THORAX - Veterian Key

Clinically, the thorax is important because of the wealth of problems that may affect the heart, lungs and trachea, esophagus and diaphragm. The regions are presternal, sternal, cardiac and costal regions.

The thorax can be examined by palpation (which is limited), auscultation and percussion. In the dog, the forelimbs can be brought forward.

The heart is situated in the 3rd to 5th intercostal spaces and occasionally as far caudally as the 7th. The cardiac notch in the lung acts as a valuable window to examine the heart more effectively, by auscultation and echocardiography. Here, there is direct access to the heart without any overlying lung tissue. It is more pronounced on the right side. Horizontally, the heart is situated at the level of the middle of the first rib and this forms the dorsal boundary. There is wide variation in the shape and size of the heart due to the tremendous variation between breeds of dog. In deep-chested dogs it is tall, narrow and upright; in barrel-chested dogs, almost globular. In the standing dog, the heart lies at the olecranon at 5th rib – therefore need to draw leg before. It occupies usually 2.5 to 3.5 intercostal spaces and the height of the heart is about two-thirds of the length of the thoracic cavity. In width, it occupies two-thirds of the width of the thoracic cavity with the apex tilted to the left of the midline. The vertebral heart score is the most useful assessment of cardiac size and is normally 10.5–12.5. The apex beat of the heart is found on the left and right in the 4th to 5th intercostal space near the mitral valve. The left atrioventricular valve (mitral) is heard best on the left in the 5th intercostal space at the costochondral junction, the aortic valves in the 4th intercostal space on the left at the level of the shoulder, and the pulmonary valves best in the 3rd intercostal space on the left above costochondral junction. On the right side, the right atrioventricular valves (tricuspid) are best heard in the 4th intercostal space at the level of the costochondral junction. The costomediastinal recess occupies the 4th to 6th intercostal spaces on the left and on the right as well.

Left-sided enlargement of the heart is important in the dog. In this you can see elevation of the trachea, the caudal border of the heart straightens. There is tensing of the left atrium and loss of the caudal heart waist. In right-sided enlargement, there is also elevation of the trachea, increased sternal contact, rounding of the cranial border, loss of cranial waist and dorsal displacement of the cranial vessels of the heart. Cardiomyopathy in the form of enlarged heart can be seen in myocardial disease, myocarditis and associated with the presence of Angiostrongylus the canine heartworm. It is particularly common in large breeds such as Dobermans.

A whole variety of heart problems both congenital and acquired can be found in dogs and are beyond this short introduction. Suffice to say that patent ductus arteriosus and persistent right aortic arch can be treated surgically. Mention of thoracotomy will be made later but for this, surgical openings are made in the left 4th or 5th intercostal space, and during repair care must be taken to protect the recurrent laryngeal nerve and thoracic duct. A patent ductus leads to a left-to-right shunt with the left ventricle continuously overloaded leading to dyspnoea (difficulty breathing), shortness of breadth and other problems with the only treatment surgical closure. Interestingly, it may produce signs of esophageal stricture because the esophagus is partially obstructed by the ductus and eventually this may lead to an extensive esophageal diverticulum, left-sided atrial and ventricular enlargement and subsequent failure.

The majority of esophageal strictures are probably associated with persistent 4th right aortic arches.

At the base of the heart, the aorta is on the left side of the esophagus so access is best from the right. In the caudal mediastinal space, the esophagus lies to the left, and the aorta is now dorsal so the right surgical approach to the esophagus is from the left. In all cases, care is needed to avoid the vagal nerves.

Esophageal surgery will also require a transthoracic esophagotomy. The blockage is found by use of a stomach tube and then you can make an entry at the appropriate intercostal space for thoracotomy. In most instances these days esophageal foreign bodies are found using flexible gastroscopes with the majority of removals via rigid endoscopes.

