Diaphragm

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CHAPTER 58 Diaphragm

The diaphragm is a curved musculofibrous sheet that separates the thoracic from the abdominal cavity (Figs 58.1, 58.2). Its mainly convex upper surface faces the thorax, and its concave inferior surface is directed towards the abdomen. The positions of the domes or cupolae of the diaphragm are extremely variable because they depend on body build and the phase of ventilation. Thus the diaphragm will be higher in short, fat people than in tall, thin people, and overinflation of the lung, as occurs for example in emphysema, causes marked depression of the diaphragm. Usually, after forced expiration the right cupola is level anteriorly with the fourth costal cartilage and therefore the right nipple, whereas the left cupola lies approximately one rib lower. With maximal inspiration, the cupola will descend as much as 10 cm, and on a plain chest radiograph the right dome coincides with the tip of the sixth rib. In the supine position, the diaphragm will be higher than in the erect position, and when the body is lying on one side, the dependent half of the diaphragm will be considerably higher than the uppermost one.

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Fig. 58.1 A, Abdominal aspect of the diaphragm. Note the apertures for the oesophagus and inferior vena cava and the aortic hiatus. The medial part of the right crus frequently consists of three components and extends further distally than the left crus. B, Trunk sectioned at the level of the tenth thoracic vertebra.

(From Sobotta 2006.)

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Fig. 58.2 Thoracic aspect of the diaphragm. Note that the fibres descend from their relatively ‘high’ anterior sternocostal attachments steeply and obliquely to their complex ‘low’ posterior attachments. The phrenic nerves are demonstrated on the diaphragmatic surfaces.

ATTACHMENTS AND COMPONENTS

The muscle fibres of the diaphragm arise from the highly oblique circumference of the thoracic outlet: the attachments are low posteriorly and laterally, but high anteriorly. Although it is a continuous sheet, the muscle can be considered in three parts, sternal, costal and lumbar, which are based on the regions of peripheral attachment. The sternal part arises by two fleshy slips from the back of the xiphoid process, and is not always present. The costal part arises from the internal surfaces of the lower six costal cartilages and their adjoining ribs on each side, and interdigitates with transversus abdominis (see Fig. 54.14). The lumbar part arises from two aponeurotic arches, the medial and lateral arcuate ligaments (sometimes termed lumbocostal arches) and from the lumbar vertebrae by two pillars or crura.

The lateral arcuate ligament is a thickened band in the fascia that covers quadratus lumborum, and it arches across the upper part of that muscle. It is attached medially to the front of the transverse process of the first lumbar vertebra, and laterally to the lower margin of the twelfth rib near its midpoint. The medial arcuate ligament is a tendinous arch in the fascia that covers the upper part of psoas major. Medially, it is continuous with the lateral tendinous margin of the corresponding crus, and is thus attached to the side of the body of the first or second lumbar vertebra. Laterally, it is fixed to the front of the transverse process of the first lumbar vertebra.

The crura are tendinous at their attachments, and blend with the anterior longitudinal ligament of the vertebral column. The right crus is broader and longer than the left, and arises from the anterolateral surfaces of the bodies and intervertebral discs of the upper three lumbar vertebrae. The left crus arises from the corresponding parts of the upper two lumbar vertebrae. The medial tendinous margins of the crura meet in the midline to form an arch, the median arcuate ligament, which crosses the front of the aorta at the level of the thoracolumbar disc; it is often poorly defined.

From these circumferential attachments, the fibres of the diaphragm converge into a central tendon. Fibres from the xiphoid process are short, run almost horizontally and are occasionally aponeurotic. Fibres from the medial and lateral arcuate ligaments, and more especially those from the ribs and their cartilages, are longer. They rise almost vertically at first and then curve towards their central attachment. Fibres from the crura diverge, and the most lateral become even more lateral as they ascend to the central tendon. Medial fibres of the right crus embrace the oesophagus where it passes through the diaphragm, the more superficial fibres ascend on the left, and deeper fibres cover the right margin. Sometimes, a fleshy fasciculus from the medial side of the left crus crosses the aorta and runs obliquely through the fibres of the right crus towards the vena caval opening, but this fasciculus does not continue upwards around the oesophageal passage on the right side.

