Some of the most complex childhood health problems are related to cardiovascular defects and anomalies. The cause of these defects and anomalies are not always known, although a range of contributing factors such as folate deficiencies, intrauterine infection and chromosomal abnormalities may play decisive roles. In general, congenital cardiovascular defects and anomalies occur in 8–10% of children born alive (Hoffman, 2003 and Sommer et al., 2008). Given the complexities involved in the development of the embryonic cardiovascular system it is not surprising that they occur this frequently. Many of the congenital cardiovascular defects and anomalies can be diagnosed in utero before birth. Other problems may be detected within the first few months of birth, usually due to the child manifesting persistent central and peripheral cyanosis often accompanied by respiratory difficulties. A small number of defects and anomalies may not be detected until later in childhood or during adult life. Irrespective of when the symptoms appear in most instances the clinical management of congenital cardiovascular problems will require specialist medical, surgical and nursing interventions.

Improved health care and better infection control are known to have reduced the incidence of acquired cardiovascular disease such as bacterial endocarditis in childhood. Nevertheless, viral myocarditis, cardiomyopathies and secondary cardiovascular disease caused by metabolic problems can arise in children and will require specific, at times life saving, evidence-based therapeutic interventions.

The normal circulatory system can be divided into two functional compartments: the systemic circulation, which is supported by the left ventricle; and the pulmonary circulation, which is supported by the right ventricle. The right and the left ventricles work in relative harmony. This ensures that the systemic circulation transports oxygenated blood and nutrients from the left ventricle through the aorta to the systemic arteries and capillaries. The pulmonary circulation receives its deoxygenated blood from the right ventricle. This blood is then pumped through the pulmonary artery to lungs. Here a complex capillary bed surrounds the alveoli and facilitates gaseous exchange across the capillary-alveolar membranes. This mechanism ensures that oxygenated blood is transported to the left atrium by way of pulmonary venules, which anastomose giving rise to four pulmonary veins which commonly attach to the posterior wall of the left atrium. The oxygenated blood flows from the left atrium to the left ventricle which ejects it into the aorta ensuring that the metabolic needs of all organs and tissue are met.

The heart and its corresponding blood vessels develop from the splanchnopleuric mesoderm (embryonic tissue adjacent to the endoderm) in the cardiogenic region. The underlying endoderm contributes to this development by signalling to the primitive angioblastic cords to converge and form a pair of lateral endocardial tubes, which are then gradually fused and reshaped as the embryo grows and folds. This gradual, but carefully orchestrated, fusion of these tubes shapes the single primitive heart tube, which is then remodelled by a sequential process of septation and folding that transforms the single lumen into the four chambers of the definitive heart. The initial chambers are primitive, and require considerable development.

The primitive heart tube consists of endothelium which is shaped by a series of definitive expansions, shallow ridges and crevices (sulci). By the 22nd day a thick mass of splanchnopleuric mesoderm surrounds this heart tube, differentiating into the loosely organised myocardium and cardiac jelly, a layer of thick acellular matrix synthesised by the developing myocardium (Larsen 2008). The visceral pericardium lies externally to the myocardium. It evolves from mesothelial cells, which are derived independently from the splanchnopleuric mesoderm. These cells are believed to migrate onto the surface of the developing heart from the regions of the septum transversum or the sinus venosus.

Approximately, from the 23rd day, the single heart tube begins to elongate, simultaneously loop and fold. This displaces the bulbus cordis to the right and adjusts the position of the primitive ventricle to the left, allowing the primitive atrium to move in an upward direction. The complex cardiac folding is believed to be completed by the 28th day of embryonic life. According to Larsen (2008), this highly organised looping of the primitive heart tube may be intrinsically motivated and possibly influenced by the state of hydration of the cardiac jelly, active migration and development of the primitive myocytes (cardiac muscle cells), and local haemodynamic forces initiated by the embryo’s circulating blood. This early cardiac morphogenesis includes the formation of the coronary vascular plexus, which later forms the coronary vessels.

The mesoderm, mesodermal growth factors and other factors expressed in the developing myocardium influence tissue differentiation and regulate the course of atrial and ventricular development. The cardiac neural crest contributes to the formation of the primitive ventricular outflow tracts. The internal septation of the heart, and the formation of the heart valves, are attributed to the development of the mesenchymal tissue at the atrioventricular and proximal outflow regions of the heart. The structural developments in the heart and blood vessels may play a role in regulating the functions of the cardiovascular system. For instance, marginal increases in blood pressure may contribute to the development of the embryonic myocardium and the smooth muscle of the blood vessels.

The primitive heart begins to contract on approximately the 22nd day (Larsen 2008) and blood begins to circulate through the primitive vessels within the next 2 days. The first blood cells found in the embryo’s circulation are formed in the yolk sac, although embryonic and later fetal haematopoiesis occurs in the liver, spleen, thymus and, of course, eventually the bone marrow. The sustained rhythmic myocardial contractility may, in part, be dependent on myocardial hypertrophy, which is a basic adaptive response of the cardiac myocytes to an increasing workload. Polin & Fox (2004) suggest that myotropin (which may play an important part in the pathogenesis of myocardial hypertrophy) could act as an important primary modulator responsible for enhancing myocyte protein synthesis and myocyte growth and differentiation.

The coordinated contraction of myocardium provides the mechanical energy for the transportation of blood throughout the entire vasculature. Both the electrodynamic controls of the heart and the force of myocardial contractility are the consequence of the sophisticated cellular mechanisms that bring about the rhythmic excitation–contraction coupling phenomena. In the fetus, the right ventricle ejects approximately 60% of blood while the left ventricle ejects only 40% of the total cardiac output (Hoffman 2003). As both ventricles pump blood against a similar vascular resistance, there is little difference in the thickness of the myocardium as the two ventricles develop. However, in some instances the right ventricular wall is slightly thicker than the left ventricular wall, and this persists for a considerable time after birth. Electrocardiographic (ECG) recordings reflect this phenomenon as a relative right ventricular dominance (Hoffman 2003 pp 1757–1762). The right ventricular hypertrophy is generally attributed to the workload of the right ventricle and associated physical development of the myocardium during fetal life. A clear distinction must, however, be made between the physiological right ventricular hypertrophy and a hypertrophy initiated by some form of pathophysiological change that may affect the competence of the entire cardiovascular and pulmonary systems in neonates and children.