FETAL CARDIOVASCULAR SYSTEM AND CONGENITAL HEART DISEASE

12


FETAL CARDIOVASCULAR SYSTEM AND CONGENITAL HEART DISEASE




Chapter objectives


After studying this chapter you should be able to:



1. Explain how the structure of the fetal heart and circulation differs from that in the adult.


2. Describe the structural changes at birth and during the neonatal period associated with the transition from the fetal to adult form of circulation.


3. Outline the changes in the functional characteristics of the circulation such as heart rate and blood pressure which occur during the growth of a child into an adult.


4. Describe some of the common differences in the pattern of ECG traces between children and adults.


5. Understand some of the common genetic and functional bases for congenital heart malformations.


6. Explain the clinical basis for the presentation, investigation and diagnosis of congenital heart defects.


7. Discuss the rationale for medical and surgical approaches to the correction of some congenital heart defects.



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.



Case 12.1   Fetal cardiovascular system and congenital heart disease: 1



A blue child


A newborn baby was noted to be cyanosed at 24-hour review. The respiratory rate was slightly increased at 60–70 breaths per minute but there was no respiratory distress. On auscultation there were no murmurs, the chest was clear and femoral arteries were easily palpable. A blood oxygen saturation check showed saturations of 78–82%. The baby was transferred to the special care baby unit (SCBU) and a hyperoxia test (increased inspired oxygen concentration) was performed. Arterial blood gases breathing room air showed pH = 7.37, Po2 = 3.5 kPa, Pco2 = 5.0 kPa, [HCO3] = 19 mmol/L, base excess = −2.3 mmol/L (see Chapter 1). When given 100% O2 for 5 minutes arterial blood gas data were pH = 7.36, Po2 = 9.5 kPa, Pco2 = 5.5 kPa, [HCO3] = 19 mmol/L, base excess = −2.2 mmol/L. A diagnosis of congenital cyanotic heart disease was made and after discussion with the regional paediatric cardiology unit dinoprostone infusion (a synthetic prostaglandin) was commenced and arrangements were made for transfer to the regional specialist centre.


On arrival echocardiography confirmed the putative diagnosis of transposition of the great arteries (TGA). The child was prepared for immediate balloon septostomy and was taken to the catheter laboratory.


Septostomy involves transvenous insertion of a catheter with a balloon at the end via the right atrium and patent foramen ovale into the left atrium. The balloon is then inflated and pulled back into the right atrium sharply in order to tear the interatrial septum. Care must be taken to avoid damaging the mitral valve or the inferior vena cava/right atrium junction.


The procedure was performed without complications. Post septostomy, oxygen saturations rose to 85–90%. The child was transferred back to the ward and after a few hours normal feeding was instituted.


Questions for consideration include:



Answers to these questions are discussed in the text of this chapter and in Case 12.1:2.



Interesting facts


Reptilian and amphibian hearts have three chambers with separation of oxygenated and deoxygenated blood by ridges within the ventricles. However the crocodile has a fully septated four chamber heart. In order to allow it to sustain dives for long periods it has a mechanical valve which diverts pulmonary blood flow to the systemic circulation whilst it is submerged.



How does the transition from fetal to adult circulation occur?



Changes at birth


On taking its first breath, the newborn infant instigates several simultaneous changes which will completely alter its circulation. Though most of these changes occur within minutes, they will take weeks to become fully complete.


The precise stimulus for the initiation of air breathing is not well understood. In utero the fetus makes some respiration-like movements but the process of birth drives the true onset of independent breathing. The clamping of the cord isolates the baby from the placenta. The associated increase in blood Pco2 and decrease in Po2 appear to be important in triggering the respiratory drive. The first few breaths are crucial, they must open the airways and complete the process of fluid clearance from the lung. Surfactant, a mixture of lipid, phospholipid and protein which lines the alveoli, is key to the reduction in surface tension necessary to allow the small airways to open. Anything which inhibits this fluid clearance can lead to failure to convert from the fetal to the neonatal pattern of circulation.


The first breath opens the airways and allows gas exchange across the alveolar membrane. The lungs expand and blood vessels open. This initiates a drop in the pulmonary vascular resistance (see Figure 12.5). Combined with the sudden rise in peripheral resistance resulting from the loss of the low-resistance placenta, there is a profound alteration in haemodynamics in the newborn infant. The blood leaving the right ventricle is preferentially directed through the now low-resistance lungs and returns to the left atrium. This increase in venous flow into the left atrium raises the left atrial pressure. At the same time there is a fall in right atrial pressure because venous return to this chamber has been reduced following the loss of the placental circulation. As left atrial pressure is now higher than right atrial pressure the foramen ovale, a flap valve, closes. Thus, in the newborn any detectable flow across the atrial septum is predominantly left to right. With the changes in pressures and the establishment of lung ventilation there is a sharp increase in blood oxygen content. This improved tissue oxygenation reduces the acidaemia to which the fetus has been adapted. Rising pH and Pao2 inhibit the synthesis of prostaglandins in the ductus arteriosus. This leads to closure of the ductus arteriosus separating the pulmonary circulation from the systemic circulation. Though generally the duct has closed within 24 hours of birth, early echocardiography demonstrates that a small left to right ductal shunt persists in a large minority of newborns for some days or even weeks. However this shunt is not usually of clinical significance.



Over the first few days of life the process of adjustment proceeds with a continued fall in pulmonary artery pressure. This single physiological process accounts for many of the clinical features of congenital heart disease and cardiovascular problems in children.



Longer term changes in the heart during childhood and adulthood


Typically the pressure in the pulmonary artery will fall from the comparatively high level in the fetus to a normal adult level (25/5, mean = 10 mm Hg) over a period of a few weeks. The lower working pressure in the right ventricle means that during childhood it becomes increasingly dominated by the left ventricle in terms of muscle mass.


Apart from the fall in pulmonary pressure with time there are a number of physiological changes with maturation. Heart rate (HR) and blood pressure (BP) both vary with age and an appropriate normal range must be used in clinical practice. An adult with a heart rate of 120 bpm and a systolic BP of 85 mm Hg might be in the early stages of hypovolaemic shock but these would be perfectly normal values for a 1 year old. It is critically important to know the normal changes in physiological variables when assessing children, as they do not conform to textbook adult values.


Table 12.2 summarizes the normal variation of HR and BP with age. An estimate of the normal systolic BP, for a given age in children, can be made from the formula: systolic BP = 80 + (2 × age in years).



The variation in heart rate reflects the relatively small size of the infant heart compared with its body mass and the relatively low compliance (increased stiffness) of the ventricles. The stiff ventricles mean that the stroke volume of the heart responds less dramatically to increasing venous return (preload effects) compared to an adult (see Chapter 4). In order to meet the demand for increased cardiac output the newborn heart rate must rise substantially, even at rest. Thus it is not unusual to see heart rates in newborns varying from 90 bpm when deeply asleep to 160 bpm when only slightly active. In the sick newborn under stress heart rates over 200 bpm may be seen with the heart still in sinus rhythm.



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.


Tags:
Jun 18, 2016 | Posted by in CARDIOLOGY | Comments Off on FETAL CARDIOVASCULAR SYSTEM AND CONGENITAL HEART DISEASE

Full access? Get Clinical Tree
Get Clinical Tree app for offline access