artificial cardiac pacemaker artificial cardiac pacemaker an artificial cardiac pacemaker (artificial pacemaker, and sometimes just pacemaker, although the term is also used to refer to the body's natural cardiac pacemaker) is a medical device, nowadays always implanted, that generates electrical pulses delivered by electrodes to the chambers of the heart, either the upper atria or lower ventricles. each pulse causes the targeted chambers to contract and pump blood, thus regulating the function of the electrical conduction system of the heart. this article is about the medical device that simulates the function. for the natural pacemaker in the heart, see cardiac pacemaker. artificial cardiac pacemakerst. jude single-lead pacemaker with ruler in cm (released in 2005)specialtycardiology, electrophysiology the primary purpose of a pacemaker is to maintain an adequate heart rate, either because the heart's natural pacemaker is not fast enough, or because there is a block in the heart's electrical conduction system. modern pacemakers are externally programmable and allow a cardiologist, particularly a cardiac electrophysiologist, to select the optimal pacing modes for individual patients. most pacemakers are on demand, in which the stimulation of the heart is based on the dynamic demand of the circulatory system. others send out a fixed rate of impulses. a specific type of pacemaker called an implantable cardioverter-defibrillator combines pacemaker and defibrillator functions in a single implantable device. others, called biventricular pacemakers, have multiple electrodes stimulating different positions within the ventricles (the lower heart chambers) to improve their synchronization. methods of pacing an ecg in a person with a single-chamber pacemaker to the atrium. note the circle around one of the sharp electrical spikes in the position where the p wave would be expected. an ecg of a person with a dual-chamber pacemaker percussive pacing percussive pacing, also known as transthoracic mechanical pacing, is the use of the closed fist, usually on the left lower edge of the sternum over the right ventricle in the vena cava, striking from a distance of 20 – 30 cm to induce a ventricular beat (the british journal of anaesthesia suggests this must be done to raise the ventricular pressure to 10–15 mmhg to induce electrical activity). this is an old procedure used only as a life-saving means until an electrical pacemaker is brought to the patient. transcutaneous pacing main article: transcutaneous pacing transcutaneous pacing (tcp), also called external pacing, is recommended for the initial stabilization of hemodynamically significant bradycardias of all types. the procedure is performed by placing two pacing pads on the patient's chest, either in the anterior/lateral position or the anterior/posterior position. the rescuer selects the pacing rate, and gradually increases the pacing current (measured in ma) until electrical capture (characterized by a wide qrs complex with a tall, broad t wave on the ecg) is achieved, with a corresponding pulse. pacing artifact on the ecg and severe muscle twitching may make this determination difficult. external pacing should not be relied upon for an extended period of time. it is an emergency procedure that acts as a bridge until transvenous pacing or other therapies can be applied. epicardial pacing (temporary) main article: epicardial ecg rhythm strip of a threshold determination in a patient with a temporary (epicardial) ventricular pacemaker. the epicardial pacemaker leads were placed after the patient collapsed during aortic valve surgery. in the first half of the tracing, pacemaker stimuli at 60 beats per minute result in a wide qrs complex with a right bundle branch block pattern. progressively weaker pacing stimuli are administered, which results in asystole in the second half of the tracing. at the end of the tracing, distortion results from muscle contractions due to a (short) hypoxic seizure. because decreased pacemaker stimuli do not result in a ventricular escape rhythm, the patient can be said to be pacemaker-dependent and needs a definitive pacemaker. temporary epicardial pacing is used during open heart surgery should the surgical procedure create atrio-ventricular block. the electrodes are placed in contact with the outer wall of the ventricle (epicardium) to maintain satisfactory cardiac output until a temporary transvenous electrode has been inserted. transvenous pacing (temporary) main article: transvenous pacing transvenous pacing, when used for temporary pacing, is an alternative to transcutaneous pacing. a pacemaker wire is placed into a vein, under sterile conditions, and then passed into either the right atrium or right ventricle. the pacing wire is then connected to an external pacemaker outside the body. transvenous pacing is often used as a bridge to permanent pacemaker placement. it can be kept in place until a permanent pacemaker is implanted or until there is no longer a need for a pacemaker and then it is removed. right atrial and right ventricular leads as visualized under x-ray during a pacemaker implant procedure. the atrial lead is the curved one making a u shape in the upper left part of the figure. permanent transvenous pacing permanent pacing with an implantable pacemaker involves transvenous placement of one or more pacing electrodes within a chamber, or chambers, of the heart, while the pacemaker is implanted inside the skin under the clavicle. the procedure is performed by incision of a suitable vein into which the electrode lead is inserted and passed along the vein, through the valve of the heart, until positioned in the chamber. the procedure is facilitated by fluoroscopy which enables the physician to view the passage of the electrode lead. after satisfactory lodgement of the electrode is confirmed, the opposite end of the electrode lead is connected to the pacemaker generator. there are three basic types of permanent pacemakers, classified according to the number of chambers involved and their basic operating mechanism: single-chamber pacemaker. in this type, only one pacing lead is placed into a chamber of the heart, either the atrium or the ventricle. dual-chamber pacemaker. here, wires are placed in two chambers of the heart. one lead paces the atrium and one paces the ventricle. this type more closely resembles the natural pacing of the heart by assisting the heart in coordinating the function between the atria and ventricles. biventricular