The central tendon of the diaphragm is a thin but strong aponeurosis of closely interwoven fibres situated near the centre of the muscle, but closer to the front of the thorax, so that the posterior muscular fibres are longer. In the centre it lies immediately below the pericardium, with which it is partially blended. Its shape is trifoliate. The middle, or anterior, leaf has the form of an equilateral triangle with the apex directed towards the xiphoid process. The right and left folia are tongue-shaped and curve laterally and backwards, the left being a little narrower. The central area of the tendon consists of four well-marked diagonal bands fanning out from a thick central node where compressed tendinous strands decussate in front of the oesophagus and to the left of the vena cava.

RELATIONS

The upper surface of the diaphragm is related to three serous membranes. On each side, the pleura separates it from the base of the corresponding lung, and the pericardium is interposed between the middle folium of the central tendon and the heart. This latter area, which is almost flat, is referred to as the cardiac plateau and extends more to the left than the right. In anteroposterior view, the superior profile of the diaphragm rises on either side of the cardiac plateau to a smooth convex dome or cupola, the cupola on the right being higher and slightly broader than that on the left. Most of the inferior surface is covered by peritoneum. The right side is accurately moulded over the convex surface of the right lobe of the liver, the right kidney and right suprarenal gland. The left side conforms to the left lobe of the liver, the fundus of the stomach, the spleen, the left kidney and the left suprarenal (adrenal) gland. In view of these differences in the profile and anatomical relationships of the right and left sides of the diaphragm, the side should always be specified in clinical descriptions. At full inspiration, the right hemidiaphragm is found at the anterior sixth rib on a posteroanterior chest radiograph; the left hemidiaphragm is 1.5–2.5 cm lower than the right (see Fig. 55.16). Unilateral paralysis may be seen as a raised hemidiaphragm on a chest radiograph, but this sign should not be relied upon uncritically in clinical practice.

APERTURES

A number of structures pass between the thorax and abdomen via apertures in the diaphragm. There are three large openings, for the aorta, oesophagus and inferior vena cava, and a number of smaller ones (Fig. 58.2).

The aortic aperture is the lowest and most posterior of the large openings, and is found at the level of the lower border of the twelfth thoracic vertebra and the thoracolumbar intervertebral disc, slightly to the left of the midline. It is an osseo-aponeurotic opening defined by the diaphragmatic crura laterally, the vertebral column posteriorly and the diaphragm anteriorly. Strictly speaking, it lies behind the diaphragm and its median arcuate ligament (when present). Occasionally, some tendinous fibres from the medial parts of the crura also pass behind the aorta, converting the opening into a fibrous ring. The aortic opening transmits the aorta, thoracic duct, lymphatic trunks from the lower posterior thoracic wall and, sometimes, the azygos and hemiazygos veins.

The oesophageal aperture is located at the level of the tenth thoracic vertebra, above, in front and a little to the left of, the aortic opening. It transmits the oesophagus, gastric nerves, oesophageal branches of the left gastric vessels and some lymphatic vessels. The elliptical opening has a slightly oblique long axis, and is bounded by muscle fibres that originate in the medial part of the right crus and cross the midline, forming a ‘chimney’ approximately 2.5 cm long, which accommodates the terminal portions of the oesophagus. The outermost fibres run in a craniocaudal direction, and the innermost fibres are arranged circumferentially. There is no direct continuity between the oesophageal wall and the muscle around the oesophageal opening. The fascia on the inferior surface of the diaphragm is continuous with the transversalis fascia and is rich in elastic fibres. It extends upwards into the opening as a flattened cone to blend with the wall of the oesophagus 2–3 cm above the oesophago-gastric (squamocolumnar) junction. Some of its elastic fibres penetrate to the submucosa of the oesophagus. This peri-oesophageal areolar tissue is referred to as the phreno-oesophageal ligament. It connects the oesophagus flexibly to the diaphragm, permitting some freedom of movement during swallowing and ventilation while at the same time limiting upward displacement of the oesophagus.

The vena caval aperture, the highest of the three large openings, lies at about the level of the disc between the eighth and ninth thoracic vertebrae. It is quadrilateral, and located at the junction of the right leaf with the central area of the tendon, and so its margins are aponeurotic. It is traversed by the inferior vena cava, which adheres to the margin of the opening, and by some branches of the right phrenic nerve.

