The lungs of mammals are structures which have evolved as part of the process of taking our delicate respiratory surface within the protection of our bodies. They are blind-ended structures with only one entrance or exit, the trachea.

This means that air must be moved into the lungs and then removed when the exchange of gas between air and blood is complete. Some of the most important diseases that interfere with this movement of air through the airways are known as chronic obstructive pulmonary diseases (COPD). There are a variety of these diseases, which make up one of the major causes of morbidity and mortality in the industrialized world.

Although many patients present with a combination of more than one type of disease, making diagnosis difficult, obstruction of the airways (other than by foreign bodies or neoplasms) can be generally attributed to:

We saw in the previous chapter that negative intrapleural pressure holds the lungs ‘stretched’ against the chest wall. The degree of stretch, and hence lung volume, depends on the compliance of the lungs and the intrapleural pressure. If intrapleural pressure remained constant the lungs would remain at constant volume and breathing would not take place. To breathe we alter intrapleural pressure, making it more negative to bring about inspiration and allowing it to become less negative to allow expiration to take place.

During the dynamic process of breathing intrapleural pressure is to all intents and purposes made up of two parts, the part required to hold the lungs open (the static component) plus a component to move the air into or out of the lungs. The latter overcomes airways resistance to flow. Pressure to overcome the viscosity of the tissues is only a small part (less than 20%) of the total, and the inertia of the system need not be considered unless the system is moving very violently, as in sneezing or coughing.

Our model from the previous chapter, which represents the respiratory system as a balloon inside a container like a syringe, is often used to illustrate the two components of intrapleural pressure mentioned above and illustrated in Figure 4.1. The plunger of the syringe represents the diaphragm, the walls of the syringe represent the chest wall, the balloon represents the alveoli, and the narrow tube represents the airways of the lung.

It is obvious that to hold the balloon inflated requires a certain effort; to draw air into the balloon (inspiration) requires more effort.

There are three further things that are not so obvious which can be deduced from this excellent model of breathing, and they all have clinical relevance:

Gas flows from regions of high pressure to regions of lower pressure. This flow may be laminar, where the movement is orderly and streamlined, or turbulent, where movement is chaotic.

Flow in most, but not all, of the respiratory system can be considered laminar as a first approximation. Important exceptions include the nose, where turbulence causes inhaled particles to be thrown out of the airstream, and the larynx, where turbulence contributes to the production of sound.

Flow in a long straight smooth tube becomes turbulent when Reynolds’ Number, which is calculated as 2rvd/η (r = radius of tube, v = velocity of flow, d = gas density, η = gas viscosity) exceeds 2000. Under turbulent conditions flow varies with the square root of the driving pressure, i.e. it is very much more difficult to produce the same flow when flow is turbulent (Fig. 4.2).

It is therefore important when designing breathing apparatus to avoid turbulence, which would increase the subject’s work of breathing.

Laminar flow has been extensively investigated by scientists, one of whom, Poiseuille, defined the relationship between driving pressure (ΔP) and flow () as:

where r is the radius of the tube, η is the viscosity of the gas, and l is the length of the tube. This relationship applies to gas flow in the long straight smooth tubes under stable conditions – hardly conditions that apply to the lungs. However, it can be roughly applied to breathing, and you may notice that the most important factor affecting airflow in this equation is the radius of the tube, which is raised to the fourth power (r⁴). This means that if you reduce the radius by half and keep everything else constant, the flow will be reduced to .

Laminar flow in a tube can be represented as a series of cylinders moving down the tube, with the central cylinder moving fastest. The outermost cylinder is stationary and is in fact a layer of the original gas in the tube left behind as the new gas moves forward, as shown in Figure 4.4.

These apparently esoteric considerations have important consequences in respiratory medicine. For example, adequate ventilation of the alveoli of the lungs can be achieved with a surprisingly small tidal volume, provided a high enough frequency of ‘breathing’ is used. This phenomenon is seen in clinical conditions when high-frequency artificial ventilation of the lungs is used where we want to avoid movements of the chest wall – in trauma victims with a crushed chest, for example. The patient is successfully artificially ventilated with a tidal volume less than his or her anatomical dead space (see p. 64) and frequencies up to 50 Hz. A ‘spear’ of fresh air in the centre of the airways penetrates deeper into the lungs than might be expected and provides adequate ventilation.

