Respiratory function tests
In secondary care, a systematic combination of lung function tests are carried out in lung function laboratories. The two tests of lung function that are often carried out routinely, in addition to spirometry, are the measurement of lung volumes and diffusing capacity. The gas held by the lungs is thought of in terms of subdivisions, or specifi c lung volumes.
Lung volume measurements
✔ One important lung volume, residual volume (RV), cannot be measured using simple spirometry because gas remains in the lungs at the end of each breath ✔ If no air was left in the lungs at the end of expiration, the lungs would collapse ✔ The RV needs to be measured so that the functional residual capacity (FRC) and total lung capacity (TLC) can be calculated ✔ In healthy subjects RV is approximately 30% of TLC ✔ In obstructive lung diseases the lungs are hyperinflated with ‘air trapping’ so that RV is greatly increased and the ratio of RV to TLC is also increased. There are three methods of measuring RV: 1. Nitrogen washout 2. Helium dilution 3. Plethysmography. ********************************** 1. Nitrogen washout • This test measures FRC which can be used in combination with plethysmography to determine the amount of trapped gas in the lungs • This technique is based on the assumption that the concentration of nitrogen (N2) in the lungs is 75–80% It consists of a rapid response gas analyser in conjunction with a spirometer, to provide breath-by-breath analysis • The patient breathes 100% O2 for several minutes • After this time N2 is gradually washed out of the lungs • There is a residual N2 remaining which is why the test is continued until alveolar concentrations of N2 reach 1% (approximately 7 minutes) • In the presence of airflow obstruction and air trapping, the test may be extended to increase accuracy. 2. Helium dilution • Helium is an inert gas that can be used to indirectly calculate FRC • A dry rolling seal spirometer is filled with a known concentration of helium (approximately 10%) room air and O2 • Measurements of the exact concentrations are recorded prior to testing, the patient then re-breathes the gas concentration until equilibrium is reached within the lungs and the re-breathe circuit • It is essential to control for the levels of CO2 during this procedure • Lung divisions such as RV, TLC, ERV and IC are derived from a relaxed VC manoeuvr. 3. Plethysmography • Determines changes in lung volumes by recording changes in pressure • The patient sits in a large airtight box and breathes through a mouthpiece At the end of a normal expiration, a shutter closes the mouthpiece and the patient is asked to make respiratory efforts • As the patient tries to inhale, box pressure increases • This method measures all intrathoracic gas including cysts and bullae.
How to interpret respiratory function tests
✔ Multiple FRC measurements by gas dilution should be made ✔ Results should be interpreted with at least two trials agreeing within 10% of the mean ✔ ERV measurements should agree within 5% or 60ml of the mean, whichever is the larger ✔ Variable measurements should be documented as not achieving the reproducibility criteria ✔ FRC varies with body size, change in posture, diurnal variation and racial or ethnic background ✔ FRC> 120% predicted is suggestive of air trapping ✔ Increased RV demonstrates that the lungs contain an abnormally increased volume ✔ Elevated RV may occur in acute asthmatic exacerbation but this is usually reversible ✔ Fixed obstructive lung diseases, especially emphysema result in an increase in RV and RV:TLC ratio caused by chronic air trapping ✔ As RV increases greater ventilation is required and the work of breathing increases for adequate gas exchange ✔ This correlates with an increase in either TV or breathing frequency or a combination of both ✔ Increased RV is also associated with type I or type II respiratory failure ✔ In severe emphysema the TLC can show a marked increase Restrictive lung disease is associated with a decrease in FRC, RV and TLC.
💡💡 Diffusing capacity test ✔ This is used for measuring the diffusion membrane capacity of the lungs ✔ It is carried out by estimating the uptake of carbon monoxide (CO) by the pulmonary gas exchange mechanism ✔ CO readily combines with haemoglobin and is a fast way of deriving diffusing capacities of the lung ✔ The method most commonly used is the single-breath CO diffusing capacity (DLCO) ✔ DLCO measures the milliliters of CO diffused per minute across the alveolocapillary membrane ✔ The test can help identify impairment in both obstructive and restrictive lung disease ✔ Diffusing capacity tests can involve the patient holding their breath for ten seconds; this may be problematic in the very breathless patient. 💡💡 Indications for and interpretation of the test ✔ Evaluation and follow-up of interstitial lung disease such as idiopathic fibrosing alveolitis. ✔ In interstitial lung disease the DLCO is impaired due to loss of lung capillaries ✔ Evaluation of patients with emphysema, which is characterized by the destruction of the alveolar membrane. ✔ The capacity for gas exchange is compromised and the DLCO will be decreased. ✔ Evaluation of other diseases associated with dust inhalation such as asbestosis, which results in a low DLCO as the airborne asbestos particles cause diffuse fi brosis ✔ Drug related reactions such as cardiac drugs (amiodarone) and rheumatoid drugs ✔ Evaluation of patients with arterial desaturation on exercise. ✔ This is found in some patients with obstructive lung disease, but is more profound in those with restrictive disease. ✔ A DLCO < 50% is usually accompanied by a clinically signifi cant fall in oxygen ✔ Following inhalation of toxic gas or organic agents. ✔ This may cause inflammation (alveolitis) which causes a reduction in diffusion which may be due to a related loss in lung volume ✔ Assessment of disability.