Chest
Volume 134, Issue 6, December 2008, Pages 1304-1309
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Topics in Practice Management
Optimizing Performance of Respiratory Airflow Resistance Measurements

https://doi.org/10.1378/chest.06-2898Get rights and content

In contrast to spirometry, airflow resistance determinations provide an effort independent measure of the airway status and allow measurement in individuals unable or unwilling to provide adequate effort during spirometry. Resistance measurements may be performed using an esophageal balloon, airflow perturbation techniques (including interrupter and oscillatory techniques), or total-body plethysmography. Esophageal balloons are invasive, and airflow perturbation techniques are becoming more widely used. Airflow perturbation methods assess small airway dysfunction using frequency dependence of resistance, a surrogate for frequency dependence of compliance. Body plethysmography remains the “gold standard” for measuring airway resistance and is based on measures of pressure changes and flows with the patient enclosed in a body plethysmograph. While plethysmographic procedures may be completed rapidly, yielding multiple trials within preset repeatability criteria, the equipment is costly and the operator must be highly trained. Plethysmographic specific resistance loops have definite shapes (morphologies) indicative of specific airway disorders, which may be interpreted in a manner analogous to spirometry. Specific resistance and conductance assess the important effects of lung volumes. Reimbursement for resistance measurements varies depending on regional guidelines.

Section snippets

Clinical Indications

The following are indications for Raw measurements16: (1) evaluating airflow limitation beyond spirometry; (2) determining bronchodilator responsiveness; (3) determining bronchial hyperreactivity to methacholine, histamine, or isocapnic hyperventilation; (4) differentiating types of obstructive lung diseases having similar spirometric configurations; (5) following the course of disease and response to treatment; and (6) distinguishing respiratory muscle weakness from obstruction as the cause of

Airflow Perturbation/Oscillatory Resistance

Forced oscillatory resistance is the most commonly used airflow perturbation method, available commercially in the United States as impulse oscillometry (IOS). IOS measures Rrs regardless of lung volume. In both IOS and simpler airflow perturbation devices (APDs), measurement of Rrs provides more useful information if end-expiratory lung volume is known. Therefore, an inspiratory capacity (IC) effort should be included prior to completion of airflow perturbation test sessions. Patients with

Plethysmography Test Performance

Performing Raw measurements by plethysmography requires enclosure of the patient in a total-body plethysmograph designed to measure pressure and flow changes. The test procedure requires < 10 min to perform multiple trials within preset acceptability and repeatability criteria. Patients are instructed in the correct techniques while the plethysmograph door is open. The instructions include the need for relaxing and breathing normally between trials, supporting the cheeks during trials,

Reporting Results

Report Rrs in centimeters of water per liter per second at the lowest frequency measured (usually 5 Hz). Report the fall in Rrs between 5 Hz and 15 Hz (R5-R15), designated f-dR in the same units. Report the IC both before and after bronchodilator. Unlike plethysmography, which measures absolute TGV, Rrs measurements during normal breathing are subject to the patient's chosen respiratory pattern. The IC effort required just before ending the test session provides an index within the vital

Clinical Evaluation

Rrs should be displayed graphically at different oscillatory frequencies (Fig 2). Determine whether Rrs decreases with increasing oscillation frequency (f-dR), demonstrating airflow obstruction, or whether Rrs is constant (normal subjects in group A, Fig 2) independent of oscillation frequency. This feature is paramount in clinical interpretation of small airway impairment.

Plethysmograhic sRaw loops have definite shapes indicative of specific airway disorders24, 25 and are as important to

Practice Management

Report Raw measurements with current procedural terminology code 94360,26 for both oscillatory and plethysmographic methods. If plethysmographic lung volumes are reported with current procedural terminology code 93720 (plethysmography, total body), then 94360 is bundled into plethysmography and cannot be reported separately.27

Raw reimbursement varies regionally in the United States based on local carrier decisions.28 Medical necessity is supported by the use of International Classification of

Summary

Raw provides useful clinical information for diagnosis and monitoring of lung disease beyond spirometry. A direct measure of airway status, Raw is useful in individuals unable to provide adequate effort during spirometry. Rrs falls with increasing oscillation frequency (R5-R15) that provides a surrogate for frequency dependence of compliance, and reflects small airway impairment. R5-R15 and sGaw are particularly useful for assessing changes in airway caliber after bronchodilator administration.

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    The authors have no conflicts of interest to disclose.

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