Doppler in Fetal Heart Failure Part I
Praca Poglądowa: „Badanie Dopplera w Niewydolności Krążenia Płodu”, cz. I
Keywords: Fetal circulation, Cardiac screening, Fetal echocardiography
Autorzy: James C. Huhta - Perinatal Cardiology, Congenital Heart Institute of Florida, Tampa Bay, Florida; Professor, Women’s Health and Perinatology Research Group, Institute of Clinical Medicine, University of Tromso, Norway;
M.D. James J. Paul, B.S. 2Pediatric Cardiology Associates/Pediatrix Medical Group, Saint Petersburg, Florida
Introduction:
The cardiovascular system provides a large volume of information about the well being of the fetus. The fetus has become the new patient of the decade due to the rapid changes in ultrasonic technologies. It is possible to detect changes in fetal well being, especially with heart failure due to abnormal heart rate (1). The decision to deliver a fetus prematurely due to cardiac changes must be made in the context of the risks both pre and post-natally. Therefore, any assessment demands a coordinated team approach between perinatalogists, cardiologists, and neonatologists.
Cardiac defects are the most common cause of infant death in United States (2). They occur in approximately 0.7-1% of liveborn children, but in a much higher percentage of those aborted spontaneously or stillborn (3-5). Although the etiology of heart defects is not known, a number of factors that may increase the risk of heart defects. One of these factors is maternal folate deficiency which has been linked to an increased rate of spontaneous abortion(6), conotruncal defects (7-9), neural tube defects (10)orofacial defects and limb defects. Therefore strategy to prevent CHD should include preconceptional supplementation of folic acid.
Non-congenital heart malformations and fetal conditions including extracardiac structural malformations can result in myocardial dysfunction and can cause heart failure. Intrauterine growth restriction, hydrops, indomethacin, tocolytic therapy and fetal anemia all can be associated with decreased effective cardiac output and fetal congestive heart failure.
Fetal Circulation
Because the pulmonary and systemic circulations are separate in the fetus, each ventricle has a stroke volume determined by the individual preload, afterload, and contractility of that chamber. Both ventricles are linked by a common heart rate and humeral environment. They are also linked by the atrial pressures which are similar due to the presence of the patent foramen ovale. They are also linked by the ventricular septum which is shared by each ventricle, and by the common arterial pressure which is the result of the widely patent ductus arteriosus. Right and left atrial pressures are almost equal because of the presence of the foramen ovale, and right and left ventricular pressures are equal due to the ductus arteriosus. The left ventricle ejects into the upper body and cerebral circulation, the right ventricle ejects into the pulmonary arteries and through the ductus arteriosus into the lower body and the placental circulation. The vascular beds of the upper and lower body are connected via the aortic isthmus.
The unique feature of the parallel nature of the ventricular ejection is that if there is increased afterload of one ventricle, the output of that ventricle will fall and the output of the contralateral ventricle will increase in a compensatory manner. This leads to the commonly observed feature associated with congenital heart disease of disproportionate growth of the normal side of the heart due redistribution of fetal blood flow. As a further consequence of the parallel arrangement of these circulations, ventricular outputs can be different and in the case of obstruction on one side of the heart, the other side is able to increase its work or even completely supply the whole circulation alone. Figure 1.
The effect of heart rate on fetal cardiovascular output (CVO) is much more pronounced than postnatally. The fetus has a range of heart rates between 50 and 200 at which the stroke volume of the ventricular chambers can adapt to maintain adequate CVO and tissue perfusion. Outside of this range, heart failure will often result. The major determinant of cardiac output is the afterload of the fetal ventricle. Any influence which raises the impedance to ejection will inversely lower the ventricular stroke volume by the effect on both the systolic and diastolic function of the heart.
Fetal Doppler-Cardiac screening vs. Fetal echocardiography
Doppler is of limited utility in fetal cardiac screening, but is essential for a complete fetal echocardiography examination (Table 1).
Cardiac and peripheral Doppler findings are integrated into the examination to screen for a wide variety of causes of fetal congestive heart failure. For example, inflow velocities in the mitral and tricuspid valves should be biphasic with an early passive filling wave E followed by a higher velocity active atrial contraction wave A. A monophasic filling pattern is a sign of advanced diastolic dysfunction of the ventricle, although it can be seen in fetuses with an accelerated heart rate.
Cardiac rate and regular rhythm should be confirmed. The normal rates range from 120-160 beats per minute. Mild bradycardia is transiently observed in normal second-trimester fetuses. Fixed bradycardia, especially heart rates that remain < 110 beats per minute, requires timely evaluation for possible heart block. Repetitive heart rate decelerations during the 3rd trimester can be caused by fetal distress. Occasional skipped beats are typically not associated with an increased risk of structural CHD. However, this finding may occur with clinically significant cardiac rate or rhythm disturbances as an indication for fetal echocardiography. Mild tachycardia (> 160 beats per minute) can occur as a normal variant during fetal movement. Persistent tachycardia, especially if greater than 220 beats per minute, however, should be further evaluated for possible fetal distress or more serious signs of heart failure.
