Advances in Pulmonary Laboratory Testing: FO techniqueTo determine Raw, one needs to know instantaneous values for flow and the pressure that drives flow through the airways at multiple points in the breathing cycle. Body plethysmographs report the average Raw of the entire airway tree from mouth to the “average” alveolus. Plethysmographs are large and expensive, and some patients find it difficult to perform the panting maneuver required for procedure. Thus, a technique to measure resistance that would involve smaller, cheaper equipment and that would be easy to use for both patients and technical personnel potentially would be useful in a clinical setting.
The FO technique has the potential to fill this need. With FO, resistance of the pulmonary system (ie, airways, lung, and chest wall) is determined by imposing known variations in flow at the mouth and by measuring the resultant pressure changes. Early studies used pure sine waves at fixed frequency, though later studies probed with multiple frequencies to find the resonant frequency (the point at which compliance effects and inertial effects cancel, giving pure flow resistance). In the early 1970s, techniques were developed to probe a frequency range of interest with a more complex forcing function, using either “white noise” (a sound wave with a uniform representation of frequencies,) or a series of brief impulses in flow at the mouth. fully

With both innovations, the patient was required only to breathe quietly while the imposed forcing function was applied and measurements were made over a period of about 1 min. Because the frequency of the imposed oscillations was much higher than that of the patients’ breathing, the effects of the normal breathing cycle could be subtracted out. White noise and flow impulses both contain a range of frequencies, and mathematical techniques were used to separate out the individual frequency components to determine frequency dependence of the resistance and compliance. The equipment, mathematical analysis, and data-handling techniques have been technically very demanding for both these techniques. Recently, several manufacturers have made impressive efforts to miniaturize the equipment and to automate the data analysis and data handling to improve the user-friendliness of the FO technique. The equipment involved is considerably smaller than the body plethysmograph, and equipment setup and calibration have been automated so that technical personnel with little training can use the equipment. However, the investigator must be aware of what resistance value these devices are reporting and of both the physiologic and technical limitations of the technique (Table 5).
Table 5—Problems and Drawbacks of the FO Technique

Problem Comments Study
Rrs Attempts to model system may help derive meaningful parameters from measurements (eg, large airway Raw, peripheral airway Raw, chest wall) Peslin and Fredberg
Effects of inhomogeneous Rrs and regional lung compliance Complicates interpretation of patterns of Rrs vs frequency; probing with lower frequency than commercial devices are capable of may help Lutchen et al
Glottic aperture may narrow during quiet breathing (expiratory braking) Resistance values dominated by upper airway rather than lung airways Stanescu et al
Peripheral airways may be “silent zone” Changes in peripheral airways may be best early marker of disease; these changes may be undetectable by commercial units Lebecque and Stanescu MacKlein et al
Low reproducibility and wide range of normal values Large changes required to attain clinical significance, possible reducing sensitivity compared to spirometry Lebecque and Stanescu Cuijpers et al Timonen et al