Am. J. Respir. Crit. Care Med., Volume 157, Number 2, February 1998, 435-446

Use of Transfer Impedance Measurements for Clinical Assessment of Lung Mechanics

KENNETH R. LUTCHEN, ANTHONY SULLIVAN, FREDERIC T. ARBOGAST, BARTOLOME R. CELLI, and ANDREW C. JACKSON

Department of Biomedical Engineering, Boston University, Boston; Warren E. Collins, Inc., Braintree; and Pulmonary Division, St. Elizabeth's Hospital, Brighton, Massachusetts

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 (), 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 (COPD), 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 COPD 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_o, the frequency at which the real part of impedance is zero; and Re₄, the real part of impedance at 4 Hz. The key model parameter was airway resistance, Raw. We found Raw, Re₄, and R_o 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_o, Raw, and Re₄ versus FVC and R_o versus FEV₁. After bronchodilator, changes in R_o, Re₄, and Raw were correlated with changes in several spirometric indices. The R_o 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.

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