Advances in Pulmonary Laboratory Testing: Plethysmography

The fact that FO measures the total pulmonary system resistance, and that peripheral Raw, obtainable only through conceptual and mathematical modeling, may be masked by effects in larger airways may seriously limit the usefulness of the method. Results from the technique can be reported in several ways. Total resistance values at particular frequencies may be the best parameters as they are independent of models, which may vary among different implementations. A frequency of 8 Hz is a good candidate because it is near the resonant frequency of the respiratory system. The resonant frequency of the system or the ratio of the resistance at low frequency to that at high frequency may also useful, but studies showing their sensitivity to disease are just starting to be produced. An alternative is to report the resistance obtained from fitting conceptual or mathematical models to the data. However, the investigator must realize that resistance at any given frequency is total respiratory system resistance and that the model-fitting procedures remain to be proven in clinical practice with large population studies.
There are several potential advantages of the FO technique over both spirometry and plethysmography (Table 6). The technique has potential advantages for challenge testing, as multiple tests at many dose levels will be easier to tolerate for the pa-tient. However, the investigator must realize that the test does not require a deep inspiration. There is increasing evidence that a major defect in asthma is the reduced or paradoxical response to deep inspirations or sighs. Challenge testing without deep inspiration reduces the difference between patients with asthma and healthy subjects. A thorough comparison study must include dose-response determination in both healthy subjects and patients with asthma using FO and spirometry on separate days (with and without deep inspirations). review

Table 6—Potential Advantages of FO Technique

Advantage Comment Study
Quiet breathing maneuver is simple to perform Some patients may still be unable to toleratemouthpiece, or have trouble keeping tongue out of the way Ducharme and Davis
Separate values for large airway and peripheral airway resistance These data come from models, which may not be standardized among implementations, and must be Lutchen et al
proven with larger studies
No deep inspiration required; avoids airway collapse and flow limitation Reversibility of resistance by deep inspiration may be defect in asthma: testing with no deep inspirations may “mask” true asthma Skloot et al
Useful in challenge studies Normal values must be determined in large population studies; adequate comparison must include separate sessions using spirometry and FO (see comments on deep inspiration) Bohadana et al Schmekel and Smith
May be useful in monitoring situations (eg, critical care or sleep) Badia et al
May be used in younger patients compared to spirometry A higher proportion of 2- to 6-yr olds could complete FO tests compared to spirometry Ducharme and Davis

Category: Pulmonary Function

Tags: flow-volume, Forced oscillation, negative expiratory pressure, nitric oxide