There are two lesser apertures in each crus: one transmits the greater, and the other the lesser, splanchnic nerve. The ganglionated sympathetic trunks usually enter the abdominal cavity behind the diaphragm, deep to the medial arcuate ligament. Openings for minute veins frequently occur in the central tendon.

On each side of the diaphragm there are small areas where the muscle fibres are replaced by areolar tissue. One, between the sternal and costal parts, contains the superior epigastric branch of the internal thoracic artery and some lymph vessels from the abdominal wall and convex surface of the liver. The other, between the costal part and the fibres that spring from the lateral arcuate ligament, is less constant; when it is present, the posterosuperior surface of the kidney is separated from the pleura only by areolar tissue.

Oesophageal reflux

Reflux of gastric contents into the oesophagus, with risk of inhalation into the lungs, is normally prevented by a physiological antireflux barrier located at the gastro-oesophageal junction. The major components of this barrier are the specialized smooth muscle of the wall of the lower oesophagus and the encircling fibres of the crural diaphragm. See also Chapter 65.

Hiatus hernia

The diaphragm lends additional power to all expulsive efforts: sneezing, coughing, laughing, crying, urinating, defecating, and expelling the fetus from the uterus are all preceded by a deep inspiration. Similarly, a deep inspiration followed by closure of the glottis is a common preliminary to powerful recruitment of the trunk muscles, e.g. in lifting heavy weights, because the increased intra-abdominal pressure provides pneumatic bracing of the vertebral column. Repeated stress may eventually compromise the integrity of the hiatus, so that the muscular hiatal tunnel widens. Concomitant laxity of the phreno-oesophageal membrane allows the gastro-oesophageal junction to slide into the thorax; this is usually termed a sliding, or type I, hiatus hernia (Kahrilas 2001). Sliding hernias are typically acquired. They commonly occur in the fifth decade of life and are found in more than 50% of patients with gastro-oesophageal reflux, a condition that induces tonic contraction of the longitudinal oesophageal muscle, which further exacerbates the hiatus hernia. When the stomach herniates into the thorax alongside the oesophagus, it is termed a para-oesophageal, or type II, hiatus hernia.

Congenital hernias/eventration

Abdominal organs, usually the stomach, may herniate through the diaphragm into the thorax. There are three sites at which such hernias can occur: posterolateral (Bochdalek), subcostosternal (Morgagni) and oesophageal.

The most common is a posterolateral (Bochdalek) hernia, which occurs as a result of a defect in the posterior diaphragm in the region of the tenth or eleventh ribs. It is more common on the left, and presents with abdominal contents in the left hemithorax at birth; clinically significant cases develop hypoxaemia and respiratory failure at birth. A chest radiograph demonstrates mediastinal shift to the contralateral side and the presence of the gastrointestinal contents in the thorax; the diagnosis should be made at routine prenatal ultrasound. Treatment involves decompression of the gastrointestinal contents and cardiopulmonary stabilization. Surgical repair is performed when the patient is stable, but prognosis is determined by the degree of accompanying pulmonary hypoplasia, severity of pulmonary vascular abnormalities and any associated congenital heart defects.

Subcostosternal hernia, first described by Morgagni, is uncommon and occurs through a defect in the anterior diaphragm just lateral to the xiphoid process. It is frequently asymptomatic.

Oesophageal hernia occurs as a result of a defect in the oesophageal aperture, so that part of the stomach herniates into the thorax. It is rarely congenital in origin, usually develops later in life, and is believed to be acquired.

Diaphragmatic trauma

Closed and penetrating thoracoabdominal injuries may result in rupture or laceration of the diaphragm. With closed injuries and diaphragmatic rupture, there may be subsequent herniation of the abdominal contents into the thorax. Spiral CT with planar reformation should be the primary investigation (Shanmuganathan et al 2000): MRI is usually performed when other imaging modalities have produced equivocal findings. Early operative repair is recommended in diaphragmatic trauma, because untreated cases are at risk of developing gastrointestinal obstruction or perforation. Patients with penetrating injuries may require additional assessment by thoracoscopy (Lowdermilk & Naunheim 2000).

