From the Department of Medicine, Division of Pulmonary, Critical Care and Sleep Medicine, New York University School of Medicine; and André Cournand Pulmonary Physiology Laboratory, Bellevue Hospital.
CORRESPONDENCE TO: Kenneth I. Berger, MD, New York University School of Medicine, 240 E 38th St, Room M-15, New York, NY 10016; e-mail: email@example.com
CONFLICT OF INTEREST: None declared.
Reproduction of this article is prohibited without written permission from the American College of Chest Physicians. See online for more details.
We agree that the holy grail of pulmonary physiologists is a test that detects early chronic airway disease. Although Dr Enright1 remains “cautiously optimistic” that forced oscillation technique (FOT) can serve this purpose, there are sufficient data to mitigate his caution.
Accumulating literature provides evidence for enhanced diagnostic capabilities of FOT vs spirometry (reviews cited in our point editorial2). Despite normal spirometry, FOT abnormalities in those studies indicate small airway dysfunction based on bronchodilator responsiveness and correlation with symptom severity, quality of life, and response to treatment. Moreover, isolated improvement in FOT metrics during treatment occurs with simultaneous improvement in airway and alveolar inflammation as well as with bronchial hyperreactivity (methacholine).
Populations exposed to dust from the World Trade Center collapse provide a unique opportunity to address the role of FOT in detecting small airway disease because histologic evaluation demonstrated distal penetration of inhaled particles with small airway injury.3 Our group’s large case-control study reaffirms the role of FOT in detection of small airway disease in subjects with normal spirometry.4 The magnitude of dust exposure was only associated with FOT abnormalities in subjects with new-onset persistent respiratory symptoms.4 Furthermore, we demonstrated an association between FOT abnormalities and systemic inflammation in other exposed subjects.
The Evaluation of COPD Longitudinally to Identify Predictive Surrogate Endpoints (ECLIPSE) study suggested that a proportion of patients with COPD and abnormal spirometric findings may have FOT measures in the normal range.5 This conclusion was based on prediction equations derived from the control group. However, the equations had poor correlation, dictating wide CIs. In fact, the normative range used was at or above the upper end of previously published values. A more recent large normative dataset has demonstrated values significantly lower than those of the ECLIPSE control group.6
Lack of correlation in the ECLIPSE study between FOT and CT imaging airway metrics is expected because smaller airways (< 2 mm) are not visible radiographically. Moreover, lack of correlation between FOT and focal emphysema is not unexpected because FOT parameters do not directly assess parenchymal destruction.5
Despite the difficulty discussed by Dr Enright1 for the most recent published normative dataset, data were successfully pooled from all institutions for parameters that reflect distal airway function.5 Moreover, for all FOT parameters (whether derived from four or five sites), the derived predictive equations have narrow CIs.
FOT resistance can be referenced to the functional residual capacity (FRC) as specific airway conductance.7,8 Postbronchodilator changes in FRC can be determined in the absence of plethysmography by assessment of inspiratory capacity.7 For subjects with low FRC (eg, those who are obese), voluntary inflation to predicted FRC may remove confounding effects of altered lung volume.7 If FOT is valid for pediatric and elderly populations, then it must also be valid for populations that perform spirometry without difficulty. Finally, even spirometry technicians require specific training and supervision.9
In conclusion, we also applaud the memory of Joe Rodarte, MD, and the role he played in the development of the field of pulmonary physiology. Although no test is perfect, data clearly demonstrate the enhanced diagnostic capability of FOT, particularly in detecting disease localized to the small airways.
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