How can a PFO (patent foramen ovale) impact DCI (decompression illness)?

A patent foramen ovale (PFO) is a small, flap-like opening between the right and left atria of the heart that is present in many people and usually closes after birth. When a diver experiences rapid decompression, nitrogen gas that has been absorbed into body tissues during the dive can form bubbles as the pressure decreases. Normally, these bubbles would be filtered out by the lungs if they enter the venous circulation.

However, when there is a PFO, any bubbles that form in the venous blood have the potential to bypass the lungs and directly enter the arterial circulation. This is significant because it can lead to the occurrence of decompression illness (DCI) in tissues and organs that would not typically be exposed to these bubbles, such as the brain, which can cause serious complications.

The correct answer highlights the vital risk associated with a PFO in divers; it provides a path for nitrogen bubbles to migrate from the venous circulation into the arterial side, where they should ideally not be. Understanding this mechanism is crucial for divers who may have a PFO, as it emphasizes the need for awareness and possible precautions when diving, especially in scenarios with a higher risk of bubble formation.