9 The Thorax

The thorax is the second largest and most anteriorly situated of the body cavities. It is shaped somewhat like a truncated cone, com­pressed laterally over its anterior part, with the base cut off obliquely at the costal arch where it adjoins the abdominal cavity. The roof is made up of the thoracic vertebrae and the asso­ciated muscle masses and ligaments. The lateral walls consist of the ribs and intercostal muscles. At its anterior, narrow portion the thorax is overlaid, on each side, by the scapula and humerus, and the mass of muscles comprising the upper part of the forelimb, above the elbow, which is attached to the lateral aspect of the ribs in this region. The posterior part of the thoracic cavity is shut off from the cavity of the abdomen by the diaphragm, which is attached at its periphery to the medial surface of the ribs and is strongly convex on its anterior aspect.

Internally the thorax is divided longitudinally by the mediastinal septum into two lateral cham­bers, the pleural cavities, which are lined by a serous membrane. The mediastinal septum is not median in position because the heart is placed more on the left side than on the right, so that as a consequence the right pleural sac is larger than the left. The important structures in the thorax include the pleura, mediastinum, lungs, heart, trachea, bronchi, oesophagus, major blood and lymph vessels and nerve trunks.

The lungs and respiration

Descriptively the respiratory system is divided into an upper and a lower portion, the greater part of the latter, comprising some of the trachea, the bronchi and the lungs, being accommodated within the cavity of the thorax. This arbitrary division has relatively little clinical merit because many of the diseases involving the upper respira­tory tract have a marked tendency to spread and involve the lungs. In suspected cases of respira­tory disease, the need to include all the associated structures in the clinical examination is patently obvious. Following a superficial appraisal of the clinical signs exhibited by an affected animal, confusion may arise as to which part of the respiratory system is involved in the disease process, e.g. acute pneumonia is not infre­quently associated with bilateral nasal discharge of varying character, suggestive of rhinitis.

The physical aspects of respiration and the clinical methods for assessing them have been discussed in Chapter 6, along with some of the more important effects of disturbed respiratory function.

Regional Anatomy of the Lungs

The lungs occupy the major part of the thora­cic cavity, where they are intimately related to the boundary structures and other organs situa­ted within the cavity. In the horse the trachea, which is large in cross-section (5-7 cm), is slightly flattened dorsoventrally. It occupies a median position in the thorax and the bifurcation occurs opposite the fifth rib, about 15 cm ventral to the sixth thoracic vertebra, or at the midway point between the roof and floor of the thoracic cavity. The right lung is larger than the left, which is reduced in surface area and thickness by reason of the mediastinum being towards the left of the median plane of the body, to accom­modate the heart. The dorsal border of the lungs, which is thick, extends posteriorly to the second last (sixteenth) intercostal space and lies in the angle formed by the bodies of the thoracic vertebrae and the ribs. The ventral border is thin and is insinuated into the narrow angle formed between the mediastinum and the ventral parts of the sternal ribs. Opposite the heart it is indented by the cardiac notch, which extends on the left side from the third to the sixth rib and on the right side from the third to the fourth inter­costal space. At these points the pericardium comes into contact with the chest wall. The basal border is also thin and is situated between the diaphragm and the costal ribs, being on a level with the costochondral junction of the seventh rib, and approximately at the midway point of the twelfth rib. The position of this border varies during the different phases of respiration.

The trachea is relatively small in diameter (4 cm) in the ox, as compared with that of the horse. The accessory bronchus to the apical lobe of the right lung originates from the trachea at the level of the third rib, while the bifurcation is opposite the fifth rib about 10 cm below the vertebral column.

In the ox the size of right lung compared with the left is proportionately even greater than in the horse. The basal border of both lungs extends upwards and backwards from near the lower extremity of the sixth rib, in a straight line, to the upper part of the second last (eleventh) intercostal space where it meets the dorsal border. The tra­chea and lungs in the sheep are proportionately smaller than in the ox and, apart from the lungs extending slightly further back at the base, the regional anatomical relationships are broadly similar to those in cattle.

The trachea in the pig is relatively short; it supplies a supplementary bronchus to the apical lobe of the right lung. The dorsal border of the lung extends posteriorly only to the third last intercostal space (eleventh or twelfth according to the number of ribs). The basal border forms an almost straight line, running from the lower end of the sixth rib to the dorsal border at its posterior extremity.

