A patent foramen ovale, patent ductus arteriosus, and patent ductus venosus are the three vital fetal channels that ensure a normal fetal circulation. Right-to-left shunting through the foramen ovale significantly contributes to the output of the left ventricle, which functions in parallel with the right ventricle to ensure normal fetal cardiac output. This chapter focuses on the important role of the patent foramen ovale in the fetus, and its function and impact in postnatal life. Of note, this chapter will not address other types of atrial defects producing interatrial shunting in neonates with congenital heart disease. This topic will be addressed in Chapter 15.

Developmental Anatomy of the Foramen Ovale

During early cardiac morphogenesis in the fetus, the first portion of the atrial septum that develops is known as the septum primum. The septum primum arises from the roof of the common atrium, and begins to divide it into the left and the right atria. Beginning from the fifth week of gestation, the septum primum develops and grows toward the endocardial cushions.¹ The progressively diminishing space between the growing septum primum and the endocardial cushions is known as the ostium primum. Prior to complete closure of the ostium primum, enlarging perforations occur in the upper portion of the septum primum. At the same time, to the right of the septum primum, a second atrial septum, the septum secundum, develops and extends, overlapping the margins of the septum primum. The slit-like gap between the septum primum and the septum secundum is known as the foramen ovale (Figure 9-1). The portion of the septum primum that covers the foramen ovale bulges into the left atrium during fetal life, allowing a large right-to-left shunt. This portion of atrial septum is known as the flap valve. The opening of foramen ovale is the same size as the inferior vena cava at 9 weeks’ gestation, and it is reduced to 40% to 55% at birth.² A small patent foramen ovale (PFO) can remain patent throughout postnatal and adult life, with an incidence of approximately 27% of cases noted in an autopsy series of 965 patients (of all age groups) with normal hearts.³ In most such cases, the PFO is clinically silent, but it can serve as a significant source of paradoxical embolism in later adult life.⁴

Eustachian Valve

The Eustachian valve⁵ is a remnant of the right valve of the sinus venosus. It functions principally as the valve of the inferior vena cava (IVC), dividing the flow of the IVC into two streams: (1) a stream that is derived mainly from ductus venosus flow, carrying a relatively higher oxygen content from the umbilical vein, is directed into left atrium; (2) the other stream will join superior vena cava (SVC) flow and will be directed into the right ventricle. After birth, with elimination of umbilical circulation and closure of the ductus venosus, any right-to-left shunt through the PFO that is facilitated by the Eustachian valve will cause systemic arterial desaturation and cyanosis.⁶

Interatrial Pressure Differences during the Normal Transitional Circulation

Right atrial pressure is higher than left atrial pressure during fetal circulation, resulting in a large right-to-left shunt through the PFO. Atrial pressures include the following wave forms: a-wave, generated during atrial systole; c-wave, generated during the closure of tricuspid valve; and v-wave, generated during rapid filling of the atrium during ventricular systole. Usually the a-wave is the highest of the three in the right atrial pressure tracing, whereas the v-wave is dominant in the left atrial pressure tracing. These pressure waveforms are higher in the right atrium during fetal life. However, following birth in normal neonates, with the rapid decrease in right atrial blood inflow and increase in left atrial blood flow, except for a brief period during atrial systole, left atrial pressure exceeds right atrial pressure, resulting in a left-to-right atrial shunt (Figure 9-2).

Two-dimensional Echocardiography of the Foramen Ovale

Subcostal coronal posterior (Figure and Video 9-3) and subcostal sagittal bicaval (Figure and Video 9-4) are the best sector scans to evaluate interatrial septal anatomy (Chapter 3). The presence and dimensions of the PFO can often be delineated from these views. Overlapping of the flap valve of the septum primum and the septum secundum is best seen in a sagittal bicaval cut, whereas the gap between the two septa is best imaged by a coronal cut. Thus, the measurement of a PFO from a single cross-sectional cut, as an indication of significance of a left-to-right shunt, is often misleading, especially when it is not highlighted by the presence of color flow. Bulging of the atrial septum into the right or left atrium gives a good indication of pressure differential between the two atria. In other sector scans, such as the parasternal long-axis (Figure and Video 9-5), apical four-chamber (Figure and Video 9-6) and parasternal short axis, the atrial septum is not fully imaged (although parts of it are included in the cross-sectional scan) due to echo dropout, since the plane of the atrial septum is parallel or has an acute angle with the plane of the ultrasound beam.

Color Flow and Pulsed Doppler Evaluation of Shunts through the PFO

Pulsed and color flow Doppler are essential for demonstrating shunts through a PFO or atrial septal defect. In contrast to the imaging of the atrial septum, which is best visualized when the ultrasound beam is oriented perpendicular to the structure, the direction of blood flow should be parallel to the ultrasound beam to obtain an optimal Doppler flow measurement. For the parasternal long-axis and apical four-chamber views, the atrial septum is parallel or forms an acute angle with the ultrasound beam, resulting in echo dropout, but atrial shunt flow directed toward the tricuspid valve might also be directed either parallel or at an acute angle with the ultrasound beam, resulting in a good visualization of atrial shunt flow (Figures and Videos 9-5 and 9-6). In a subcostal imaging of the atrial septum, atrial shunt flow is often either parallel or at an acute angle with the ultrasound beam, enabling adequate color and directional Doppler flow recording. However, one complicating factor is the constant presence of SVC flow, which has the same velocity and direction as PFO flow, and can therefore mask small atrial left-to-right shunts. It is often necessary to advance color flow recordings frame by frame to distinguish small PFO flows (Figure and Video 9-7). Large PFO flows are, however, often well visualized and easy to distinguish from SVC flow (Figure 9-8 and Video 9-8). One important consideration when evaluating venous flows (including shunts through the atrial septum) is to set color flow gain to between 30 and 70 cm/sec and Doppler range gain to less than 100 cm/sec in order to detect low-velocity flows. Usually, pulsed Doppler with the region of interest gated to the area of the shunt is preferred for low-velocity flows.