Cardiac Abnormalities and Arrhythmias

Normal Size

The heart occupies one-third of the thorax or one-half of the thoracic circumference. The size of the fetal heart can be assessed by eye. However, the cardiothoracic ratio can be easily measured in the four-chamber view should there be any doubt. The approximate normal range is 0.55 ± 0.05 ( Figure 14-5 ).

Normal Position

The heart lies mainly in the left chest with the apex pointing to the left. The angle between the ventricular septum and the midline of the thorax is approximately 45 degrees ( Figure 14-6 ).

Normal Structure

• 1.

  There are two equally sized atria and ventricles, with equal wall thickness. The right ventricle appears more trabeculated than the left and its apex contains the moderator band ( Figure 14-7 ).

  

• 2.

  The atrioventricular valves meet the ventricular and atrial septum forming the centre of the heart or crux. The right sided tricuspid valve is a little closer to the apex than the left sided mitral valve, creating characteristic off-setting at the crux ( Figure 14-8 ).

  

• 3.

  The atrioventricular valves are equal in size. They open simultaneously in ventricular diastole and close in systole.

• 4.

  The ventricular septum should appear intact ( Figure 14-4 ).

• 5.

  There is an interatrial communication (the foramen ovale ) occupying about one-third of the atrial septum ( Figure 14-9 ). The foramen ovale is guarded by a flap valve, which oscillates in the left atrium as a result of blood flow from the right to left atrium in the normal fetal heart.

  

• 6.

  Usually two pulmonary veins are seen at the back of the left atrium. However, they should be confirmed by colour Doppler ( Figure 14-9 and Figure 14-10 ).

  

• 7.

  In the normal four-chamber view, there is only one vessel, the descending aorta, seen in the posterior mediastinum lying in front and slightly to the left of the spine ( Figure 14-3 ).

• 8.

  The appearance of a black rim around the ventricles represents fluid in the pericardial sac and is normal ( Figure 14-11 ).

  

• 9.

  In the last 10 weeks of gestation, the normal fetal heart becomes asymmetric to some extent as the right ventricle ( Figure 14-12 ) and pulmonary artery become slightly larger than the left-sided structures.

  

Function

Two equally opening atrioventricular valves and equally contracting ventricles should be identified. It is necessary to use the colour Doppler in order to demonstrate equally filling ventricles and competent atrioventricular valves ( Figure 14-13 ).

Obtaining the Views

The great arteries and their ventriculo-arterial connections are best seen in a series of transverse planes, imaged in a continuous sweep moving cranially from the four-chamber plane. The aorta is the first vessel seen sweeping towards the right as it arises just above the level of the four-chamber plane ( Figure 14-14 ). In order to image the left ventricular outflow tract perfectly, it might be necessary to rotate the transducer slightly in order to ‘open out’ the aorta. It is very important to image the left ventricular outflow tract sufficiently clearly to ensure that the anterior wall of the aorta is continuous with the ventricular septum ( Figure 14-15 ).

Moving sequentially towards the fetal head, the next transverse section will demonstrate the pulmonary artery ( Figure 14-16 ). The pulmonary artery arises anteriorly, close to the chest wall and is directed straight back towards the spine. This plane demonstrates the three vessel view (3VV). It images the right ventricular outflow tract and duct, with the aorta and superior vena cava cut in short axis and lying to the right of the pulmonary artery. A further sweep cranially will demonstrate the transverse arch ( Figure 14-17 ). Slight angling of the transducer on this plane will allow the duct and arch to be seen in the same section ( Figure 14-18 ). The initial direction of the aorta, at its origin from the heart, is cranial and towards the right shoulder, then it turns posteriorly and towards the left shoulder to form the aortic arch. In contrast, the pulmonary artery course is straight, just to the left of the midline and directed posteriorly towards the spine.