In the dog the trachea is almost circular in cross-section at both ends, but the middle part is slightly flattened dorsoventrally. The bifurca­tion, which is opposite the fifth rib, forms a very wide angle. The costal surface of the lungs is more convex than in other species, conforming to the contour of the thoracic wall. The right lung is much larger (25 %) than the left. The pericar­dium is in contact with the chest wall at the ventral part of the fourth and fifth intercostal spaces opposite the cardiac notch of the right lung. On the left side, the pericardium is in contact with the chest wall along a narrow area at the ventral part of the fifth and sixth intercostal spaces; there is no distinct cardiac notch in the left lung. The dorsal border of the lung extends posteriorly to the second last (eleventh) intercos­tal space. The basal border forms a shallow cavity and extends upwards from the ventral extremity of the seventh rib to the dorsal border near the upper part of the twelfth rib.

Physical Examination of the Thorax (Lung Area)

Consideration has already been given to res­piration in respect of frequency, rhythm, and types of respiratory movements in the different species of domestic animals. Abnormal sounds that are associated with respiration, and which have already been discussed, include sneezing, wheezing, snoring, roaring and whistling, grunt­ing, coughing and yawning.

More accurate information about the condi­tion of the thoracic portion of the respiratory system can be obtained by means of physical examination of the lung area. This consists of palpation, percussion and auscultation, which should be routine procedure when the evidence obtained during the general clinical examination of the patient suggests the presence of respiratory disease. The information obtained by means of physical examination varies in value according to the particular method, the experience of the clinician, and the conformation and species of animal.

Palpation

Palpation of the lung area is of limited value because very little worthwhile evidence is ob­tained by this means. Increased sensitivity of the chest wall might suggest a pain reaction arising from pleurisy. Differentiation between a pain reaction and that evinced by a nervous animal is essential. Palpation may reveal the presence of a pleuritic thrill arising from the movement of fluid, in which case the intercostal spaces are bulging in character. Decreased rib movements, with narrowing of the intercostal spaces, which occur in the early stages of pleurisy, in tetanus and in extensive collapse of a lung, are recog­nizable by palpation.

Percussion

Percussion is a useful diagnostic procedure to apply to the chest wall, more particularly in rela­tion to the lungs. By this means some of the anatomical relationships and position of the normal lung, and the presence of abnormal states, can be recognized by noting the variations in the percussion sounds (resonance) so pro­duced.

Types of Resonance

Percussive resonance is the sound that is pro­duced at the place percussed by the blow itself, by the vibration of the body wall and by the column or body of air or gas contained beneath the point of impact. Any of these three factors may influence the percussive resonance and so change the nature of the sound produced by percussion.

Resonant sound (ringing). This usually indi­cates the presence of a large volume of air or gas beneath the site of percussion. It is the sound produced during percussion over a normal large lung (a smaller lung normally gives a tympanic response, see below). The thinner the wall of the thorax and the larger the lung, the more resonant the sound, and the smaller the lung and the thicker the wall, the less ringing the sound.

The ringing sound is exaggerated when there is an excessive quantity of air or gas present at the site of percussion. Such a sound, therefore, is induced in conditions such as pulmonary emphysema, pneumothorax (air or gas in a pleural cavity), subcutaneous emphysema, when a gas-filled viscus occupies a diaphragmatic hernia and over an area limited to the left posterior part of the thorax in ruminal tympany.

When the percussion sound is less clear and loud than the ringing sound it is said to be abbreviated. It represents the transition to the next type.

Dull sound. This is heard when there is no air or gas beneath the part percussed. If dull sounds are heard on percussion over the lung they must be taken to indicate the presence of disease; it is not possible, however, to decide by means of percussion alone which structure (lung, other thoracic organ, chest wall) is diseased and what type of disease (inflammation, neoplasia, fluid effusion) is present. In order to ascertain these points, other signs and findings (fever, cough, findings on auscultation, etc.) have to be taken into consideration. The dull sound itself indicates only that, in that particular part, air-containing tissue is absent or reduced in amount. Dull sounds can be recognized only when the airless area of the lung is at least as large as the palm of the hand, and is superficially situated.

Dull percussion sounds occur when there is an increase in density of pulmonary tissue as a result of congestion, neoplasia or collapse; in hydro-thorax and effusive pleurisy; with thickening of the chest wall or pleura; in subcutaneous oedema when it impinges on the lung area; or because of a thick subcutaneous layer of fat.