The thoracotomy can be used for all intrathoracic surgery. You have to cut mm latissimus dorsi, so be careful with the thoraco-dorsal nerve. It can be done in several ways. The lateral intercostal is usually used and gives good exposure and a good repair. Approximately one-third of the half of the thorax can be viewed from your incision. The incisions are made equidistant from the vertebrae to avoid the vessels and nerves. There is nowadays perceived to be no higher incidence of postoperative complications with median sternotomy than intercostal methods. In median sternotomy the manubrium or the xiphoid is left intact to aid stabilization of the ribcage when it is closed. Occasionally, transcostal is performed and the last method involves midline trans-sternal incision and this may be used for repair of the ruptured diaphragm where entrapment or adhesion of abdominal viscera within the thorax is anticipated. It is possible to remove a rib completely and then approach through the rib cage. Cranial thoracotomy requires incision through the M. scalenus and caudal thoracotomy through the external abdominal oblique and serratus ventralis muscles. Hiatus hernia of the esophagus is also repaired through a left thoracotomy or through a midline celiotomy (laparotomy). Pulmonary lobectomy for removal of a lung is usually for neoplasia, pneumothorax or emphysema, torsion of a lobe lung or foreign body infection can be carried out at the 5th intercostal space on either the left or right side or via a median sternotomy. For thoracolumbar disc protrusion, incision can be made over the intercostal muscle which is then transected and further incisions can be made to the spine.

Thorascopic techniques are now increasingly used to examine the thoracic contents with minimal postoperative problems.

Thoracentesis or puncturing of the chest is carried out to empty fluid (transudate, exudate, blood, pus or chyle) or air from the chest when a pneumothorax has occurred. The key is to make sure that the skin wound does not overlay the puncturing through the intercostal region. If it does, there is danger of creating a pneumothorax.

Dyspnoea or difficult breathing is a major clinical entity and is investigated in several ways. The respiratory pattern is studied, the mucous membrane color is noted and auscultation and percussion of the chest carried out. Radiography, thoracentesis, fluoroscopy and other techniques can be used to aid diagnosis including broncho-alveolar lavage where sterile fluid is introduced into the lungs and collected for analysis. Ultrasound scanning techniques are useful for cardiac pathology but with Doppler techniques it can be very useful. A cough is a similar entity and may originate in the larynx, pharynx or trachea or main bronchi, and is investigated by a similar range of techniques.

Thoracic respiratory sounds may originate from the bronchi, pulmonary parenchyma or the pleura. Auscultation of these sounds may indicate pathological processes.

In practice, the cranial border of the lung is the caudal border of the long head of the triceps. The dorsal border is a line from the coxal tuberosity lateral to the iliocostalis line and the caudal border is to the 11th intercostal space from the olecranon.

Fig. 5.1 Surface features of the shoulder and thorax: left lateral view. The palpable bony ‘landmarks’ and those muscles whose contours are clearly recognizable on palpation are indicated. In the normal standing posture the scapula and its associated muscles cover ribs 1 to 4 completely. The broken green lines indicate the position of structures as they are seen in subsequent dissections. It is extremely important to appreciate that these are not the normal positions in the live animal (see Fig. 1.3). In the live animal, the heart is more cranially placed and the cupola of the diaphragm may extend as far cranially as rib 6.

Fig. 5.2 Skeleton of the shoulder and thorax: left lateral view. Palpable bony features shown in the surface view are colored green for reference. In addition, the more caudal ribs and the entire length of the sternum within the median pectoral groove are palpable. Passing caudally from the small cranial thoracic aperture (thoracic inlet) between the first pair of ribs, the sternum and vertebral column diverge, and the ribs lengthen (up to rib 9) and increase in their outward curvature.

Fig. 5.3 Radiograph of the thorax: lateral view. Air within its lumen highlights the trachea. The heart is clearly visible. The lungs can be seen by virtue of their air content. The majority of other structures are soft tissue and mediastinal and so cannot be clearly distinguished from one another. The position, shape and relationship of structures may alter quite markedly according to breed type, body condition and stage of respiration.

Fig. 5.4 Radiograph of the thorax: dorsoventral view. Only the lungs and the heart can be easily appreciated in this view as other structures are mediastinal and thus superimposed on the spine and sternum.

Fig. 5.5 Radiograph of the thorax: lateral view, immediately after oral administration of barium mixed with dog food. (1), (2) & (3) A bolus of food mixed with contrast medium can be seen within the esophagus. The longitudinal mucosal folds can be seen in the distal esophagus. (4) Streaks of barium remaining within the esophagus highlight its path and the mucosal folds.