An advanced study may include examination of origin of the head and neck vessels from the aortic arch. Normally the most proximal major branch of the aortic arch is the right common carotid and right subclavian artery arising as a common vessel (the right brachiocephalic artery). Normally the right subclavian artery is seen as a tortuous vessel directed towards the right arm, in a course which passes anterior to the trachea ( Figure 14-19 ). The left common carotid and then the left subclavian arteries arise more distally from the arch in separate origins.

An experienced sonographer might find it useful to use long-axis or sagittal planes of the fetus, as it may be possible to demonstrate the pattern of branching of the head and neck vessels from the aortic arch, as well as crossover of the pulmonary artery ( Figure 14-20 ). In a sagittal plane, the systemic venous connections to the right atrium, the inferior and superior vena cava can be seen in longitudinal cut ( Figure 14-21 ). However, all these structures can be well seen and analyzed in the transverse views, which are easier to obtain and understand.

The great arteries should be examined using the same principles applied to the four-chamber view by evaluating the four features of normality: size, position, structure and function.

Size

The two arteries are almost equal in size. The pulmonary artery is slightly bigger than the aorta from the second trimester onwards. The aortic arch (isthmus) is slightly bigger than the duct in a normal mid-trimester fetus ( Figure 14-18 ).

Position

The aorta arises wholly from the left ventricle. It lies in the centre of the chest, unlike the pulmonary artery, which arises entirely from the anterior or right ventricle and is close to the anterior chest wall. The pulmonary trunk crosses over the aortic origin, so that the pulmonary valve lies to the left of the aortic valve ( Figure 14-18 ).

Structure

The aorta gives rise to the aortic or upper arch with characteristic head and neck vessels arising superiorly. These are the first visible branches of the aorta during fetal life, since the coronary arteries are too small to be seen. In contrast, the pulmonary artery branches early and laterally into the left and right pulmonary arteries and the arterial duct ( Figure 14-22 ). The arterial duct is a straight continuation of the main pulmonary trunk and forms the lower or ductal arch. The left aortic arch crosses the midline in front of the trachea and meets the arterial duct in front of the spine creating the so-called ‘V’ shaped appearance ( Figure 14-18 ).

Both arterial valve leaflets appear thin and mobile. They both consist of three leaflets, an anatomical feature which is not possible to see in the fetus during the second-trimester scan.

Function

It should be possible to see the arterial valves opening freely, disappearing in ventricular systole and appearing in diastole. Colour Doppler should demonstrate unaliased forward flow in both great arteries, as well as in the duct and arch ( Figure 14-18 ). There is no flow regurgitation seen on colour flow mapping or on pulsed Doppler of the arterial valves, as they should be competent in the normal fetal heart. The normal flow velocity in the great arteries should not exceed 80cm/sec in the second trimester.

Disproportion of the Ventricles

Disproportion is a descriptive term with several different causes. However, it does not refer to mitral or tricuspid valve atresia where the abnormality is immediately obvious.

The disproportionate heart usually presents with both left sided chambers appearing smaller than the right. Both atrioventricular valves are patent. The appearance of disproportion may be physiological in the last 10 weeks of pregnancy ( Figure 14-12 ). On the other hand, this appearance can be associated with extracardiac causes such as:

• diaphragmatic hernia;

• absent ductus venosus;

• intrauterine growth retardation ( Figure 14-23 );

  

• uncommon genetic syndromes;

• arteriovenous malformation (vein of Galen aneurysmal malformation);

• anaemia.

However, in the last two, usually there is a global cardiomegaly.

The most common cause of disproportion is coarctation of the aorta. Examination of the great arteries and aortic arch confirms this diagnosis, but suspicion is first raised during four-chamber assessment ( Figure 14-24 ). The disproportion varies, as it is dependent on the severity of coarctation. The most likely explanation for disproportion of the aorta in coarctation is that the increased afterload of the left ventricle results in a diminished right to left shunt at the foramen ovale. This reduces the left heart flow and therefore the size of the ventricle. The implications of this malformation are described under malformations of the great arteries.