The diaphragm is a membranous muscle separating the abdominal cavity and chest, and in adults has a total surface area¹¹ of approximately 900 cm². It is the most important inspiratory muscle, with motor innervation solely from the phrenic nerves (C3, 4, 5). In comparison with other skeletal muscles, the diaphragm is extremely active. Muscle fibres within the diaphragm can reduce their length by up to 40% between residual volume and total lung capacity,¹¹ and spend 45% of each day contracting, compared with only 14% for the soleus muscle.¹² The diaphragm has considerable reserve of function, and unilateral phrenic block causes little decrement of overall ventilatory capacity. Despite the importance of the diaphragm to respiration, bilateral phrenic interruption is still compatible with good ventilatory function.

Mechanics of diaphragmatic function. The origins of the crural part of the diaphragm are the lumbar vertebrae and the arcuate ligaments, whilst the costal parts arise from the lower ribs and xiphisternum. Both parts are inserted into the central tendon. Recent studies of human subjects using MRI scans, illustrated in Figure 6.1, have enabled the in vivo actions of the diaphragm to be better defined.^11,13,14 Under normal circumstances, a zone of apposition exists around the outside of the diaphragm where it is in direct contact with the internal aspect of the ribcage, with no lung in between, but the parietal pleura still allowing free movement of the diaphragm. At upright FRC in humans, approximately 55% of the diaphragm surface area is in the zone of apposition.¹¹

Fig. 6.1 Three-dimensional reconstructions of the human diaphragm at functional residual capacity using fast computed tomography scanning (dimensions in cm). (A) Normal subject showing extensive zone of apposition and normal curvature of the diaphragm domes. (B) Patient with hyperinflated chest as a result of chronic obstructive pulmonary disease (page 411). Note the reduced zone of apposition and the flattened diaphragm domes.

(After reference 14 by permission of the authors and the publishers of American Journal of Respiratory and Critical Care Medicine.)

There are many ways by which diaphragm contraction may bring about an increase in lung volume,¹⁵ and these are illustrated schematically in Figure 6.2. These may be considered using a ‘piston in a cylinder’ analogy, the trunk representing the cylinder and the diaphragm the piston (Figure 6.2A). Figure 6.2B illustrates the first possible mechanism, involving downward movement of the diaphragm simply by shortening the zone of apposition around the whole cylinder and leaving the dome shape unchanged. This is a pure ‘piston-like’ action and has the advantage of very efficient conversion of diaphragm muscle fibre shortening into changes in lung volume. Figure 6.2C illustrates ‘non-piston-like’ behaviour in which the zone of apposition remains unchanged but an increase in the tension of the diaphragm dome reduces the curvature, so expanding the lung. This is likely to be less efficient than piston-like behaviour because much of the muscle tension developed simply opposes the opposite side of the diaphragm rather than moving the diaphragm downwards, such that in theory, when the diaphragm becomes flat, further contraction will have no effect on lung volume. Finally, Figure 6.2D incorporates both types of behaviour already described but also now includes expansion of the lower ribcage (known as ‘piston in an expanding cylinder’) that occurs with diaphragmatic contraction, particularly in the supine position, and so represents a simple description of the in vivo situation.

Fig. 6.2 “Piston in a cylinder” analogy of the mechanisms of diaphragm actions on the lung volume. (A) Resting end-expiratory position. (B) Inspiration with pure piston-like behaviour. (C) Inspiration with pure non-piston-like behaviour. (D) Combination of piston-like and non-piston-like behaviour in an expanding cylinder, which equates most closely with inspiration in vivo. ZA, zone of apposition.

In the supine position, diaphragm action is a combination of all the above mechanisms as well as a change in shape involving a tilting and flattening of the diaphragm in the antero-posterior direction.¹¹

Ribcage Muscles¹⁶

As already described, the ribcage may be regarded as a cylinder with length governed primarily by the diaphragm and secondarily by flexion and extension of the spine. The cross-sectional area of the cylinder is governed by movement of the ribs. This movement involves mainly rotation of the neck of the rib about the axis of the costovertebral joints, and their shape is such that elevation of the ribs in this way increases both the lateral and anteroposterior diameter of the ribcage. Elevation of the ribs by the intercostal muscles tends to result in a ‘bucket handle’ action, whilst elevation of the anterior ribcage by, for example, the sternomastoid muscle elevating the sternum results in a ‘pump-handle’ type of movement. These two actions tend to occur together, and depend also on other requirements such as posture and upper limb movements. Upper ribs are inserted into the sternum and do not necessarily behave in quite the same way as the lower ‘floating’ ribs, which are inserted into the more flexible costal cartilage.

