I feel like there would be a lot of confounding factors here, I'd like to see the survey. People who are belly breathers are going to be people who are either avid practitioners of relaxation techniques (known to decrease incidents of heart attacks), people who are health aware enough to make it a priority, people who are physically fit enough to have found the technique naturally, or people who are engaged heavily in yoga (which will overlap with the relaxation techniques and fitness), and thus be in better shape than the average population.