94726

Plethysmography for Lung Volumes and Airway Resistance Determination

Plethysmography, often referred to as body plethysmography or the 'Body Box,' is a sophisticated pulmonary function test (PFT) used to measure the volume of air within the lungs that cannot be measured by standard spirometry alone. While spirometry measures the volume of air a patient can inhale or exhale, it cannot quantify the air remaining in the lungs after a full exhalation, known as the residual volume (RV). The procedure is fundamentally based on the application of Boyle's Law, which states that the volume of a gas is inversely proportional to its pressure at a constant temperature. During the test, the patient is seated inside a sealed, airtight chamber equipped with high-precision pressure transducers. The patient breathes through a mouthpiece connected to a shutter system and a pneumotachograph. The technician instructs the patient to perform specific breathing maneuvers, most notably 'panting' against a momentarily closed shutter. This action causes changes in the volume of the chest and the pressure within the chamber. By measuring the changes in pressure within the cabin relative to the pressure at the mouth, the thoracic gas volume (TGV) can be accurately calculated. Once TGV, specifically Functional Residual Capacity (FRC), is established, other lung volumes like Total Lung Capacity (TLC) and Residual Volume (RV) are derived by combining these results with standard spirometry data. Additionally, 94726 is used to measure airway resistance (Raw) and airway conductance (Gaw), providing critical data on the ease with which air flows through the respiratory tract. This test is essential for differentiating between obstructive and restrictive lung diseases and for assessing the severity of pulmonary impairment.

Clinical Indications

  • Diagnosis of restrictive lung disease by measuring Total Lung Capacity (TLC)
  • Differentiating between obstructive and restrictive lung patterns when spirometry is inconclusive
  • Quantification of air trapping and lung hyperinflation in patients with COPD or Emphysema
  • Pre-operative evaluation prior to lung resection or major thoracic surgery
  • Monitoring patients on drugs with known pulmonary toxicity (e.g., Amiodarone, Bleomycin, Methotrexate)
  • Evaluation of dyspnea that is out of proportion to standard spirometry findings
  • Assessment of respiratory muscle weakness or chest wall deformities
  • Management and staging of Interstitial Lung Disease (ILD) and Sarcoidosis
  • Monitoring of lung transplant recipients for rejection or bronchiolitis obliterans

Procedure Steps

  1. Calibrate the body plethysmograph cabin pressure and the flow sensors (pneumotachograph) according to manufacturer standards.
  2. Perform a safety check of the airtight seal and ensure the patient is comfortably seated inside the box with the door securely closed.
  3. Instruct the patient to use a nose clip and breathe through the mouthpiece to establish a stable tidal breathing pattern.
  4. Perform the thoracic gas volume (TGV) maneuver where the patient pants gently against a closed shutter at the end of a normal expiration (FRC level).
  5. Simultaneously record the changes in mouth pressure and box pressure to calculate the volume of gas in the thorax.
  6. Perform the airway resistance (Raw) maneuver by having the patient pant with an open shutter to measure the relationship between flow and pressure.
  7. Conduct at least three acceptable and reproducible trials to ensure data validity as per ATS/ERS guidelines.
  8. Follow the plethysmography maneuvers with a slow vital capacity (SVC) maneuver to determine the relationship between FRC, RV, and TLC.
  9. Clean and disinfect the mouthpiece and internal components of the plethysmograph according to infection control protocols.

Coding Guidelines

  • CPT 94726 is a standalone code for lung volume and/or resistance determination via plethysmography.
  • Do not report 94726 in conjunction with 94727 (gas dilution or nitrogen washout) during the same session, as these are alternative methods for lung volume determination.
  • If spirometry (e.g., 94010 or 94060) is performed during the same encounter, it should be reported separately as it measures different physiological parameters.
  • Use modifier 26 if only the professional component (interpretation and report) is provided by the physician.
  • Use modifier TC if only the technical component (equipment and technician) is provided by the facility.
  • For pediatric patients, ensure the patient is old enough (typically 5-6 years and older) to cooperate with the panting maneuvers required for a valid test.
  • Check NCCI edits to ensure bundling compliance with other pulmonary diagnostic procedures like bronchodilator responsiveness (94060).

Associated ICD-10 Codes