The heart is the first organ to become fully functional in the developing embryo, providing the circulatory system necessary for embryogenesis and subsequent fetal development when growth can no longer be sustained by diffusion of nutrients from the surrounding tissue. The rapid advances in genetics and molecular biology have revolutionised our knowledge of the developing embryonic heart. In similar fashion, technical improvements in imaging and non-invasive physiological recording of the early human fetus have enabled us to study the human heart non-invasively from the first trimester, and build on information from studies of the chick embryo model. 1
The drive towards a greater understanding of the molecular aspects of cardiac development during the previous decade temporarily pushed the physiologist to the sidelines in cardiovascular research. 2 The pendulum is now swinging back to a combined approach that will allow translation of information obtained from basic science into clinical practice, and provide a unique picture of human cardiovascular development along with the long-term cardiovascular responses to intra-uterine and postnatal challenges. In this chapter, I review our current understanding of the physiology and pathophysiology of the human heart in fetal life, and determinants of a successful transition in the perinatal period.
EMBRYONIC CIRCULATION
In the chick embryo, rhythmic pulsations of approximately 50 Hz begin in the ventricle, coincident with fusion of cushions in the ventriculo-arterial segment. These pulsations, although rhythmic, are insufficiently forceful to set blood in motion, or to generate recordable pressures, the onset of electrical activity preceding myofibrillogenesis. 3 The elaboration of intracellular contractile proteins is incomplete at this stage, the functional contractile units are not fully assembled, and the matrix of collagen has not yet formed. 4 Once cardiac rhythm is established, nonetheless, the myofibrils within the myocytes become aligned and, as the heart rate rises, the direction of flow of blood is established to provide a circulation for the growing embryo. Growth of the atriums and ventricles is associated with an increase in the rate of pulsation of the primitive heart tube. This establishes the direction of propagation of the peristaltic waves of contraction from atrium to ventricle.
Cardiac myocytes isolated from the venous sinus, atrium, and ventricle at this developmental stage in the chick embryo all exhibit automaticity with different intrinsic rates of contraction. The ventricle is slowest, at approximately 50 to 60 Hz, while cells from the venous sinus have the fastest rate, with the atrium being intermediate. The earliest recordings of human fetal cardiac activity have been obtained at 25 days after fertilisation by high-frequency trans-vaginal ultrasound. At this stage, only the amniotic sac is visible, and no embryonic poles are identifiable. The mean heart rate at this stage of gestation is approximately 90 beats per minute and regular. This most likely represents atrial rhythm. The mechanism responsible for the characteristic early increase in heart rate between the fifth and eighth weeks of gestation is uncertain. The initial rapid increase in the frequency of contraction is comparable to that occurring in the chick embryo. It is associated with the transition of the pacemaker, first from ventricle to atrium as fusion occurs between the two, and then to the venous sinus as this segment becomes incorporated into the right atrium. The precursor of the sinus node, which assumes the role of the cardiac pacemaker subsequently, is formed at the junction of the developing superior caval vein with the atrium.
By 8 to 10 weeks, the mean heart rate in the human fetus varies between 160 and 170 beats per minute, declining to an average of 150 beats per minute at 15 weeks. After this, the rate declines progressively towards term. This pattern of change in heart rate, seen during embryonic and fetal life in the human, parallels that occurring in the chick, in which cardiac action begins between 33 and 36 hours at a rate of 60 beats per minute, and increases to 220 beats per minute by the eighth day of gestation.
In the human, there is little variation of the mean heart rate at any particular gestational age up until 15 weeks, as the pre-innervated immature cardiovascular system does not rely on heart rate acutely to control cardiac output. The maximum cardiac output occurs at the intrinsic heart rate at each embryonic stage, suggesting that, as in the chick, cardiac function is optimised by the systolic and diastolic time intervals. 5
Alterations in heart rate significantly affect cardiac performance, 6 and there is no compensatory change in cycle length in response to preload as is seen in the more mature heart. Human embryos die if their heart rate falls, or if they experience tachycardia. 7 This indicates that, during cardiogenesis, extremes of heart rate are not commensurate with long-term viability. 8
AUTONOMIC CONTROL
The subsequent decline in heart rate after 10 weeks, and the increased variability in heart rate after 15 weeks of gestation, may be explained by a combination of maturational changes. These include development of the nervous control of the heart, stresses associated with cardiogenesis, and changes in the handling of calcium by the myocyte.
