Flow and Volume Dependence of Respiratory System Flow Resistance in Patients with Adult Respiratory Distress Syndrome

Using a simple and rapid technique, we studied the flow and volume dependence of the total resistance of the respiratory system (Rrs) in six patients with ARDS. At any given inflation volume, Rrs decreased progressively with increasing flow (V) according to the following hyperbolic function: Rrs = a/V + b, where a and b are constants. At any fixed inflation flow, Rrs increased progressively with increasing inflation volume. The observed flow and volume dependence of Rrs is probably mainly due to the viscoelastic properties of the pulmonary and chest wall tissues. The flow and volume dependence of Rrs found in the patients with ARDS is qualitatively similar to that previously observed in normal anesthetized paralyzed subjects. In ARDS, however, Rrs was considerably greater than in the normal subjects, indicating that besides a low respiratory compliance ARDS is characterized by a high flow resistance. This mainly reflects increased effective flow resistance of the pulmonary and chest wall tissues, although airway resistance is also higher than normal.