Introduction

Though the main functions of the fetal cardiovascular system are the same as in the adult, i.e. the distribution of oxygen and nutrients to the body and the transfer of carbon dioxide and waste products to the organ of excretion, there are some fundamental differences. Before birth the lungs are collapsed and filled with fluid and the organ of oxygenation, nutrient delivery and waste excretion is the placenta. The fetus is essentially a parasite and must extract oxygen and nutrients from the maternal circulation. Despite adaptations, such as an increased affinity of fetal haemoglobin (HbF) for oxygen compared with adult haemoglobin (HbA), the fetus exists in a state of relative hypoxia. Haemoglobin in the blood returning from the placenta is generally around 80% saturated with oxygen compared to above 98% in the pulmonary venous blood of a child or adult (see Chapter 1). Carbon dioxide levels are correspondingly elevated in the fetus. It is important to remember that the fetus is adapted to tolerate these conditions and aspects of this adaptation initially persist after birth.

The placenta consists of a system of villi inserted into lacunae containing maternal blood. The fetal blood vessels are contained within the villi which are bathed in maternal blood but there is no direct communication between fetal and maternal blood supplies. There is however only a short distance for diffusion of oxygen, carbon dioxide and nutrients between mother and fetus. The placenta is not simply an organ of passive diffusion, it also has synthetic and catabolic functions and provides active transport mechanisms for nutrients.

Some aspects of the evolution of the circulatory system from infant to adult help us to understand the fetal circulation. In the fetus the fluid-filled lungs pose a high resistance to blood flow and, as they are not involved in gas exchange, there is little value in directing blood flow to them.

As in adult life the brain, after the heart, is the dominant oxygen demanding organ. In contrast to the adult, in the fetus the oxygen supply comes from the placenta and so the most highly oxygenated blood arrives at the heart in the inferior vena cava rather than in the pulmonary veins. This blood is partially desaturated at around 80% despite the high affinity of HbF for oxygen. In order to ensure optimum supply of oxygen to the brain the fetal circulation has two major structural differences compared to that in the adult, the ductus arteriosus and the foramen ovale. Figure 12.1 summarizes the fetal circulation.

The foramen ovale, in conjunction with the Eustachian valve, directs most of the maximally oxygenated blood entering the right side of the heart directly into the left atrium. From here it passes via the left ventricle into the aorta and predominantly supplies the head. The Eustachian valve is essentially a fetal structure, though its remnants can be seen on an echocardiogram in up to 5% of adults. The valve maintains a separation between the inferior and superior vena caval flows in the fetus and thus preserves the relatively high saturation of blood passing into the left atrium and to the brain. Normal growth of the brain is a high priority for the fetus. This is illustrated by the preservation of head growth under conditions of stress such as placental insufficiency. This leads to asymmetric intrauterine growth retardation with the head being large with respect to the body.

Desaturated blood returning in the superior vena cava is streamed preferentially through the tricuspid valve into the right ventricle. It then passes out into the pulmonary artery where it is divided on the basis of flow resistance with the majority (approximately two thirds) passing through the ductus arteriosus and into the descending aorta. Roughly 2/3rds of the blood in the aorta supplies the abdominal organs and lower limbs and the remaining 1/3rd passes to the placenta. The key point regarding the placenta is that it has a low resistance to blood flow and so takes a high proportion of the aortic blood flow. Overall the peripheral resistance to flow is low. The collapsed lungs have a high resistance to flow in utero but this will change in the baby after birth.

The division of blood flow at the level of the right atrium has an important impact on the flow of blood around the aortic arch. Since the major recipient of the relatively well oxygenated blood is the brain, only a small proportion (about 10%) of the left ventricular output passes through the section of the aortic arch between the origin of the left subclavian artery and the opening of the ductus arteriosus.

A further important feature of the fetal circulation is the flow of blood from the right ventricle into the systemic circulation via the ductus arteriosus. In the fetus the duct is comparable with the arch of the aorta. This can be visualized in fetal echocardiography where the ‘ductal arch’ must be distinguished from the aortic arch. In the fetus the two ventricles eject blood in parallel rather than in series, as in the adult.

To re-cap, the main features of the fetal circulation compared with the adult circulation are as shown in Table 12.1.

Sometimes transition from fetal to adult circulation at birth is incomplete, leading to so-called persistent fetal circulation. An example of a case history is shown in Case 12.1:1.

How does the transition from fetal to adult circulation occur?

The normal ECG in childhood

Broadly speaking the ECG of the child has the same components as that of the adult, i.e. P waves, QRS complexes and T waves (see Chapter 7). However the relative contribution of the right and left ventricles to the shape of the ECG trace is different in children compared to adults. There is therefore a progressive change in the pattern of the ECG through childhood which reflects the declining contribution of the muscle mass of the right ventricle compared to the left ventricle. The large muscle mass of the newborn right ventricle affects the axis of the QRS complex in the frontal plane. The axis of the frontal QRS vector in the infant may be well past the upper limit of normal for the adult with the normal range being as high as +180°. Again, with time and the involution of the right ventricle, the axis ‘swings’ leftwards until it reaches the normal range for adults −30° to +90° (see Chapter 7). This change over time means that the acceptable axis for children in the frontal plane is very wide ranging. Broadly speaking, the axis should be inferior, i.e. between 0° and 180°. Deviation above the horizontal axis should be considered abnormal in the absence of evidence to the contrary. In the presence of physical evidence to support it a rightwards axis more extreme than usual may be suggestive of a residual load on the right ventricle. Put simply if the QRS vector is positive (i.e. pen deflection is upwards) in lead aVF then the vertical plane axis of the heart is probably normal.

The chest leads are particularly informative. In V₁ the QRS complexes are initially dominantly positive with a dominant R wave reflecting depolarization in the relatively hypertrophied septum. T waves in V₁ may be positive in the first 2 weeks of life but then become negative and remain inverted up until late childhood. The initial solely, or dominantly, positive deflection in V₁ slowly becomes equiphasic and by adulthood will be purely negative in the majority of normal ECGs. Figure 12.2 shows the typical variation.

Persistence of infantile ECG patterns may reflect a failure in the decline of RV pressures, as for example in the case of significant obstruction to the right ventricular outflow caused by pulmonary stenosis. Other causes of persistence of high pulmonary pressure (pulmonary hypertension) are a large ventricular septal defect (VSD), a patent ductus arterious (PDA) or, more rarely, primary pulmonary hypertension. Atrial septal defects (ASD) rarely cause pulmonary hypertension even though there may be a substantial left to right shunt. The persistence of infantile ECG patterns a month after birth should prompt urgent discussion with a specialist paediatric cardiology centre particularly in the presence of symptoms or signs supporting a diagnosis of congenital heart disease.

Broadly, the normal range for the intervals between successive components of the ECG for children are shorter than for adults and generally the younger the child the shorter the interval. This largely reflects the physical size of the heart, as a wave of electrical activity spreading through a smaller heart has a shorter distance to travel and so takes less time. This generalization applies for everything but the index referred to as the corrected QT interval (QTc). This is defined as the QT interval/square root (preceding R–R interval). Generally this falls with increasing age. Identification of the QTc has become clinically more important with increasing understanding of the role that abnormal repolarization plays in sudden death in young people. This may be evident in prolongation of the QTc above the normal range for an adult on the resting ECG.

Congenital heart disease

Congenital heart disease (CHD) affects around 1% of all children. The degree to which the heart can be distorted yet remain capable of supporting life is remarkable. One of the most frequently asked questions by parents is ‘Why did it happen?’ The frequently given answer is that, for the most part, we do not know.