Figure 1 . The fetal circulation. Pathways of the fetal heart and representative oxygen saturation values (in numbers). Red - well-oxygenated blood , Blue - De-oxygenated blood FOV, foramen ovale valve; LHV, left hepatic vein; LP, left portal branch; MHV, medial hepatic vein; MP, portal main stem; PV, pulmonary vein, RHV, right hepatic vein; RP, right portal branch. (Reproduced with permission from: Kiserud T, Acharya G. The fetal circulation. Prenat Diagn. 2004 Dec 30;24(13):1049-59. Review)
Table1
| Cardiac screening | Fetal echocardiography | |
| Duration | 2 minutes | 45 minutes |
| Responsible personnel |
1) Pediatric cardiologists 2) Obstetrical or radiology personnel |
Specialists who are familiar with the prenatal diagnosis of congenital heart disease: 1) Pediatric cardiologists - with expertise in fetal and pediatric echocardiography and knowledgeable about the nuances of congenital heart disease including the medical and surgical outcomes and knowledgeable about structural, functional and rhythm-related fetal heart disease, 2) Obstetrical or radiology personnel with training in fetal echocardiography but with assistance from pediatric cardiologists for interpretation of the findings and prenatal counseling. |
| Evaluation |
1) 4 chamber view of heart 2) Heart is seen in the left chest 3) Cardiac apex points approximately 45 degrees to left 4) Left ventricle to right ventricle ratio approximately 1:1 Left and right outflow tracts must cross each other and are similar in size with the pulmonary being slightly larger |
As in cardiac screening and: 1) Assess the heart using all cardiac views – Apical 4 chambers, parasternal long axis, parasternal short axis, subcostal and arch views 2) Establish situs to confirm IVC and Aorta are in correct relationships 3) Aortic outflow arises from left ventricle and is in continuity with the mitral valve 4) Pulmonary outflow arises from right ventricle and branches into the pulmonary branches 5) Dimensions of aortic and pulmonary branches 6) Aortic and ductal arches with the head and neck vessels arising from the aorta. Identify left or right sidedness of aortic arch and patency of ductus arteriosus 7) Pulmonary venous connections to the LA 8) IVC and SVC connecting to the right atrium 9) Tricuspid and mitral valves of similar size, with the tricuspid inserting slightly more apical than mitral 10) Integrity of intraventricular and intra-atrial sseptum’s 11) Foramen ovale bulging to the left atrium with fflow from right to left 12) Assess for effusions, ascites or hydrops 13) Umbilical venous and arterial wave forms, PI aand S/D ratio of artery 14) Ductus arteriosus color flow, wave form, peak velocity and PI 15) IVC, ductus venosus and SVC wave forms 16) AV and semilunar valves – wave forms, integrity, color flow and peak velocities 17) Color flow and peak velocities to assess integrity of pulmonary and aortic valves 18) Normal sinus rhythm – M-mode, in-flow, out-flow for arrhythmias 19) Wave form and PI of branch pulmonary arteries 20) Wave form of pulmonary veins 21) Color flow, wave forms and peak velocities of aortic arch including isthmus 22) Wave form and PI of medial cerebral artery |
Color Doppler in the Fetal Echocardiography examination
The principle of color Doppler echocardiography is that information on blood flow velocity is computed, color encoded, and simultaneously superimposed on the two-dimensional echocardiographic image. The two-dimensional echocardiographic system assigns different colors to the blood flow, depending on the direction and velocity of the blood flow. Most commonly, flow moving toward the transducer probe is displayed in shades of red and flow moving away from the transducer is in shades of blue. Color Doppler echocardiography assesses mean flow velocities, the higher velocities being shown in brighter shades and the lower velocities in darker shades. Turbulent blood flow can be displayed as a mosaic with green superimposed on the other colors. Thus, color Doppler gives information regarding direction, velocity, and turbulence of blood flow to provide visualization of blood flow patterns in the fetus.
Color-flow mapping is an important adjunct to cross-sectional scanning. The correct direction of flow throughout the cardiac chambers and arch vessels can be demonstrated. It can be used to exclude regurgitation at any valve. Unaliased color flow throughout the heart indicates that the flow is at normal velocities. If color shows aliasing at any point in the heart, pulsed Doppler should be used to obtain an accurate velocity. The peak velocity at the atrioventricular valves is 30–60 cm/sec and is fairly constant throughout gestation. The peak velocity of flow at the arterial valves is approximately 25 cm/sec at 12 weeks, increasing to 60–100 cm/sec by term. Turning on color flow when the ultrasound beam is perpendicular to the ventricular septum helps to exclude a significant ventricular septal defect. Color-flow mapping is essential to confirm a normal pulmonary venous connection. The ultrasound beam must be positioned “in-line” with the flow under observation when using pulsed or color-flow mapping.
Today, no fetal echo exam would be complete without the use of color flow Doppler. The addition of color flow Doppler adds further information concerning the function of the atrioventricular and semilunar valves (stenosis or regurgitation), and flow within important fetal flow pathways such as the DV (Figure 1), FO and DA. Color directed pulsed wave Doppler assessment should include: spectral Doppler of ventricular inflows and outflows, inferior vena cava and ductus venosus, aortic arch isthmus and ductal arch, umbilical artery and middle cerebral artery.
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