A pressure–flow transducer measures inspiratory and expiratory flow and pressure, which are then separated from the breathing pattern by “signal filtering”. A useful analogy here is radio waves: radio waves of high frequency, such as FM radio travel shorter distances, while radio waves of lower frequency, such as AM radio travel long distances. Higher frequencies (>20 Hz) travel shorter distances (generally up to the large airways), while lower frequencies (<15 Hz) travel deeper into the lung and reach the small airways and lung parenchyma ( fig. The impulses generated by the loudspeaker travel superimposed upon the normal tidal breathing through the large and small airways. What are the parameters that FOT/IOS measure? The differences between spirometry and FOT/IOS are described in table 1. More recently, the within-breath analysis of R rs and X rs has been shown to help differentiate between asthma and COPD and also offer more useful information about the pathophysiology of asthma and COPD, which the spirometer does not. FOT/IOS are therefore more sensitive instruments to detect small airways obstruction in patients with asthma and chronic obstructive pulmonary disease (COPD). In most cases, spirometry does not provide a clear indication of peripheral airway obstruction regardless of the information contained in the flow–volume curve and the forced expiratory flow at 25–75% of forced vital capacity (FEF 25–75%). One of the most remarkable features of FOT/IOS in relation to spirometry is that it has much greater sensitivity to detect peripheral airways obstruction. Moreover, it can be performed in patients on ventilators and also during sleep. The main advantage of FOT/IOS is that the patient needs to perform simple tidal breathing manoeuvres that require less effort and co-operation than spirometry, meaning that children and the elderly can therefore perform this test easily. Both FOT and IOS are widely used in paediatric clinics across the world as well as in several lung physiology laboratories as a valuable clinical research tool. This new technique was subsequently refined over the years by Jaeger and became commercially available in 1998. Moreover, the mixed multi-frequency waveform provides improved signal-to-noise characteristics. However, the IOS provides extensive description of oscillatory pressure–flow relationships over a range of frequencies between 4 and 32 Hz and gives better mathematical analyses of resistance and reactance using the fast fourier transform (FFT) technique. The temporal resolution of IOS is slightly inferior to FOT and it sends pulses of pressure waves inside the lungs that can be a bit uncomfortable. This improvised technique of FOT that could use multiple sound frequencies at one time was called the impulse oscillometry system (IOS). developed a computer-driven loudspeaker output to apply bursts of square wave oscillatory pressures (5 times⋅s −1) of multiple sound frequencies and analysed the pressure–flow relationship using spectral analysis. The main advantage of FOT is that it provides very good time resolution with measures of respiratory resistance. Some of the more recent FOTs now use sound waves of two or three different frequencies at one time. To measure Z rs over a range of sound frequencies therefore took a long time.
The earlier FOT instruments allowed only one sound frequency to be passed at a time. The main advantage of this device was that the procedure was easy to perform and provided information about the lung which was different from that given by the spirometer. These parameters provided valuable information about the mechanical properties of the airways and lung parenchyma. The output was a measure of respiratory impedance ( Z rs), which included the respiratory resistance ( R rs) and respiratory reactance ( X rs) measured over a range of frequencies (usually from 3 to 35 Hz). described the forced oscillation technique (FOT) as a tool to measure lung function using sinusoidal sound waves of single frequencies generated by a loud speaker and passed into the lungs during tidal breathing. Children aged <5 years, elderly people and those with physical and cognitive limitations cannot perform spirometry easily. However, spirometry is not an easy test to perform because the forceful expiratory and inspiratory manoeuvres require good patient co-operation. Spirometry is currently the most commonly performed lung function test in clinical practice and is considered to be the gold standard diagnostic test for asthma and COPD. Not only does it help in arriving at a specific diagnosis, but it also helps in evaluating severity so that appropriate pharmacotherapy can be instituted, it helps determine prognosis and it helps evaluate response to therapy. Measuring lung function is an important component in the decision making process for patients with obstructive airways disease (OAD).
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