Lung Function Tests Flashcards
Why do we measure lung function?
Screening for COPD or occupational lung disease
Evaluation of the breathless patient
Lung cancer - fitness for treatment
Pre-operative assessment
Disease progression and treatment response
Monitoring of drug treatment toxic to the lungs
Pulmonary complications of systemic disease
What lung function tests can be performed at home?
Peak flow (oximetry)
What lung function tests can be performed at the GP surgery?
Spirometry test and oximetry test
What lung function tests can be done in a specialist lab?
Spirometry, transfer factor, lung volumes, blood gases, bronchial provocation testing, respiratory muscle function testing
Describe spirometry.
Forced expiratory manoeuvre from total lung capacity followed by a full inspiration
- take a big breath in as far as possible and blow out as hard as you can for a long as possible, then take a big breath all the way in
- best of 3 acceptable attempts
What are the problems with spirometry?
Requires an appropriately trained technician
Depends on the effort and technique of the patient
Patient frailty
Can be performed poorly if the patient is in pain or very unwell
What is the reserve volume?
When the patient has fully expired, this is the amount left in their lungs
What is the functional resistance capacity?
The amount left in the patients lungs after they have exhaled naturally (end tidal volume)
What is the total lung capacity in terms of a spirometry measurement?
The difference between an empty/collapsed lung and when the patient has fully inspired
What is the vital capacity of the lungs?
The difference between the fully inspired and the fully expired lungs
What are the inspiratory and expiratory reserve volume?
IRV - the difference between the tidal inspiration volume and the fully inspired volume
ERV - the difference between the end tidal volume and the fully expired lung
What should the normal FEV1/FVC ratio be?
greater than 70%
- less than this indicates airflow obstruction
What is the peak expiratory flow?
The maximum speed at which a person can exhale
What is the FEV (forced vital capacity)?
How much air a person can exhale during a forced breath
What is the FEV1 (forced expiry volume in one second)?
The volume of air blown out by a person in one second
What is a normal FEV1 (the normal reference range)?
85% of the predicated value is still considered normal
- this is adjusted for age, race, gender, height and atomspheric values
What are the different severities of COPD and their associated %predicated FEV1?
Mild COPD - more than 80%
Moderate COPD- 50-80%
Severe COPD - 30-50%
Very severe COPD - less than 30%
Describe what happens to a flow/volume loop when a person has COPD?
Inspiratory limb - normal
Expiratory limb
- slower expiration speed (forcing out air passed the obstruction
- scalloping - this is because the air in the large vessels is expired normally, but the the air in the smaller vessels is expired slower
General - vital capacity is reduced (residual volume increases)
How does COPD cause problems when expiring air from the small air vessels in the lungs?
The damage to the lungs is normally caused within the lungs so that the small vessels are effected. The inflammation/scarring causes the pressure outside the vessels to be greater than the pressure inside the vessels, so they collapse, and air has to be forced out.
How can COPD be differentiated from asthma on spirometry?
Reversibility testing
- a person with a suspected obstructive lung disease is given a spirometry test
- then they are given nebuliser salbutamol, and the spirometry test is performed again after 15 mins
- 15% and 400ml reversilbitiy in FEV1 is suggestive of asthma
What investigations can be done to diagnose asthma?
Reversibility testing PEFR testing - looking for diurnal variation and variation over time - response to inhaled corticosteroids - occupational asthma Bronchial provocation
What spirometry results are suggestive of resistive lung disease?
FEV1 and FVC are reduced - but has a normal curve
FEV1/FVC ratio will be greater than 70%
The goal lung volume is reduced - which is why the FVC is low
What are some forms of restrictive lung disease?
Interstitial lung disease (stiff lungs)
Chest wall abnormality
Previous pneumonectomy
Neuromuscular disease (Myasthenia Gravis or Guillain Barre syndrome)
Obesity
Some people have restrictive spirometry if it is done with poor effort or technique
What are the first three things you should look at when interpreting spirometry?
First look at the FEV1/FVC ratio, if it less than 70%, it is obstructive
- if obstructed, look at the % predicated FEV1 (severity) and any reversilbity (COPD vs asthma)
If the FEV1/FVC is normal, then you need to look at the % predicated FVC. If this is low it suggests restrictive abnormality
What is a transfer factor test?
A single breath of a very small concentration of carbon monoxide
- CO has very high affinity to Hb
- measure concentration in expired gas to derive uptake in the lungs
What factors affect the uptake of carbon monoxide in the lungs?
Alveolar surface areas
Pulmonary capillary blood volume
Haemoglobin concentration
Ventilation perfusion mismatch
When would CO uptake be decreased in the lungs?
Emphysema - reduces the surface area
Interstitial lung disease - this causes a ventilation/perfusion mismatch
Pulmonary vascular disease
Anaemia (increased in polychtaemia)
Why is it important to be able to measure lung volumes when diagnosing lung disease?
Lung volumes are reduced in restrictive lung disease
Residual volume and total lung capacity are increased in obstructive lung disease
As you are unable to measure lung volumes by spirometry, how can it be done?
Helium dilation - inspire a known quantity of inert gas
Body plethysmography
- respiratory manoeuvres in a sealed box lead to changes in air pressure can from this, lung volumes can be worked out
What is oximetry and how is it performed?
A non-invasive measure of saturation of haemoglobin by oxygen
- oxyhaemoglobin and deoxyhaemoglobin absorb infrared light differently, so the amount of oxygenated haemoglobin
- depends upon adequate perfusion
What are the drawbacks of oximetry testing?
Doesn’t measure carbon dioxide, so you have no measurement of ventilation
Perfusion is still normal is patients with COPD and acute asthma, so gives false reassurance
What are the main causes of hypoxaemia?
Hypoventilation - e.g. Drugs, neuromuscular disease
Ventilation/perfusion mismatch - COPD, pneumonia
Shunt, causes deoxygenated haemoglobin to bypass the lungs - e.g. Congenital heart disease
Low inspired oxygen - altitude
What is ventilation/perfusion mismatch?
Main cause of hypoxaemia
Areas of lungs that are well perfumed, but not well ventilated (e.g. Pneumonic consolidation)
- this deoxygenated blood then mixes with blood from well ventilated parts of the lungs, which causes hypoxaemia
As no air is meeting this part of the lung, it does not correct with oxygen administration
How can a ventilation perfusion mismatch be diagnosed?
The difference between the arterial and the alveolar oxygen partial pressures should be less than 2-4kPa - any more suggests mismatch
- arterial pO2 can be directly measured by arterial blood gas
- alveolar pO2 can be found with this equation
Inspired oxygen concentration - (1.25 x PaCO2)
When should a person in respiratory failure be given additional oxygen?
When their arterial blood gas shows a pO2 of less than 8
How can you tell the difference between type 1 and type 2 respiratory failure?
Type 1 - normal PCO2
Type 2 - high PCO2
What happens to arterial blood gases in acute respiratory acidosis?
Elevated pCO2
Normal bicarbonate
Elevated hydrogen
What happens to arterial blood gases in compensated respiratory acidosis - suggestive of a chronic condition?
Normal hydrogen levels
Elevated pCO2
Elevated bicarbonate (renal compensation)
What happens to arterial blood gases in decompensated respiratory acidosis?
Elevated hydrogen levels
Elevated bicarbonate levels
Elevated pCO2 levels
