Studies have shown that practising yoga on a regular basis can have a very positive effect on asthmatics. Yogic breathing exercises the lungs and encourages a full diaphragmatic breath. Learning to breathe deeply in a calm controlled way, helps to reduce symptoms when they arise.
Many yoga poses bring relief to the thoracic area (upper back and chest). Twisting and some back bending, lengthen and open these tight areas, caused by shallow breathing and wheezing .
On a personal note, I found that as an asthmatic myself - and having had a respiratory arrest - yoga transformed my life and enabled me to drastically reduce prescribed medication.
Always seek your doctors advise before adjusting your medication.