The respiratory system presents a uniquely vulnerable aspect of our bodies to the outside world. It is, for example, the only place where capillaries continuously come into direct contact with the outside air. Admittedly, that air is well conditioned before being presented to the respiratory surface, but attacks on the respiratory system and its function are extremely common. It is to be hoped that our readers will never suffer from coronary heart disease or renal failure, but which of us will go through life without a disease of the respiratory system, if only the common cold?

Most malfunctions of the respiratory system are reliably diagnosed by clinicians after taking a history of the patient’s complaint and making a clinical examination. Radiological examination at appropriate levels of sophistication, microbiological and cytological investigations, blood tests, lavage, bronchoscopy, and a pantheon of other ingenious tests are available to confirm or refute the original diagnosis.

Such a diagnosis is, however, usually qualitative. It does not quantitatively measure the effect the disease is having on the functioning of the respiratory system, and hence on the quality of life. This is the province of the lung function test.

These tests range from the relatively simple to those that are the province of the specialized lung function laboratory. We have placed the tests roughly in increasing order of exoticism.

A simple spirometer (Fig. 5.2, p. 63) will provide much useful information about a patient’s lungs. Large people have larger lungs than small people and age exerts its malign effect. Extensive study of these relationships has provided us with tables which, for example, relate vital capacity to height (see Appendix).

Measurements made on a spirometer may be classified as:

Although such measurements as inspiratory reserve volume (IRV) and expiratory reserve volume (ERV) can be informative, the most usual and useful static spirometric test is the forced vital capacity (FVC). This is ‘forced’ because the subject is enthusiastically urged to breathe in as far as he can and out as far as he can (Fig. 11.1). This test, which can be classed as static because it does not involve an element of time, is often combined with a dynamic test, the FEV₁:

FEV₁ is commonly expressed as a percentage of FVC. This takes into account the problem that a very small person (with very small, perfectly healthy lungs) would never be able to breathe out the same amount in 1 second as a very large person, whose lungs may not be so healthy. You can expect a healthy person to force out at least 70% of his vital capacity in 1 second. Using this percentage alone can create problems in restrictive lung diseases, which restrict the expansion of the lungs: both VC and FEV₁ are reduced, therefore in those cases that percentage may be normal. For this reason both absolute values and percentage are measured. Many years ago a ratio of 70% VC was considered acceptable, but that was when smoking was considered normal. A higher percentage is required today.

Characteristic traces in normals and patients with chronic obstructive (emphyzematous/bronchitic) or restrictive (fibrotic) lung disease are shown in Figure 11.1.

Although emphysema is the ‘classic’ obstructive lung disease it can only be diagnosed with certainty at post mortem (pathologists are the only people who invariably make the perfect diagnosis, but by then it’s too late). We therefore describe obstructive patterns of lung disease as asthma (reversible) or chronic obstructive pulmonary disease (COPD, irreversible).

In the helium dilution method the principle is simple. The patient breathes out to FRC or RV, whichever is being measured, and is connected to a spirometer of known volume containing helium (He) at known concentration. The patient breathes normally for an appropriate length of time and the dilution of the He by the RV or FRC in his lungs is measured. The level of the trace of his breathing is carefully watched and oxygen added at the same rate as it is used up to keep the overall volume in lungs + spirometer constant (Fig. 11.2).

An interesting disparity is often seen between RV measured by plethysmography and by dilution. This arises because air trapped in the lungs, which is not in contact with the mouth, is measured by the plethysmographic method but does not take part in the dilution of He.

Restrictive lung diseases decrease TLC, FRC, RV and VC. Frequently RV is first to be affected. Care should be taken in interpreting results from obese patients, where the outward recoil of the chest wall is reduced, resulting in lower FRC.

Obstructive lung diseases show an increasing RV as gas is trapped behind the collapsed airways (see above). Increased FRC and TLC in these patients is the result of reduced lung recoil and breathing at increased lung volumes in an instinctive attempt to keep the airways open.

Peak expiratory flow is the maximum expiratory flow (L.s²¹) that a subject can produce. Of course this is dependent on the subject’s motivation even with healthy lungs. The advantage of this measurement is that it can be made with simple apparatus – where the subject blows against a paddle or propeller which records flow. Although not precise this has been found to be a useful domiciliary measurement in asthma with the patient keeping a diary of his progress.

Flow-volume loops. With the subject breathing through a pneumotachograph (Fig. 4.6, p. 45) which measures flow, and by integrating that flow to provide volume, loops of inspiratory and expiratory flowvolume relationships can be recorded. (Fig. 11.3). These loops are constructed by having the patient breathe from total lung capacity down to residual volume several times. They are particularly useful in assessing chronic obstructive pulmonary disease (COPD) where the inspiratory part of the loop has a normal shape, although being of reduced volume, while the expiratory part of the loop has a characteristic ‘scooped out’ shape as flow is restricted by airway collapse.

This instrument is described in Chapter 4 and consists of an airtight box in which the subject sits. To understand the principles on which this instrument works, consider the subject’s chest as a syringe with the diaphragm represented by the plunger.

The subject first pants against a closed shutter – the neck of the syringe is blocked.

In terms of gas law the situation is as in Figure 11.4A, where a large enclosed volume of gas (the contents of the box) surrounds a small enclosed volume of gas (the air in the lungs) which increases and decreases in volume as it is compressed or decompressed. This change in volume of the syringe compresses and decompresses the air in the box, and so the pressure in the box changes proportionately and in the opposite sense to the pressure in the lungs.

Measuring the pressure changes in the box while the subject pants against a closed shutter enables us to calculate pressure within the lungs (Fig. 11.4) and therefore from the gas laws calculate lung volume, and is usually used to measure functional residual capacity (FRC) and residual volume (RV).

Plethysmography is also used to measure lung volumes (TLC) in patients with severe airflow obstruction in preference to the usual helium (He) dilution technique. This is because in these patients air trapping is so bad that the He cannot get to closed off volumes in the lungs, which are therefore not registered. In plethysmography, however, these closed off volumes are still subject to the gas laws (see the Appendix) on which this technique is based.

The principle of the relationship between box pressure and lung airway pressure does not depend on the ‘syringe’, which represents the lungs, being closed. In Figure 11.4B