Tympanic sound. This is distinguished from the preceding types of percussion sound by its characteristic musical ring (like a kettle-drum). It occurs only as an accompaniment to the fore­going types of sound, when it is known as a ringing tympanic sound or a dull tympanic sound. It occurs when, in addition to the con­ditions necessary for the production of ringing or dull sounds, there exists any comparatively small column of air, or a very large column that communicates with the air outside the body through a large opening. This type of percussion sound is produced from very small lungs, and may be noted in miniature breeds of dogs and in cats. Depending upon the musical tone, the tympanic sound is described as being high—or low—pitched.

Tympanic percussion sounds are associated with the early and late stages of pneumonia, pul­monary cavitation (rare in animals), pneumo­thorax, subcutaneous emphysema and the presence of loops of gas-distended intestine in the thoracic cavity (diaphragmatic hernia or rupture).

Metallic ring. This is similar to the tympanic sound, but its musical character is more pro­nounced. The tone is a very high-pitched tym­panic one, and resembles the response obtained on striking an empty metal jug. It occurs in diseases similar to those in which the tympanic sound is obtained, particularly where there are small cavities containing air or gas under pressure.

The 'cracked pot' sound simulates that pro­duced by striking a cracked pot. It arises when air escapes through a narrow opening, and is produced during percussion where there are cavi­ties communicating with a bronchus through a narrow aperture (a very rare circumstance in animals), in subcutaneous emphysema and some­times in pneumonia if an air-containing portion of lung has been surrounded by other areas devoid of air; it also occurs at the margin of the dull areas in exudative pleurisy.

Tracheal Percussion

In addition to the methods of performing per­cussion already described, certain other methods may be of value when applied to the thorax and some other parts of the body. Tracheal percus­sion, one of these procedures, is a combination of percussion and auscultation. In its perfor­mance an assistant places the pleximeter on the skin overlying the trachea in the mid-neck region and strikes it with the hammer, using single blows of medium strength. Immediate percussion with the fingers of one hand will serve the same purpose adequately. Simultaneously, the clinician auscultates the lungs in turn, and observes the manner in which sounds produced in the trachea are heard in the respiratory area. An air-contain­ing lung is a poor conductor of sound, hence in a normal lung the percussion sounds are distant and indistinctly heard. Over consolidated areas, on the other hand, the sound of the percussion impact is loud and distinct, because dense tissue is a good conductor of sound. The value of the method is that its application makes it possible to distinguish between dullness caused by inflam­matory infiltration of the lung and that resulting from the presence of free fluid in one, or both, pleural sacs. In pneumonia the referred sound is very loud and distinct, and seems to originate from directly beneath the chest-piece of the stethoscope. In exudative pleuritis, hydrothorax and other similar conditions the sound, although in fact equally loud, seems in comparison to be coming from a remote point.

Tuning fork-stethoscope method. The advan­tage of this method is that it can yield very accurate information concerning the underlying organs. With experience it is possible, in an extensive dull area, to distinguish the dullness of the lungs from that caused by the heart, and also from that caused by the liver, whereas in the more usual methods of percussion the area of cardiac, pulmonary and hepatic dullness appear to be continuous. The method is also particu­larly suitable for determining the exact limits of individual organs.

The instruments used are a stethoscope and a long-handled tuning fork. The procedure is to place the vibrating fork against the skin, where its tone is picked up by the stethoscope. When

both instruments lie over the same organ or medium the tone is loud and distinct, but as soon as the tuning fork passes beyond the border of the organ or medium over which the stethoscope is placed, the tone becomes soft and indistinct. If a tuning fork is not available, a process of continuous scratching with the fingers may be substituted for the vibration of the fork (frictional auscultation).

Percussion of the Lung

In most species, percussion of the healthy lung evokes a resonant (ringing) sound, but in very small animals, e.g. miniature dogs, cats and rabbits, the sound is normally somewhat tym­panic on account of the short length of the vibrat­ing column of air in the lung. It should be appreciated that changes in the character of the percussion sound are detectable only when any lesion present is of considerable size and is superficially situated. It therefore follows that a lung which evinces normal percussion sounds is not necessarily healthy or free from consolidated foci. If such foci are small, as in bronchopneumonia, or deep-seated, as is often the case in the early stages of contagious bovine pleuropneumonia, tuberculous bronchopneumonia and metastatic pulmonary neoplasia, their presence cannot easily be demonstrated by percussion. The limitation applies, therefore, to the lungs of large animals as well as to those of small animals such as the dog, cat, etc. In the latter species, pulmonary lesions are frequently undetected during percussion, because in the small lung even relatively comprehensive changes are sel­dom extensive enough to produce a recognizable change in resonance.