Innervation of the mammalian heart is similar to that in the chick heart, consisting of parasympathetic, sympathetic, and sensory components, all of which derive from the neural crest. 9 Although parasympathetic and sympathetic innervation of the heart occurs early during cardiac development, the time period between innervation, neuroeffector transmission, and functional neurotransmitter reactivity of both cholinergic and adrenergic receptors, varies greatly between species. Functional adrenergic and muscarinic cholinergic receptors have been detected in the heart of very early chick embryos. 10 These differences in the timing to achieve a balance between parasympathetic and sympathetic reflex neuroeffector transmission in varying mammalian species relates largely to the maturity and independence of the individual animal species at birth. The variability in heart rate reflects the changing status of the autonomic nervous system as the heart becomes more sensitive in response to internal and external stimuli. Spectral analysis of the variability of heart rate in the normal human fetus demonstrates gestational-related changes ascribed to the imbalance between sympathetic and parasympathetic neuroactivity consistent with cardiovascular maturity of the fetus. Maturation of autonomic control has been difficult to assess in the human fetus until relatively recently, as the cardiotocograph has been the only non-invasive tool available to measure the variability in heart rate. This system provides limited information, as it uses a mean of three fetal heartbeats, compared with beat-to-beat recordings of cardiac activity. Furthermore, it does not produce an electrical signature. New tools, such as magnetocardiography, have permitted a more detailed assessment of the developing electrophysiology in the human fetus, and will be discussed later in this chapter.
THE BIOPHYSICAL PROPERTIES OF FETAL MYOCARDIUM
The biophysical characteristics of fetal, neonatal and adult myocardium have been investigated in a number of mammalian species, but the studies in the sheep and rabbit have provided the majority of information. 11,12 (See also Chapter 4 .) These studies have consistently demonstrated that active development of tension is lower in fetal than adult myocardium at all lengths, including the optimal length. 11 In addition, in the ovine fetus, resting tension was greater than that in the adult animal; consequently, operational development of peak tension was less at all lengths ( Fig. 5-1 ). The velocity and extent of shortening in fetal myocardium were lower than in adult myocardium at every level of developed tension ( Fig. 5-2 ). Explanations for this have been sought. Sarcomeral length is optimal, and not significantly different, in fetal and adult myocardium. The difference in developed tension, therefore, cannot be accounted for entirely by the greater proportion of non-contractile protein per unit cross sectional area of fetal myocardium. It may be explained in part by the different sensitivity of the fetal contractile proteins troponin and myosin to cytosolic calcium. 12
In the early human fetus, handling of calcium depends on diffusional gradients through the sarcolemma in the absence of a developed sarcoplasmic reticulum. Sarcoplasmic calcium ATPase is expressed in a downstream gradient along the primitive heart tube, resulting in increased contraction duration in the outlet portions of the heart. By the 38th day of gestation, the early human myocardium may be divided into primary and working functional components. The primary components are characterised by slow conduction of the cardiac impulse, owing to the low density of gap junctions and the presence of slow voltage-gated calcium ion channels. The working components, found in the atriums and ventricles, permit fast conduction through the development of gap junctions and of fast voltage-gated sodium channels. The sarcoplasmic reticulum later regulates calcium release in the cell and is known to play an important role in the frequency-dependent facilitation of the L-type calcium current in the rat ventricular myocyte. 13
The three major connexins, 40, 43, and 45, are present in cardiac myocytes and are developmentally regulated. Immunoconofocal microscopy has been used to compare the distribution of these within the developing mouse and human heart. In the human, connexin 45 is most prominent in the conduction tissues; connexin 40 is also abundant in conduction tissues, particularly in the Purkinje fibres, and in the atrial rather than in the ventricular muscle; and connexin 43 is distributed in the ventricular myocardium, and plays an important role in conduction across gap junctions. 14 Downregulation of gap junctional conduction has been demonstrated in cardiomyopathy caused by tyrosine phosphorylation of connexin 43. 15 Altered patterns of calcium ionic fluxes, and of abnormal β-adrenoreceptor stimulation in diseased adult myocardium, 16 may also occur in the developing heart exposed to chronic hypoxaemia associated with restriction of growth, or in conditions with abnormal volume loading.
