What is peak flow?

PEFR (peak expiratory flow rate) is a form of pulmonary function test. It measures the fastest rate of air that can be expired.

The convenient size of the peak flow meter makes regular measurement of forced expiration possible at home. The significance of its moderate 'pocketability' is that patients can write down multiple readings in a day, recording them in a peak flow diary. This provides information on:

  • diurnal variability
  • response to bronchodilators
  • monitoring severity of disease

The main use of peak flow is in asthma. Although it reveals some information about COPD it tends to underestimate the severity and therefore is unreliable.

What is spirometry?

Spirometry is a form of pulmonary function test. Patients blow into a tube as strongly as possible, expiring until they have no breath left.

What do all the acronyms mean?

FEV1: forced expiratory volume in one second

VC: vital capacity - how much air you can breath out after as deep a breath as possible. Crucially it is not 'forced'...

FVC: forced vital capacity. Essentially the vital capacity but blowing out as strongly as possible.

*In normal young people VC and FVC are most likely to be equal. However, in emphysematous lungs elasticity of the airways is lost. This elasticity is what normally keeps the airways patent during expiration. If the elasticity is not present the airways can become closed on forceful expiration and so not all the air is expelled. The key word to describe this phenomenon in OSCEs is 'trapping'. It is important to be aware of this disparity between VC and FVC in emphysematous patients.

When is spirometry indicated?

Spirometry is a cheap, non-invasive, valuable investigation, available in both primary and secondary care. It is also a dynamic investigation, with the following uses:

  • Confirm diagnosis of obstructive (e.g. COPD) or restrictive lung disease;
  • Assessment of disease severity
  • Measuring deterioration of disease

 

Pulmonary function tests

Most general practices have their own spirometer. In secondary care formal request may be necessary but many thoracic departments have a spirometer in each ward. If this is the case in your teaching hospital take the opportunity to do patients' pulmonary function. It is not uncommonly requested by the consultant. It is a basic piece of equipment so you only need to do it once to be covered for any potential OSCE station.

Interpretation - Part 1

1. Details

  • Name
  • Age
  • Sex
  • Height
  • Ethnicity

2. Reproducibility

Look at the variability (Var). BTS states that there must be at least two values within 3% of each other for the result to be reliable.

Patient A (figures)

3. Is it technicaly acceptable?

  • Did they cough during the expiration?
  • Did they take an extra breath during expiration?
  • Maybe they had a slow start?
  • Was it a sub-maximal effort?

All of these produce dents and artefacts in the cruve produced. It may be difficult to achieve a perfectly accurate graph but the result should be as smooth as possible.

Patient A (graphs)

Interpretation - Part 2

Is the ratio FEV1/FVC <70%?

Yes: obstructive lung disease

No: may be restrictive lung disease

 

Our patient above appears to have an obstructive pattern. Another clue towards this is the shape of the curve, which will have a flatter gradient the more obstruction there is.

Interpretation - Part 3 (Obstructive pattern)

Is this pattern reversible? Or is it fixed?

To test for reversibility either:

a) Repeat test 15 minutes after: salbutamol inhaler through spacer (400mcg)

b) Repeat test 15 minutes after: salbutamol nebuliser (5mg)

To determine reversibility it is the FEV1 figure we are interested in. Has the amount of air the patient can forcefully blow out in one second increased after a bronchodilator? Is there 'reversibility'? An increase of 400ml is said to be significant. Look in the 'Best spirometry result' section below (same patient again) and determine whether his obstructive disease is reversible or not.

 

Our patient's FEV1 increased from 1.21L to 1.63L, a difference of greater than 400ml. His airway disease follows a reversible obstructive pattern, e.g. asthma.

Interpretation - Part 3 (Restrictive pattern)

It is important to check for restrictive lung disease regardless of whether an obstructive pattern is present or not. Clearly COPD patients and other obstructive disease individuals can have an element of restrictive pattern as well.

Therefore:

Is the % predicted FVC < 80%?

 

Our 72 year old Caucasian man with reversible obstructive disease has a % predicted FVC of 91%. This is greater than 80% and so there is no additional restrictive element.

Clearly, in other situations there will be patients with no obstructive disease but a restrictive pattern. These can be divided into pulmonary problems and non-pulmonary causes:

1. Pulmonary: fibrosis; pneumoconiosis; pulmonary oedema; parenchymal lung tumours; lobectomy/pneumectomy.

2. Non-pulmonary: neuromuscular disorders; postural problems (e.g. thoracic cage deformity or kyphoscoliosis); obesity; pregnancy.

Additional example

Is there satisfactory reproducibility?

Is it technically acceptable?

Is the pattern obstructive or restrictive, or combined?

If there is an obstructive pattern, is it reversible or fixed?

If the obstruction is fixed, what grade of severity is their COPD?

FEV1 ≥ 80%   Stage 1 - Mild

FEV1 50-79%   Stage 2 - Moderate

FEV1 30-49%   Stage 3 - Severe

FEV1 < 30%    Stage 4 - Very severe

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