After reading your correspondence (stress-induced tachychardia and high BP that does not respond to medication), I really stand by my first post.
Doing alternate nostril breathing without retention will give you all the benefits of relaxation without the risk of inducing more stress. Just one of the reasons for avoiding retention is that many people with stress tend to become less sensitive to their bodies and therefore may not even be able to tell when they are introducing more stress.
You are just fine with alternate nostril breathing. Breathe comfortably. If you like, you can add a mantra (Soham), with SO on the inhalation and HAM on the exhalation. Try this for three months, say 10 - 15 min in the morning before breakfast and 10 - 15 min in the evening before your dinner. Anything beyond that should be supervised directly by a competent teacher (in my opinion).
If you want to progress in pranayama or apply yoga therapeutically, you could try The Yoga Institute (www.yogainstitute.org) in Mumbai. You went to see a doctor. Why not see a yoga expert to "administer" such a powerful technique as pranayama?
I believe that yoga can help you to control stress and therefore your symptoms. In terms of asana, I would recommend gentle movement in harmony with your breath (pavanmuktanasana), gentle breathwork (abdominal breathing, ujjayi, alternate nostril breathing), and some meditation (such as yoga nidra). It would also help to examine your lifestyle for stressors and try to reduce these. And move to regular sleeping, eating, working and relaxation habits. It helps to have a teacher to guide you on this path.