PROTEIN COMPONENTS
Troponin
The differences between fetal and adult myocardial contractile function are, in large part, related to their regulatory and structural protein components. Different isoforms of troponin and myosin heavy chain have been identified in fetal and adult myocardium from a number of mammalian species, including humans. These are genetically programmed during early embryonic development, and are modulated by specific neurohormones. Troponin T has been studied extensively and cloned. It regulates contraction in response to the concentration of ionic calcium. 17 Multiple isoforms have been recognised, the gene TNNT2 being identified on chromosome 1q23. Slow skeletal muscle troponin T is the predominant isoform throughout fetal life, but this is lost after birth. Only troponin I is detectable by 9 months of postnatal life. 18 The genes coding for these two isoforms lie in close apposition, but show independent tissue-specific expression, although this close arrangement may complicate investigation of mutations implicated in cardiomyopathy. 19 A knock-out model of myocardial troponin I has shown that, while affected mice are born healthy, they begin to develop heart failure by 15 days. They have an isoform of troponin I that is identical to slow skeletal troponin I, and permits survival, but this isoform disappears after birth despite the lack of compensatory myocardial troponin I. Cardiac muscle is abnormal in the absence of troponin I. The ventricular myocytes have shortened sarcomeres, and elevated resting tension under relaxing conditions, and they show reduced sensitivity of their myofilaments to calcium under activating conditions. 20 Calcium is released more readily in fetal isoforms of SSTN1 than adult muscle cardiac troponin T isoforms. In its presence, troponin T shows increased calcium sensitivity of force development. 21 In contrast, a reduction in calcium sensitivity of force development is seen in individuals with dilated cardiomyopathy caused by mutation of troponin T1 in the R141W and delta K210 regions. 22
Beta-myosin
The β-myosin heavy chain isoform predominates in all fetal mammals thus far examined, including humans. This isoform is more efficacious in the fetus, conferring biochemical and mechanical advantages on fetal myocardium, in that the β-isoform utilises less oxygen and ATP than the adult α-isoform in generating the same amount of force. Recent investigations have shown re-expression of fetal genes that downregulate adult, but not fetal, isoforms in response to increased cardiac work and subsequent mechanical unloading. This response appears to result in mechanical improvement, and may be important in future strategies for the management of cardiac failure. 23
The myosin heavy chain carries the ATPase site. The enzymic kinetics of the ATPase are specific for each isoform. This has importance, because it is the rate at which ATPase hydrolyses ATP that primarily determines the force–velocity relationship during myocardial contraction. This explains the fundamental differences in active and passive mechanics between fetal and adult myocardium. 24 The transition from fetal to adult isoforms of myosin heavy chain around birth is similar to the controlled switch from fetal to adult haemoglobin, and represents stage-specific regulation of genetic expression of the proteins prior to birth While a number of hormones, and particularly thyroid hormone, 25,26 are known to modulate the phenotypic expression of the myosin heavy chain, the factors responsible for the precise timing of the transition from fetal to adult isoforms remain unknown.
Chronic Heart Failure
Chronic heart failure in the human adult is characterised by left ventricular remodelling and reactivation of a fetal gene programme, with alterations of expression of micro RNA closely mimicking those observed in fetal cardiac tissue. 27 Furthermore, transfection of cardiomyocytes with a set of fetal micro RNAs induced cellular hypertrophy, as well as changes in gene expression comparable to that seen in the failing heart. Re-expression of fetal genes may also be seen in mice having obstruction to the left ventricular outflow tract. The myocardium achieves stable mural stress by hypertrophic response in the presence of pressure overload. Once compensated cardiac hypertrophy had occurred in these experiments, most of the genes returned to basal levels of expression. Thus, it appears that pressure overload results in transient early changes in genetic expression, and this may reflect a beneficial response, as there is no evidence of deterioration of haemodynamic function or heart failure. 28 This response, nonetheless, may result in a decompensated hypertrophic phenotype. There is some evidence in the Wistar rat model that hearts responding in a decompensated form show activation of pro-apoptotic pathways, in contrast to those showing compensated hypertrophy, who appear to block this using p38-MAPK. In these experiments, the response occurred early after the induction of the phenotype, and thus might be one helpful early predictor of clinical outcome, potentially allowing early interventional therapy. 29
CIRCULATORY PHYSIOLOGY IN THE NORMAL HUMAN FETUS
The Fetoplacental Circulation
Knowledge of the physiological changes in the circulatory system beyond the period of cardiogenesis and embryonic life, including growth of the cardiac chambers, haemodynamics and oxygen saturation of the fetal pathways, ventricular interaction, and distribution of the cardiac output, are largely based on studies of the ovine fetus. 30–32 These experimental investigations have provided important insights into ovine fetal circulatory dynamics, which are similar to those in the human fetus, even though there are quantitative differences in the distribution of flows of blood, especially to the brachiocephalic and uteroplacental circulations in the human fetus, that caution against unconditional extrapolation.
