implantable pacemakers and defibrillators are devices that apply electric shocks to maintain the rhythm of the heart and if necessary restart it as the technology improves and the list of treatable conditions grows the number of devices being implanted is increasing steadily and now exceeds half a million a year the heart is made up of four chambers two atria and two ventricles on each side the atrium is connected to the ventricle by a one-way valve blood is pumped as these chambers contract and relax in turn the beating of a healthy heart is regulated by electrical impulses the sequence begins as the atria fill with deoxygenated blood from the body on the right and oxygenated blood from the lungs on the left an electrical signal from the sinoatrial node then causes the atria to contract forcing blood into the ventricles the electrical signal is then picked up by the atrioventricular node and directed into the Purkinje fibers in the ventricle walls causing the ventricles to contract and the blood is then pumped through the pulmonary valve on the right to the lungs and the aortic valve on the left to the rest of the body these valves closed and the cycle then restarts when the sinoatrial node fails to function correctly an artificial pacemaker can be fitted to help regulate the heartbeat with small evenly timed electric shocks this involves implanting electrodes into one or more of the heart's chambers by inserting leads into a vein near the collarbone and implanting a device called the generator just under the skin for more severe heart conditions an implantable defibrillator or ICD can be used which is also capable of sensing a stopped heart and delivering an electric shock powerful enough to restart it for some conditions an even more sophisticated device called a CRT ICD can be implanted this uses a third lead inserted into the left ventricle to resynchronize the ventricles when necessary however all these leads can cause problems of their own patients with ICD s have a 20% chance of a lead failure within 10 years and replacing leads can require open-heart surgery in about 2% of cases this is resulted in several efforts to develop new pacemakers that do not depend on leads inside the heart one design the subcutaneous ICD places the lead just outside the heart under the patient's skin and wireless designs are now being developed that may eventually do away with the need for leads altogether