EFFECT OF LUNG-VOLUME ON LUNG RESISTANCE AND ELASTANCE IN AWAKE SUBJECTS MEASURED DURING SINUSOIDAL FORCING

Background. Although lung volume may be changed by certain procedures during anesthesia and mechanical ventilation, dependence of the dynami c mechanical properties of the lungs on lung volume are not clear. Bas ed on studies in dogs, the authors hypothesized that changes in lung m echanics caused by anesthesia in healthy humans could be accounted for by immediate changes in lung volume and that lung resistance will not be decreased by positive end-expiratory airway pressure if tidal volu me and respiratory frequency are in the normal ranges. Methods. Lung r esistance and dynamic lung elastance were measured in six healthy, rel axed, seated subjects during sinusoidal volume oscillations at the mou th (5 mL/kg; 0.4 Hz) delivered at mean airway pressure from -9 to +25 cmH2O. Changes in lung volume from functional residual capacity were m easured with inductance plethysmographic belts. Results. Decreases in mean mean airway pressure that caused decreases in lung volume from fu nctional residual capacity comparable to those typically observed duri ng anesthesia were associated with significant increases in both dynam ic lung elastance and lung resistance. Increases in mean mean airway p ressure that caused increases in lung volume from functional residual capacity did not increase lung resistance and increased dynamic lung e lastance only above about 15 cmH2O. Conclusions. Increases in dynamic lung elastance and lung resistance with anesthesia can be explained by the accompanying, acute decreases in lung volume, although other fact ors may be involved. Increasing lung volume by increasing mean airway pressure with positive end-expiratory pressure will decrease lung resi stance only if the original lung volume is low compared to awake, seat ed functional residual capacity.