Pulmonary Function Tests

Pulmonary function testing (PFT) includes spirometry, static lung volumes, and gas exchange (diffusing capacity).

They are used as a screening tool for lung disease, to assess respiratory impairment, monitor disease course or response to treatment, identify baseline lung function for occupation, or to provide preoperative assessment.

 

Factors influencing normal values include:

 

 

 

Indications and Contraindications for PFTs

 

Commonly evaluated diseases include:

asthma: measure at least q6-12 months for diagnosis, severity, and response to treatment

COPD: use PFTs for early detection, assessment of severity, and monitoring response to therapy

The following people should also be screened:

 

Contraindications

PFTs should not be performed if patients have:

PFTs are less accurate if there is chest, abdominal, face, or mouth pain; headache, stress incontinence; in children; and with dementia or confusion.

 

 

Measures of Lung Function and Volume

  • spirometry
  • lung volumes

Spirometry

FVC = forced vital capacity = volume exhaled after maximal inspiration through to maximal expiration

FEV1 = forced expiratory volume in 1 sec

FEV1/FVC = ratio = percent exhaled within first second

FEF25-75 = forced expiratory flow: the % exhaled between 25%-75%

PEFR = peak expiratory flow rate: highest at first because of mechanical advantage and traction of airways; also a measure of effort

 

Lung Volumes

  • VC = vital capacity
  • IRV = inspiratory reserve volume
  • TV = total volume
  • ERV = expiratory reserve volume

 

 

 

 

Obstructive and Restrictive Disease Patterns

Lung disease, both obstructive and restrictive, cause FEV1 to be below 80% of expected.

Aide Memoire
oBstructive = Below

 

Obstructive disease is characterized by reduced expiratory airflow and not a loss of lung capacity. Accordingly, they have a higher FVC than restrictive diseases. Examples of obstructive diseases include:

  • COPD
  • asthma
  • cystic fibrosis
  • upper airway obstruction

Obstructive disease severity is determined by FEV1.

Restrictive diseases also reduce FEV1 and FVC but have a normal or increased ratio, as FVC can be markedly reduced through a restriction in avalaible lung volumes. These patterns are caused by diseases in the:

  • lung parenchyma (ie interstitial lung disease)
  • pleura or chest wall (scoliosis)
  • neuromusculature (muscular dystrophy)
  • increased BMI

Restrictive disease severity is determined by TLC.

 

If the ratio is below 70% and the FVC is low, ie below 80% predicted, it suggests a combination of obstructive and restrictive disease, or obstructive disease with hyperinflation.

 

In early obstructive impairments, FEF 25%-75% may be the first value to become abnormal, and obstructive diseases also have a smaller expired volume (hyperinflation).

 

Large airway obstruction appears as a flattened inspiratory (intrathoracic) or expiratory (extrathoracic) loop.

 

 

Flow Volume Loops

Expiratory flow limitation causes the linear portion of expiratory flow in the normal loop.

 

Scalloping occurs with obstructive diseases, such as asthma or COPD, due to increased lung compliance.

 

Values are compared with normal values on the basis of age, sex, height, and race.