Does anybody enlighten me how the brahmari pranayama helps in preventing the heart blocks and how it cleans up the clogged arteries
In the pranayama phase there should be no such conditions.
Unclogging arteries is best done through moderate exercise and diet with no processed foods, no foods of animal origin (no dairy, no meats including fish, no eggs) no fast assimilating carbohydrates (no sugar, no honey) no refined oils of any kind except the cold pressed ones, and those with moderation, too. No acidifying foods.
The reason for clogged arteries is not unproper breathing, but unproper feeding habits. Naturally, the real cause needs to be adressed first.
After the good feeding habits are acquired, and a new balance reached, some pranayama, emulating the increased breathing rythm of moderate physical exercise would help those who are restricted from it. But in case of those who are not, exercise is way better.