Inspiratory flow dynamics during mechanical ventilation in patients with respiratory failure

We studied the effect of inspiratory flow rate on respiratory resistance during mechanical ventilation in 15 patients with acute respiratory failure (ARF). Resistance was measured by both constant flow inflation and occlusion methods as inspiratory flow rates were increased from 0.66 to 2.0 L/s. Endotracheal tube resistance was subtracted from total resistance to obtain respiratory resistance. In contrast to the flow-dependent increase in endotracheal tube resistance, respiratory resistance decreased continuously as flow rate and airway pressure increased, except in four of six patients with asthma in whom respiratory resistance increased as flow increased. Mechanical airway dilatation, tissue resistance, stress relaxation, and time-constant inequalities may contribute to the decrease in respiratory resistance. In status asthmaticus, however, the effects of turbulence, noncompliant airways, and/or "reflex" bronchoconstriction may be sufficient to cause a flow-dependent increase in resistance.