed in the breathing circuit to produce increased resistance to breathing. Acute lung injury was produced with oleic acid. Ventilatory demand was increased by increasing the CO2 concentration in inspired gas to produce an increase of 20 torr (2.7 kPa) in end-tidal CO2 tension. During airway obstruction, pressure support was adjusted to reduce the inspiratory decrease in intrathoracic pressure to the level present during unobstructed breathing. During acute lung injury, pressure support was applied to reduce the maximum negative deflection of intrathoracic pressure by 50%.

MEASUREMENTS AND MAIN RESULTS

The ventilator effort was estimated by calculating the pressure-time integral of proximal airway pressure; the spontaneous ventilatory effort was estimated in a similar manner from esophageal pressure. The pressure support averaged 10 +/- 3 cm H2O during airway obstruction and 7 +/- 2 cm H2O during lung injury. The CO2 challenge effected an average increase in mean minute ventilation of 78% during airway obstruction (p < .001) and 120% during acute lung injury (p < .01). The augmentation of minute ventilation was accomplished by increasing the ventilatory rate and the transpulmonary pressure while inspiratory time remained unchanged. The pressure-time integrals measured using both airway (p < .001) and esophageal pressure (p < .01) increased significantly during each CO2 challenge, reflecting an increase in the contribution of both the ventilator and the animal to the required breathing effort. Significant decreases in the relative magnitude of the ventilator effort both during airway obstruction (p < .05) and during lung injury (p < .01) indicated that the increase in the spontaneous effort was predominant over the increase in mechanical ventilatory support.

CONCLUSIONS

A ventilatory rate-dependent adaptation of pressure support to increased ventilatory demand occurs in an experimental setting both during airway obstruction and lung injury. The results of this study confirm an advantage of breath-to-breath inspiratory pressure support over techniques designed to supply a predetermined mechanical minute volume.