Fetal Cardiac Function

Images provided by Dr Wadi Mawad, Pediatric Cardiologist at the Montreal Children's Hospital

Watch a wonderful presentation by Dr Mawad on Fetal Echocardiography Overview here.

Function during fetal life may be assessed by the “eye-ball” technique (subjective): 

• Myocardial thickening 

• Cavity area change 

• Annular excursion 

• Heart size 

Other signs reflective of fetal cardiac function and output:

• Dilated cavities 

• Effusions / Hydrops / Skin edema / Ascites 

• Fetal activity

• Doppler in ductus venosus, umbilical arteries, umbical vein and cerebral arteries may also inform on fetal cardiac function (see article below)

• Mitral Inflow-Outflow Doppler

• Atrio-ventricular valve inflow velocities

  • Inflow patterns 

  • IVCT, IVRT, MPI 

Assessment of Fetal Cardiac Function by Fetal Echocardiography

Fetal echocardiography has become an indispensable tool for the prenatal diagnosis of congenital heart diseases (CHDs). It allows for the analysis of the anatomy of the fetal heart and the evaluation of the hemodynamic impact of malformations using increasingly advanced ultrasound techniques. This assessment is crucial for prenatal counseling, planning postnatal management, and identifying potential candidates for fetal interventions. While many severe malformations are well-tolerated in utero, some can lead to significant hemodynamic consequences that may necessitate intervention.

Fetal echocardiography uses various modalities, including two-dimensional (2D) imaging, M-mode, and Doppler (pulsed wave, continuous wave, and color flow). 2D imaging provides detailed anatomical views, M-mode is useful for measurements and timing of cardiac motion, and Doppler evaluates blood flow direction, velocity, and can estimate pressure gradients. Color Doppler specifically visualizes turbulent flow, suggesting obstruction. The diagnosis of most cardiac malformations relies primarily on the 2D anatomical analysis, while Doppler helps assess the hemodynamic consequences.

General Assessment of Fetal Cardiac Function

Evaluating fetal cardiac function goes beyond just anatomical structure. It involves assessing ventricular size and development, valvular function, flow patterns, and signs of increased pressure or volume load on the cardiac chambers. It is important to remember that fetal hemodynamics differ significantly from postnatal circulation. The fetal right ventricle (RV) is dominant in utero, handling about 60% of the combined cardiac output (CVO), while the left ventricle (LV) handles about 40%. The lungs receive only a small percentage of CVO (about 8-10% near term), with most RV output flowing through the ductus arteriosus to the descending aorta. Well-oxygenated blood is preferentially directed from the ductus venosus across the foramen ovale to the LV and ascending aorta.

Assessment of Left Ventricular (LV) Function

Normal LV development depends on sufficient blood flow entering and exiting the chamber. Echocardiographic assessment includes evaluation of chamber size, wall thickness, contractility, and inflow/outflow patterns.

Markers of Worsening LV Function or Hypoplasia:

• Decreased Size or Hypoplasia: A relative decrease in LV size with advancing gestation may be observed. Severe cases show a small or absent LV cavity. Quantitative assessment like LV diastolic length Z-score or LV length Z-score can indicate reduced size.

• Impaired Systolic Function: Reduced stroke volume and decreased contractility. Assessed by measures like shortening fraction (SF) (<0.28 abnormal) or ejection fraction (EF). Tissue Doppler indices may also show reduced myocardial function.

• Endocardial Fibroelastosis (EFE): Increased echodensity of the endocardial surface, particularly in the region of the mitral valve or papillary muscles. Severe fibroelastosis has been observed in fetal hearts.

• Limited Inflow: Reduced flow into the ventricle due to obstruction at the mitral orifice.

• Increased Afterload: Increased pressure load on the ventricle due to outflow obstruction. This can reduce ejection.

• Abnormal Flow Patterns:

  • Retrograde flow in the transverse aortic arch (TAA) or ascending aorta (AAo). This is a sign that flow through the aortic valve is severely restricted and the head, brain, and upper extremities are receiving blood supply from the ductus arteriosus. The observation of retrograde systolic blood flow in the arch and ascending aorta indicates no significant flow across the aortic valve and predicts the unlikelihood of survival with attempted two-ventricle repair.

  • Monophasic mitral valve inflow. Monophasic filling of ventricles is a sign of compromised diastolic function and fetal heart failure.

