Thursday, 1 December 2011

FEV1/FVC ratio

This is a ratio used to diagnose OBSTRUCTIVE and RESTRICTIVE lung disease.

Obstructive pattern

In an obstructive lung disease like COPD most of the air can squeeze its way out eventually but takes a long time to do so (due to an obstruction!) This might be a sputum plug or floppy airways seen in conditions such as emphysema or tracheomalacia.

  • Problems with airflow into and out of the lungs
  • Increased RV (residual volume of air left in the lungs after expiration)
  • This air-trapping can lead to hyperinflation (barrell chest etc)

Effect of FEV1/FVC
  • Reduced FEV 1
  • FEV1/FVC ratio less than 70% of predicted value (ability to exhale 70% of breath in 1 sec)

Obstructive pattern

Due to conditions in which the airways are obstructed eg asthma or COPD. The FEV1 and FVC are reduced disproportionately.

Restrictive pattern

In an restrictive lung condition getting the air out isnt an issue, its the fact that there's not a whole lot of air to get out in the first place! If you imagine stiff hardened lungs that have difficulty expanding, you're not going to be able to inhale a lot of air to expel. Pathologies like pulmonary fibrosis, sarcoidosis, and certain types of pneumonia can have this effect.

  • Lung expansion/compliance is RESTRICTED...
  • Which causes decreased  lung volumes...
  • Which causes increase WOB...
  • Leading to reduced ventilation (reduced V of the V/Q ratio)
  • Annnd if breathing is harder work, patient is more likely to fatigue and start retaining CO2 which is never good.

Effect of FEV1/FVC
  • Reduced FEV1 and FVC equally

Restrictive pattern

Due to conditions in which the lung volume is reduced eg fibrosing alveolitis, scoliosis. The FEV1 and FVC are reduced proportionately.

National guidelines vary:

FEV1 as a % of predicted

  • <80% = mild
  • <60% = moderate
  • <40% = severe

Diagrams referenced from:
  • <80% = mild
  • <50% = moderate
  • <30% = severe
    (NICE guidelines)

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