The intercostal muscles are divided into the external group, fibres of which run in a caudal–ventral direction from their upper rib and are deficient anteriorly, and the less powerful internal group which have fibres running caudal–dorsal from their upper rib and are deficient posteriorly. Internal intercostal muscles of the upper ribcage become thicker anteriorly where they are known as the parasternal intercostal muscles. In 1749, mechanical considerations led Hamberger to suggest that the external intercostals were primarily inspiratory, and the internal intercostals primarily expiratory.¹⁷ Though an oversimplification,¹⁶ this has generally been confirmed by electromyography. The parasternal portion of the internal intercostals are inspiratory in both humans and animals, and the inspiratory activity of external intercostals, though minimal during quiet breathing, becomes increasingly important during stimulated breathing. Posture plays an important role in intercostal activity in humans. For example, during the rather extreme postural challenge of rotating the trunk, which changes the mechanical properties of the ribs, the respiratory activity of internal and external intercostals is reversed with internal intercostals becoming expiratory and vice versa.¹⁸

Scalene muscles are active in inspiration during quiet breathing in humans¹⁹ particularly when upright. Their role is to elevate the ribcage and this counteracts the tendency of the diaphragm to cause inward displacement of the upper ribs. Innervation is from C1 to C5.

Accessory muscles. These are silent in normal breathing in humans but as ventilation increases, the inspiratory muscles contract more vigorously and accessory muscles are recruited. Considerable hyperventilation (about 50 1.min⁻¹) or severe increases in respiratory loading are usually present before the accessory muscles become active. Accessory muscles include the sternocleidomastoids, extensors of the vertebral column, pectoralis minor, trapezius and the serrati muscles. Many of these muscles, for example the pectorals, reverse their usual origin/insertion and help to expand the chest, provided the arms and shoulder girdle are fixed by grasping a suitable support.

With the exception of gas within the bowel lumen, the abdomen is an incompressible volume held between the diaphragm and the abdominal muscles. Contraction of either will cause a corresponding passive displacement of the other. Thus abdominal muscles are generally expiratory.

Rectus abdominis, external oblique, internal oblique and tranversalis muscles are the most important expiratory muscles, whilst the muscles of the pelvic floor have a supportive role. Contraction of these muscles results in an increase in abdominal pressure displacing the diaphragm in a cephalad direction. In addition, their insertion into the costal margin results in a caudad movement of the ribcage, so assisting expiration by opposing the ribcage muscles. Gastric pressure is a valuable index of their activity because their contraction will always cause an increase in intra-abdominal pressure.

In the supine position, the abdominal muscles are normally inactive during quiet breathing and become active only when the minute volume exceeds about 40 1.min⁻¹, in the face of substantial expiratory resistance, during phonation or when making expulsive efforts. When upright, their use in breathing is complicated by their role in the maintenance of posture.

Figure 6.3 shows the radiographic appearance of the ribcage at residual volume, the normal expiratory level and at maximal inspiration, and illustrates the enormous range of movement within the semi-rigid ribcage. Expiration normally proceeds passively to the functional residual capacity (FRC), which may be considered as the equilibrium position governed by the balance of elastic forces, unless modified by residual end-expiratory tone in certain muscle groups. Inspiration is the active phase, entering the inspiratory capacity but normally leaving a substantial volume unused (the inspiratory reserve volume). Similarly, there is a substantial volume (the expiratory reserve volume) between FRC and the residual volume (see Figure 3.9). By voluntary effort it is possible to effect a satisfactory tidal exchange anywhere within the vital capacity, but the work of breathing is minimal at FRC.

Fig. 6.3 Outlines of chest radiographs of a normal subject at various levels of lung inflation. The numbers refer to ribs as seen in the position of maximal inspiration.

(With thanks to Dr R. L. Marks who was the subject.)

Although we tend to think of the respiratory muscles individually, it is important to remember that they act together in an extraordinarily complex interaction that is influenced by factors including posture, minute volume, respiratory load, disease and anaesthesia. Figure 6.4 illustrates some features of the interaction.^18,20

Fig. 6.4 A model of the balance of static and dynamic forces acting on the respiratory system. The central bar, attached to the lungs, is floating freely, held in equilibrium by the elastic forces at the end-expiratory position as shown. It may be displaced by the actions of the various muscles shown, with movement to the right generally indicating inspiration and vice versa. Action of the various inspiratory or expiratory muscles causes changes, not only in the lung volume but also in the inclination of the bar, which represents relative changes in the cross sectional area of the rib cage and abdomen. See text for details.

(Derived from references 20 and 21.)

Inspiration. In Figure 6.4 it can be seen that the ribcage inspiratory muscles (external intercostals and scalenes) and diaphragm act in parallel to inflate the lungs, with posture affecting which muscle group is dominant (see below). In either position, diaphragm activity alone results in a widening of the lower ribcage and an indrawing of the upper ribcage (often seen with spontaneous respiration during general anaesthesia), which must be countered by the intercostal and neck muscles contracting simultaneously.

Expiration. Requires no muscular activity during quiet breathing in the supine position, because the elastic recoil of the lungs provides the energy required, aided by the weight of the abdominal contents pushing the diaphragm in a cephalad direction. In the upright posture and during stimulated ventilation the internal intercostal muscles and the abdominal wall muscles are active in returning the ribcage and diaphragm to the resting position. In extreme hyperventilation, for example following exercise, the expiratory muscles become progressively more important until ventilation assumes a quasi sine wave push-pull pattern.

Konno & Mead originally proposed that the separate contributions to tidal volume of changes in ribcage (RC) and abdominal (AB) compartments could be measured.²² Essentially similar results may be obtained by measuring either anteroposterior distance (magnetometers), circumference (strain gauge) or cross-sectional area (respiratory inductance plethysmography, RIP²³

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