Deep Breathing

For past 9 months I feel I hold my breath at times and do not deeply inhale.  Feels like someone sitting on my chest at times (not always) so that I can't get a deep breath in.  At times yawning helps to get a deep breath.  Had a calcium score of 18 for Right Coronary Artery all others were zero. When I lay on right side it seems it is harder to deep breath than laying on left side.  I was also diagnosed with a beart murmer during annual checkup 4/2007, saw a cardiologist (9/07) for an Echocardiogram who confirmed it was benign heart murmer (no antibiotics required for dental procedures, etc.). Just saw an Internal Medicine physician and he suggested I have a EKG and the Excercise Stress Test with Thalium.  Cardio CPR, Homocysteine blood tests were normal.  I am concerned with radioactive being sent through an IV to determine if there if any blockage.  What are the downside of having this EKG Talium test, issues having it, etc?