PFO – That Perfectly Formed Hole

During the early 1980s, eminent British cardiologist and enthusiastic scuba diver Dr. Peter Wilmshurst decided to investigate a commonly encountered heart defect that might be responsible for divers getting decompression sickness while diving within the limits of the Royal Navy decompression tables. These deco tables were popular with British divers and more conservative than the U.S. Navy tables often trusted elsewhere in the world.

Dr. Wilmshurst employed a doppler bubble test to track the stream of tiny bubbles introduced into a volunteer subject's bloodstream to determine if they had a PFO, a patent foramen ovale, or as he liked to describe it to us laymen, a "perfectly formed hole."

A PFO is a common heart defect. It's a small opening between the left and right atrium of the heart that usually closes after birth. If it stays open, arterial blood can flow between the chambers, avoiding the lungs and causing nitrogen-loaded blood to be more easily recirculated. Therefore, nitrogen absorbed during a dive is less easily off-loaded on ascent.

Decompression theory is built on the assumed speed at which nitrogen can be discarded via the lungs.

Dr. Wilmshurst engaged volunteers from the British Sub Aqua Club in London and members were later alarmed to discover that the vast majority had a PFO (estimates are that one third of the population has some form of PFO). This was exacerbated when he suggested at the time they give up diving.

Most didn't. Rob Bryning and Sam Harwood took their PFOs with them when they set up Maldives Scuba Tours (a successful liveaboard company for more than two decades), worked as its first dive guides, and never suffered ill effects.

Of course, it all depends on the size of the PFO and the sort of diving done, whether it can lead to a DCS event. A big personality in the British diving scene, Mike Calder suffered dramatic skin bends for years but nothing worse. Another diver, Dee Buxton, suffered skin bends attributed to a PFO, resorted to surgery to close it, later went diving in the Galapagos Islands, and got severely bent on her first dive (at less than 60-feet deep). She was trapped in the islands for weeks until it was safe for her to fly. So, it's not as simple as it first appears.

Dive computers have introduced a layer of safety into diving by being more cautious than earlier tables and controlling a dive and the diver's ascent in a more precise manner. Safety stops have become de rigueur too.

For divers with a PFO, the absorbed nitrogen sheds more slowly, increasing the risk of decompression illness. There are three options available: Stop diving; be cautious and limit depth and ascent speeds; or have the PFO surgically closed. Some divers who suffer minor DCS events thought to be caused by a PFO prefer to resort to surgery and have them closed up. There was even talk a few years ago of including screening for PFO in diving medicals.

Bill Schlegel, MD, a cardiologist and long-time Undercurrent contributor, sent us a recent medical article about the association of PFOs with unprovoked DCS hits. The article in JACC: Cardiovascular Imaging reports that in a recent study, 829 divers were enrolled in the Decompression Illness Prevention in Divers with a Patent Foramen Ovale registry between January 2006 and December 2018. An analysis showed that those who were screened and then underwent PFO closure were less likely to experience unprovoked decompression sickness (DCS) than those who took a conservative approach to management.

Schlegel, who is also associated with DAN, tells us in 30 years of diving medicine, he's seen three people who had undeserved DCS hits where a thorough medical workup found that the cause could have been a PFO. "We plugged two of them. One declined the procedure and just decided to stop diving. She was a nurse anesthetist. She has done well. Two others, both of whom had serious but transient neurologic 'hits,' had the closure, and both did well. One of them has returned to occasional diving, conservatively."

Closing the hole these days is much easier than in the past. Medtronic Corp makes a closure device to plug PFOs. It is delivered into the heart on the end of a catheter. The device is pushed through the PFO and deployed. It expands, locks into place, and plugs the hole. The catheter is then removed. The patient ends up only with a small puncture at the site where the catheter went in.

- John Bantin