Aetna considers total body plethysmography medically necessary as an adjunct to complete pulmonary function testing (including residual volumes and diffusion) for any of the following indications:

1. For determination of bronchial hyper-reactivity in response to methacholine, histamine, or isocapnic hyperventilation; or
2. For determination of response to bronchodilators in patients who show a clinical response but fail to show an improvement in forced expiratory volume in 1 second (FEV1) by spirometry; or
3. For evaluation of obstructive lung diseases, such as bullous emphysema and cystic fibrosis, which may produce artifactually low results if measured by helium dilution or nitrogen washout; or
4. For evaluation of resistance to airflow in persons with obstructive processes, where plethysmography is necessary for accurate calculation of true lung volumes; or
5. For measurement of lung volumes to distinguish between restrictive and obstructive processes, where a restrictive process is suggested by a low vital capacity (less than 80 % predicted) on a spirometry test; or
6. For measurement of lung volumes when multiple repeated trials are required, or when the subject is unable to perform multi-breath tests.

Aetna considers total body plethysmography experimental and investigational for all other indications (e.g., pectus excavatum, primary pulmonary hypertension, scoliosis, and systemic sclerosis).

This policy is based on the American Academy of Respiratory Care (AARC, 1994) Clinical Practice Guideline on Body Plethysmography.  Body plethysmography is a very sensitive lung measurement used to detect lung pathology that might be missed with conventional pulmonary function tests.

Spirometry is the standard method for measuring most relative lung volumes; however, it is incapable of providing information about absolute volumes of air in the lung.  Thus, a different approach is required to measure residual volume, functional residual capacity, and total lung capacity.  Two of the most common methods of obtaining information about these volumes are gas dilution tests and body plethysmography.

In body plethysmography, patients sit inside an airtight chamber equipped to measure pressure, flow, or volume changes, inhales or exhales to a particular volume (usually the functional residual capacity [FRC]), and then a shutter drops across their breathing tube.  The subjects make respiratory efforts against the closed shutter, causing their chest volume to expand and decompressing the air in their lungs.  The increase in their chest volume slightly reduces the box volume and thus slightly increases the pressure in the box.  The most common measurements made using the body plethysmograph are thoracic gas volume (TGV) and airways resistance (Raw).  Airways conductance (Gaw) is also commonly calculated as the reciprocal of Raw.  Specific airways conductance (i.e., conductance/unit of lung volume) is routinely reported as SGaw.  Other tests that can be measured in the body plethysmograph include spirometry, bronchial challenge, diffusing capacity, single-breath nitrogen (N2), multiple-breath N2 washout, pulmonary compliance, occlusion pressure, and cardiac output, including pulmonary blood flow.

The American Academy of Respiratory Care's guidelines base their recommendations for body plethysmography on the test's advantages as compared with gas dilution techniques.

Lung volumes provide useful information that confirms the presence of restrictive lung disease suggested by a low vital capacity on a spirometry test.  Both plethysmography and gas dilution techniques measure the FRC, the residual air in the lung at the end of exhalation during tidal breathing.  This value is not obtainable with spirometry.

Unlike gas dilution tests (e.g., helium dilution and nitrogen wash-out techniques), body plethysmography has the ability to measure non-communicating or poorly communicating air spaces such as blebs or bullae, which usually are present in conditions involving air trapping such as cystic fibrosis or bullous emphysema.  Thus, plethysmography is preferred over gas dilution techniques in measuring lung volumes in obstructive conditions, when air trapping may occur, or where there is co-existing restriction and obstruction.

In addition to this advantage, body plethysmography allows multiple determinations of lung volumes to be made rapidly.

Although determination of the response to bronchodilators is a listed indication for body plethysmography based on the AARC guidelines, spirometry is the standard of care for evaluating response to bronchodilators.  Airflow limitation from asthma should usually demonstrate some degree of reversibility following acute treatment with a beta-agonist.  The currently recommended criteria for a significant response to a bronchodilator in adults are that either forced vital capacity (FVC) or forced expiratory volume in 1 second (FEV1) should increase by 12 % and by at least 200 ml, although complete consensus on this is lacking.  In patients with baseline airflow limitation, failure to improve following bronchodilator administration suggests an alternate diagnosis (e.g., chronic obstructive pulmonary disease) or airways inflammation that requires additional therapy (e.g., glucocorticoids).  It has also been suggested that clinicians measure other parameters of airflow obstruction in addition to spirometry, such as specific airway conductance (SGaw) obtained with a plethysmograph.  This approach increases the likelihood of observing airflow reversibility.  If a patient fails to show an improvement in FEV1 with albuterol treatment and a clinical response appears to occur, body plethysmography can be used to document this change.

Patients with mild or no airflow limitation may not show reversal after bronchodilator administration.  In such cases, a bronchial challenge with inhaled methacholine, histamine or other agents would be indicated to demonstrate reversible airflow obstruction.  Although response to bronchial challenge can be assessed by measurement of FEV1 with spirometry, it has been reported that sensitivity is increased by measurements of changes in FVC, SGaw, and TGV.  This result is in keeping with the known axial heterogeneity of the response of airways of difference caliber to bronchoactive agents.

Airway resistance can be measured directly using whole-body plethysmography, but is more commonly inferred from spirometric measurements of dynamic lung volumes and expiratory flow rates, which can be obtained more easily.  Airways resistance (Raw) and Gaw, the reciprocal of Raw, provide an effort independent measure of airway status.  They are a more sensitive measurement and will detect airways disease earlier than forced expiratory flow with spirometry.

According to the guideline on body plethysmography provided by the AARC (2001), the frequency with which plethysmography is repeated should depend on the clinical question(s) to be answered.