It is not possible to examine the whole lung by percussion. That part of the lung situated beneath the shoulder is inaccessible on account of the heavy musculature overlying the area. Here, on percussion, only the completely non-resonant response of muscular tissue is obtained. Neither is it possible, in a large lung, to obtain a satisfactory percussion response from the deep-seated areas surrounding the bifurcation of the trachea, because the thickness of the lung in this and in similar areas prevents the penetration of the percussion impulses.

Immediately posterior to the shoulder, above the heart base, the pulmonary tissue extends, except for the intervening mediastinum, from one wall of the thorax to the other. Farther back, behind the mid-point of the seventh rib, this situation no longer prevails on account of the dome-like anterior projection of the diaphragm. The posterior border of the lung is normally situated a variable distance in front of the line of attachment of the diaphragm to the ribs. During inspiration, of course, and more mar­kedly in the horse, the border of the lung moves closer to the anterior surface of the diaphragm at this point. This can be demonstrated occasionally by percussion, since at inspiration, percussion immediately anterior to the attachment of the diaphragm evokes a ringing sound, whereas during expiration the sound is dulled, because the posterior border of the lung moves away from the insertion of the diaphragm.

In the horse, the area of the thorax suitable for percussion of the lung comprises a triangle, the points of which are situated at the posterior angle of the scapula, the olecranon process of the ulna, and the second last intercostal space at a point on a horizontal line from the scapula to the external angle of the ilium. The ventral boundary of the area extends upwards and back­wards from the olecranon process of the ulna to join the dorsal one at the second last intercostal space, forming a shallow concavity on its anterodorsal aspect (Figs 120, 121), The part of the chest wall within the limits of these boundaries is frequently referred to as the respiratory area.

In the ox, the ventral boundary of the area is a straight line extending from the point of the elbow to join the dorsal border at the second last intercostal space (Fig. 122). In addition to the thoracic percussion area, there is also a smaller prescapular area, more or less oval in shape with its long axis roughly perpendicular, which is situated in the lower part of the neck in front of the shoulder (Fig. 120). Percussion in this area normally gives an abbreviated sound. In adult cattle in good physical condition the area is roughly 3 cm wide, but in lean animals it is twice as wide. In this area it is possible to percuss the lungs in the region of the first, second, and even the third rib, particularly when the foreleg is drawn backwards.

In the dog and cat, the area available for lung percussion has the same general outline as in the horse. The area in the pig shows some variation in that the ventral boundary, which forms a straight line, joins the dorsal border at the third or even fourth last intercostal space.

The posterior boundary of the respiratory area, as indicated, is the approximate position of the posterior border of the lung midway between inspiration and expiration. To determine the position of this boundary, the thorax is percussed at various levels, along horizontal lines extending from the central or most resonant part of the respiratory area towards the abdomen (Fig. 123). By drawing the foreleg well forwards, it is possible to extend the area of percussion anteriorly. Care must be taken that percussion is uniform, and not too vigorous, particularly in the area approximating the posterior border of the lung, on account of the reduced thickness of the organ at that part. The point at which a distinct change in sound occurs is the margin of the lung. A finger is placed at this point while, with the other hand, the intercostal spaces are counted from the last space forward to the point indicated. As a general rule it is sufficient to determine the posterior margin of the lung only in the upper and middle thirds of the respiratory area; it is only in special cases that determination of the lower part of this boundary is of impor­tance. As the number of ribs in a particular species (see Table 9) is not always applicable to the individual animal—more (supernumerary) or fewer ribs may be present—accuracy is better served by recording the number of ribs posterior to the ventral border of the lung (counting anteriorly from the last rib).

The change in sound at the periphery of the normal lung consists usually of a transition—in some instances this is sudden—from a ringing sound to one that is dull. This occurs in those parts of the respiratory area where the thin border of the lung overlies organs such as the liver, heart, spleen and rumen, stomach and in­testine when filled with food material. If, however, the adjacent parts of the rumen, stomach or intestine are distended with gas a tympanic sound or, more rarely, even a ringing sound, simulating that of the lung, may be obtained in the vicinity of and behind the normal respiratory area, thus rendering it impossible to determine the lung boundary (e.g. in the ox, on the left side in the upper posterior part of the thorax over the rumen, and at an intermediate point over the abomasum on the right side).

