Here is the answer from Divers Alert Network.
An atrial septal defect (ASD) results from the incomplete closing of the wall that separates the right and left atria (the two upper chambers of the heart) during embryonic development. This is not an uncommon phenomenon in the general population, and, if the hole is small enough, the average person will experience minimal physiologic consequences. Women are affected more commonly than men.
Surgical correction of the defect may be undertaken, especially if the person is experiencing symptoms secondary to blood flowing from the normally higher pressure left atrium to the right atrium. Early in life, symptoms may be few, but over a period of years, complications, such as abnormal heart beats and shunting (bypassing) of blood from left to right may occur.
On examination, the person with an ASD may have a significant murmur.
Fitness and Diving Issue: While the normal pressures in the chambers of the heart favor blood flowing from left to right through an ASD and VSD, periods in which this flow is reversed can occur, particularly for ASD. Although individual variations exist, Doppler studies have shown that most divers will have venous bubbles after a dive of significant depth and bottom time. These usually pose no significant threat, and the diver remains symptom-free.
Having a defect that allows bubbles to cross from the right side of the heart to the left is a whole different matter, however: once in the left side of the heart, bubbles may then be transported through the arteries to areas of the body where they can do some harm (e.g., to the brain, kidneys, and spinal cord). Several studies have demonstrated that a rate of ASD (and other defects in the wall separating the right and left sides of the heart) in divers treated for decompression illness was higher than expected, compared to the general population.
Someone with an ASD or VSD who wants to take up scuba diving should be discouraged from doing so. The diver with a known ASD or VSD should know of the potential increased risk of decompression illness and make an educated decision whether to continue diving. Individuals with a VSD, where the shunt is small and runs uniformly from left to right as determined by an echocardiogram, may be able to dive if it is determined to be safe by a physician knowledgeable in diving medicine.