Cardiac pacemaker is a device meant to give regular electrical pulses to the heart when the normal pacemaker function of the heart is defective. The normal pacemaker of the heart is the sinoatrial node (SA node) situated in the upper part of right atrium (upper chamber of the heart). It gives out regular electrical pulses to the heart at a rate between 60 – 100 per minute. These pulses are conducted down the right atrium to the atrioventricular node (AV node, situated at the junction between the atrium and the ventricle), which is the relay station for the pulses. In the AV node the pulses are delayed and sent further down the conduction system known as the bundle of His and bundle branches (right and left) into the ventricles (lower chambers of the heart). The delay ensures completion of atrial emptying prior to the onset of ventricular contractions. This helps in better filling of the ventricles.
The basic electronic pacemaker consists of an electronic circuitry to generate the pulses (an oscillator), a battery with long storage life of the order of several years and a set of leads (electrodes) to convey the pulses to the heart. The battery and circuitry are housed together in a hermetically sealed (water tight) metallic container made of biocompatible material which will not induce a foreign body reaction in the body. This part known as the pulse generator is usually implanted beneath the skin under local anaesthesia. Modern pacemakers have highly complex circuitry capable of monitoring the body’s requirement and varying the pulse rate accordingly. They are also communicating devices which can transmit the stored information to a device known as programmer kept over the surface of the skin. Programmer can interrogate the pacemaker to retrieve important information regarding pacemaker function and malfunction. Latest programmers can sent the data to a server over the internet making it feasible for the specialised physician to evaluate complex pacemaker function sitting in another country or continent! The pacemaker lead is a passive component which transmits the pulses to the heart and receives the heart’s electrical signals for the pacemaker to analyse. The current day demand pacemakers will sense the heart’s intrinsic activity and give pulses only when they are not present. There are pacemakers available which can pace one, two or three chambers of the heart – they are known as single chamber, dual chamber and biventricular pacemakers respectively. The biventricular pacemakers are mainly used for synchronizing the contraction of the two ventricles in patients with heart failure. They are known as heart failure devices used for cardiac resynchronization therapy (CRT).
Pacemaker is not a device which can be left alone once implanted. They require regular evaluation in a pacemaker clinic and periodic reprogramming as the needs of the body change. In the pacemaker clinic the programmer head is kept over the device implanted under the skin and interrogated using radiofrequency signals. The result is printed out or seen on the programmer screen and analysed by the cardiac electrophysiologist, a physician trained in the treatment of electrical disorders of the heart. Many malfunctions can be detected and managed by reprogramming the device. Battery status is assessed so that elective replacement time can be detected well in advance, usually several months ahead.
Implantation and replacement is done under local anaesthesia. A small pocket is created below the skin and the pulse generator implanted under the skin. The lead is introduced into the chambers of the heart through a vein running through or near the pocket. The lead is guided to the destination chamber under x-ray fluoroscopic guidance. The number of leads depends on the number of chambers paced – usually one lead per chamber. The lead is connected to the socket on the pacemaker and screwed tightly. Most of the leads are attached to the cardiac chambers using passive fixation structures known as tines. In some instances where lead instability is expected, active fixation screw in leads are used.