Fig. 5.6 Superficial fascia of the shoulder thorax: left lateral view. The superficial fascia is not attached to the skeleton and may be variably infiltrated with fat. The cutaneous muscle of the trunk is visible embedded in the fascial layer.

Fig. 5.7 Cutaneous muscle of the trunk and cutaneous nerves of the thorax: left lateral view. Limited cleaning of superficial fascia has exposed the cutaneous muscle. It has been cut into and reflected ventrally, revealing two series of lateral cutaneous nerves, with their accompanying small cutaneous blood vessels which originate from the intercostal vessels.

Fig. 5.8 Superficial structures of the shoulder and thorax (1) cutaneous nerves: left lateral view. The remaining superficial fascia and most of the cutaneous muscle have been cleaned. The lateral thoracic vessels and nerve emerge from between the diverging pectoral and latissimus dorsi muscles which extend from the axilla caudally onto the chest.

Fig. 5.9 Superficial structures of the shoulder and thorax (2) extrinsic forelimb muscles: left lateral view. Extrinsic muscles of the forelimb are exposed after removal of the superficial fascia and trimming of lateral cutaneous nerves. The basis of the caudal boundary of the upper arm, the tricipital margin, is clearly displayed. Deep thoracic fascia passes internal to the latissimus dorsi and pectoral muscles and medial to the upper arm, and forms the medial lining layer of the axilla (see Figs 3.43 and 5.82).

Fig. 5.10 Thoracic wall (1) after removal of the latissimus dorsi muscle: left lateral view. The latissimus dorsi and trapezius muscles have been removed to begin exposure of the thoracic wall, displaying the entire thoracic origin of the external abdominal oblique muscle from rib 13 cranially to rib 6; the caudal attachments of the thoracic ventral serrate muscle from ribs 5 to 8; the thoracic rhomboid at the dorsal end of the scapula related to its caudal angle and vertebral border. Part of the deep thoracolumbar fascia covering the epaxial musculature has been removed in order to demarcate those areas of the fascia which also act as aponeurotic attachments for the cranial and caudal dorsal serrate muscles.

Fig. 5.11 Thoracic wall (2) after removal of the forelimb: left lateral view. The forelimb was removed by severing the rhomboid muscle at its attachment to the supraspinous and nuchal ligament, and the mid dorsal tendinous raphe of the neck; cutting the ventral serrate muscle from its attachment onto the serrated face of the scapula; freeing the superficial and deep pectoral muscles from their humeral attachments and severing axillary vessels and nerves arising from the brachial plexus.

Fig. 5.12 Thoracic wall (3) dorsal serrate and scalene muscles: left lateral view. The ventral serrate muscle has been removed (bar its ventral rib attachment) exposing the cranial dorsal serrate muscle. Left pectoral musculature has been removed exposing the medial surface of the right pectoral muscles abutting onto the left pectoral muscles ventral to the sternum; the small rectus thoracis muscle; the aponeurosis of origin of the rectus abdominis muscle; and the manubrium of the sternum.

Fig. 5.13 Thoracic wall (4) external abdominal oblique muscle: left lateral view. The thoracolumbar fascia has been removed exposing the origin of the splenius muscle (see also Chapter 3), the supraspinous ligament and the spinalis and semispinalis thoracis muscles. Component parts of the scalenus muscle (dorsal and middle) which extended caudally onto ribs 2-6 have been removed leaving only the rib 1 attachment. This has exposed rib 1, parts of ribs 2-5 and the external intercostal muscles in the intervening intercostal spaces.

Fig. 5.14 Thoracic wall (5) longissimus and iliocostalis thoracis muscles: left lateral view. The rest of the ventral serrate and cranial dorsal serrate muscles and the component of the scalene muscle which attached to rib 1 have been removed. The overall extent of the ribcage is now becoming apparent with all 13 ribs at least partly exposed. Splenius and semispinalis capitis muscles have been removed from the neck (see Chapter 3) with much of the longissimus cervicis muscle. This has exposed the spinalis thoracis and semispinalis thoracis muscles, the longissimus thoracis extending cranially to cervical vertebra 6, and the iliocostalis thoracis muscle extending cranially onto cervical vertebra 7.