An increase in the size of the area over which lung percussion sounds can be evoked occurs in overdistension of the lung (pulmonary emphy­sema), and in accumulation of air in the thoracic cavity (pneumothorax). In large animals this increase in size, or backwards displacement of the lung, may amount to between 2 and 10 cm. In established cases of pulmonary emphysema characteristic expiratory dyspnoea will be ob­served, and in pneumothorax both inspiration and expiration are obviously laboured.

A reduction in size of the area over which lung percussion sounds are heard exists when the lungs are prevented from being fully inflated during inspiration because of increased intra-abdominal pressure. This occurs in pregnancy, tympany of the stomach, intestines or rumen, large abdominal tumours, ascites, etc. Enlarge­ment of the heart, caused by hypertrophy or dilatation, and fluid distension of the pericar­dium will also, locally, reduce the area of lung resonance as determined by percussion. This reduction may only be detectable in large animals.

Changes in the percussion note at the posterior margin of the area must always be interpreted with caution, since they may result from changes elsewhere than in the lung itself. Gas-filled loops of intestine in this area will evoke a confusing tympanic sound, and a full rumen, stomach or intestine a correspondingly dull sound.

A general reduction in the resonance of the percussion sound occurs when, because of exten­sive pulmonary disease, the volume of air in the lungs is reduced. This situation prevails in acute congestion of the lungs, pneumonia associated with consolidation, collapse of the lung and chronic tuberculous pleurisy with adhesions. Local reduction in resonance may be caused by bronchopneumonia, congestion of dependent parts of the lung, or neoplasia. There may be a unilateral reduction in resonance as the result of hypostatic congestion which often follows from prolonged lateral recumbency. In many small animals, percussion yields disappointing results because any area of dullness is too small to be readily defined.

A pain reaction produced by percussion is indicated by the animal becoming restive, with­drawing the body, biting or kicking, e.g. in fracture of a rib, acute pleurisy and other painful conditions of the parts percussed. Percussion may also induce coughing, more usually when there is acute disease of the lung and/or pleura, e.g. in pneumonia, bronchitis and acute pleurisy.

Loops of intestine that have moved forwards into a pleural cavity following rupture of the diaphragm, give a tympanic percussion sound that may change position during the course of the examination as a result of the motility of the largely gas-filled loops.

Differentiation between the reduced resonance caused by increased density of the lungs and that resulting from the presence of fluid in the pleural sacs, is determined as follows:

1. Increased density of the lung, as in pneu­monia: the area of dullness has an irregular outline; the cardiac impulse is palpable; heart sounds are clearly audible outside the cardiac area; abnormal bronchial or other sounds are often heard during auscultation (rales or frictional sounds); on percussion over the trachea, strong, loud sounds are heard directly beneath the chest-piece of the stethoscope when it is applied to the chest wall.

2. Presence of fluid in the pleural sac (e.g. exudative pleurisy, hydrothorax); the area of percussion dullness has a horizontal delimita­tion; the position of the area of dullness changes when the posture of the animal is altered (the fluid always gravitates to the lowest possible site); as the heart is pushed away from the chest wall, the cardiac impulse is correspondingly less distinct; no rales or frictional sounds are heard during auscultation; on percussion over the trachea, the sounds heard on auscultation are distant, but strong and loud; fluid can be ob­tained by exploratory puncture.

Auscultation of the Lung

The area of auscultation is the same as that of percussion (Figs 120-122, p. 117). Auscultation requires much practice and application in order to yield worthwhile results. The chest-piece of the stethoscope should be held firmly against the thorax to minimize the crackling sounds caused by friction against the hair of the animal's coat. When the coat is long or matted together it is necessary, before applying the chest-piece, to expose the skin by separating the hair or wool so that they do not form a barrier to sound waves. The whole of the respiratory area should be methodically auscultated, commencing in the upper, anterior part and moving horizontally backwards, and then forwards again, at a slightly lower level, until the entire area has been ex­plored. The stethoscope should be applied to each point on the chest wall for the duration of at least one complete respiratory cycle. It is advisable to compare obviously diseased areas with those that are healthy, because this makes it easier to identify any lung abnormality. Com­paring corresponding portions of lung on both sides, in the same animal, or in different animals of the same species with similar physical charac­ters, may assist in recognizing relatively minor abnormalities. It should be remembered that the respiratory sounds may be masked by the thick­ness of the chest wall, and by the presence of subcutaneous oedema or emphysema involving the thoracic region.