Measuring respiratory resistance with the APD

Inspiratory and expiratory flow rates are a function of driving pressure (i.e. the pressure difference between the alveoli and the atmosphere) and airway resistance. For this reason it would seem that airway resistance should be one of the most commonly performed pulmonary function tests but instead it is the outcome of airway resistance and driving pressure, i.e. the expiratory and inspiratory flow rates that are measured almost exclusively. One reason for this is that resistance measurements requires relatively expensive equipment such as a body plethysmograph or an impulse oscillometer as well as a fair amount of technical expertise.

The airflow perturbation device (APD) is a potentially inexpensive system for measuring respiratory resistance during tidal breathing. The device itself is mechanically simple, the concepts and mathematics that permit it to work are, however, a bit more complicated.

The APD consists of a mouth pressure transducer and a pneumotach whose end is attached to a rotating wheel. The wheel has open segments and segments with a mesh that partially obstructs airflow through the pneumotach. The rotation of the wheel causes a series of perturbations to the airflow through the pneumotach.

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Social Security Disability Evaluations

I was contacted recently by an individual with some questions about the pulmonary function testing needed for a Social Security Disability evaluation. With a small amount of research I was able to answer their questions but this brought up an interesting point and that is that despite the number of patients we see every year my lab only rarely performs any pulmonary function testing for disability evaluations. The reason I know this is because the Social Security Administration (SSA) has very specific requirements for the content and form of pulmonary function reports and we are very rarely asked for these reports.

The pulmonary function tests the SSA uses as part of a disability evaluation are:

  • Spirometry
  • Diffusing Capacity (DLCO)
  • ABG
  • Pulse Oximetry

Interestingly, lung volume measurements are not included. This is not specifically explained but it appears to be because evaluation for restriction is covered by the criteria for FVC and FEV1.

For all pulmonary function tests the SSA requires that the individual be medically stable, which they define as not:

  • Within 2 weeks of a change in prescribed respiratory medication.
  • Experiencing, or within 30 days of completion of treatment for, a lower respiratory tract infection.
  • Experiencing, or within 30 days of completion of treatment for, an acute exacerbation of a chronic respiratory disorder.
  • Hospitalized, or within 30 days of a hospital discharge, for an acute myocardial infarction (heart attack).

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CPT Codes

About a month or so ago I was corresponding with the manager of a small PFT lab and in response to one of their questions I had mentioned that there were no CPT codes for MIP/MEP. They responded with “what’s a CPT code?” so I guess this means that CPT codes aren’t as well known as I thought they were.

CPT stands for Current Procedural Terminology and is managed by the American Medical Association. CPT codes are a relatively universal way to classify and describe all medical tests and procedures. They are also used by all insurance companies for medical billing so one downside to this is if there isn’t a CPT code for a test or a procedure, you can’t bill for it. CPT codes also include conditions that limit performing (or at least billing for) some tests in various combinations and to some extent this drives the way PFT tests are ordered and performed.

The CPT codes are reviewed, revised and updated annually. There have been a number of additions and changes to PFT CPT codes during the last five to ten years, and I’d say that with a few notable exceptions, most current PFT testing is adequately covered by the CPT codes. The current PFT CPT codes are:

CPT: Description: Exclusions:
94010 Spirometry, including graphic record, total and timed vital capacity, expiratory flow measurement(s), with or without maximum voluntary ventilation. Do not report in conjunction with 94150, 94200, 94375, 94728.
94011 Measurement of spirometry forced expiratory flows in an infant or child through 2 years of age
94012 Measurement of spirometry forced expiratory flows, before and after bronchodilator, in an infant or child through 2 years of age.
94013 Measurement of lung volumes (i.e., functional residual capacity (FRC); forced vital capacity (FVC), and expiratory reserve volume (ERV) in an infant or child through 2 years of age.
94014 Patient-initiated spirometry recording per 30 day period of time; includes reinforced education, transmission of spirometry tracing, data capture, analysis of transmitted data, periodic recalibration and review and interpretation by a physician or other qualified health professional.
94015 [patient-initiated spirometry] recording (includes hook-up, reinforced education, data transmission, data capture, trend analysis, and periodic recalibration).
94016 [patient-initiated spirometry] review and interpretation only by a physician or other qualified health professional.
94060 Bronchodilator responsiveness, spirometry as in 94010, pre- and post-bronchodilator administration. Do not report in conjunction with 94150, 94200, 94375, 94728. For prolonged exercise test for bronchospasm with pre- and post-spirometry use 94620.
94070 Bronchspasm provocation evaluation, multiple spirometric determination s as in 94010, with administered agents (eg. antigen(s), cold air, methacholine).

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