Regional lung impedance from forced oscillations through alveolar capsules

We developed a technique for applying forced oscillations in flow between 5 and 200 Hz to the lung through an alveolar capsule. This provided measures of the complex alveolar input impedance of the lung, which we modelled in terms of an elastic alveolar unit connected to the rest of the lung by a peripheral airway. The estimated elastance of the alveolar compartment in 6 normal dogs was about 5 orders of magnitude greater than the elastance of the entire lung. The resistance of the airway leading into the compartment from the rest of the lung was about 5 orders of magnitude greater than the resistance of the entire airway tree. The capsule oscillation technique thus gives mechanical information about an extremely small region of the lung (in the order of 10 mm3 in volume) just under the capsule. A bolus i.v. injection of methacholine caused a variable response in the alveolar regions studied, with some regional elastances and resistances increasing while others decreased, despite the fact that overall lung resistance and elastance always increased. We conclude that our capsule oscillator technique yields precise mechanical information about very small distal regions of the lung, and that these regions respond heterogeneously to injected methacholine.