What Causes Jaw To Virtually Locked Up, Shallow Breaths After Collapsing?

Tonite my 51 year old mother in law collapsed, we thought it was a seizure and in 5 minutes we were doing cpr because she stopped breathing and had no pulse. While doing cpr her jaw was virtually locked closed and we were having a hard time opening her airway, we had three of us working cpr on her, two working and one prying her mouth open. She started breathing very short, very shallow breaths, like a fish out of water. One us is a cardiac nurse so immediatly she was being worked on and the ambulance was fast, from start to hospital was probably 30 minutes if that. I think we ve heard she has heart disease. What does that sound like to you?