  • Abnormal pulmonary venous flow patterns: Decreased systolic velocity and exaggerated flow reversal, indicative of restricted foramen ovale. A prominent a-wave reversal in PV Doppler suggests smaller foramen ovale size.

• Chamber Dilation: Left atrial (LA) dilation, often due to mitral regurgitation. LV dilation can occur with significant AV valve insufficiency despite outflow obstruction.

• Valvular Regurgitation: Mitral regurgitation (MR) can develop. Severe MR can lead to a gigantic LA.

Assessment of Right Ventricular (RV) Function

Normal RV development is influenced by outflow. Echocardiographic assessment includes size, wall thickness, contractility, and inflow/outflow patterns.

Markers of Worsening RV Function or Hypoplasia:

• Decreased Size or Hypoplasia: The RV may be identifiable but qualitatively small, or markedly hypoplastic. Tricuspid valve (TV) annulus size, particularly a Z-score below -2 or -3 or less than 8 mm in a newborn, correlates with RV size. RV size and morphology (tripartite, bipartite, monopartite) are important.

• Impaired Systolic Function: RV dysfunction can occur. This is difficult to evaluate fully due to the RV's shape. Measures like TAPSE, fractional area change, myocardial performance index, and tissue Doppler can be used.

• Hypertrophy: RV wall thickening. Thickness of the ventricular wall is proportional to stenosis severity [Source not explicitly stating this for PV in fetus, but mentioned for RVH in general in postnatal context]. Marked localized hypertrophy in the infundibulum can occur.

• Increased Afterload: Increased pressure load due to outflow obstruction (pulmonary stenosis). High pulmonary vascular resistance (PVR) exacerbates this in the newborn.

• Increased Volume Load: Due to tricuspid regurgitation.

• Valvular Regurgitation: Tricuspid regurgitation (TR) is frequent and can be severe. TR severity correlates with RV pressure. Severe TR can lead to a greatly enlarged RV and potentially hydrops. Tricuspid insufficiency is almost constant in some severe forms [Source not explicit, but implied in clinical descriptions].

• Chamber Dilation: Right atrial (RA) and RV dilation. Enlargement of the RA is a sign of impending cardiac failure.

• Interventricular Septal (IVS) Bowing: Bowing into the LV, suggesting RV pressure is higher than LV pressure. Severe bowing can limit LV filling.

• Ventricular–Coronary Connections (VCCs): High RV pressure can lead to development of VCCs, potentially causing coronary artery stenosis/interruption. Identification can be difficult by ultrasound.

Markers of Severe Fetal Cardiac Compromise

Severe dysfunction of either ventricle, or significant valvular regurgitation or shunting, can lead to fetal cardiac failure.

• Hydrops Fetalis: This is the manifestation of cardiac failure in the fetus. It involves the accumulation of fluid in at least two compartments (e.g., ascites, pleural effusion, pericardial effusion, skin edema, polyhydramnios, placental edema). Hydrops is associated with increased systemic venous pressure.

• Severe Valvular Regurgitation: Severe TR or MR. Regurgitation of multiple valves may signify myocardial compromise.

• Abnormal Venous Doppler Patterns: Increased pulsatility, increased atrial reversal, or flow reversal in the inferior vena cava (IVC), hepatic veins, ductus venosus, portal vein, or umbilical vein. These patterns reflect increased central venous pressure and reduced ventricular compliance. Umbilical venous pulsations correlate with increasing degrees of cardiac compromise and predict perinatal mortality.

• Decreased Cardiac Output: Can be assessed indirectly. Severe reduction can lead to redistribution of blood flow to vital organs (brain sparing).

• Cardiomegaly: Enlargement of the heart chambers. Cardiothoracic ratio > 0.35 is cardiomegaly.

• Fetal Death: Severe cardiac malformations can lead to fetal demise.

Differential Diagnoses and Key Indicators

Cardiac dysfunction or hypoplasia in the fetus can be caused by a variety of conditions:

• Pulmonary valve stenosis/atresia with intact ventricular septum (PA/IVS, CPS/IVS): Characterized by a thickened, doming or immobile pulmonary valve, turbulent or absent flow across it, and RV hypertrophy or hypoplasia. RV size, TV size, and the presence/extent of VCCs are key.

