NUR 643E Pulmonary, Cardiac, Peripheral Vascular, Or Lymphatic System Disorder
NUR 643E Pulmonary, Cardiac, Peripheral Vascular, Or Lymphatic System Disorder
DQ1 Select one pulmonary, cardiac, peripheral vascular, or lymphatic system disorder. Discuss the clinical characteristics and identify the appropriate laboratory, imaging, and other diagnostic and screening tools that apply to this disorder. Explain why these tests or tools were chosen as appropriate to the conditions. Select different topics from that of your classmates.
DQ2 You are called to the bedside of a critical patient as a rapid response team member. Upon initial inspection the patient is cyanotic, has labored breathing, and is not responding well. Develop an SBAR (situation, background, assessment, recommendation) for this patient. Use references to substantiate your approach to this clinical scenario.
Myocardium
The myocardium is the muscular layer of the heart. It consists of cardiac muscle cells (cardiac myocytes [also known as cardiac rhabdomyocytes] or cardiomyocytes) arranged in overlapping spiral patterns. These sheets of cells are anchored to the fibrous skeleton of the heart, which surrounds the atrioventricular valves and the origins of the aorta and pulmonary artery. The myocardial thickness is related to the pressure present in each chamber; thus the atria are thin walled and the ventricles are thicker. In adult animals, the thickness of the left ventricular free wall is approximately threefold that of the right ventricle, measured in a transverse section across the middle of the ventricles, because the pressure is greater in the systemic circulation than in the pulmonary circuit.
The arterial supply to the heart is the left and right coronary arteries, which arise from the aorta at the sinus of Valsalva behind the left and right cusps of the aortic valves. The arteries course over the heart in the subepicardium and give off perforating intramyocardial arteries that supply a rich capillary bed throughout the myocardium. Extensive anastomoses occur between the capillaries that tend to run parallel to the elongated cardiac muscle cells. The ratio of the area of capillaries to that of muscle cells is approximately 1 : 1, a fact evident when the myocardium is viewed histologically in cross section. Cardiac myocytes are dependent on oxidative phosphorylation for energy requirements. This requires a constant supply of oxygen delivered by coronary arteries.
Cardiac Conduction System
The heart is a muscular four-chamber pump that simultaneously supplies blood to the pulmonary and systemic circulatory beds (see ). Mechanical pumping is composed of sequential contraction (systole) and relaxation (diastole) that must be preceded by an electrophysiologic process that triggers a coordinated chronologic sequence of electrical events that result in muscle contractions. This electrophysiologic process is made possible by a network of special conducting fibers that are collectively referred to as the cardiac conduction system.
The cardiac conduction system is infrequently examined in animals because it is a labor-intensive process. Exceptions are cases with documented electrocardiographic alterations of undetermined origin. Components include (1) the sinoatrial node (SAN) at the junction of the cranial vena cava and the right atrium, (2) the atrioventricular node (AVN) located above the septal leaflet of the tricuspid valve and the atrioventricular (AV) bundle traversing the lower atrial septum onto the dorsal portion of the muscular interventricular septum, and (3) the right and left bundle branches that descend on each side of the muscular interventricular septum and eventually ramify in the ventricular myocardium as the Purkinje fiber network.
The major pacemaker of the cardiac conduction system is the SAN. This disk-shaped structure lies between the wall of the cranial vena cava and the external wall of the right auricular appendage. Four internodal pathways connect the SAN with the AVN. The AVN is present in the wall of the right atrium dorsal to the septal cusp of the tricuspid valve. For the atria to be electrically insulated from the ventricles so that an unwarranted ectopic conduction wave will not activate the ventricles (or vice versa) and disrupt the synchronous events of the cardiac cycle, a fibrous cardiac skeleton composed of a layer of dense collagen (central fibrous body [CFB]), as well as occasional plates of chondroid and osseous metaplasia, separates the atrial from the ventricular myocardium. This skeleton forms two fibrous rings around the AV orifices and the aortic and pulmonic orifices. Conduction fibers arising from the AVN, known as the bundle of His or AV bundle, pierce through the CFB into the ventricles and continue along the subendocardium of the interventricular septum. The AV bundle then splits into the right and left bundle branches, which further split and ramify into many other smaller branches that blend into the ventricular myocardium. Purkinje fibers constitute the AV bundle and downstream conduction pathways.
Endocardium and Heart Valves
The endocardium is the innermost layer of the heart and lines the chambers and extends over projecting structures such as the valves, chordae tendineae, and papillary muscles. The endocardium of the atria is thicker than that of the ventricles and thus normally appears white to gray on gross examination. The surface of the endocardium is endothelium that lies on a thin layer of vascularized connective tissue; the subendocardial layer contains blood vessels, nerves, and connective tissue. Purkinje fibers are distributed in the subendocardium throughout both ventricles. The heart valves (tricuspid valve [right AV valve], mitral valve [left AV valve], aortic valve, and pulmonary valve) are attached to fibrous rings and have thin avascular cusps. The valves open and close to regulate blood flow through the heart. During embryogenesis, endocardial cushions (mesenchymal tissue covered by endothelium) are precursors of the valve cusps. By remodeling, growth, and elongation, the cushions become thin mature cusps composed of connective tissue with an endothelial covering.
