Low-frequency assessment of airway and tissue mechanics in ventilated COPD patients

Abstract

Lorx A, Szabó B, Hercsuth M, Pénzes I, Hantos Z. Lowfrequency assessment of airway and tissue mechanics in ventilated COPD patients. J Appl Physiol 107: 1884 -1892, 2009. First published October 15, 2009 doi:10.1152/japplphysiol.00151.2009.-Low-frequency forced oscillations have increasingly been employed to characterize airway and tissue mechanics separately in the normal respiratory system and animal models of lung disease; however, few data are available on the use of this method in chronic obstructive pulmonary disease (COPD). We studied 30 intubated and mechanically ventilated patients (COPD, n ϭ 9; acute exacerbation of COPD, n ϭ 21) during short apneic intervals at different levels of positive endexpiratory pressure (PEEP), with small-amplitude forced oscillations between 0.4 and 4.8 Hz. In 16 patients, measurements were made before and after inhalation of fenoterol hydrobromide plus ipratropium bromide (Berodual). Newtonian resistance and coefficients of tissue resistance (G) and elastance (H) were estimated from the respiratory system impedance (Zrs) data by model fitting. Apart from some extremely high Zrs data obtained primarily at relatively low PEEP levels, the model yielded a reasonable partitioning of the airway and tissue parameters, and the inclusion of further parameters did not improve the model performance. With increasing PEEP, Newtonian resistance and the ratio G/H decreased, reflecting the volume dependence of the airway caliber and the improved homogeneity of the lungs, respectively. Bronchodilation after the administration of Berodual was also associated with simultaneous decreases in G and H, indicating recruitment of lung units. In conclusion, the measurement of lowfrequency Zrs can be accomplished in ventilated COPD patients during short apneic periods and offers valuable information on the mechanical status of the airways and tissues. forced oscillation; respiratory resistance; respiratory elastance; mechanical ventilation; bronchodilation THE FORCED OSCILLATION TECHNIQUE (FOT) has increasingly been used as a lung function test method in both health and disease (26). A variant of the FOT, involving low oscillation frequencies comparable with the breathing rate (LFOT) and a modelbased evaluation of respiratory system impedance (Zrs), has been shown to provide a noninvasive method for partitioning of the total respiratory or pulmonary mechanics into airway and respiratory tissue properties. The LFOT has been employed extensively in infants during Hering-Breuer reflex apnea (e.g., Refs. 14, 32) and in perioperative studies on adults (2,3,8,25) and children with normal lung function (28). In these studies, the interpretation of the measured Zrs data via a mechanical model consisting of a simple airway compartment connected to a constant-phase tissue impedance (16) resulted in physiologically meaningful parameters. Although this approach has been implemented in numerous investigations on different animals both under normal conditions and in disease models, few LFOT measurements have been...

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