Introduction
In contrast to the adult, who is surrounded by air with a changing environmental temperature, the developing fetus is surrounded by amniotic fluid at 37°C. The fetus relies upon the maternal circulation for provision of nutrients, removal of metabolites and respiration, including oxygen supply and carbon dioxide removal. The fetus is a rapidly developing organism, but it exists in a state of relative hypoxia. The developing brain is the most sensitive organ to hypoxia, and the fetal cardiovascular system ensures delivery of the highest oxygenated blood to the brain, whilst blood is distributed to the remaining organs depending on local requirements.
Comparison of the Adult and Fetal Circulations
In health, the adult circulation contains approximately 5 litres (66 mL/kg) of blood, equivalent to 6 to 8 per cent of body weight. Eighty per cent of the circulating volume is in the systemic veins, right side of the heart and pulmonary circulation. The cardiac output is approximately 5 litres/min (~66 mL/kg/min) secondary to a left ventricle contracting 70 times per minute and ejecting a stroke volume of 70 mL with each contraction. Deoxygenated blood returns to the right ventricle, and the entire cardiac output passes via the pulmonary arteries to the lungs for the purposes of gas exchange. Oxygenated blood returns via the pulmonary veins to the left ventricle and systemic circulation. The right and left sides of the heart are separate, and the pulmonary and systemic circulations are in series. The pulmonary circulation is low pressure (25/10 mmHg) compared to the systemic circulation (120/80 mmHg).
In the fetus, gas exchange occurs at the placenta. For oxygenated blood to return to the systemic circulation and deoxygenated blood to the placenta there are several communications or shunts present, and the two circulations are in parallel with both ventricles contributing to the total cardiac output. The fetal circulation is illustrated in Figure 1.1.
The fetal cardiac output is approximately 300 mL/min/kg, with the right ventricle contributing approximately two-thirds of the total cardiac output. The heart rate is 130 to160 beats/min. Approximately 40 per cent of the cardiac output perfuses the placenta and returns to the heart via the umbilical venous system with an oxygenation saturation of approximately 80 per cent. This is the most highly saturated fetal blood in the circulation. Half this blood supplies the liver, and the rest passes via the ductus venosus to the inferior vena cava (IVC), where it meets the desaturated systemic venous drainage from the lower body. Selective streaming of these two flows minimizes mixing with the well-oxygenated blood from the ductus venosus, which is directed posterior and leftward in the IVC. The blood from the IVC enters the right atrium, and further streaming occurs via the anatomical configuration of the Eustachian valve and the upper margin of the foramen ovale to split the stream of blood into an anterior rightward stream that enters the right atrium and a posterior leftward stream (well-oxygenated ductus venosus blood) to the left atrium. Despite this arrangement, some mixing does occur, but the oxygen saturation of the left atrial blood is approximately 70 per cent (Table 1.1). This blood is ejected by the left ventricle to supply the heart and brain. Desaturated blood returning from the upper body via the superior vena cava (SVC) is directed through the tricuspid valve along with the desaturated blood from the coronary sinus into the right ventricle. This accounts for approximately 60 per cent of the venous return to the heart and explains why the right ventricle contributes to two-thirds of the cardiac output. The right ventricular blood is approximately 55 per cent saturated. Only 8 per cent of the combined ventricular output passes to the pulmonary circulation; the remainder passes directly via the ductus arteriosus to the descending aorta. The pulmonary vascular resistance is very high due to the presence of relatively few arteries and because the lungs are not expanded. The right atrial pressure is higher than the left, reflecting the greater blood flow through the right atrium. The ductus arteriosus creates little resistance, and the right ventricular and pulmonary artery pressure is 1 to 2 mmHg higher than that of the aorta (55/35 mmHg) and left ventricle (55/2 mmHg).
