Use of Transfer Impedance Measurements for Clinical Assessment of Lung Mechanics

Abstract

Respiratory transfer impedance (Ztr) 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 Ztr 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 Ztr from 1-80 Hz for assessing the degree and type of impaired lung function. Spirometry and Ztr 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 Ztr features and model estimated mechanical properties. The COPD and smokers groups showed significant differences in portions of their Ztr spectra from that of the healthy group. Key Ztr spectral features included R0, 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 R0 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 R0, Raw, and Re4 versus FVC and R0 versus FEV1. After bronchodilator, changes in R0, Re4, and Raw were correlated with changes in several spirometric indices. The R0 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.

Respiratory input impedance measurement: Forced oscillation methods

“…Normative studies of Z~,in confirm strong regression relationships between R~ and lung volume and height, as expected from the hyperbolic relationship between airway resistance and total lung capacity (PESLIN et al, 1992;PASKER et al, 1994;GIMENO et al, 1992;JIEMSNIPONG et al, 1976;LANDSEN et al, 1982;DUIVERMAN et al, 1985;JACKSON et al, 1996;LUTCHEN et al, 1998;MACLEOD 1997). ideally Z~,i~, ZrsHG and Z~,t ~ should always be referred to lung volume and flow (aOSTVEEN et al, 1989).…”

“…Additional work on transfer impedance Z~,t ~ (see below) has also been published (LUTCHEN et al, 1998). The selection ofoptimun FOT frequency and signal processing and within-breath Z~,i .…”

“…Z~s,t ~ data have also been evaluated in adults, but, until recently, less used in the clinical setting (LUTCHEN et al, 1998). Pressure oscillations are superimposed around the thorax, while the subject sits in an enclosed chamber (Fig.…”

“…Zr~,t ~ has good modelling ability to partition tissues from airway components . Further consideration ofZ~s,t ~ is beyond the scope of this review of FOT, and the reader is referred to other studies (LUTCHEN et al, 1998). In summary, Z~,i ~ is determined by airway, tissue and chest wall components and requires strict measurement conditions for adequate evaluation.…”

Assessment of Respiratory Mechanics in Patients with Sarcoidosis Using Forced Oscillation: Correlations with Spirometric and Volumetric Measurements and Diagnostic Accuracy

“…Although this technique has been successfully applied to investigate abnormal respiratory mechanics in diseases such as asthma [11,12,13], chronic bronchitis [12], emphysema [12, 14] and COPD [10, 11], there are few data in the literature about the use of FOT in sarcoidosis. Lutchen et al [15 ]described changes in lung mechanics by FOT measurements, including a sarcoidosis patient, amongst others. To the best of our knowledge, there are no studies in the literature exclusively dedicated to the evaluation of the clinical usefulness of FOT for detecting changes in respiratory mechanics in patients with sarcoidosis.…”