The fetal circulation is characterised by four shunts. These are, first, within the placenta, second, across the venous duct, which connects the intrahepatic portion of the umbilical vein to the inferior caval vein, third, through the oval foramen, which is essential for filling of the fetal left ventricle, and fourth, across the arterial duct, which directs the majority of flow through the right ventricular outflow tract into the descending aorta below the level of the isthmus in the structurally normal heart. The patterns of flow through all these structures have been studied non-invasively in health and disease states using echo Doppler.
The Placenta
The placenta plays a major role in the fetal circulation, fulfilling the functions of the lung for exchange of gases, and for the kidney and gastrointestinal tract in delivery of nutrients and excretion of metabolites. The fetal side of the placenta, which develops from the chorion, receives blood from paired umbilical arteries, which take origin from the internal iliac arteries of the fetus. The umbilical arteries within the cord spiral around the umbilical vein, and then divide into branches at the junction of the cord and the placenta. These branches have a radial disposition. The terminal branches perforate the chorionic plate, and form anastomotic plexuses within the main stem of each chorionic villus. Each main stem possesses a derivative of the umbilical artery, which penetrates the thickness of the placenta, dividing to form a huge network of capillary plexuses. These project into the inter-villus spaces that contain maternal blood. 32 As a result, there is a very extensive surface area within each chorionic villus, across which exchange of gas occurs down gradients for both oxygen and carbon dioxide. There is, essentially, no mixing of maternal and fetal blood. Following oxygenation within the chorionic villi, the blood enters the venous radicals within each main stem. These efferent venules become confluent at the junction of the placenta and umbilical cord to form the umbilical vein.
The Venous Duct
The umbilical vein carries oxygenated blood, with an oxygen saturation of between 80% and 90%, from the placenta to the umbilical cord ( Fig. 5-3 ). The cord enters the fetal abdomen, where it divides to form the portal sinus and the venous duct. The portal sinus joins the portal vein, while the venous duct carries oxygenated blood to the inferior caval vein. The origin and proximal part of the venous duct act as a physiological sphincter, which, during hypoxaemia or haemorrhage, results in an increased proportion of oxygenated blood passing through the duct to the inferior caval vein and to the heart, with less exiting to the portal sinus and the liver. 33 The oxygenated blood from the venous duct can be demonstrated coursing along the medial portion of the inferior caval vein after their confluence. Flow in the inferior caval vein continues towards the head into the inferior aspect of the right atrium, where a proportion of the oxygenated blood is slipstreamed by the lower border of the infolded atrial roof, also known as the dividing crest or crista dividens. This structure, therefore, acts as a baffle diverting blood into the atrium, a process which can readily be visualised during echocardiography ( Fig. 5-4 ).
A proportion of the oxygenated blood is diverted to the heart, and this proportion varies in different mammalian species. The remainder of the mainly desaturated blood mixes with the desaturated blood from the mesenteric, renal, iliac, and right hepatic veins, and with that from the coronary sinus and the brachiocephalic veins.
The Oval Foramen
Patency of the oval foramen is essential to enable filling of the left side of the heart in the fetus, as pulmonary venous return is low. The proportion of oxygenated blood returning to the heart via the inferior caval vein that crosses the oval foramen to reach the left side of the heart also varies between species. This oxygenated blood mixes with the desaturated blood returning to the left atrium via pulmonary veins such that, after complete mixing in the left ventricle, the oxygen saturation is approximately 60%, compared with levels between 50% and 55% in the right ventricle. Blood from the left ventricle is directed to the brachiocephalic circulation, thus supplying the most oxygenated blood to the brain, which grows at a disproportionately greater rate in the human fetus compared with the rest of the body. The majority of blood ejected into the ascending aorta by the left ventricle is directed cephalad to the head and upper limbs, and only about one-third of the left ventricular stroke volume crosses the aortic isthmus to reach the descending thoracic aorta and lower body. Although the arterial saturation of oxygen is comparatively low, extraction of oxygen by the tissues is facilitated by the leftward displacement of the dissociation curve for oxygen of fetal haemoglobin compared with that of the adult.
The Arterial Duct
The mixed venous return has an oxygen saturation of approximately 40%. This blood passes through the tricuspid valve into the right ventricle, where mixing occurs before it enters the pulmonary trunk, where the oxygen saturation is between 50% and 55%. The majority of blood in the pulmonary trunk passes through the arterial duct to the descending thoracic aorta, with only a small proportion continuing to the lungs via the right and left pulmonary arteries. The arterial duct enters the descending aorta immediately distal to the origin of the left subclavian artery, and blood is directed to the descending thoracic aorta by the geometry of its insertion, and also by a shelf-like projection at its upper insertion. The degree of patency of the arterial duct is regulated by the periductal smooth muscle cells, which produce prostaglandins. In the human fetus, ductal flow may be compromised by maternal ingestion of inhibitors of prostaglandins, such as the non-steroidal anti-inflammatory agents. Blood from the arterial duct mixes with that crossing the aortic isthmus from the aorta. This produces a saturation of oxygen between 50% and 55% in the descending aorta, from which a major proportion returns to the placenta via the umbilical arteries for reoxygenation.
