Chapter 9: Shunts Through a Patent Foramen Ovale^*

INTRODUCTION

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.

ANATOMY AND PHYSIOLOGY OF THE FORAMEN OVALE

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 ...