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2006 What are forced Oscillation Technique and Impulse Oscillmetry?

Dr. Yuk-lung Kwok, Department of Medicine and Geriatrics, Tuen Mun Hospital

Forced oscillation technique (FOT) was first described by DuBios in 1956 as a method to characterize respiratory impedance (Z). In contrast with the conventional pulmonary function tests (PFT) that require specific respiratory actions by the subject, FOT determines Z by the pressure (P)-flow (V’) relationship of artificial impulse-shaped test signals superimposed on the tidal breathing of the patient, independent of the underlying respiratory pattern. Mastery of the mathematical infrastructure of FOT may be challenging to clinicians. Nevertheless, by understanding the basic principles, useful information can be obtained while interpretation is uncomplicated.

Methodology of Impulse Oscillation Technique
Flow oscillations generated by a loudspeaker are superimposed on the tidal breathing. The resulting pressure and flow signals are recorded and analyzed. (Figure 1) The ratio of pressure to flow of each of the frequencies of the waveforms is computed as the impedance (Z). Respiratory system impedance (Zrs) embodies both the in-phase and out-of-phase relationships between P and V’. The in-phase component is called the resistance (Rrs), whereas the out-of-phase relationship is expressed as reactance (Xrs), both appear as functions of the frequency of oscillation (f). (Appendix A)

Figure 1. Principle of impulse oscillometry for determining pulmonary impedance Z.

The simplest form of FOT is mono-frequency sinusoidal waveform, previously used to measure Rrs in patients with sleep apnoea. Multi-frequency FOT, by means of pseudo-random noise (PRN) or impulse oscillations (IOS), provides more detailed characterization of respiratory mechanics. PRN is a mixture of sinusoidal waveforms of discontinuous spectra with improved time resolution compared with mono-frequency FOT. However, the noise of Z calculation is increased and the discontinuous nature of PRN requires post-processing to smooth the overall spectral range that may diminish information contained in characteristic peaks and plateaus in airway obstruction.

IOS uses a rectangular waveform composed of a continuous spectrum of frequencies of short duration (30-40 ms). The advantage of continuous spectra is important in abnormal respiratory systems with regional non-homogeneities. In the oscillation frequency range of 5-35 Hz, a healthy respiratory system exhibits a frequency-independent Rrs. Xrs undergoes the transition from negative values (elastic reactance dominates) to positive values (inertial reactance dominates) increasing with f. (Figure 2) Oscillation frequencies <5 Hz are affected by higher harmonics of lung periphery whereas higher oscillation frequencies are increasingly affected by the properties of upper airways.

Figure 2. Illustration of respiratory impedance of adults in the medium oscillation frequency range. Respiratory resistance (R) is higher (a) and respiratory reactance (X) is lower (b) in patient with COPD (----) than normal subject (). Both R and X exhibit frequency-dependence. Arrow indicates resonant frequency (fres) and the shaded area is the reactance area (AX). (See text)

Interpretation of Oscillation Mechanics (Rrs, Xrs, fres, AX)
In proximal airway obstruction, Rrs is elevated independent of oscillation frequency. In distal airway obstruction Rrs is highest at low frequencies and falls with increasing frequencies. Xrs is expressed in inertance (I) and capacitance (Ca). (Appendix B) Respiratory Ca is most prominent at low frequencies while I dominates at higher frequencies. Low frequency capacitive Xrs expresses the ability of the respiratory tract to store capacitive energy, primarily resident in the lung periphery. In both fibrosis and emphysema this ability is reduced. Resonant frequency (fres) is the point at which the magnitudes of capacitive and inertive reactance are equal, i.e. the frequency when Xrs crosses the reactance zero axis. (Figure 2, appendix C) It is a marker to separate low- (peripheral airway) from high-frequency (central airway) Xrs. Both obstructive and restrictive respiratory diseases cause fres to increase.

Reactance area (AX), the area between the reactance zero axis and Xrs, is a quantitative index of total respiratory reactance Xrs at all frequencies between 5Hz and fres. (Figure 2) AX includes changes in the low-frequency Xrs, fres and the curvature of the Xrs(f)-tracing (Appendix D) and is reported to be more sensitive than Xrs in detecting bronchial challenge responses.

Clinical application of oscillometry
Data acquisition is easy in FOT, as the technique requires only tidal breathing around a mouthpiece for 20-30 seconds. Children, elderly or patients with neuromuscular disease can be easily studied. Published normative FOT data in Chinese children is now available. However, the adult normative database is small, and Asian population is not included. Yet concern over precise definition of “normal vs. abnormal” should not preclude the application of FOT to supplement standard PFT. In particular, measuring bronchial challenge responses by FOT is easy, timely, and saves the patient from exhaustion when compared with spirometry, and may detect false negative spirometric responses to inhaled -agonist in children with asthma. Nonetheless, there is no consensus to the FOT cutoff value to define broncho-reactivity, and the generally used >45% fall in Rrs is based on a study in COPD patients. Figures 3 - 7 show the typical IOS pattern in normal subjects and various respiratory diseases.

In asthmatic patients, both high- and low-frequency Rrs may decrease after -agonist, with relatively greater decrease in low-frequency Rrs. (Figure 8a) In COPD receiving inhaled bronchodilators, a decrease in low-frequency Rrs with little or no change in high-frequency Rrs is commonly observed. (Figure 8b) In particular, AX is reported to be more sensitive to changes in peripheral airway obstruction as small increments in resistance are amplified by the large cross-sectional diameter of all lower generations airways. (Figure 9)

FOT is a useful supplement to standard PFT in diagnosis and monitoring therapeutic changes in various respiratory diseases. IOS is currently available in a number of hospitals in Hong Kong.

A. Respiratory impedance (Zrs) is the ratio of effective pressure (Prs) and flow (V’rs), derived from the superimposed forced oscillations. Multi-frequency oscillations (PRN or IOS) provide measures of Zrs as a function of frequency (f). In engineering terms, Zrs is described as resistance (Rrs) and reactance (Xrs)
Multi-frequency input impedance Zrs(f) = [Prs(f)]/[V’rs(f)]
Zrs(f) = Rrs(f) + jXrs(f)
B.   Respiratory reactance (Xrs) is expressed in terms of inertance (I) and capacitance (Ca).
Xrs(f) = w×I – 1/(w×Ca)       w = 2×p×f
C.   Resonant frequency (fres) is defined as the point at which the magnitudes of capacitive and inertive reactance are equal.
w0×I = 1/(w0×Ca)                   w0 = 2×p×fres
D. Reactance area as the area under the zero axis of the reactance graph above the Xrs(f) tracing.
AX = ∫ Xrs (f) × df
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