FETAL DEVELOPMENTAL PHYSIOLOGY
Cardiac Growth
The first systematic study of cardiac growth in the human fetus was made using a large series of normal hearts obtained at postmortem. This established the relationship between total body weight, total heart weight, and the variability of heart weight with gestational age. 34 Detailed examination of the heart is now possible during the first trimester using trans-vaginal and transabdominal ultrasound. 35,36 These scans can provide excellent imaging of the fetal heart from 12 gestational weeks, even in multiple pregnancies. Diagnostic views at normal obstetric scanning depths can be obtained using modern ultrasound transducers with limits of resolution of about 50 μm in the axial plane at 6 MHz, and less than 100 μm in the lateral plane. As a result, morphological and physiological data have become easier to record and more reliable. Z -scores have been derived to take account of the effects of fetal gestation and growth on the size of vessels, valves and chambers. These are particularly useful for quantitative comparison when cardiac structures are very hypoplastic. 37,38 They are downloadable from references 37 and 38. Models derived from anatomical specimens have been superseded by three-and four-dimensional technology, which now permits assessment of volumes and morphology non-invasively in larger cohorts. 39,40 Magnetic resonance imaging, however, is still not sufficiently robust to provide cardiac imaging of comparable quality to ultrasound in the fetal heart.
Assessment of the Fetal Circulation
The fetal circulation is assessed using pulsed wave Doppler. Initial studies used blind continuous wave ultrasound of the umbilical cord. Technical improvements, including newer colour Doppler modalities such as energy and directional power, have enabled the visualisation and interrogation of smaller vessels in regional circulations and indicators of fetal wellbeing have been derived. 41,42
In the healthy placenta, the copious villous bed allows exchange of oxygen and metabolic products. When placental function is severely reduced, as in fetuses with restricted growth, increased placental resistance leads to a reduction in total delivery of arterial oxygen to the fetus because of the reduction in mean placental return, even though the content of oxygen of the umbilical venous blood is often near normal. The fetal brain, heart and adrenal glands respond to this pathological state by drawing increased flow, thus requiring an increase in combined ventricular output to provide it. In the human fetus, the brain is the largest organ, and the healthy, responsive fetus is able to reduce cerebral resistance by arteriolar dilation.
The pulsatility index was derived in the 1970s to quantify waveforms in the umbilical cord, and assess fetal compromise. It uses the ratio of flow velocities as shown in Figure 5-5 . Abnormalities of flow in the cord are characterised firstly by a reduction, and then a reversal, of diastolic velocities, thus increasing the pulsatility index. This may be accompanied by abnormalities of the umbilical vein as seen in Figure 5-6 .
Initial experimental work in the fetal sheep model demonstrated a redistribution of flow in response to hypoxaemia. 43 With the availability of non-invasive Doppler techniques, the observed low diastolic flow was associated with uteroplacental insufficiency. 44 On the fetal side of the placenta, an increased resistance to flow in growth-restricted pregnancies was described. 45 Evidence of redistribution of flow in the growth-restricted human fetus during the same period was provided by a comparison of Doppler waveforms in the carotid, aortic and umbilical arteries, and also in the middle cerebral artery. 46–48 Animal work has supported the concept that, in the presence of uteroplacental insufficiency, the cerebral circulation becomes the vascular bed with the lowest impedance in the fetoplacental circulation. As systemic impedance rises, flow is directed in a retrograde manner about the arch towards the cerebral circulation. 49 Increased flow to the brain results in a decreased pulsatility index recorded usually in the middle cerebral artery. Auto-regulatory arterioles are sensitive to the local concentration of metabolic products, but will not function if the surrounding tissue is metabolically inactive. This may misleadingly manifest as a normalisation of cerebral Doppler flow waveforms in terminally sick fetuses just prior to their intra-uterine death. 50
Arterial and venous Doppler waveforms have become incorporated as standard measurements in surveillance of the high-risk pregnancy, 51 with the latter shown to provide a more specific predictor of fetal compromise. 52 Of the Doppler measurements used in evaluation, absence or reversal of end-diastolic flow in the descending aorta or umbilical artery of the fetus is seen first, and may be tolerated for a period of weeks in the compromised growth-restricted fetus, but once changes are seen in the systemic veins, imminent delivery is required. 53
