TESTS OF GAS EXCHANGE
For efficient respiratory gas exchange, the major processes of lung ventilation, diffusion across the alveolar-capillary memebrane and lung perfusion, should be intact. Normal distribution of the inspired air depends on an adequate thoracic ‘bellows’ action followed by unimpaired flow through the bronchial tree to the lung periphery where molecular diffusion occurs into the alveolar compartments.
The efficiency and uniformity of ventilation can be assessed by measuring nitrogen concentration at the mouth while breathing 100% oxygen. If gas mixing is perfect the expired nitrogen concentration will fall by the same proportion with each breath but, in generalised airways obstruction, wash-out is much slower and more uneven. The single breath nitrogen test or closing volume is a variation in which a single full inspiration of 100% oxygen is followed by a slow full expiration with monitoring of nitrogen concentration at the mouth.
This test and radioactive gas techniques (for estimating topographical inequality of ventilation) will not be discussed further. Oxygen and carbon dioxide cross the alveolar-capillary membrane by diffusion. Molecules of oxygen have to cross alveolar epithelium, interstitium, endothelium, blood plasma, red cell membrane and then chemically combine with haemoglobin (Fig. 3/5). Carbon monoxide (CO) is also avidly taken up by haemoglobin and serves asa useful inhaled gas in estimating the ease of transfer across these barriers - carbon monoxide transfer factor (TLCO).
The patient takes a vital capacity breath of CO and helium, holds the breath for 10 seconds and then exhales. As helium is not taken up by the pulmonary blood it gives the dilution of the inspired gas with alveolar gas and thus the initial alveolar Pco. It is possible, using a simple equation, to calculate the volume of CO taken up per minute, per mmHg alveolar Pco, i.e. TLCO. As a bonus, the helium dilution techi^ue also gives an estimate of the alveolar volume or, at least, that volume that the inspired CO ‘sees’ while the breath is held.
In a patient with generalised airways obstruction this volume will be much less than the true alveolar volume because of the impaired distribution of the inspired volume, and CO transfer will also be reduced because of the reduced area ‘seen5. To allow for this reduced volume effect the transfer coefficient (or transfer factor divided by the alveolar volume obtained during the test) is estimated. This helps to differentiate between a real reduction in CO transfer or a reduction solely due to the CO being presented to an abnormally small volume or surface area. A patient who has had a pneumonectomy may have a; perfectly functioning single lung but his transfer factor will be reduced by about 50 per cent because of the halving of his total lung capacity. The transfer coefficient, or KCO as it is known, corrects for this volume effect.
The transfer factor is determined by the thickness and area of alveolar membrane, the size of the pulmonary capillary volume and how well ventilation is matched to pulmonary perfusion. It will therefore be affected by such diverse disorders as anaemia, pneumonectomy, emphysema and pulmonary embolus.
EXERCISE TESTING
In the same way that any piece of machinery cannot be considered fully tested until maximally stressed, the lungs and heart need to be stressed with exercise to assess their efficiency. Minor dysfunction which is not apparent at rest, may come to light on exercise. Asthma may be induced, known disability graded, myocardial insufficiency diagnosed and on occasions an objective assessment will contrast with the patient’s own expectation of exercise performance. Whatever the choice of exercise, a doctor should be present with resuscitation equipment to hand and the patient should always be able to stop the test if symptoms become distressing.
In those already known to be moderately limited by respiratory disease, a simple walking test will help to grade severity. The 12-minute walking test (McGavin, 1976) This is a test of free corridor walking in which the patient is asked to cover as much distance as possible during 12 minutes with stops for a breather when necessary. At least two practice walks are required in order to improve subsequent reproducibility of the test and, like most exercise studies, motivation is an important factor. Free walking can be replaced by treadmill walking at a fixed rate and zero gradient for 12 minutes in a more controlled environment and with continuous EGG monitoring. In more sophisticated testing using a treadmill or cycle ergometer, either during progressive increasing workload or at a steady state of exercise, measurements of tidal volume, ventilation, heart rate, EGG, oxygen consumption and carbon dioxide output, can be monitored continuously. In this way, the overall capacity to perform exercise can be assessed and compared with predicted values.
If arterial blood is also sampled during exercise, information on the efficiency of pulmonary gas exchange can be obtained. If dyspnoea on exertion is due to respiratory rather than cardiac disease, the patient will tend to stop near to maximum predicted voluntary ventilation or even exceed this without reaching predicted maximum heart rate. The interpretation of such tests is not always clear.
Running on a treadmill for 6 minutes with regular monitoring of PEFR is one way of diagnosing exercise-induced asthma. The typical response being a slight rise in PEFR during exercise and a marked drop after exercise, usually reaching lowest levels about 4 minutes after stopping. This is one of the few diagnostic tests of lung function.
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