Inherent coupling of elastic and dissipative behavior of the lung through a viscoelastic time constant

The simple model best able to describe the viscoelastic behavior of lung tissues contains a Kelvin body. Some facets of the viscoelastic behavior of the model can be characterized by a time constant. We have performed a series of experiments to demonstrate this viscoelastic time constant in open-chest mechanically ventilated puppies by recording the stress recovery after midexpiratory flow interruption. In 19 8- to 10-wk-old puppies, the mean time constant was 0.64 +/- 0.01 (SE) s. This time constant remained unchanged after inhalation challenge with histamine, methacholine, or hypertonic saline, despite marked changes in tissue mechanics (resistance and elastance) and in hysteresivity. The constancy of the viscoelastic time constant demonstrates a tight coupling of the parameters of the Kelvin body model, effectively reducing the viscoelastic parameters to a single degree of freedom, and may explain the coupling between the dynamic elastic and resistive properties of the lung tissues previously demonstrated.