VENTILATION OF THE RESPIRATORY SYSTEM

THE IMPORTANCE OF ITS LACK OF UNIFORMITY IN DISEASE

Chapter objectives

After studying this chapter you should be able to:

1. Define ventilation (differentiating it from lung volume).

2. Define the common lung volumes and capacities and how they are changed in restrictive and obstructive diseases.

3. Explain the importance of respiratory tract structure (as blind-ended tracts in parallel) on ventilation.

4. Explain the alveolar gas equation.

5. Differentiate between physiological and anatomical dead space and relate increased dead space to emphysema.

6. Describe the physiological factors influencing the distribution of ventilation.

7. Explain the composition of the parts of a single expirate and why these are changed in disease.

Introduction

So far we have talked about breathing as if it is simply a uniform repeated action of inhalation followed by exhalation. We now begin to explore the detail of this far from uniform phenomenon.

Ventilation of a room or building can be measured as flow of air, in litres per minute through that room. Normal breathing involves about 12 breaths per minute, each of about 0.5 L. The volume of air passing into the lungs per minute in this case (minute ventilation, ) is:

The symbol for ventilation, , has a dot over the V to show it is a rate. The volume breathed out is approximately equal to the volume breathed in (tidal volume, V_T), therefore the net flow over a complete cycle is zero. This is not a very helpful way of expressing ventilation if we want to express changes in breathing, as the result of exercise or disease, for example. We therefore measure the flow in one direction only – conventionally the volume breathed out per minute () – to give us minute ventilation.

In respiratory medicine ventilation is the rate of flow of air into or out of the lungs, and results from the expanding and contracting of the lungs by the changes in intrapleural pressure described in Chapter 4 (pp. 41, 42) and illustrated in Figure 5.1.

Fig. 5.1 A single respiratory cycle. Airflow would be measured using a pneumotachograph and integrated to give tidal volume. Recoil pressure has a negative sign because it is measured relative to intrapleural pressure.

The part of the air ventilating our lungs which is of paramount functional importance is that which forms alveolar ventilation, that is which ventilates those parts of the lung where gas exchange with the blood takes place. Insufficient (hypoventilation) or excess (hyperventilation) alveolar ventilation occurs in many lung pathologies.

We all know we can consciously alter the volume of our lungs, breathing in or breathing out more than normal: what is frequently not realized is that we cannot totally empty our lungs.

At the end of a normal quiet expiration average intrapleural pressure (P_PL) is approximately −0.5 kPa (below atmospheric pressure) and lung volume (V_L) 3 L. This volume is called the functional residual capacity (FRC). When you breathe in as hard as you can and hold your breath, P_PL decreases to −2 kPa and V_L increases to about 6 L.

Alternatively, if you breathe out as hard as possible P_PL will be −0.2 kPa and V_L 1.5 L. This residual volume (RV) cannot be expelled.

The anatomy (size) of an individual’s chest, the elasticity of his lungs and chest wall and the strength of his respiratory muscles determine these volumes. Changes in lung volume can easily be measured using a spirometer, as illustrated in Figure 5.2. This instrument, which comes in many forms, consists of a closed space from which the subject breathes. In the type illustrated a hollow bell is supported in a trough of water; as the subject breathes in, air is drawn from the bell and it sinks slightly; when he breathes out the bell rises. Movements of the bell are recorded as changes in lung volume on a moving chart.

Fig. 5.2 A spirometer record of breathing. Average adult volumes are shown. Because the lungs cannot be completely emptied, residual volume (RV) and functional residual capacity (FRC) cannot be measured by direct spirometry.

Much information about lung properties and diagnosis of disease can be obtained by measuring changes in lung volume. When a maximum inspiration is taken the increase in lung volume (inspiratory reserve volume, IRV) to reach total lung capacity (TLC) is about 3 L. A maximal expiration from TLC will expel the IRV, V_T and the expiratory reserve volume (ERV), the total of all these volumes being the vital capacity (VC). In the vocabulary of lung volumes a capacity is the sum of two or more volumes.

Except for RV and FRC (which depends on RV), these volumes can be measured using a spirometer in the living subject. If the lungs are taken out of the body and allowed to collapse there will still be a little air left in them: the minimal air; these lungs will float (see ‘lights’ Chapter 2, p. 19). The lungs of a stillborn baby who has not taken a breath will not float because they contain no air; this test is important in forensic investigations.

The names of these volumes and their abbreviations are intimidating to students, but reference to Figure 5.2 should make all clear.

The changes in intrapleural pressure that bring about these volume changes do not vary much between individuals in health or disease in either humans or animals. The volumes themselves, however, do vary with:

• body size – all are larger in large people.

• age – all volumes are smaller in children, only partially due to their smaller body size. In old age VC is decreased and RV increased because of degenerative changes.

• sex – all volumes are smaller in women than in men the same size.

• muscle training – increases all the lung volumes and allows greater maximal ventilation during exercise.

• disease – changes in these lung volumes from the normal values, which have been measured in numerous extensive surveys, are used in the diagnosis of many diseases of the lungs and respiratory system.

Because RV cannot be breathed out, it and FRC (which is made up of RV + ERV) cannot be measured directly by a spirometer. They are measured by inhaling from RV a known volume of a non-absorbable tracer gas (e.g. helium) and measuring its dilution by the unknown volume of air in the lungs. Alternatively, the subject breathes out to RV and then breathes in and out a few times from a bag containing a known volume of pure oxygen. He then breathes out again to RV into the bag. The air in his RV was approximately 80% nitrogen, and the dilution of this by the known volume of pure oxygen in the bag enables RV to be calculated.