Flow in the Venous Duct
The venous duct has been investigated at length because it occupies a unique physiological position as a regulator of oxygen in the fetoplacental circulation. Animal studies first demonstrated streaming of oxygenated blood from the umbilical vein through the oval foramen into the left side of the heart, estimated at half of the returning flow. The degree of shunting in the human fetus is less, being estimated at between one-quarter and two-fifths. 33 The determinants of shunting include the differing resistances of the portal vasculature and venous duct, along with other influences such as blood viscosity, umbilical venous pressure, and mechanisms of neural and endocrine control. The waveforms measured within the venous duct have been found to remain normal for long periods during placental compromise, reflecting its essential role in the fetal circulation. 54 Patterns in the umbilical vein also act as a barometer of fetal wellbeing. These are caused by the dilation of the venous duct in response to fetal hypoxaemia that reduces impedance, and allows pressure waves to travel in a retrograde fashion from the right atrium to the umbilical vein resulting in venous pulsations. Absent or reversal of flow in the venous duct is usually an ominous sign ( Fig. 5-7 ). It reflects fetal hypoxaemia, and may result in emergency delivery by caesarean section. An alternative explanation for absent or reversed end-diastolic flow in the venous duct is increased central venous pressure, seen particularly where there is obstruction within the right heart, such as pulmonary atresia with severe tricuspid regurgitation ( Fig. 5-8 ). This may also result in fetal hydrops and intra-uterine death. 55–57
A combination of Doppler parameters and assessment of fetal myocardial function has been combined to create a cardiovascular profile. 58 This scoring system includes the size of the heart and venous Doppler parameters. The best predictor of adverse outcome remains abnormal venous Doppler, and may be predictive when used in isolation. 52,53
Flow Across the Aortic Isthmus
Only approximately one-third of left ventricular output, or one-tenth of total cardiac output, flows through the aortic isthmus. One consequence of this is that the isthmal diameter is less than that of the transverse arch, and shows a characteristic Doppler pattern ( Fig. 5-9 ). Experimental increases in systemic impedance in the lamb, mimicking placental insufficiency in the human, have been shown to reduce or stop isthmal flow. 49 In the human, where flow to the brain is 8 to 10 times that of the lamb, the hypoxic-mediated increase allows a reduction in cerebral impedance, with reversal of flow about the aortic arch that can be demonstrated on pulsed wave Doppler. 59
In fetuses with intra-uterine restriction of growth, abnormal arterial and venous Doppler findings influence perinatal outcome. At delivery, brain sparing was associated with hypoxaemia and abnormal venous flows with acidaemia. Abnormal flow in the venous duct predicts fetal death. 51–53 Growth restricted fetuses with abnormal venous flow have worse perinatal outcome compared to those where the abnormality in flow is confined to the umbilical or middle cerebral arteries. In fetuses with low middle cerebral arterial pulsatility, an abnormal venous Doppler signal indicates further deterioration. 60
A combination of Doppler indexes shown to be predictive of the optimal time for delivery of the sick fetus has been studied to identify whether they are predictive of early cerebral injury if combined with imaging techniques. 61 Abnormal fetoplacental flow did not appear to correlate with cerebral injury, but evidence of cerebral redistribution, as measured by the ratio of the pulsatility indexes of the umbilical artery compared to the middle cerebral artery, was associated with reduced total volume of the brain. The significance of this in neurodevelopmental outcome remains uncertain, and requires further investigation.
Flow of Blood to the Lungs
The flow of blood in the lungs of the normal human fetus has been calculated non-invasively from the difference in estimated volumes in the arterial duct and in the pulmonary trunk using Doppler ultrasound. In this way, an increase with age for pulmonary flow from 13% to 25% has been described in a cross-section of normal fetuses studied from 20 to 30 weeks of gestation, and an increase in the proportion of the cardiac output from the right side to 60% at term. 62 Pulmonary vascular resistance increases during the last trimester. This again changes the balance of the cardiac output, increasing flow into the systemic circulation.
