These tests are mainly used to assess the degree of airflow limitation present during expiration.
PEAK EXPIRATORY FLOW RATE (PEFR). This is an extremely simple and cheap test. Subjects are asked to take a full inspiration to total lung capacity and then blow out forcefully into the mini-Wright peak flow meter, which is held horizontally; the lips must be placed tightly around the mouthpiece. The best of three tests is recorded.
Although reproducible, PEFR is not a good measure of airflow limitation since it only measures the expiratory flow rate in the first 2 ms of expiration and overestimates lung function in patients with moderate airflow limitation. PEFR is best used to monitor progression of disease and its treatment. Regular measurements of peak flow rates on waking, during the afternoon, and before bed demonstrate the wide diurnal variations in airflow limitation that characterize asthma and allow an objective assessment of treatment to be made.
SPIROMETRY. The Vitalograph spirometer measures the FEV I and the forced vital capacity (FVC). Both the FEV I and FVC are related to height, age and sex. The instrument used is shown. The technique involves a maximum inspiration followed by a forced expiration (for as long as possible) into the dry bellows spirometer. The act of expiration triggers the moving record chart, which measures volume against time. The record chart moves for a total of 5 s, but expiration should continue until all the air has been expelled from the lungs, as patients with severe airflow limitation may have a very prolonged forced expiratory time. This is demonstrated on the record chart The FEV, expressed as a percentage of the FVC is an excellent measure of airflow limitation. In normal subjects it is around 75%. With increasing airflow limitation the FEV, falls proportionately more than the FVC, so that the FEV,/FVC is reduced. With restrictive lung disease the FEV, and the FVC are reduced in the same proportion and the FEV /FVC remains normal or may even increase because of the enhanced elastic recoil. In chronic airflow limitation (particularly in emphysema and asthma) the total lung capacity (TLC) is usually increased, yet there is nearly always some reduction in the FVC. This is the result of disease in the small airways causing obstruction to airflow before the normal RV is reached. This trapping of air within the lung (giving an increased Rv) is a characteristic feature of these diseases.
OTHER TESTS. Tests such as the measurement of airways resistance in a body plethysmograph are more sensitive but the equipment is expensive and the necessary manoeuvres are too exhausting for many patients with chronic airflow limitation.
FLOW-VOLUME LOOPS. The ability to measure flow rates against volume (flow-volume loops) enables a more sophisticated analysis to be made of the site of airflow limitation within the lung. At the start of expiration from TLC, the site of maximum resistance is the large airways, and this accounts for the flow reduction in the first 25% of the curve. As the lung volume reduces further, so the elastic pressures within the lung holding open the smaller airways reduce, and disease either of the lung parenchyma or the small airways themselves becomes readily apparent. For example, in diseases such as chronic bronchitis and emphysema, where the brunt of the disease falls upon the smaller airways, expiratory flow rates at 50% or 25% of the vital capacity may be disproportionately reduced when compared with flow rates at larger lung volumes.
LUNG VOLUME. The subdivisions of the lung volume are shown in Fig. 12.16. Tidal volume and vital capacity can be measured using a simple spirometer, but the TLC and RV need to be measured by an alternative technique. TLC is measured by connecting the lungs to a reservoir containing a known amount of non-absorbable gas (helium) that can readily be measured. If the concentration of the gas in the reservoir is known at the start of the test and is measured after equilibration of the gas has occurred (when the patient has breathed in and out of the reservoir), the dilution of the gas will reflect the TLC. This technique is known as helium dilution. RV can be calculated by subtracting the vital capacity from the TLC.
The TLC measured using this technique is inaccurate if large cystic spaces are present in the lung, because the helium cannot diffuse into them. Under these circumstances the thoracic gas volume can be measured more accurately using a body plethysmograph. The difference between the two measurements can be used to define the extent of non-communicating air space within the lungs.
Measurement of blood gases
The measurement of the partial pressure of both oxygen and carbon dioxide within arterial blood is an extremely useful test in diseases of the respiratory and circulatory systems. It is essential in the management of cases of respiratory failure and severe asthma, when repeated measurements are often the best guide to therapy.
TRANSFER FACTOR. This measures the transfer of gas across the alveolar-capillary membrane and reflects the uptake of oxygen from the alveolus into the red cell. A low concentration of carbon monoxide is inhaled and is avidly taken up in a linear fashion by circulating haemoglobin, the amount of which must be known when the test is performed. In normal lungs the transfer factor is a true measure of the diffusing capacity of the lungs for oxygen and depends on the thickness of the alveolarcapillary membrane. In lung .di~ease the diffusing capacity (Dca) also depends on the VIQ relationship as well as on the area and thickness of the alveolar membrane. To control for differences in lung volume, the uptake of carbon monoxide is related to the lung volume; this is known as the transfer coefficient (Kea).
Gas transfer is usually reduced in patients with severe degrees of emphysema and fibrosis. Overall gas transfer can be thought of as a relatively non-specific test of lung function but one that can be particularly used in the early detection and assessment of progress of diseases affecting the lung parenchyma (e.g. cryptogenic pulmonary fibrosis, sarcoidosis and asbestosis).