Fig. 5.15 Thoracic wall (6) rectus abdominis muscle and rectus sheath: left lateral view. The rib attachments of the external abdominal oblique muscle have been removed leaving that part of the ventral aponeurosis which contributes to the external lamina of the rectus sheath closely adherent to the rectus muscle surface. Caudally the contribution of the internal abdominal oblique aponeurosis to the rectus sheath remains intact. The rectus abdominis muscle is now exposed along the length of the thorax.

Fig. 5.16 Ribcage (1) external intercostal muscles: left lateral view. The rectus thoracis muscle has been removed and the rectus abdominis muscle is reflected caudally, although a fleshy attachment of the rectus abdominis muscle to the sternal portion of costal cartilage 9 prevents total reflection. Practically the entire ribcage is now exposed with the external intercostal muscles occupying much of the intercostal spaces.

Fig. 5.17 Ribcage (2) internal intercostal muscles: left lateral view. The rectus abdominis muscle is fully reflected following the severance of its fleshy attachment (see Fig. 5.16). Exposed by this procedure are the costal arch, the continuation of the rectus sheath and the cranial epigastric vessels. The external intercostal muscles have also been removed from all of the intercostal and caudal interchondral spaces exposing the internal intercostal muscles.

Fig. 5.18 Ribcage (3) transverse thoracic muscle and endothoracic fascia: left lateral view. The internal intercostal muscles and their interchondral components have been removed, bar those small portions in interchondral spaces 11 & 12 lying caudal to the costodiaphragmatic line of pleural reflection. In the last four intercostal spaces the endothoracic fascia has also been removed. In those intercostal spaces in which the fascia is intact, an intercostal artery, vein and nerve ‘triad’ lies embedded in fat against the caudal border of a rib.

Fig. 5.19 Ribcage (4) topography of the thorax: left lateral view. Intercostal spaces were cleared by removing the endothoracic fascia, transverse thoracic muscle, internal thoracic vessels, and intercostal arteries, veins and nerves. The ribs are outlined and the costal surface of the left lung is viewed through the intercostal spaces. An accurate representation of the position and extent of the lungs is sometimes difficult to obtain in embalmed material. The broken green line indicates the approximate contour of the caudal and ventral borders of the lung in a ‘resting’ position.

Fig. 5.20 Ribcage (5) topography of the thorax: right lateral view. The lung has been preserved in a fairly normal ‘resting’ condition, its outer contour conforming quite closely to the proposed borders – the broken green line on the drawing. This line passes cranioventrally from the upper end of the penultimate intercostal space at the lateral border of the iliocostalis muscle and parallels the costodiaphragmatic line of pleural reflection (broken blue line) down to the costochondral junction of rib 6 where it continues cranially parallel with the sternum.

Fig. 5.21 Thoracic viscera in situ: left lateral view. Ribs 1, 3 and 6 are left in place to show the costal relationships of the viscera. The lungs in this specimen had hardened on fixation rather than becoming ‘waterlogged’ (unlike Fig. 5.19), presenting a more ‘normal’ outer contour. The costodiaphragmatic line of pleural reflection is indicated on this, and accompanying drawings by the broken blue line. It follows the attachment of the diaphragm to the ribcage from the midpoint of rib 13, through the distal part of rib 12 and the costochondral junction of rib 11 and follows the costal cartilages of ribs 10 and 9.

Fig. 5.22 Thoracic viscera in situ: right lateral view. Removal of ribs 4 and 5 has exposed the cardiac notch in the ventral border of the lung between cranial and middle lobes. Through the notch the fat-infiltrated pericardium is exposed on the surface of the heart. The cranial extent of the pleural cavity does not show at all well in any of the dissections – it is represented on either side by a pleural pocket at the thoracic inlet medial to rib 1.

Fig. 5.23 Caudal ribs, costal arch, intercostal arteries and nerves: left lateral view. This is an enlarged view of the caudal end of Fig. 5.18. Intercostal arteries and nerves in intercostal spaces 9, 10 and 11 are displayed after removal of the endothoracic fascia. Continuations of intercostal nerves in spaces 10 and 11 penetrate between interdigitating fibers of the diaphragm and transverse abdominal muscle dorsal to the costal arch. Some internal intercostal muscles remain between the ribs caudal to the costodiaphragmatic line of pleura reflection.

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