Normal Physiology
Figure 3. Illustration of fetal blood flow or circulation (2019)
- The cardiopulmonary circuit in a developing fetus differs from that of a newborn infant or an adult. Anatomic alterations in the cardiac structure exist to decrease blood flow to the lungs and increase blood flow to the placenta, which is the primary oxygen source in a fetus (McCance & Huether, 2019). The foramen ovale and ductus arteriosus are two openings in the fetal heart that help increase placental blood flow. The foramen ovale is an opening between the atria that allows blood to flow directly from the higher pressure right atrium to the lower pressure left atrium, therefore bypassing the pulmonary circuit. Once an infant is born the primary oxygen source must switch from the placenta to the lungs; therefore, more blood should be sent to the lungs for oxygenation. During birth, the higher pressure in the systemic vascular system (increased pulmonary venous return and decreased inferior vena caval return) from the clamping of the placenta and beginning of respiration normally causes the ductus arteriosus, ductus venosus, and foramen ovale to close. This physiologic closure is termed “functional” because it prevents further blood flow between the atria. Blood flow to the lungs increases, allowing the infant to respire (McCance & Huether, 2019).
Pathophysiology
- The foramen ovale should “anatomically” (permanently) close within the first month of birth as a result of fibrin and connective tissue. Until this more permanent closure, an event that increases right-sided pressures or dilates the right atrium such as pulmonary hypertension, right ventricular failure, or tricuspid atresia could reopen the foramen ovale (McCance & Huether, 2019).
- When the foramen ovale fails to anatomically close after birth, the opening is termed a patent foramen ovale (McCance & Huether, 2019). Approximately one quarter of the global population has a patent foramen ovale, whether or not they experience symptoms (AHA, 2017). In some cases, a right-to-left shunt may develop in the heart as blood can leak from the right atrium across the opening and into the left atrium. This shunt may exist during the Valsalva maneuver, or rarely at rest (Kim, Kim, Kang, Kim, & Kwon, 2014). The blood then pumps into the left ventricle and into the systemic circulation. A patent foramen ovale can increase the risk of stroke in few patients by allowing some blood to bypass the pulmonary circulation. Typically, small blood clots that form in the veins of the legs that travel up to the heart are filtered out by the lungs. In the circumstance of a patent foramen ovale, a microclot can travel across the opening in the atrial septum and enter the systemic circulation. This creates a risk for stroke or transient ischemic attack (TIA) should the microclot lodge in the brain. However, the risk of having a stroke due to a patent foramen ovale is very low (AHA, 2017). Due to the typically mild nature of the defect, most patients with a patent foramen ovale are asymptomatic (Mayo Clinic, 2018).
Key Diagnostic Criteria
Figure 4. Echocardiogram (2019)
- While patients of all ages are usually asymptomatic; the following symptoms are possible:
- Heart murmur
- Dizziness
- Unexplained TIA or stroke
- Mimicry of symptoms with bearing down
- Confirmation with transthoracic or transesophageal echocardiogram (TTE or TEE)
- Rule out other causes for TIA or stroke (Mayo Clinic, 2018)