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Measuring Lung Function Using Sound Waves: Role of the Forced Oscillation Technique and Impulse Oscillometry System

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Measuring Lung Function Using Sound Waves: Role of the Forced Oscillation Technique and Impulse Oscillometry System Bill Brashier, Chest Research Foundation, Sundeep Salvi, Chest Research [email protected] Marigold Complex, Kalyaninagar, Foundation, Marigold Complex, Sundeep Salvi Pune, India Kalyaninagar, Pune 411014, India Measuring lung function using sound waves: role of the forced oscillation technique and impulse oscillometry system Introduction was a measure of respiratory impedance Measuring lung function is an important (Zrs), which included the respiratory resis- Conflict of interest component in the decision making process tance (Rrs) and respiratory reactance (Xrs) None declared for patients with obstructive airways disease measured over a range of frequencies (usually (OAD). Not only does it help in arriving at a from 3 to 35 Hz). These parameters provided specific diagnosis, but it also helps in evalu- valuable information about the mechanical ating severity so that appropriate pharmaco- properties of the airways and lung paren- therapy can be instituted, it helps determine chyma. The main advantage of this device was prognosis and it helps evaluate response that the procedure was easy to perform and to therapy. Spirometry is currently the most provided information about the lung which commonly performed lung function test in was different from that given by the spirome- clinical practice and is considered to be the ter. The earlier FOT instruments allowed only gold standard diagnostic test for asthma and one sound frequency to be passed at a time. COPD. However, spirometry is not an easy To measure Zrs over a range of sound frequen- test to perform because the forceful expi- cies therefore took a long time. Some of the ratory and inspiratory manoeuvres require more recent FOTs now use sound waves of good patient co-operation. Children aged two or three different frequencies at one time. <5 years, elderly people and those with phys- The main advantage of FOT is that it provides ical and cognitive limitations cannot perform very good time resolution with measures of spirometry easily. respiratory resistance. In 1956, DUBOIS et al. [1] described the In 1975, MICHAELSON et al. [2] developed a ERS 2015 forced oscillation technique (FOT) as a tool computer-driven loudspeaker output to apply to measure lung function using sinusoidal bursts of square wave oscillatory pressures sound waves of single frequencies gener- (5 times⋅s−1) of multiple sound frequencies and HERMES syllabus link: ated by a loud speaker and passed into the analysed the pressure–flow relationship using module D.1 lungs during tidal breathing. The output spectral analysis. This improvised technique DOI: 10.1183/20734735.020514 Breathe | March 2015 | Volume 11 | No 1 57 Measuring lung function using sound waves of FOT that could use multiple sound fre- flow sensor-based spirometer does. Both the quencies at one time was called the impulse FOT and IOS devices measure Rrs and Xrs at oscillometry system (IOS). The temporal res- multiple frequencies, but they do not neces- olution of IOS is slightly inferior to FOT and sarily show similar values. it sends pulses of pressure waves inside the lungs that can be a bit uncomfortable. How- ever, the IOS provides extensive description What are the parameters of oscillatory pressure–flow relationships over that FOT/IOS measure? a range of frequencies between 4 and 32 Hz and gives better mathematical analyses of The impulses generated by the loudspeaker resistance and reactance using the fast fou- travel superimposed upon the normal tidal rier transform (FFT) technique [3]. Moreover, breathing through the large and small airways. the mixed multi-frequency waveform provides Higher frequencies (>20 Hz) travel shorter improved signal-to-noise characteristics [4]. distances (generally up to the large airways), This new technique was subsequently refined while lower frequencies (<15 Hz) travel deeper over the years by Jaeger and became commer- into the lung and reach the small airways and cially available in 1998 [5]. Both FOT and IOS lung parenchyma (fig. 1). A useful analogy here are widely used in paediatric clinics across the is radio waves: radio waves of high frequency, world as well as in several lung physiology lab- such as FM radio travel shorter distances, oratories as a valuable clinical research tool. while radio waves of lower frequency, such as The main advantage of FOT/IOS is that AM radio travel long distances. A pressure– the patient needs to perform simple tidal flow transducer measures inspiratory and breathing manoeuvres that require less effort expiratory flow and pressure, which are then and co-operation than spirometry, meaning separated from the breathing pattern by “sig- that children and the elderly can therefore nal filtering”. Measured Zrs is the sum of all perform this test easily. Moreover, it can be the forces (Rrs and Xrs) opposing the pres- performed in patients on ventilators and also sure impulses (oscillations) and is calculated during sleep. One of the most remarkable fea- from