VASCULAR SUPPLY

ARTERIES

The lower five intercostal and subcostal arteries supply the costal margins of the diaphragm while the phrenic arteries supply the main central portion of the diaphragm. The anastomotic arrangement (see p. 1010) ensures profuse blood supply.

Phrenic arteries

Two small phrenic vessels help to supply the diaphragm via the abdominal surface. They may arise separately from the aorta, just above its coeliac trunk, or by a common aortic stem, or from the coeliac trunk. Occasionally, one is from the aorta, the other from a renal artery. Each artery ascends laterally anterior to a crus of the diaphragm, near the medial border of the suprarenal gland. The left passes behind the oesophagus and then runs forwards on the left side of its diaphragmatic opening. The right phrenic artery passes posterior to the inferior vena cava, and then along the right side of its opening. Each phrenic artery divides into medial and lateral branches near the posterior border of the central tendon. The medial branch curves forwards to anastomose with its contralateral fellow in front of the central tendon and with the musculophrenic and pericardiacophrenic arteries. The lateral branch approaches the thoracic wall, and anastomoses with the lower posterior intercostal and musculophrenic arteries. The lateral branch of the right artery supplies the inferior vena cava, whereas the left sends ascending branches to the oesophagus. Invariably there is a communicating artery between the left gastric and inferior phrenic arteries.

VEINS

Phrenic veins

The phrenic veins follow the corresponding arteries on the inferior diaphragmatic surface. The right phrenic vein ends in the inferior vena cava. The left phrenic vein is often double: one branch ends in the left renal or suprarenal vein, the other passes anterior to the oesophageal opening to join the inferior vena cava.

INNERVATION

The diaphragm receives its motor supply via the phrenic nerves (Fig. 58.3). Sensory fibres are distributed to the peripheral part of the muscle by the lower six or seven intercostal nerves. The right crus of the diaphragm, the fibres of which divide to the right and left of the oesophageal opening, is innervated by both right and left phrenic nerves. There is some evidence that the crural fibres contract slightly before the costal part, and this may be functionally significant for non-respiratory tasks (Sharshar et al 2005).

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Fig. 58.3 The distribution of the right and left phrenic nerves.

(From Sobotta 2006.)

Phrenic nerve

The phrenic nerve is a mixed nerve that provides the sole motor supply to the diaphragm. It is derived mostly from the fourth cervical ramus, but also receives contributions from the third and fifth cervical rami (see Fig. 43.6). The course of the cervical part of the phrenic nerve is described on page 456. Within the thorax, the phrenic nerve descends anterior to the pulmonary hilum, between the fibrous pericardium and mediastinal pleura, to the diaphragm, accompanied by the pericardiophrenic vessels. In its thoracic course, each phrenic nerve supplies sensory branches to the mediastinal pleura, fibrous pericardium and parietal serous pericardium. The right and left phrenic nerves differ in their intrathoracic relationships (Rajanna 1947).

Right phrenic nerve

The right phrenic nerve is shorter and more vertical than the left, and is separated at the root of the neck from the second part of the right subclavian artery by scalenus anterior. It subsequently descends lateral to the right brachiocephalic vein, the superior vena cava and the fibrous pericardium that covers the right surface of the right atrium and inferior vena cava, and divides just above or at the level of the diaphragm (Fig. 58.2).

Left phrenic nerve

At the root of the neck, the left phrenic nerve is commonly stated to leave the medial edge of scalenus anterior and to pass anterior to the first part of the left subclavian artery and behind the thoracic duct. However, sometimes the right and left nerves are symmetric in their cervical course, and so the left phrenic nerve may cross anterior to the second part of the left subclavian artery, separated from it by scalenus anterior at the level of the thoracic inlet. Thereafter, the left phrenic nerve crosses anterior to the left internal thoracic artery, descending across the medial aspect of the apex of the left lung and its pleura to the first part of the subclavian artery, which it crosses obliquely to reach a groove between the left common carotid and subclavian arteries. It passes anteromedially, superficial to the left vagus nerve just above the aortic arch and behind the left brachiocephalic vein, and then passes superficial to the aortic arch and the left superior intercostal vein, anterior to the left pulmonary hilum, to lie between the fibrous pericardium covering the surface of the left ventricle and the mediastinal pleura (see Fig. 56.1).

