As I live and breathe is an everyday English expression which is used to state something unbelievable. The expression is similar to saying "Oh my God! I can't believe this!" The exclamation is conveyed through the tone of voice and very revealing facial expressions (a stunned expression), sometimes holding up one's hands to one's cheeks or over the mouth.
In short, the everyday English expression As I live and breathe conveys total disbelief.
1. When Raju topped in the exams, the class teacher was astonished and exclaimed to the other teachers "As I live and breathe, Raju proved he is serious about his studies!"
2. Mahendra rushed home and told his brother, "Come quick! As I live and breathe Shahrukh Khan is dining at the restaurant at the corner!"
3. As I live and breathe, the neighbour's teenage son rescued the baby from the fire.