Use of transfer impedance measurements for clinical assessment of lung mechanics

Respiratory transfer impedance (Z(tr)) measured using the forced oscillation technique requires virtually no patient cooperation and provides a noninvasive approach for acquiring data reflective of lung mechanics. Also, model analysis of Z(tr) provides reliable estimates of separate airway and tissue properties (1), but only if data out to 64 Hz are acquired. The current study evaluated the clinical utility of Z(tr) from 1-80 Hz for assessing the degree and type of impaired lung function. Spirometry and Z(tr) measurements were made on 37 individuals: 11 healthy subjects and 26 patients with lung disease including chronic obstructive pulmonary disease (CORD), asthma, lung cancer, and sarcoidosis. Over the entire patient group, 12 were also smokers. We first established normal ranges for several Z(tr) features and model estimated mechanical properties. The CORD and smokers groups showed significant differences in portions of their Z(tr) spectra from that of the healthy group. Key Z(tr) spectral features included R-0 the frequency at which the real part of impedance is zero; and Re-4 the real part of impedance at 4 Hz. The key model parameter was airway resistance, Raw. We found Raw, Re-4, and R-0 to be significantly elevated during disease (p < 0.0005) and to significantly decrease with bronchodilator therapy (p < 0.025). Moreover, we found moderate to strong correlations between R-0, Raw, and Re-4 versus FVC and R-0 versus FEV1. After bronchodilator, changes in R-0, Re-4, and Raw were correlated with changes in several spirometric indices. The R-0 feature has not been previously evaluated since it is typically above 32 Hz (well above 32 Hz in diseased individuals) and not encompassed in previous clinical studies.