Diaphragmatic relationships

The right phrenic nerve passes through the central tendon of the diaphragm, either by the caval aperture or just lateral to it. The left phrenic nerve passes through the muscular part of the diaphragm anterior to the central tendon, just lateral to the left cardiac surface and more anterior than the right phrenic nerve (Fig. 58.2). At the diaphragm or slightly above it, each phrenic nerve supplies fine branches to the parietal pleura above, and the parietal peritoneum below, the central diaphragm. Typically, the trunk of each nerve then divides into three branches as it passes through the diaphragm. These are commonly arranged as follows (with some variation): an anterior (sternal) branch, which runs anteromedially towards the sternum and connects with its fellow; an anterolateral branch, which runs laterally anterior to the lateral leaf of the central tendon; a short posterior branch, which divides into a posterolateral ramus coursing behind the lateral leaf and a posterior (crural) ramus which supplies the crural fibres. Posterolateral and crural branches may arise separately from the phrenic nerve. These main branches may be submerged in diaphragmatic muscle or lie below it. They supply motor fibres to the muscle and sensory fibres to the peritoneum and pleura related to the central part of the diaphragm. They also relay proprioceptive fibres from the musculature. The right phrenic nerve supplies the part of the right crus to the right of the oesophagus, and the left phrenic nerve supplies the left crus and the part of the right crus that lies on the left of the oesophagus. Phrenic rami connect with branches of the coeliac plexuses on the inferior surface of the diaphragm, and there is a small phrenic ganglion on the right, at the junction of the plexuses. Rami from the plexuses supply the suprarenal glands and, on the right, the hepatic falciform and coronary ligaments, the inferior vena cava and, possibly (via connections with coeliac and hepatic plexuses), the gallbladder.

Lesions of the phrenic nerve

An appreciation of the location of the main branches of the phrenic nerves is important to avoid iatrogenic surgical damage. Radial incisions in the diaphragm from the costal margin to the oesophageal hiatus lead to diaphragmatic paralysis, whereas thoracoabdominal incisions in a circumferential manner in the periphery of the diaphragm do not involve any significant branches of the phrenic nerves and preserve diaphragmatic function. Similarly, incisions of the central tendon are safe.

Division of the phrenic nerve in the neck completely paralyses the corresponding half of the diaphragm, which atrophies. If an accessory phrenic nerve exists, section or crushing of the main nerve as it lies on scalenus anterior will not produce complete paralysis. The phrenic nerve may be involved with traumatic lesions of the upper brachial plexus. Historically, it was deliberately injured in order to collapse, and hence rest, the lung in patients with pulmonary tuberculosis. Cardiac surgery is one of the most common medical causes of phrenic nerve injury, especially as a result of the instillation of saline slush for myocardial preservation. Other causes include thoracic surgery, tumours of the lung or mediastinum, and infections such as polio. Respiratory muscle weakness of rapid onset is a feature of Guillain–Barré syndrome.

Phrenic nerve damage leads to paradoxical movement of the diaphragm that is best observed fluoroscopically, with the patient first in the upright position (diaphragm unloaded), and then supine with a small weight on the abdomen (diaphragm loaded). Diaphragmatic paralysis can also be assessed by ultrasound examination with a sniff manoeuvre. The ‘gold standard’ for assessment of the phrenic nerve/diaphragm unit is electrical or magnetic stimulation of the phrenic nerve with recording of the compound muscle action potential and/or the transdiaphragmatic pressure. Electrical stimulation of the diaphragm, by ‘pacing’ of one or both phrenic nerves, has been used with some success in infants with central alveolar hypoventilatory syndrome (‘Ondine’s curse’) and in patients with high cervical lesions of the spinal cord, in whom the diaphragm is paralysed but the lower motor neurones are intact. Electrodes are placed adjacent to the nerves, sometimes in the neck but more usually in the chest, and a ventilatory rhythm is established by trains of stimuli delivered by an implanted device. Because this is an unphysiological way of recruiting the muscle, the fibres must be ‘conditioned’ during the initial period of stimulation, so that they acquire the necessary resistance to fatigue.

Referred pain

Diaphragmatic pain is frequently felt at the tip of the shoulder, reflecting common nerve root origins in the neck. It usually occurs when there is inflammation of the diaphragmatic pleura, e.g. in basal pneumonia, pleural effusions or malignant disease.

