4.2 Surgical Anatomy of the Thorax and Cardiovascular System

Surgical Anatomy of the Thorax and Cardiovascular System

Quick Answer: Cardiothoracic and vascular surgeries demand an exact knowledge of thoracic wall anatomy, the bronchopulmonary segments, mediastinal compartments, and the intricate pathways of the heart and great vessels.

The Thoracic Wall and Cavity

The thoracic wall forms a protective cage around the vital organs of the chest. It consists of the sternum anteriorly, twelve pairs of ribs laterally, and twelve thoracic vertebrae posteriorly. The first seven ribs are "true ribs" articulating directly with the sternum via costal cartilages. Ribs 8-10 are "false ribs" attaching to the cartilage of the rib above, and ribs 11 and 12 are "floating ribs" with no anterior attachment. The intercostal spaces contain the intercostal muscles and the neurovascular bundle, which runs along the inferior border of each rib (in the costal groove) in the order of Vein, Artery, Nerve (VAN) from superior to inferior. Because of this arrangement, surgical needles, chest tubes, and thoracentesis needles are always passed over the superior border of the rib to avoid injuring the neurovascular bundle.

The thoracic cavity is lined by the pleura, a serous membrane. The parietal pleura lines the inner surface of the thoracic wall, diaphragm, and mediastinum, while the visceral pleura tightly adheres to the surface of the lungs. The potential space between these layers is the pleural cavity, normally containing only a thin film of lubricating fluid. Introduction of air (pneumothorax) or blood (hemothorax) into this space disrupts the negative pressure, leading to lung collapse.

The Trachea and Lungs

The trachea is a cartilaginous and membranous tube extending from the larynx to its bifurcation into the right and left mainstem bronchi at the level of the sternal angle (Carina). The right mainstem bronchus is wider, shorter, and more vertical than the left, making it the more common site for aspirated foreign bodies and accidental mainstem intubation.

The lungs are the organs of respiration. The right lung is larger and divided into three lobes (superior, middle, inferior) by the oblique and horizontal fissures. The left lung is smaller, accommodating the heart, and is divided into two lobes (superior and inferior) by the oblique fissure. It features the cardiac notch and the lingula, a projection of the superior lobe homologous to the middle lobe of the right lung. Surgically, each lung is further divided into bronchopulmonary segments (typically 10 in the right, 8-10 in the left). Each segment has its own tertiary bronchus and branch of the pulmonary artery, functioning as a discrete anatomical unit. This allows thoracic surgeons to perform segmentectomies (removal of a single segment) for localized disease, sparing the healthy surrounding lung tissue.

The Mediastinum

The mediastinum is the central compartment of the thoracic cavity, situated between the two pleural cavities. It is arbitrarily divided into the superior and inferior mediastinum by the transverse thoracic plane (a horizontal line extending from the sternal angle to the T4/T5 intervertebral disc). The inferior mediastinum is further subdivided into the anterior, middle, and posterior compartments.

• Superior Mediastinum: Contains the thymus, trachea, esophagus, aortic arch and its three branches (brachiocephalic trunk, left common carotid, left subclavian), superior vena cava (SVC), and the vagus and phrenic nerves.
• Anterior Mediastinum: A narrow space between the sternum and pericardium, containing thymic remnants, fat, and lymph nodes.
• Middle Mediastinum: The most important surgical compartment, containing the pericardium, the heart, the ascending aorta, the lower half of the SVC, the pulmonary trunk, and the main bronchi.
• Posterior Mediastinum: Located behind the pericardium and diaphragm, containing the descending thoracic aorta, esophagus, thoracic duct, azygos and hemiazygos veins, and sympathetic trunks.

The Heart and Coronary Circulation

The heart is a muscular pump enclosed within the pericardium (an outer fibrous layer and an inner serous layer). It consists of four chambers: two atria and two ventricles. Deoxygenated blood returns from the systemic circulation via the SVC, inferior vena cava (IVC), and coronary sinus into the right atrium. It passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary trunk and arteries to the lungs. Oxygenated blood returns via four pulmonary veins to the left atrium, passes through the mitral (bicuspid) valve into the thick-walled left ventricle, and is ejected through the aortic valve into the ascending aorta.

The myocardium itself is supplied by the coronary arteries, the first branches of the ascending aorta.

• The Right Coronary Artery (RCA): Arises from the right aortic sinus, traveling in the right atrioventricular groove. It supplies the right atrium, right ventricle, SA node, and AV node in most individuals. It typically terminates as the posterior descending artery (PDA), characterizing a "right-dominant" circulation.
• The Left Main Coronary Artery (LMCA): Arises from the left aortic sinus and quickly bifurcates into the Left Anterior Descending (LAD) and the Circumflex (Cx) arteries. The LAD travels down the anterior interventricular sulcus, supplying the anterior wall of the left ventricle and the anterior two-thirds of the interventricular septum. It is often referred to as the "widow-maker" due to the catastrophic consequences of its occlusion. The circumflex artery travels in the left atrioventricular groove, supplying the lateral and posterior walls of the left ventricle. During coronary artery bypass grafting (CABG), the internal mammary arteries (IMA) or saphenous veins are frequently used as conduits to bypass occlusions in these vessels.