Spirometry

Good link for normal values here.

Remember:

  • FEV1/FVC ratio is 70% or higher in restrictive disease and <70% in obstructive.
  • Restrictive disease will show reduction in both FEV1 and FVC, proportionately (FEV1 <80% but FEV1/FVC ratio 70% or higher)
  • Reversibility testing can be done with 2.5 mg Salbutamol, 2 weeks of 30 mg oral pred, or six minutes of exercise. Positive reversibility is >15% FEV1.
  • Spirometry can underestimate the FVC in patients with severe COPD as it takes them longer than 15 seconds to empty their lungs.

NICE guidance on COPD classification:

  • Mild (Stage 1) – FEV1 80% or greater
  • Moderate (Stage 2) – FEV1 50 – <80% predicted
  • Severe (Stage 3) – 30 – FEV1 <50% predicted
  • Very severe (Stage 4) – FEV1 <30% predicted, or FEV1 <50% predicted with associated respiratory failure

all of which should also have FEV1/FVC <70%

Further notes: 21.4.18

I’m just redoing the BMJ Learning module on spirometry (the GP in our practice who specialises in respiratory problems is leaving, so it seemed a good time to brush up) and thought that making notes here to keep it all in one place would be better than starting a new post.

Spirometry quality: A good-quality spirometry test should have at least three good blows, of which at least two are within 100 ml or 5% of each other. This may take a lot of blows.

Terminology refresh:

  • Tidal volume is the amount of air that moves in or out of the lungs when breathing without conscious effort.
  • Inspiratory reserve volume is the extra air that can be drawn in when filling the lungs to maximum capacity.
  • Inspiratory capacity is the sum of the above two.
  • Expiratory reserve volume is, obviously, the extra air that can be exhaled when expiring as much as possible after finishing a normal breath.
  • Vital capacity is the sum of tidal volume + IRV + ERV; in other words, the amount of air that can be produced by taking the deepest breath possible and blowing as much as possible of it out.
  • Residual lung volume is the amount of air that remains in the lungs even after exhaling as fully as possible.
  • Functional residual capacity is the amount of air remaining in the lungs after a normal, unforced expiration; in other words, the sum of residual lung volume plus expiratory reserve volume.
  • Total lung capacity is the sum of vital capacity + residual lung volume.

Easy, right? Those physiology lectures are coming back nicely.

Also remember FEV1, which is the amount of air exhaled in one second of forced maximal exhalation after inhaling to maximum capacity and, along with FVC (forced vital capacity) is one of the two key measurements in spirometry.

FEV1/FVC ratio should be >70%; <70% is classically considered diagnostic of obstruction.However, it’s now more complicated than that; BTS/SIGN guidelines, as well as GOLD (Global Initiative for Chronic Obstructive Lung Disease) guidelines, now acknowledge that this may lead to overdiagnosis in older adults and less frequent diagnosis in the under-45s. So shouldn’t we be going with age-adapted figures, in that case? Well, it’s complicated; BTS/SIGN wants us to use LLN values in asthma diagnosis, even though they classify the bottom 5% of the healthy population as abnormal, NICE says to use this value if available, or the 70% cut-off if not. For COPD diagnosis, we’re still meant to use the fixed ratios (GOLD). Give it a few years and they’ll all get their stories straight on that one and find something else to disagree over and confuse us about.

Normal volume-time graphs should have a sharp rise before levelling off; kind of curved like a camper van corner. Obstructive spirometry gives a graph that slopes upwards much more gradually. Restrictive spirometry gives a similar shape to the normal spirometry, but shorter, with the flattening off coming sooner.

Note that bronchiectasis can cause obstruction, although one or more airways will be irreversibly dilated; there is chronic airway inflammation and excess mucus production.

If patients with a typical COPD history have obstructive spirometry, NICE advises that reversibility testing is not necessary.

In assessing suspected asthma, the BTS/SIGN algorithm now allows for spirometry to be omitted in cases where the probability of asthma is high; in such cases, it is reasonable to go for a trial of treatment and monitor the results with peak flows or ‘validated questionnaires’ (the module doesn’t say which these are or where to get them). However NICE guidelines still advise always using an objective test, starting with spirometry (how things proceed depends on whether the patient is under or over 17).

When the probability of asthma is intermediate (the patient has some of the features but not all), BTS/SIGN guidelines do recommend spirometry, preferably using LLN, as above. In cases where it’s obstructive, BTS/SIGN recommend reversibility testing either with bronchodilators or with repeat of spirometry after six weeks of treatment with ICS (or both). An FEV1 improvement of 12% or more with 200 ml or more increase in volume is regarded as positive. However, note that some people with COPD have significant reversibility. An improvement of >400ml in FEV1 strongly suggests asthma.

Aha – and, in the midst of all this confusing debate, the Primary Care Respiratory Society of the UK has helpfully published a consensus statement for GPs which can be downloaded here. Thank you, guys!

Indications for specialist referral

  • Abnormal spirometry with uncertain diagnosis
  • Suspected occupational asthma
  • Symptoms of COPD reducing quality of life even on optimal treatment
  • Rapidly progressive disease
  • Signs of cor pulmonale
  • COPD in <35-year-olds
  • Symptomatic with spirometry suggestive restrictive airways disease
  • Mixed restrictive and obstructive pattern
  • O2 <92% on air
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About Dr Sarah

I'm a GP with a husband and two young children.
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