Accessory phrenic nerve

The accessory phrenic nerve is described in Chapter 28.

ANATOMY OF BREATHING

Breathing is a highly coordinated abdominal and thoracic process. The diaphragm is the major muscle of inspiration, responsible for approximately two-thirds of quiet breathing in healthy humans. The external intercostal muscles are most active in inspiration, and the internal intercostals, which are not as strong, are most active in expiration. Increasing the vertical, transverse and anteroposterior dimensions of the chest increases the volume of the pleural space, and the resulting decrease in intrapleural pressure draws air into the lungs. During expiration, the diaphragm relaxes and moves superiorly. Air is expelled from the lungs and the elastic recoil of the lung creates a subatmospheric pressure that returns the lateral and anteroposterior dimensions of the thorax to normal (De Troyer & Estenne 1988, Celli 1998).

During inspiration, the lowest ribs are fixed, and contraction of the diaphragm draws the central tendon downwards. In this movement, the curvature of the diaphragm is scarcely altered. The cupolae move downwards and a little forwards almost parallel to their original positions. The associated downward displacement of the abdominal viscera is permitted by the extensibility of the abdominal wall, but the limit of this extensibility is soon reached. The central tendon, its motion arrested by the abdominal viscera, then becomes a fixed point from which the fibres of the diaphragm continue to contract. This causes the second to tenth ribs to be elevated and the inferior portions of the ribs are turned outwards as a result of direct transmission of pressure through the zone of apposition (Fig. 58.4). The medial aspect of the rib is elevated and this increases the transverse dimension of the chest in the same manner as a bucket handle swinging outwards (Fig. 58.5A). This effect is most evident in the lower ribs (seventh to tenth ribs). Movements at the costovertebral joints cause elevation of the anterior ends of the ribs that push the body of the sternum and the upper ribs forwards. This ‘pump handle’ movement is most evident in the superior ribs (second to sixth ribs) and increases the anteroposterior dimension of the thorax (Fig. 58.5B). The right cupola of the diaphragm, which lies on the liver, has a greater resistance to overcome than the left, which lies over the stomach, and so the right crus and the fibres of the right side are more substantial than those of the left. The balance between descent of the diaphragm, protrusion of the abdominal wall (‘abdominal’ breathing), and elevation of the ribs (‘thoracic’ breathing) varies in different individuals and with the depth of ventilation. The thoracic element is usually more marked in females, but increases in both sexes during deep inspiration.

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Fig. 58.4 Inspiratory movements: pressure changes during inspiration.

(Adapted from Drake, Vogl and Mitchell 2005.)

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Fig. 58.5 Movements of the ribs during inspiration (A) increase the transverse diameter of the chest by the ‘bucket handle’ movement, and (B) increase the anteroposterior dimension of the thorax by the ‘pump handle’ movement.

Diaphragmatic excursion is typically 1.5 cm in quiet breathing. During deep ventilation, the maximum movement ranges from 6 to 10 cm. After a forced inspiration, e.g. when breathing is partially obstructed, the right cupola of the diaphragm can descend to about the level of the eleventh thoracic vertebra, while the left cupola may reach the level of the body of the twelfth thoracic vertebra. After a forced expiration, the right cupola of the diaphragm is level anteriorly with the fourth costal cartilage, laterally with the fifth, sixth and seventh ribs, and posteriorly with the eighth rib, and the left cupola is a little lower.

The level of the diaphragm is affected by the phase and depth of ventilation, and by the degree of distension of the stomach and intestines and the size of the liver. Radiographs show that the height of the diaphragm within the thorax also varies considerably with posture. It is highest when the body is supine, and in this position it performs the greatest ventilatory excursions with normal breathing. When the body is erect, the diaphragm is lower, and its ventilatory movements become smaller. The diaphragmatic profile is still lower in the sitting posture, and ventilatory excursions are smallest under these conditions. When the body is horizontal and on one side, the two halves of the diaphragm do not behave in the same way. The uppermost half sinks to a lower level than that seen when sitting, and moves little with ventilation. The lower half rises higher in the thorax than it does even in the supine position, and its ventilatory excursions are considerably greater. Changes in the level of the diaphragm with alterations in posture explain why patients with severe dyspnoea are most comfortable, and least short of breath, when sitting up.

