<s>[INST] A patient is complaining of a shocklike sensation in the left precordial area (breast) lasting for several seconds. It is on/offf, and more frequent when active. He has a pacemaker (not new), and interogation showed no problems. What is the etiology?? [/INST] Hello and thank you for using HCM.I carefully read your question and I understand your concern. I will try to explain you something and give my opinion. This shock like sensation has to do with a rhythm issue. There are some electrical impulses that generates from  parts of the heart different from normal sinus rhythm.They cause a premature heart contaction that its not a normal one. This electrical issues might be extra sistolic beats, atrial fibrillation or different arythmia. In this cases a person might experience a palpitation, flattering or shock like sensation.This might happen even in a patient that has a pacemaker. The pacemaker is a safe batery in cases when the heart rhythm falls down.The pacemaker might function very well and the patient might experience arythmia. This rhythm issue might have different causes like simple emotional stress to different pathologys like anemia,  hyperthyroidism, active infections,  ischemic heart disease. So, if it happens once a wile it is nothing to worry about. If it persist,  if I was the treating doctor I will recommend some examination like an electrocardiogram,  a cardiac echo to evaluate heart function and the most important one a holter rhythm monitoring to see what happens with the rhythm during day and night. Hope I was helpfull. Best regards. </s>