Cardiologists specialize in diseases of the heart, lungs, and circulatory system and treat patients with all aspects of heart disease. Cardiologists diagnose, treat and manage complex conditions such as heart attacks, arrhythmias (abnormal heartbeat) and angina. An interventional cardiologist is skilled in the techniques that improve coronary circulation and alleviate narrowing or blockage of the heart valves. These include cardiac catheterization, balloon angioplasty, stent placement and permanent pacemaker insertions.