Pacemaker
A pacemaker, also known as an artificial cardiac pacemaker, is an implanted medical device that generates electrical pulses delivered by electrodes to one or more of the chambers of the heart. Each pulse causes the targeted chamber to contract and pump blood, thus regulating the function of the electrical conduction system of the heart.
The primary purpose of a pacemaker is to maintain an even heart rate, either because the heart's natural cardiac pacemaker provides an inadequate or irregular heartbeat, or because there is a block in the heart's electrical conduction system. Modern pacemakers are externally programmable and allow a cardiologist 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 to improve their synchronization.
Methods of cardiac pacing
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. This is an old procedure used only as a life-saving means until an electrical pacemaker is brought to the patient.Transcutaneous pacing
Transcutaneous pacing, 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 until electrical capture 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 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 to maintain satisfactory cardiac output until a temporary transvenous electrode has been inserted.Permanent epicardial pacing leads can be implanted surgically and tunneled to the pulse generator pocket. These leads are either passively touching the heart and sewn in place, or have a screw mechanism to actively fix to the heart.
Transvenous pacing (temporary)
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.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 under the skin below 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.
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 as small as a capsule and 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
Pacing modes
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.
| I | II | III | IV | V |
| Chamber paced | Chamber sensed | Response to sensing | Rate modulation | Multisite pacing |
| O = None | O = None | O = None | O = None | O = None |
| A = Atrium | A = Atrium | T = Triggered | R = Rate modulation | A = Atrium |
| V = Ventricle | V = Ventricle | I = Inhibited | V = Ventricle | |
| D = Dual | D = Dual | D = Dual | D = Dual |
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 or sick sinus syndrome. Where the problem is atrioventricular block the pacemaker is required to detect the atrial beat and after a normal delay 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 and ventricle. 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.