• Critical aortic stenosis with evolving hypoplastic left heart syndrome (HLHS): Features thickened aortic valve, limited opening, turbulent flow, and potentially retrograde flow in the AAo/arch. The key progression is the development of LV hypoplasia, EFE, reduced function, and reversed arch flow. HLHS is diagnosed by a combination of findings including a hypoplastic LV, and mitral/aortic atresia or severe stenosis.

• Ebstein anomaly: Defined by inferior displacement of the TV leaflets into the RV cavity, leading to a large "atrialized" portion of the RV and significant TR. Characterized by RA and often RV dilation. Severe cases may have functional pulmonary atresia.

• Total Anomalous Pulmonary Venous Connection (TAPVC): Pulmonary veins drain abnormally to the right atrium or systemic veins instead of the left atrium. Key features include a small LA and dilated RA and RV, and inability to see pulmonary veins entering the LA. Obstruction to the abnormal drainage is a severe complication.

• Atrioventricular Septal Defect (AVSD) (Endocardial Cushion Defect): Involves a common atrioventricular valve and defects in the atrial and ventricular septa. Often associated with Trisomy 21. AV valve regurgitation is common.

• Transposition of the Great Arteries (TGA): Discordant ventriculo-arterial connection (aorta arises from RV, pulmonary artery from LV). Fetal diagnosis requires specialized views. Postnatal findings may include a narrow mediastinum ("egg on a string"). Restrictive atrial septum can lead to severe hypoxia.

• Tetralogy of Fallot (TOF): Characterized by a VSD, overriding aorta, RV outflow tract obstruction (pulmonary stenosis or atresia), and RV hypertrophy. Retrograde flow in the ductus arteriosus suggests severe PS/atresia.

• Tricuspid Atresia: Absence of the tricuspid valve, resulting in a small RV and a large LV. Usually associated with a VSD and pulmonary stenosis/atresia or TGA. Postnatal ECG typically shows left axis deviation.

• Coarctation of the Aorta (CoA) / Aortic Arch Interruption: Narrowing of the aorta, often near the ductus arteriosus. Suspected in fetuses with right dominant heart (large RV/MPA, small LV/AAo). Arch interruption is a more severe form.

• Double Outlet Right Ventricle (DORV): Both great arteries arise predominantly from the RV. VSD location and presence of pulmonary stenosis are key features that determine physiology.

• Cardiomyopathies: (Dilated, Hypertrophic, Noncompaction): Abnormal myocardial appearance or function in the absence of significant primary valvular or outflow obstruction. Myocardial hypertrophy can be due to maternal diabetes, certain syndromes, or fetal ductus closure. RV noncompaction has been noted in the fetus.

• Myocarditis: Inflammation of the heart muscle, often viral, leading to dysfunction.

• Cardiac Arrhythmias: Severe or sustained bradycardia or tachycardia can cause fetal heart failure. Heart block can be associated with maternal autoantibodies.

• Arteriovenous Malformations (AVMs): High-output lesions (cerebral, hepatic, pulmonary) that cause increased cardiac workload and potential heart failure/hydrops. Hepatic AVMs are seen as echogenic areas in the liver with color Doppler flow.

• Anemia: Fetal anemia causes increased cardiac output and can lead to heart failure/hydrops. Assessed by increased flow velocities in the fetal aorta and middle cerebral arteries.

• Intracardiac Tumors: Rhabdomyomas (often multiple, associated with tuberous sclerosis) or fibromas. Can obstruct flow.

Conclusion

Fetal echocardiography provides crucial insights into fetal cardiac function, identifying normal development and detecting deviations that may indicate CHDs or other issues. Careful assessment of ventricular size, morphology, and function, along with detailed Doppler evaluation of flow patterns and velocities across valves and vessels, is essential. Recognizing markers of worsening function, such as chamber dilation or hypoplasia, valvular regurgitation, abnormal flow patterns (especially reversal), abnormal venous Doppler, and ultimately hydrops, helps predict the severity and potential progression of the disease. Integrating these findings allows for a differential diagnosis and helps pinpoint key indicators for specific cardiac conditions, guiding prenatal counseling and postnatal care planning. It is important to note that accurate interpretation requires significant expertise and should be performed by experienced specialists.

M-mode use in fetal life to assess cardiac wall motion. This may be used to assess septal and posterior wall hypertrophy, as well as the shortening fraction of the LV. This may be obtained in the long axis or short axis view. However, it is important to remember that during fetal life, the RV function is an important contributor to the combined cardiac output. M-mode may not be the best indicator of RV function, but may provide some insight on cardiac wall motions.