We have established that it takes a greater pressure to inflate the lungs than simply to hold them in a steady inflated state. This extra pressure is required to produce flow in the airways. We have also seen that the relationship between pressure and volume on slowly inflating and deflating the lungs is not a straight line, as you would get with a rubber balloon, but rather a loop, with a greater pressure needed to inflate the lungs to a given volume than that which exists at the same volume when they are being allowed to deflate (Fig. 4.3).

The small dotted loop in Figure 4.3 was produced by inflating and deflating the lungs extremely slowly, a situation that is called pseudostatic. If we were to carry out this inflation and deflation at a normal breathing rate (12 times per minute, say) the loop would be wider (the solid line), with inflation pressures required for a given volume being greater than under the static conditions and deflation pressures being less.

This situation arises because energy is used up to propel the air along the airways; the airways can be said to resist flow in a phenomenon known as airways resistance. This resistance during laminar flow can be thought of as the friction between the layers of air as they are pushed down the airway. Pushing a pile of typing paper or a pack of cards across a table gives a good idea of what is happening (Fig. 4.4).

As its name implies, airways resistance is analogous to electrical resistance. To measure the electrical resistance of a length of wire you need to know two things, the potential difference (voltage) between the two ends of the wire, and the current flowing in the wire. Using reasonable currents (that do not overheat the wire) we find that the relationship between voltage (V) and current (I) is a straight line (Fig. 4.5).

This is Ohm’s Law, and the slope of the line (V/I) is the resistance of the wire in ohms.

Just as when describing the static properties of the lung we use the term compliance, rather than its reciprocal elastance, when considering airways resistance we frequently use its reciprocal, conductance. Airways conductance = 1/Airways resistance.

To measure the airways resistance of a tube you also need to know two things, the driving pressure (the difference in pressure) between the two ends of the tube, and the airflow of the tube. Using flows that do not produce turbulence, we find that the relationship between pressure (P) and flow () is also a straight line, like electrical resistance.

The slope of the line is the airway resistance of the tube in kPa 1⁻¹ s.

The airways resistance of an adult breathing quietly is about 0.2 kPa 1⁻¹ s, which means that a normal sort of flow of 0.5 1⁻¹ s is brought about by a pressure difference of 0.1 kPa between the lips and the alveoli.

Two important points arise from this apparently innocuous statement:

Airways resistance changes in disease and its measurement has been the focus of interest of doctors for many years. Measurement of airflow is not difficult and is usually accomplished using an instrument known as a pneumotachograph, which itself illustrates the principle of airways resistance. The pneumotachograph consists of a tube through which the subject breathes (Fig. 4.6).

The tube contains a resistance to flow. The pressure difference across the resistance is measured and is proportional to the flow.

Flow can then be instantaneously integrated (which is adding up moment by moment all the measurements of flow) to give volume.

Measurement of the driving pressure from alveoli to lips, or lips to alveoli, in a subject or patient is more difficult.

One way of getting round the problem of measuring the driving pressure is to use the fact that it is the change in alveolar pressure that brings about flow and stretches the lungs against recoil pressure. Consider the changes in intrapleural pressure and recoil pressure due to the elasticity of the lungs in a single respiratory cycle (Fig. 4.7).

Subtract changes in recoil pressure from changes in intrapleural pressure and you are left with the changes in alveolar pressure that produce changes in airflow – the two variables we are interested in to measure resistance. This subtraction process can be done instantaneously electronically, and so we need to know the changes in recoil pressure, which can be worked out from lung compliance, and the changes in intrapleural pressure, which we obtain by measuring changes in the pressure inside the oesophagus passing through the intrapleural space.

Measuring pressure inside the oesophagus involves swallowing a pressure-measuring device, which initially goes down into the stomach and is then pulled back into the oesophagus. You can imagine that this is not very comfortable, and other techniques are generally used with patients.