Lung Function Testing Flashcards
What are different ways of measuring lung function?
- Peak flow (home)
- Spirometry or oximetry (gp)
- In specialist labs they can do spirometry, transfer factor, lung volumes, blood gases, bronchial provocation testing, resp muscle testing and exercise testing
Explain spirometry and its features
It is a forced expiratory manoeuvre from total lung capacity followed by full inspiration. Best of 3 attempts. Pitfalls of spirometry include the need for trained technicians, test is effort dependant, and may not be as good in frail or poorly patients
What are some of the values measured in spirometry?
Tidal volume (tv) – normal volume breathed at rest.
Forced vital capacity (FVC) – Litres of air expired
FEV1 – Forced expiratory volume in one second. This allows us to calculate the FEV1/FVC ratio. Normal is over 70%
An FEV1/FVC ration under 70% indicates what?
An obstructive lung disease, generally asthma or COPD. Mild COPD has over 80% of predicted FEV1. Moderated is 50-80% of predicted FEV1. Severe has 30-50% of predicted FEV1 and very severe which has less than 30% of the predicted FEV1
What is predicted FEV1 and its uses?
Patient is given a percentage of how their FEV1 compared to the predicted. Predicted values are corrected for age, gender, race, height and atmospheric values. It is used to determine the severity of COPD
What is reversibility testing in spirometry?
When a patient is given nebulised/inhaled salbutamol and their spirometry is taken before and 15mins after salbutamol. A reversibility of 15% and 400ml of FEV1 is suggestive of asthma.
Other than reversibility testing, what are other investigations for asthma?
- Peak flow testing to look for diurnal variation/variation over time, response to inhaled corticosteroids or occupational asthma.
- Bronchial provocation
What are the spirometry results of restricted lung disease?
Both FEV1 and FVC are reduced but the ratio is still over 70%, it is just small.
What are some causes of restrictive spirometry results?
- Interstital lung disease
- Kyphoscoliosis/chest wall abnormalities
- Previous pneumonectomy
- Neuromuscular disease
- Obesity
- Poor effort/technique
Briefly explain a method to interpreting spirometry results
- Look at FEV1/FVC ratio, if less than 7-% then obstructive.
- If obstructive then look at % predicted FEV1 to determine severity and any reversibility to determine whether COPD or asthma.
- If FEV1/FVC ratio is normal then look at % predicted FVC (if low then restrictive)
What is transfer factor for carbon monoxide (DLCO, TLCO)
It is a spirometry test which involves the patient inhaling a small (safe) breath of a low conc of carbon monoxide and then measures the concentration of carbon monoxide in expired gas to derive uptake of CO in the lungs. As CO has a very high affinity for Hb. It is affected by alveolar surface area, pulmonary capillary blood flow, haemoglobin concentration and ventilation perfusion mismatch.
Transfer factor for carbon monoxide is reduced in what?
Emphysema, interstitial lung disease, pulmonary vascular disease and anaemia.
What are the methods of measuring lung volumes?
Helium dilution (isn’t used much anymore) or body plethysmography (patient does respiratory manoeuvres in a sealed box which causes a change in air pressure. Lung volumes will be reduced in restrictive lung disease and the residual volume will be increased.
What is oximetry and describe some of its features
Non-invasive measurement of haemoglobin saturation by oxygen. It is dependant on adequate perfusion so not reliable in shock/cardiac failure. Does not measure carbon dioxide levels so isn’t a measure of ventilation.
What are the main causes of hypoxaemia?
Hypoventilation (drugs or neuromuscular disease), ventilation/perfusion mismatch, shunt (congenital heart disease) and low inspired oxygen (high altitudes).