Spirometric abnormalities in disease

Lung disease changes many of the lung volumes in Figure 5.2. It is usual, for diagnostic purposes, to exaggerate these changes by stressing the respiratory system by asking the patient to breathe in as deeply as he can and out as hard as he can for the single breath of a test. The forced expiratory volume in 1 second (FEV₁) is frequently abbreviated to forced expired volume (FEV), but is still the same creature: the volume of air forced out in the first second of such a test. Similarly, forced vital capacity is the total volume of air a patient can breathe out after a maximal inspiration. It is usual to express FEV₁ as a percentage of FVC: this takes into account the fact that larger people normally have larger lungs and therefore a larger FVC. Often VC is used in place of FVC in this ratio.

Much careful work has gone into preparing tables that relate spirometric measurements to a normal subject’s height and weight. Deviations from values predicted by these tables are diagnostic of lung disease.

Diseases of the thoracic cage, such as ankylosing spondylitis, diseases of the nerves and muscles of respiration, e.g. poliomyelitis, diseases that restrict expansion of the lungs, such as fibrosis, or diseases that cause airway collapse during expiration all limit these spirometric measurements. Examples of the modifications produced by diseases of the lungs on spirometric traces are shown in Chapter 11, and can be summarized in a very general way as follows:

Variable	Restrictive disease	Obstructive disease
FVC	− −	−
FEV₁	− −	− −
FEV₁/FVC	0	− −
FRC	−	+
RV	−	+
TLC	−	+

0 = no change; + =increase; − =decrease.

NB: some of these changes are not seen to any degree until the disease is very advanced.

Uneven distribution

These considerations of the various volumes that make up breathing still give an impression of uniformity of distribution which is far from true. In Chapter 2 we described the anatomy of the bronchial tree as blind-ended sacks connected to the outside through a system of tubes. Some thought enables us to see that the composition of gas in this arrangement may be different at its entrance (the lips) from that at its ends (the alveoli) – differences in series with each other; also, the composition in different alveoli may be different – differences in parallel; or a combination of both. Differences in the composition of air in different parts of the lung depend largely on how well that part is ventilated and how much gas exchange between air and blood and blood and air takes place there. In the ideal situation just the right amount of both air and blood are supplied to a particular region, so there is no ‘waste’ of either. One particular kind of inequality between air and blood supply is known as dead space.

Dead space

Essentially all exchange of gas between air and blood only takes place at the alveolar surface. The system of tubes connecting this surface to the atmosphere takes little part in this exchange and can be considered anatomical dead space. These tubes are essential to bring air to the respiratory surface, but ventilating these connecting tubes is an inescapable waste of effort as far as gas exchange is concerned. To understand anatomical dead space you must understand that the lungs fill and empty in a sequential fashion (Fig. 5.3).

Fig. 5.3 Distribution of dead space gas. At the end of expiration dead space is filled with ‘used’ alveolar gas. When the lungs inspire a volume of fresh air equal to dead space, the alveolar region has expanded but the composition of the gas it contains is not changed, there is fresh air in the dead space and ‘used’ air in the alveoli. The alveolar air will be diluted by further inspiration, but the composition of the dead space air will remain that of fresh air. If alveoli fill at different rates they are said to have different time constants and receive different amounts of dead space gas. The alveoli that expand first will receive most dead space gas.

Case 5.1 Ventilation in the respiratory system: 1

Pneumothorax: a failure of lung ventilation

Mr Price is a 21-year-old man who went to the Accident and Emergency department of his local hospital complaining of chest pain. The pain had come on suddenly while he was playing football. The pain was stabbing, on the right-hand side of his chest, and was very much worse when he took a breath in. He also felt rather short of breath.

After taking a history, performing a physical examination and taking a chest X-ray, the doctor in the emergency department made a diagnosis of pneumothorax.

In this chapter we will consider:

1. What causes a pneumothorax.

2. Diagnosis of a pneumothorax.

3. Treatment of a pneumothorax.

4. Tension pneumothorax – a rare medical emergency.

At the end of inspiration the contents of the alveoli have been diluted by inspired room air, which now also fills the anatomical dead space (Fig. 5.3C). As the lungs then empty during expiration, the rule of ‘last in first out’ applies and the dead space containing unmodified room air is exhaled first. At the end of expiration the anatomical dead space is filled with alveolar air, and this partly used air is inhaled first in the next inspiration (Fig. 5.3A,B). If some regions of the lung expand before others in the process of inspiration they will receive an inappropriately large part of this dead space gas, and the regions receiving air later in inspiration will receive more fresh air (Fig. 5.3C,D). So the timing of inflation of a part of the lung during inspiration will affect the composition of the gas it receives.

This type of dead space is called ‘anatomical’ because it measures the anatomical volume of the conducting airways. The strict definition of anatomical dead space is ‘the volume of an inspired breath which has not mixed with the gas in the alveoli’. Because gas exchange effectively only takes place in the alveoli there is no CO₂ excreted into the dead space, and a scientist called Fowler pointed out that anatomical dead space can be measured as the volume of expired gas leaving the mouth and nose before CO₂ appears at the lips (Fig. 5.4).