It is clinically helpful to predict infants with important fetal pulmonary hypoplasia, both to aid counselling of parents, and to prepare the resources required for neonatal resuscitation and support. Doppler parameters such as the pulsatility index have not proven to be discriminatory for pulmonary hypoplasia, for which there is currently no good antenatal test. 63 Early echocardiographic ratios, such as the ratio of sizes of the lungs and head, were derived to predict pulmonary hypoplasia in cases with diaphragmatic hernia. A ratio below 0.6 has been associated with poor outcome, whereas one above 1.4 has been associated with survival. 64 Alternative indexes of the ratio of fetal lung volume to fetal body weight using magnetic resonance imaging in combination with ultrasonography have been devised. 65 Others have used magnetic resonance imaging alone to assess the total lung volume by comparing the signal intensity of lung to that of spinal fluid. 66 These may prove to be more promising methods for predicting fetal pulmonary hypoplasia than Doppler indexes alone.
Coronary Arterial Flow
In normal fetuses, flow in the coronary arteries is not usually seen until the third trimester. Reference ranges for velocities have been described, and do not appear to change with gestational age in the structurally normal heart. 67 Visible flow was first described in terminally sick fetuses, and proposed as an additional predictor of adverse outcome 68 ( Fig. 5-10 ). Animal studies of myocardial flow show that coronary reserve is mediated by nitric oxide and, therefore, changes during hypoxaemia. 69 This finding has been termed fetal cardiac sparing 70 Accordingly, visible flow in the coronary arteries is attributed to an increased volume of flow secondary to low fetal arterial content of oxygen.
Fetal ultrasound may demonstrate visible coronary arterial flow in conditions associated with restriction of growth, anaemia, constriction of the arterial duct, and bradycardia, thus demonstrating short-term auto-regulation and long-term alterations in myocardial flow reserve in the human fetus. It can be demonstrated in growth-restricted fetuses earlier in gestation than in appropriately grown fetuses and at higher velocity. Fetuses with anaemia show the highest velocities in the coronary arteries, perhaps reflecting increased left ventricular output due to a reduction in cerebral impedance in response to both pathological situations. Coronary arterial flow is no longer visible once the underlying cause has been treated, for example by intra-uterine fetal transfusion for anaemia, or by stopping any causative medication such as indomethacin in the case of constriction of the arterial duct. Visualisation of flow coincides with important increases in the Doppler velocity Z-scores in the umbilical artery, inferior caval veins, and venous duct. The greatest change was observed in the venous duct Z-score occurring 24 hours before visible coronary arterial flow was identified. These changes were associated with adverse perinatal outcomes. 71
It is relatively easy to demonstrate abnormal vascular connection between the coronary arteries and ventricular cavities, particularly in association with obstructed outflow tracts. Postnatal coronary arterial steal may be predicted by reversal of flow in the aortic arch, and coronary stenoses or occlusion by the finding of retrograde flow at high velocity ( Fig. 5-11 ). These findings are important to discuss during antenatal counselling, as outcomes for these babies may be poor, and associated with postnatal death.
Intracardiac Flows
In the early embryo, gradients across the atrioventricular orifices act as a resistance to, and regulate, the flow of blood, thus influencing ventricular development. 72 As the ventricular mass becomes trabeculated, so its mass increases while stiffness decreases, thus optimizing ventricular filling and ejection. Increasing cardiac efficiency is associated with increasing myocardial mass and competence of the atrioventricular and arterial valves. 73 Trans-vaginal Doppler ultrasound of the human fetal heart has confirmed that ventricular inflow waveforms are monophasic before 9 weeks of gestation, becoming biphasic by 10 weeks. Atrioventricular valvar regurgitation is a common finding from 9 weeks onwards. 74 Tricuspid regurgitation is commonly found in the first trimester, and is thought to be more common in fetuses suffering from aneuploidy, particularly trisomy 21. It has been incorporated into early screening programmes, and is used to adjust the age-related risk for trisomy 21. 75 It is not clear why tricuspid regurgitation is more common in these fetuses, but it may be associated with delayed development of the atrioventricular cushions, as it resolves spontaneously in most cases, and has no physiological consequences later in pregnancy or after birth. Most cases of tricuspid regurgitation reported in later gestation are also transient and trivial. They are associated with a normal outcome. 76
In the absence of an obstructed right ventricular outflow tract, non-invasive estimates of the systemic fetal pressures can be estimated from the peak velocity of the jet of tricuspid regurgitation. Important tricuspid regurgitation is holosystolic, often with increased duration of the systolic Doppler envelope, with a compensatory shortening of the diastolic filling time. It may be associated with abnormal waveforms in the peripheral arterial and venous circulations, for example reversal of flow in the venous duct at end-diastole.
DEVELOPMENTAL CHANGES IN SYSTOLIC FUNCTION
In common with observations in more mature fetuses, mean velocities through the outflow tract increase with gestational age. Isovolumic relaxation and contraction times decrease, thus improving the cardiac function. The second half of pregnancy is associated with a rising ventricular stroke volume, and reduction in afterload, which affects the left side more than the right. The peak velocities in the ascending aorta are generally higher than in the pulmonary trunk, and a linear increase with increasing gestation is observed in cross sectional studies.
