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It is not very clear from your mail what exactly you mean by "hard breath". I consider it "difficulty in breathing" and my answer is based on difficulty in breathing. Difficulty in breathing is a non-specific symptom and can be due to a wide variety of reasons. As the symptom is related to lungs, my explanation is based on that. So, if your symptoms are due to a problem with the lungs, then you can follow these guidelines for yoga:
Useful pranayama (breathing exercise) are Anuloma viloma (27 rounds), sectional breathing (5 breaths each), bhramari (9 rounds) and kapalbhati.
Useful asanas (yoga poses) are basically chest openers like bhujangasana, ardhachakrasana, matsyasana, ustrasana, vakrasana, ardhamatsyendrasana,simple inverted postures like sarvangasana, and ardhasirsasana. Complementary postures needed to be followed are a mandatory part of yoga practices. Each asana is to be maintained for five normal breaths.
All the practices should be done slowly.
Kapalbhati has to be started at a slow pace and then gradually increase the pace. If your disease is stable, then start at 30 strokes/minute.
Meditation and chanting are especially beneficial for allergic bronchitis and asthma patients.
Start yoga practices under supervision.
No yoga session is successful without yogic understanding and yogic counseling.
Start with these yoga practices for one hour per day.
You can report back to a yoga specialist online for further follow up --> https://www.icliniq.com/ask-a-doctor-online/yoga-specialist