Respiratory mechanics and bronchodilator responsiveness in patients with the adult respiratory distress syndrome

Objective: To study the effects of salbutamol (a selective beta2-adrenergic receptor agonist) on respiratory mechanics in patients with the adult respiratory distress syndrome (ARDS). Design: Prospective study. Setting. ICU in a university hospital. Patients: Seven mechanically ventilated, paralyzed ARDS patients. Main Outcome Measurements. Measurements of respiratory system compliance, maximum, and minimum inspiratory resistance (by the end-inspiratory occlusion method during constant flow inflation) were performed at 0, 5, 10 cm H2O positive end-expiratory pressure, both before and at least 30 mins after the start of a continuous iv infusion of salbutamol (15 mug/min). Minimum inspiratory resistance represents the ohmic air flow resistance, while maximum inspiratory resistance includes minimum inspiratory resistance plus the effective additional resistance due to stress adaptation and to time constant inhomogeneities. Air flow was measured at the airway connector and tracheal pressure near the central end of the artificial airway. Results. Maximum inspiratory resistance, minimum inspiratory resistance, and additional resistance were higher than the values reported for normal anesthetized subjects. On average, salbutamol caused a decrease in maximum and minimum inspiratory resistances (from 6.48 +/- 2.56 to 4.67 +/- 1.74 and from 4.06 +/- 2.12 to 2.07 +/- 0.95 cm H2O/L/sec, respectively). Positive end-expiratory pressure increased additional resistance, whereas it decreased minimum inspiratory resistance. No interaction was found between positive end-expiratory pressure and salbutamol. Respiratory system compliance was not significantly affected by salbutamol nor by positive end-expiratory pressure. Conclusions. In ARDS patients, salbutamol decreases the abnormally high airway resistance, by reducing minimum resistance, but has no effect on the effective additional resistance.