The primary role of the intercostal muscles is to stiffen the chest wall, preventing paradoxical motion during descent of the diaphragm in inspiration. This becomes most obvious immediately after high spinal injury, when there is flaccid paralysis of the entire trunk and only the diaphragm is left functioning. In a healthy adult with a vital capacity of 4.5 L, some 3 L is accounted for by diaphragmatic excursion. Immediately after high spinal injury, the vital capacity decreases to about 300 mL, even though the diaphragm is moving maximally, because some 2.7 L is lost by paradoxical incursion of the flaccid chest wall as the diaphragm descends. With time (usually several weeks), the paralysis becomes spastic, stiffening the chest wall, and the vital capacity increases towards its phrenic limit of about 3 L.

In the same way, high spinal injury reveals the role of the abdomen in inspiration and expiration. The abdominal musculature plays a major role in active expiration in man. During the flaccid stage of high spinal paralysis, the only mechanisms available for returning the relaxed diaphragm into the thorax on expiration are passive recoil of the lungs and chest wall, and the weight of the abdominal viscera. The latter is the most important, and operates only when patients are lying down. If they are sat up or raised upright, they are unable to breathe out. Trussing the abdomen with an elastic binder can help such patients. Conversely, when paralysis becomes spastic, the stiff abdominal wall opposes inspiration.

The role of the abdomen in breathing is often underestimated. If, for example, the anterolateral wall were made of steel and linked the pelvic rim rigidly to the costal margins, inspiration would be impossible. The diaphragm could not descend (because the abdominal contents are incompressible), and the ribs could not rise (because the links to the pelvis would be inextensible). During normal breathing, the abdomen relaxes as the diaphragm contracts. It is possible to oppose this motion by tensing the abdomen, as in the ‘beach posture’ adopted to exaggerate the size of the chest. In this case, the abdominal contents fix the central tendon of the diaphragm, so that it raises the ribcage as it contracts, but it is a condition of that manoeuvre that the gap between the ribs and the pelvic rim widens.

The ventilatory muscles must also work during sleep, when the pharyngeal muscles relax and upper airway resistance increases. It is now appreciated that in some people, particularly the obese, this relaxation can lead to periodic apnoea and marked hypoxia during sleep, implying that the pharyngeal muscles play an important ventilatory role in waking life. It is also clear that although ventilatory muscles rarely tire in normal life, they do fatigue when placed under abnormal loads, e.g. in chronic obstructive pulmonary disease.

The different pulmonary regions do not all move equally in ventilation. In quiet ventilation, the juxtahilar part of the lung scarcely moves and the middle region moves only slightly. The superficial parts of the lung expand the most, and the mediastinal surface, posterior border and apex move less, because they are related to less movable structures. The diaphragmatic and costomediastinal regions expand most of all. Most of the volumetric change during ventilation occurs in the alveoli.

REFERENCES

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Celli B. The diaphragm and respiratory muscles. Chest Surg Clin N Am. 1998;8:207-224.

De Troyer A, Estenne M. Functional anatomy of the respiratory muscles. Clin Chest Med. 1988;9:175-193.

Kahrilas PJ. Supraesophageal complications of reflux disease and hiatal hernia. Am J Med. 2001;111(suppl 8A):51-55.

Reviews gastro-oesophageal reflux and type I sliding hiatus hernia..

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Discusses the additional benefit and role of video-assisted thoracoscopy in thoracic trauma..

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Describes how the normal anatomy of the distal oesophagus, proximal stomach and the diaphragm serves as an antireflux barrier..

Rajanna MJ. Anatomical and surgical considerations of the phrenic and accessory phrenic nerves. J Int Coll Surg. 1947;60:42-52.

Describes the anatomical course of the phrenic nerve in 203 cadaveric dissections..

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Reviews the radiological techniques available in the assessment of the diaphragm after trauma, and the extent and anatomical sites of coexisting thoracoabdominal injuries..

Sharshar T, Hopkinson NS, Ross ET, et al. Motor control of the costal and crural diaphragm – insights from transcranial magnetic stimulation in man. Respir Physiol Neurobiol. 2005;146:5-19.

Evaluates the neural control of the diaphragm..

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