pacemaker

pacemaker In the healthy heart the cells of the sino-atrial (SA) node constitute the natural pacemaker, generating regular electrical signals which spread through the heart and cause it to beat. An artificial pacemaker is required if for any reason, usually heart block, this natural system is compromised, resulting in an irregularly or consistently slow heart rate (bradycardia).

An artificial pacemaker contains a battery and circuitry which produces electrical pulses of short duration capable of stimulating the heart. The device was invented by Wilson Greatbatch of the USA and patented in 1960 after surgeons in New York had made the first clinically successful implant in a 77-year-old man. The pulses are delivered via an electrode which makes direct contact with the heart muscle. Pacemakers can be made to stimulate the heart at a fixed rate irrespective of the intrinsic heart rate, or a demand pacemaker may be used which is capable of sensing the native rhythm and pacing the heart only when the sensed rate falls below a certain value. Recent advances in design have produced pacemakers that are capable of re-synchronizing atrial and ventricular activity, thus functioning as a defibrillator.

A temporary pacemaker can be fitted by placing the pacing electrode within the right ventricle. The electrode is introduced through a needle inserted into a large vein in an arm or the neck. The electrode is advanced, under X-ray monitoring, within the vein following its course back to the heart. Once the electrode is in contact with the inner surface of the heart it is connected to the pacemaker, which remains outside the body. A temporary pacemaker may be required in the short term for certain individuals after a heart attack, during cardiac surgery or general anaesthesia. A permanent pacemaker is fitted in people requiring long-term pacing. The electrode in a permanent pacemaker is also introduced into a vein, but the vein in this case is surgically exposed. The permanent pacemaker is sufficiently small to be placed in a small pouch formed within the muscle under the skin; it is con-nected to the pacing electrode. Less commonly, pacemakers may be implanted that can detect the onset of abnormal tachycardias (fast heart rate). The pacemaker can stimulate the heart in competition with the abnormal beat in an attempt to return it to a normal rhythm. More recently, power supplies capable of delivering high energies for defibrillation have been introduced. The pacemaker may last five to fifteen years, depending on the lifetime of the battery and the frequency of stimulation. More modern versions can retain a microchip memory of their activity for periods of up to a year; this information can then be routinely ‘downloaded’ for analysis so that the physician has access to a detailed electrical history of the patient's heart. Pacemaker batteries can usually be recharged via an induction coil outside the skin so no further surgery is required. Modern pacemakers thus have increasingly sophisticated microprocessor-controlled ‘brains’. Like all such equipment, there is a risk of electrical interference by very powerful electrical devices; these risks are often signposted.

David J. Miller, and Niall G. MacFarlane

See also defibrillator; heart; heart block.