the ratio of pressure and flow at each fre- tures of FOT/IOS in relation to spirometry is quency [6]. The FOT/IOS is therefore an accu- that it has much greater sensitivity to detect rate and powerful method that measures Rrs peripheral airways obstruction. In most cases, and Xrs from input Zrs measurements made spirometry does not provide a clear indication over a range of frequencies. of peripheral airway obstruction regardless of the information contained in the flow–vol- Respiratory resistance ume curve and the forced expiratory flow at 25–75% of forced vital capacity (FEF25–75%). Rrs measured by FOT and IOS includes the FOT/IOS are therefore more sensitive instru- resistance of the oropharynx, larynx, trachea, ments to detect small airways obstruction in large and small airways, lung and chest wall patients with asthma and chronic obstructive tissue. However, the use of multiple oscilla- pulmonary disease (COPD). More recently, the tion frequencies permits a dissection of large within-breath analysis of Rrs and Xrs has been airway behaviour from that of peripheral small shown to help differentiate between asthma airways. Sound waves at frequency <15 Hz and COPD and also offer more useful infor- travel more distally and those >20 Hz are mation about the pathophysiology of asthma damped out in the intermediate sized airways. and COPD, which the spirometer does not. The resistance at 5 Hz (R5) represents the The differences between spirometry and FOT/ total airway resistance, and the resistance at IOS are described in table 1. 20 Hz (R20) represents the resistance of the Choosing between FOT and IOS to mea- large airways. Subtracting R20 from R5 (R5−R20) sure respiratory resistance and reactance is reflects resistance in the small airways. like choosing between a volume– displacement In healthy adult subjects, R is nearly inde- spirometer and a flow sensor-based spirometer pendent of oscillation frequency (i.e. resis- respectively. Although the volume–displacement tance is more or less the same at frequencies spirometers offer more accurate measures of between 5 and 20 Hz). When airway obstruction lung volumes than the flowsensor-based spi- occurs, either central or peripheral, R5 is rometers, they are bulky, difficult to maintain increased above normal values. Central airway and do not offer important readouts that a obstruction elevates R evenly independent 58 Breathe | March 2015 | Volume 11 | No 1 Measuring lung function using sound waves Table 1 Differences between spirometry and FOT/IOS Parameter Spirometry FOT/IOS Main principle Flow sensor/volume displacement helps Forced oscillations of single frequency measure flow rates and lung volumes sound waves (FOT) or impulses of multiple frequency sound waves (IOS) are pushed into the lungs as pressure waves to measure respiratory resistance and reactance Main parameters Volumes: FEV1, FVC Zrs, Rrs, Xrs, Fres, Ax Flows: PEFR, FEF25–75% Patient co-operation required +++ + Type of breathing manoeuvre Forced exhalation Tidal breathing Variability (intra-subject) 3–5% 5–15% Sensitivity to airway location Central + +++ Peripheral ++ +++ Cut off for bronchodilator response 12–15% for FEV1 40% for R5 or X5 Cut off for bronchoconstrictor response 20% for FEV1 50% for R5 Insight into lung mechanics + +++ Standardised methodology +++ ++ Availability of robust reference values +++ + FEV1: forced expiratory volume in 1 s; FVC: forced vital capacity; PEFR: peak expiratory flow rate; FEF25–75%: forced expiratory flow at 25–75% of FVC; Zrs: respiratory impedance; Rrs: respiratory resistance; Xrs: respiratory reactance; Fres: resonant frequency; Ax: reactance area; R5: respiratory resistance at 5 Hz; X5: respiratory reactance at 5 Hz. of oscillation frequency, whereas peripheral the respiratory system to store energy and is airways obstruction increases R at low frequen- primarily located in the lung periphery. cies, an effect that diminishes with increasing C and I are in opposite phase with each frequency (fig. 2). Therefore, in small airways other, and unlike resistive properties of the obstruction, R becomes frequency depen- normal respiratory system, they are depen- dent and is considered to be a characteristic dent on oscillation frequency. At low fre- feature. Small children normally present fre- quencies, the magnitude of the oscillatory quency dependence of resistance, and this capacitative pressure loss is relatively large may be greater than in adults in the presence and that of inertive pressure loss is relatively of peripheral airflow obstruction. Resistance is small. Therefore, at low frequencies, the −1 −1 −1 −1 measured in cmH2O⋅L ⋅s or kPa⋅L ⋅s . capacitative properties of the small peripheral airways dominate. As oscillation frequency increases, the magnitude of the capacitative Respiratory reactance pressure dissipation decreases, while that Xrs is the imaginary part of Zrs and includes of inertive pressure increases. Therefore, at the mass-inertive forces of the moving air col- high frequencies, the inertive properties of umn expressed in terms of inertance (I) and the large airways dominate. By convention, the elastic properties or compliance of lung capacitative pressure losses are designated periphery expressed as capacitance (C).
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