Pericardium and Epicardium
The pericardium, which normally contains a small amount of clear, serous fluid, is composed of an outer fibrous component and an inner serous layer, which form the sac surrounding the heart. The outer component is continuous with the mediastinal pleura. The base of the fibrous pericardium surrounds and blends with the adventitia of the greater arteries and veins exiting and entering the heart. The serous pericardium forms a closed sac surrounding the heart and the roots of the great vessels.
The epicardium (also known as visceral pericardium), the outermost layer of the heart, is continuous at the cardiac base with the parietal pericardium. The parietal pericardium is fused with the fibrous pericardium. The entire inner surface of the pericardial cavity is covered by mesothelium. The subepicardial layer is attached to the myocardium and consists of a thin layer of fibrous connective tissue, variable but generally abundant amounts (in well-nourished animals) of adipose tissue, and numerous blood vessels, lymphatic vessels, and nerves.
Blood and Lymphatic Vascular Systems
Blood Vessels.
The aorta originates from the left ventricle and provides oxygenated blood to the entire body via arteries. In a treelike manner, arteries branch and become smaller arterioles as they approach capillary beds (see ; also see Chapter 2). These beds and postcapillary venules provide the site for exchange of oxygen, carbon dioxide, nutrients, and waste. Small venules return the exchanged fluid and blood to larger veins, and eventually the postcava and precava drain into the right atrium. The poorly oxygenated blood enters the pulmonary artery from the right ventricle. Oxygen exchange occurs in the capillaries of the lung, and oxygenated blood is returned to the heart via the pulmonary veins into the left atrium.
Lymphatic Vessels.
Lymphatic vessels are thin-walled, endothelial-lined channels that originate near the capillary beds and serve as a drainage system for returning interstitial tissue fluid and inflammatory cells to the blood. Afferent lymphatic vessels drain lymph into regional lymph nodes, which then filter and provide immunologic surveillance of the lymph, its cells, and the foreign matter it contains. The filtered lymph continues into larger efferent lymphatic vessels, which eventually drain into the caval blood via the thoracic duct. Both lymphatic vessels and veins have valves to prevent backflow of fluid. A more complete description can be found in Chapter 2.
Microscopic Structure
Myocardium
The myocardium consists of cardiac muscle cells surrounded by interstitial components that include blood and lymphatic vessels, nerves, and connective tissue cells, such as fibroblasts, histiocytes, mast cells, pericytes, primitive mesenchymal stem cells, and extracellular matrix elements of connective tissue, including collagen fibrils, elastic fibers, and acid mucopolysaccharides. Cardiac muscle cells can be divided into two populations: the contracting myocytes and the specialized fibers of the conduction system. The contracting myocyte is a cross-striated branching fiber of an irregular cylindric shape that measures 60 to 100 µm in length and 10 to 20 µm in diameter, with centrally located, elongated nuclei. Myocytes in young animals are smaller and have less sarcoplasm. Atrial myocytes are smaller than ventricular myocytes. Adjacent myocytes are joined end-to-end by specialized junctions known as intercalated disks and less frequently by side-to-side connections termed lateral junctions. Multinucleated fibers with nuclei arranged in central rows are frequently seen in hearts of young pigs ( ). The myocytes of old animals commonly have large polyploid nuclei. The cytoplasm (sarcoplasm) of myocytes is largely occupied by the contractile proteins that are highly organized into sarcomeres, the repeating contractile units of the myofibril (see Figs. 15-3 and 15-8). Myofibrils are formed by end-to-end attachment of many sarcomeres. The cross-striated or banded appearance of myocytes is the result of sarcomere organization into A bands composed of myosin in the form of “thick” filaments (12 to 16 nm in diameter), I bands composed of actin in the form of “thin” filaments (5 to 8 nm in diameter), and dense Z bands at the end of each sarcomere. Thick and thin filaments interdigitate and provide the basis for the sliding mechanism of muscle contraction. Myocytes are enclosed by the sarcolemma, which consists of the plasma membrane and the covering basal lamina (external lamina). Other important components of cardiac muscle cells are generally only apparent in electron micrographs and include abundant mitochondria, a highly organized network of intracellular tubules termed the sarcoplasmic reticulum, cylindric invaginations of the plasma membrane called T tubules, ribosomes, cytoskeletal filaments, glycogen particles, lipid droplets, Golgi complexes, atrial granules (contain atrial natriuretic factor), lysosomes, and residual bodies
