ABSTRACT

Although patent foramen ovale (PFO) was anatomically depicted in 1513 by Leonardo da Vinci and described as a thromboembolism route in 1877, it has been ignored for a long time as a potential way to produce pathological conditions. The unifying hypothesis associated with multiple clinical issues, such as cryptogenic stroke, migraine and decompression sickness is that a particle, inert gas bubbles or chemical substance in the venous circulation bypasses the lungs and enters to the systemic circulation via PFO. In this review, current data on the status of PFO in diving medicine are discussed.

Introduction

Foramen ovale is an interatrial connection that enables rapid transiting of umbilical blood to the brain and vital organs without any further oxygen loss during intrauterine period. After birth, the foramen ovale flap (septum primum) is closed on the septum secundum, physiologically when pulmonary vascular resistance and right atrium pressure drop. Fusion, which begins with contact, is completed irreversibly, in the first two years of life. Foramen ovale remains patent in 25% of the population (1,2). The patent foramen ovale (PFO) was drawn and depicted by Leonardo Da Vinci in the form of a “channel” centuries after the physiological closure was defined by Galen. Use of the term “channel” is unique in that century in terms of predicting the complex structure of PFO pathophysiology and pointing out that it is more than just a simple hole (3).

While individuals with PFO are generally identified incidentally in autopsies, antemortem diagnosis is often made during the etiological investigations of clinical pictures associated with PFO. In an autopsy study consisting of 965 people, PFO sizes were measured between 1-19 mm (4.9 mm on average) and the mean size was 3.4 mm in the first decade and was 5.8 mm in the tenth decade. This is interpreted as the fact that small-size PFOs are closed over time and that large-sized ones remain open (4). The combining hypothesis for the association of PFO with numerous clinical conditions such as cryptogenic stroke, migraine, sleep apnea, pulmonary edema due to high altitude, platypnea-orthodeoxia, decompression sickness (DCS), is based on the passage of a particulate, gas bubble, or chemical substance in venous circulation to systemic circulation without being exposed to the lungs through a right-to-left shunt. The left atrium pressure is higher than the right atrium, which prevents passage by holding down the septum primum flap to septum secundum. Even if the flap is partially open, the blood flow will be from left to right. However, daily activities such as lifting, coughing, vomiting, and pushing which increase intrathoracic (ITP) and intraabdominal pressure may reverse the interatrial pressure gradient, creating a temporary right-to-left shunt. One of the most effective methods is an extended and forced Valsalva maneuver (VM) (1,2,5). The maneuver, originally described in 1704 by Mario Antonio Valsalva in detail in his work “De Aure Humana Tractatus” (Treatise on the Human Ear), is simply an effort of forced expiration against a closed airway. In order to prevent middle ear barotrauma, the VM is frequently used during diving and hyperbaric oxygen treatment. The response to the maneuver is related to the duration of the maneuver, the level of strain, the position of the body and the respiratory pattern (5). Astonishingly, while gentle VMs during diving do not increase the ITP at all, much larger increases are observed if maneuver is performed during challenging and crouching (6).

Another issue about PFO-mediated transition that has recently been discussed is the blood flow dynamics in the right atrium and its relationship with fossa ovalis. At the right atrium, the currents from the caval veins do not collide head to head, they turn forward and contribute to the rotation of the blood in the clockwise. This filling pattern associated with directing the atrial volume towards the tricuspid valve entry is extremely important in maintaining the continuous activity of the heart with minimal energy. This vortex formed at the right atrium entrance is thought to remove the blood out of PFO which carries the majority of thrombus material, bubble, vasoactive chemicals and which is coming with the inferior caval current directed at almost to the fossa ovalis at the beginning which (7,8).

The PFO-mediated shunt can be determined by different echocardiographic techniques, including transthoracic echocardiography (TTE), transesophageal echocardiography (TEE), and transcranial Doppler (TCD). Having superior image resolution, having ability to distinguish shunt localization, having ability to define morphology, presence of accompanying defects, number and size of these defects, completeness of septum apart from defect and the presence of anatomic structures that will affect the placement of the device and visualizing the three-dimensional appearance of PFO in mind make TEE the gold standard in the diagnosis of PFO (1,2,9,10). However, it comes after TTE or TCD in the evaluation hierarchy because it is a semi-invasive procedure with well defined staff training criteria, it has life-threatening complications such as esophagus hemorrhage, perforation and it is contraindicated in patients with severe bleeding risk. TTE is the most frequently used initial screening test because of its low cost, non-invasive nature and easy accessibility (9,10).

The most common contrast used in echocardiography routine is saline which is agitated by mixing with air. Air bubbles are cleared as they pass through the lungs, and those who can pass are dissolved in the blood. Because tissues are not supersaturated with nitrogen, DCS symptoms are not observed even if there is a high bubble passage (11). However, during echocardiographic studies with contrast agents, for safety reasons, oxygen should be available and the diver should not have dived within the last 24 hours (6). It has been shown that TCD has similar sensitivity to shunt detection, but it fails to differentiate cardiac and pulmonary localizations (2,9,10).

Provocative maneuvers that increase the ITP, also significantly increase the sensitivity of TTE and TEE. Because conscious sedation is applied during TEE, the effect to be achieved with a challenging VM is tried to be performed by abdominal compression. The timing of the bubbles in the left heart is very important in the separation of intracardiac and transpulmonary shunts. In the presence of a shunt at the cardiac level, the bubbles are expected to be seen in the left heart in three cardiac cycles. In the presence of a large pulmonary shunt, it should be noted that contrast in the left heart can be seen in three cardiac cycles, and a more detailed imaging of the shunt to clarify localization should be performed with TEE. The systems used in grading are subjective and are usually based on semi-quantitative factors that focus on the number of bubbles seen in the left atrium. Therefore, there is no widely accepted schema (9,10). Another important point is the localization of contrast injection. Femoral injections have been shown to be more effective than conventional brachial injections. This effect is related to the blood flow dynamics in the right atrium and its relation with PFO, which is described above, and to the rapid bolus and shorter venous transit time provided by a larger diameter femoral catheter, and thus to the reduction of the dissolved bubbles (9,12).