Cardiac output is traditionally calculated from the right and left ventricles using the velocity time integral of the maximum velocity envelopes through the valves, and a static assessment of valvar diameter measured at the hinge-points. The mean total cardiac output calculated in this way is approximately 550 mL/min/kg body weight. Various investigations in humans, supported by animal data, have shown right cardiac output to be greater than left by at least two-fifths. The major source of error in the calculation of cardiac output results from measurement error of the diameter of the vessel, particularly for clinical studies, and failure to account for pulsatility of the vessel in the equation. In one study, the upper 95% confidence limits for intraobserver variation were reduced to 0.04 mm and 0.09 mm for diameters of 0.6 mm and 6 mm, respectively, by making six repeated measurements of the vessel. 77
DEVELOPMENTAL CHANGES IN DIASTOLIC FUNCTION
Atrial pressure exceeds ventricular pressure throughout filling, and from early gestation there is a clear distinction between passive and active filling, referred to as the E and A waves, respectively. 5 The active velocities are higher than passive velocities in the fetus and in the newborn period, resulting in a ratio between the E and A waves which is below 1 in the normal fetus. This ratio, nonetheless, is highly dependent on preload. It cannot provide a load-independent assessment of ventricular function. It is, therefore, a particularly unsuitable measure in fetal life, when direct pressures cannot easily be measured. The patterns of ventricular filling change with age, with a relative increase in early diastolic filling, represented by the E wave, compared with the late diastolic component, or A wave, reflecting increasing ventricular compliance. 78–80 Reference ranges between 8 and 20 weeks of gestation show a greater volume of flow passing through the tricuspid than mitral valve at all gestational ages. Maturational changes in ventricular properties in human fetuses accelerate after mid-gestation as diastolic filling increases mainly after 25 weeks. They are associated with a decrease in the ratio of the area of the myocardial wall to the end-diastolic diameter of the left ventricle. Thus, the decrease in left ventricular wall mass related to gestational age may be one important mechanism responsible for the alterations in diastolic properties noted in the fetal heart. These are co-incident with the reduction in placental impedance associated with normal adaptation of the spiral arteries. 81,32
METHODS FOR NON-INVASIVE FUNCTIONAL ASSESSMENT OF THE FETAL HEART
Diastolic function of the fetal ventricles may be examined using pulsed wave Doppler, but results thus far have prompted differing conclusions. Longitudinal studies have shown that peak E and A waves both increase with gestational age, reflecting the increasing preload of the normally growing fetus, and also, more speculatively, improved maturation of ventricular function. 82–84 Some studies have reported a similar gestational increase in the waves across the mitral valve, resulting in no significant change in the ratio between them in individuals studied longitudinally. 82 A significant increase was found, however, in the ratio of the waves through the tricuspid valve in both normal and growth-restricted fetuses. 83 An alternative parameter, the velocity–time integral of flow into the ventricles during early and late diastole, has been thought to reflect more accurately changes in diastole. The ratio of the velocity time integral of the A wave to the total velocity time integral has been evaluated, 82 but no significant change was found, implying no change in diastolic function with increasing gestation. Others, however, have suggested there is an increase in right ventricular compliance. 83,85 Differences between studies may be explained, in part, by the imprecise nature and variability of these parameters, and the fact that they are poor reflectors of ventricular diastolic function. Furthermore, studies have failed to correlate changes in downstream impedance with patterns of ventricular filling. 86 Better methods may include measurement of long-axis function of the fetal heart using amplitude of displacement of the atrioventricular ring, and Doppler tissue velocities.
The Tei Index
First described in 1997 from measurements made in adult hearts, 87 the Tei index has been used in fetal echocardiography to describe changes in myocardial performance with gestational age, and in conditions of altered loading such as twin-twin transfusion syndrome. 88–90 This index uses pulsed wave Doppler of the mitral inflow and aortic outflow waveforms. It is technically easy to record and reproducible in serial examination of the fetus ( Fig. 5-12 ), particularly when modified by using the valvar ejection clicks, 88 and by other authors using myocardial tissue velocities to produce a Doppler tissue Tei index. 91 A reduction in the index is seen with increasing gestational age due to a reduction in isovolumic contraction and relaxation times. Although it has been shown ineffective in animal models to reflect global myocardial function, it is sensitive to afterload changes and fetal conditions complicated by differential loading can be predicted and monitored using this technique. 89,90