pacemaker. this pacemaker has three wires placed in three chambers of the heart. one in the atrium and two in either ventricle. it is more complicated to implant. rate-responsive pacemaker. this pacemaker has sensors that detect changes in the patient's physical activity and automatically adjust the pacing rate to fulfill the body's metabolic needs. the pacemaker generator is a hermetically sealed device containing a power source, usually a lithium battery, a sensing amplifier which processes the electrical manifestation of naturally occurring heart beats as sensed by the heart electrodes, the computer logic for the pacemaker and the output circuitry which delivers the pacing impulse to the electrodes. most commonly, the generator is placed below the subcutaneous fat of the chest wall, above the muscles and bones of the chest. however, the placement may vary on a case-by-case basis. the outer casing of pacemakers is so designed that it will rarely be rejected by the body's immune system. it is usually made of titanium, which is inert in the body. leadless pacing leadless pacemakers are devices that are small enough to allow the generator to be placed within the heart, therefore avoiding the need for pacing leads. as pacemaker leads can fail over time, a pacing system that avoids these components offers theoretical advantages. leadless pacemakers can be implanted into the heart using a steerable catheter fed into the femoral vein via an incision in the groin. basic function single-chamber vvir/aair pacemaker dual-chamber dddr pacemaker modern pacemakers usually have multiple functions. the most basic form monitors the heart's native electrical rhythm. when the pacemaker wire or "lead" does not detect heart electrical activity in the chamber – atrium or ventricle – within a normal beat-to-beat time period – most commonly one second – it will stimulate either the atrium or the ventricle with a short low voltage pulse. if it does sense electrical activity, it will hold off stimulating. this sensing and stimulating activity continues on a beat by beat basis and is called "demand pacing". in the case of a dual-chamber device, when the upper chambers have a spontaneous or stimulated activation, the device starts a countdown to ensure that in an acceptable – and programmable – interval, there is an activation of the ventricle, otherwise again an impulse will be delivered. the more complex forms include the ability to sense and/or stimulate both the atrial and ventricular chambers. the revised naspe/bpeg generic code for antibradycardia pacing iiiiiiivv chamber(s) pacedchamber(s) sensedresponse to sensingrate modulationmultisite pacing o = noneo = noneo = noneo = noneo = none a = atriuma = atriumt = triggeredr = rate modulationa = atrium v = ventriclev = ventriclei = inhibitedv = ventricle d = dual (a+v)d = dual (a+v)d = dual (t+i)d = dual (a+v) from this the basic ventricular "on demand" pacing mode is vvi or with automatic rate adjustment for exercise vvir – this mode is suitable when no synchronization with the atrial beat is required, as in atrial fibrillation. the equivalent atrial pacing mode is aai or aair which is the mode of choice when atrioventricular conduction is intact but the sinoatrial node of the natural pacemaker is unreliable – sinus node disease (snd) or sick sinus syndrome. where the problem is atrioventricular block (avb) the pacemaker is required to detect (sense) the atrial beat and after a normal delay (0.1–0.2 seconds) trigger a ventricular beat, unless it has already happened – this is vdd mode and can be achieved with a single pacing lead with electrodes in the right atrium (to sense) and ventricle (to sense and pace). these modes aair and vdd are unusual in the us but widely used in latin america and europe. the dddr mode is most commonly used as it covers all the options though the pacemakers require separate atrial and ventricular leads and are more complex, requiring careful programming of their functions for optimal results. automatic pacemakers are designed to be over-ridden by the heart's natural rate at any moment that it gets back to a non-pathologic normal sinus rhythm and can reinitiate influencing the electric activity in the heart when the pathologic event happens again. a "ventricular-demand pacemaker" produces a narrow vertical spike on the ecg, just before a wide qrs. the spike of an "atrial-demand pacemaker" appears just before the p wave. comparably, a triggered pacemaker is activated immediately after an electrical activity is commenced in the heart tissue by itself. a "ventricular triggered pacemaker" produces the impulse just after a pulse is created in the ventricular tissue and it appears as a simultaneous spike with qrs. an "atrial triggered pacemaker" is the mode in which an impulse is produced immediately after an electrical event in the atrium. it appears as a discharge following the p wave but prior to the qrs which is commonly widened. biventricular pacing three leads can be seen in this example of a cardiac resynchronization device: a right atrial lead (solid black arrow), a right ventricular lead (dashed black arrow), and a coronary sinus lead (red arrow). the coronary sinus lead wraps around the outside of the left ventricle, enabling pacing of the left ventricle. note that the right ventricular lead in this case has two thickened aspects that represent conduction coils and that the generator is larger than typical pacemaker generators, demonstrating that this device is both a pacemaker and a cardioverter-defibrillator, capable of delivering electrical shocks for dangerously fast abnormal ventricular rhythms. main article: cardiac resynchronization therapy cardiac resynchronization therapy (crt) is used for people with heart failure in whom the left and right ventricles do not contract simultaneously (ventricular dyssynchrony), which occurs in approximately 25–50% of heart failure patients. to achieve crt, a biventricular pacemaker (bvp) is used, which can pace both the septal and lateral walls of the left ventricle. by pacing both sides of the left ventricle, the pacemaker can resynchronize the ventricular contractions. crt devices have at least two leads, one passing through the vena cava and the right atrium into the right ventricle to stimulate the septum, and another passing through the vena cava and the right atrium and inserted through the coronary sinus to pace the epicardial wall of the left ventricle. often, for patients in normal sinus rhythm, there is also a lead in