Howdy whaleshark:
Welcome to the board! Your question brings up a couple of things. First, there is a difference between a patent foramen ovale (PFO) and a ventricular septal defect (VSD). The heart is divided into four chambers- the two "upper" chambers, the right and left atria, and the two "lower" chambers, the right and left ventricles. The right side of the heart accepts blood from the venous circulation and pumps it to the lungs for gas exchange, and the left side takes the blood from the lungs and pumps it to the body through the arteries. The right and left sides of the heart are separated by muscular walls- the atrial and ventricular septums*- that keep the blood from the right and left sides of the circulation from mixing. Normally, you won't have holes in those septums so any hole is usually called a "defect".
A PFO is a specific kind of atrial septal defect (ASD- a hole in the septum between the atria). A VSD is a hole in the septum between the ventricles. The problem with septal holes in divers is that they can allow blood from the right (venous) side of the circulation that might contain bubbles from decompression to mix with blood from the left (arterial) side of the circulation. It's common for divers to develop a few venous bubbles on ascent, but usually these bubbles cause no problems because they're "filtered" by the lungs. If the bubbles are able to bypass the lung filter and enter the arterial circulation through a septal defect, they can cause problems- perhaps serious problems.
Now, atrial septal defects are generally considered more concerning in divers than ventricular septal defects. The pressure in the atria is much lower than in the ventricles, and is pretty close to equal in the right and left atria. If a diver with an ASD strains, performs a Valsalva maneuver, etc. he can increase pressure in the right atrium shunting bubble-rich blood into the left (arterial) circulation. PFO's come in various "degrees" and are often less concerning that other types of ASD's. PFO's aren't necessarily "open" all the time like some other kinds of ASD's.
The normal pressure in the ventricles is much higher than in the atria, and higher on the left (arterial) side than on the right (venous) side. People with a VSD will generally shunt blood from the left side to the right side which is no problem as far as bubbles from decompression are concerned. But in diastole- the heart's relaxation phase between beats- the pressure in the two ventricles is closer to the same and there can be one of those dangerous right-to-left shunts of bubble rich blood.
OK, what does this mean for a diver with a VSD? Well, it depends on who you ask. A VSD with a large left-to-right (arterial-to-venous) shunt can cause health problems sometimes like heart failure and lung damage that can interfere with diving. I would think that any diving medicine authority would say that a diver with serious heart or lung issues from their VSD should not dive.
But what about the problem of a possible right-to-left (venous-to-arterial) shunt in a diver with no other problem from their VSD? This is where it gets controversial. In his chapter on "Cardiovascular Disorders" in Bove and Davis' Diving Medicine Dr. Bove describes the "diving problem" with a "small" VSD as "None". But in Bennett and Elliott's The Physiology and Medicine of Diving Drs. Mebane and McIver take a more conservative approach. They say, "The ventricular shunt flows from left to right, but there may be transient reversals of pressure during diastole allowing right-to-left flow. The ventricular septal defect, therefore, is a disqualification for diving."
So who do you believe? Dunno, not enough information.
HTH,
Bill
The above information is intended for discussion purposes only and is not meant as specific medical advice for any individual.
*OK, "septa" not "septums". You'd think I could at least be consistent.
