If somebody has already experienced a heart attack and recovered, can adopting a vegetarian or vegan diet help to prevent a second heart attack? If yes, is one more effective than another, or is there a specific variation of vegetarian diet that has shown to be most successful? Scientific studies would be appreciated.
Disclaimer: This answer does not rely on my own experience with veganism but only on what I found in the literature.
TL;DR to be found in the conclusion section.
Heart attack is a broad term that is frequently misused. Heart attacks are part of a broader category called Acute Coronary Syndrome and can be split into three main types:
ST segment elevation myocardial infarction (STEMI)
non-ST segment elevation myocardial infarction (NSTEMI)
coronary spasm, or unstable angina
STEMI is often the one people talk about when they say they had a heart attack. It happens when muscles stop receiving blood because a coronary artery got completely blocked. The block is usually due to the presence of fats that stick to the artery walls and prevent the blood from flowing and the effects on the human body are sometimes irreversible.
It seems important to recall the different types of heart attacks because there are many other coronary diseases that may affect a person's life with the same magnitude, but also because studies tend to focus on coronary diseases as a whole when they question the affect of diet on them.
This study2 from 1990 studied the effects of lifestyle changes on coronary diseases (so they didn't only focus on diet), but it's interesting to notice that they invited patients to have a low-fat vegetarian diet. Here are their results:
Overall, 82% of experimental-group patients had an average change towards regression. Comprehensive lifestyle changes may be able to bring about regression of even severe coronary atherosclerosis after only 1 year, without use of lipid-lowering drugs.
The diet changes were combined to stress management, moderate exercise and smoking avoidance so that gives up hope, but no concrete proof yet.
Luckily this is a trendy topic in research and many studies were conducted since the previously linked one.
This one from 20103 does not say that a vegetarian diet definitely improves the heart condition but indicates it may help and is a safe way of improving your health:
A carefully planned vegetarian diet with adequate supplementation may be effective for primary prevention of heart disease. The vegetarian diet is cost effective, safe, and relatively easy to implement.
I've also found this study from 20175, that makes a clear connection between a vegan diet and low levels of cholesterol ... which are partially responsible for artery thickening and by extension, heart issues.
We've seen so far that a combination of healthy habits does induce a regression in coronary diseases and that vegetarian diets might help reducing heart diseases on their own. As often in science the answer is not a definite "yes" or "no" but so far, it seems safe to assume that a vegetarian diet does nothing but good to the human coronary system. Let's be good scientists and dig further, shall we? ;)
If you don't know about it yet, Cowspiracy and What The Health filmmakers have these great "Facts" pages available online that references every single study they've used to make the movies. I therefore checked what WTH had to say about vegetarianism and heart diseases. They talk about the effects of eating egg yolk on the carotid walls4, dairy being the #1 source of saturated fats6 (which can be responsible for higher cholesterol levels), the concerning amounts of mercury in fish7 (mercury can induce cardiac arrythmia and hypertension, among others) ... I didn't mention all of them but the list goes on.
Conclusion (and TL;DR)
can adopting a vegetarian or vegan diet help to prevent a second heart attack?
If combined with other lifestyle changes such as quitting smoking, stress management and exercise, yes (cf src #2). With a diet change only, maybe. Science says that it's safe to try (cf src #3).
If yes, is one more effective than another, or is there a specific variation of vegetarian diet that has shown to be most successful?
It appears veganism would be more effective, as dairy and egg contain fats that might induce heart and artery conditions.
[Study] - Egg yolk consumption and carotid plaque