// This isn't medical advice. It is a starting point to chat with your doctor about if you have questions. //
As you pointed out, a VSD is a hole in the septum of the heart, which separates your left-sided, oxygen-rich circulation (from lungs to heart through aorta to body to deliver oxygen) from your right-sided, oxygen-poor circulation (from body to heart through pulmonary artery to lungs to go get oxygen).
It's important to note that for a normal human being, left-sided pressures are significantly higher than right-sided pressures, because the left heart is pumping blood to the high-resistance body tissue, while the right heart is pumping blood into the low-resistance lungs.
That means, if you have a VSD, you are going to get a "left-to-right shunt". Notably, this does NOT cause a drop in oxygen saturation. It does increase the amount of work your heart has to do a little bit, because the left heart is effectively "losing" some of its output to the shunt. But it doesn't result in any oxygen-poor, right-side blood winding up on the left side. It just results in a bit of extra oxygen-rich, left-side blood winding up on the right. That's generally not a big deal (in the short term).
If a VSD persists into adulthood, the extra left-side blood on the right side becomes a bigger deal. Over time, the extra work the right heart has to do to pump that extra blood can cause pulmonary hypertension -- higher right-side pressures. If the right-side pressure comes to match or exceed left-side pressure, then you CAN get a right-to-left shunt. This does cause cyanosis, hypoxemia, etc. But it's a late complication of a VSD that doesn't close on its own. Unless your son has other cardiac issues (and the doctor would know about them), it's not going to happen in an 8-year-old. Now, if "VSD" was one thing in a list of a few things wrong with your son's heart, then it could be a lot more complicated than this, and you need to talk in detail to a pediatric cardiologist about what's going on.
So, if a VSD doesn't cause a right-to-left shunt, why shouldn't your son dive? The issue with diving isn't so much about oxygen utilization. It's about bubbles. When you breathe compressed air at depth, that compressed air is dissolved into your blood and tissues. When you ascend from that depth, the bubbles come out of the blood like opening a soda bottle. By ascending very slowly, you can ensure that the bubbles come out very slowly. Because a lot of the gas is dissolved in tissue, the bubbles are tiny and show up in right-sided circulation.
Those tiny bubbles might wind up in tiny blood vessels in your lungs. That's absolutely fine. The bubbles will diffuse out into the atmosphere via the lungs, and cause you no harm. But if you have a hole between your left-side and right-side circulation (ie a VSD), there's a chance one of the bubbles will cross over from the right side to the left side before they get to the lungs. If that happens, and the bubble lodges in a coronary artery or brain artery, it can cause some real damage.
(And even though the left-side pressures are higher, so blood isn't moving left-to-right, bubbles in turbulent flow don't follow the rules as nicely as significant quantities of blood. Though they do follow the rules a little bit, which is why an ASD is more dangerous than a VSD in diving -- atrial pressures are more similar than ventricular pressures.)
So, if what your son has is just a VSD, you don't have to worry about giving him supplemental oxygen early or anything like that. You just have to be extra careful that he doesn't get debris or bubbles in his blood. (This is why if he's in a hospital, I would hope nurses would be extra-careful to get all of the air bubbles out of IV lines for him, while for the rest of us, little bubbles will just wind up in the lungs and diffuse away.)
So the question is: Will normal flying in a GA airplane cause bubbles in your son's veins? If you're climbing from sea level to 6,000 feet at 500 feet/minute, you're going from about 1 atmosphere pressure to 0.8 atmosphere pressure in 12 minutes, or on average 0.017 atmospheres per minute.
Compare that to diving, where safe divers ascend at 30 feet per minute. The pressure dynamics are a little different because water is a relatively incompressible fluid, but long story short that is an average of about 1 atmosphere per minute.
So, long story short, the rate of pressure change in GA airplanes is about 2% of that of diving, and I've never heard of anyone getting decompression issues from a GA flight (unless they'd been scuba diving immediately before). That being said, crazy situations like U-2 pilots operating at 70,000 feet (pressurized to 24,000 feet) can cause deco issues. But it doesn't sound like that's what you're proposing.
In short, in my completely non-expert, non-educated opinion, which you should definitely check with your son's doctor, normal GA flying maneuvers would affect him no differently than anyone else, if a VSD is his only issue. If he's needed oxygen for any reason in the past couple of years, or has been told he shouldn't be doing strenuous exercise, or has actually turned chalky/blue/cyanotic at some point in the past (that wasn't immediately after birth), or if he has other cardiac things going on (pulmonary artery stenosis, overriding aorta, pulmonary hypertension, right ventricular dysplasia), then perhaps it's more complicated.
// This is not medical advice. It is a starting point to chat with your doctor about if you have questions. //
