A pacemaker is an artificial pulse generator that delivers an electrical current to the heart muscle when the heart fails to deliver this impulse on its own. Pacemakers may be permanent (implanted) or temporary (external).
Pacing is used to initiate myocardial contractions when the heart’s intrinsic stimulation is:
- the native impulses are not being conducted,
- the heart rate is too slow to maintain an adequate cardiac output.
As per ACLS guidelines, when available, pacing is a first line, and the most likely beneficial, treatment for bradycardiac arrhythmias.
Key Pacing Terms and Concepts
Asynchronous (fixed-rate): a pacing mode that paces the heart at a programmed rate regardless of intrinsic cardiac activity. This may result in competition between the pacemaker and the heart’s own electrical activity. Asynchronous pacing of the ventricle is dangerous. If the pacing stimulus occurred during the vulnerable period of repolarization, ventricular fibrillation may result. Asynchronous pacing of the right ventricle is less dangerous but may cause atrial fibrillation.
Atrial pacing: Pacing from a lead attached to the right atrium; designed to correct abnormalities in the SA node.
Atrial tracking: A state of pacing in which sensed atrial activity triggers a ventricular pacing output at the end of the programmed atrioventricular interval. In other words, when the pacemaker senses an intrinsic P wave it will start counting (tracking) the AV interval. If the set time passes without the ventricle depolarizing, the pacemaker will deliver the electrical stimuli to the ventricles.
AV interval: In dual-chamber pacing, the length of time between an atrial sensed/paced event and the delivery of a ventricular paced event; equivalent to the PR interval of an intrinsic rhythm. If no intrinsic QRS occurs within the AV interval the pacemaker is triggered to pace the ventricle. However, if an intrinsic QRS wave occurs within the AV interval the pacemaker will be inhibited from firing.
AV (sequential) pacing: Occurs when the pacemaker stimulates the atrium and then the ventricle, mimicking normal cardiac physiology.
Base Rate: Rate at which the pulse generator is set to pace when no intrinsic activity is detected. Determines the minimum rate at which the pacemaker paces in the absence of intrinsic activity.
Bipolar lead: Pacing lead with two electrical poles that are external from the pulse generator; bipolar pacing is used in transvenous or permanent pacing systems.
Capture: This is both an electrical and a mechanical event. A pacing spike followed by a corresponding P wave/QRS complex indicates electrical capture.
Cardiac Resynchronization Therapy (CRT): Also called a biventricular pacemaker, or dual chamber pacemaker, CRT is used to treat the delay in ventricle contractions that occur in some people with advanced heart failure. An implanted pacemaker sends electrical signals to the chambers on both sides of the heart, making sure they contract at the same time. The device has 2 or 3 leads which are implanted through a vein in the right atrium and right ventricle and into the coronary sinus vein to pace the left ventricle. CRT defibrillators (CRT-D) incorporates defibrillating therapy to quickly terminate life-threatening dysrhythmias.
Demand Pacing: A pacing stimulus is delivered to the myocardium if the intrinsic rate falls below the base rate on the pacemaker.
Epicardial Pacing: Type of temporary pacing where epicardial pacing wires are fixed directly to the myocardium (ventricular and often atrial) and are exposed through the skin on the chest wall.
Fixed Rate Pacing: A pacing stimulus is delivered to the myocardium at a programmed fixed rate regardless of the underlying rate and rhythm. This is also known as asynchronous ventricular pacing.
Fusion beat: the ECG waveform that results when an intrinsic depolarization and a pacing stimulus occur at the same time, both contributing to depolarization of the chamber.
P = paced F = fusion
Inhibited: When an intrinsic cardiac event is sensed, the pacemaker suppresses electrical output and resets the timing cycle of the pacemaker.
Interval (automatic, demand or paced): The period, measured in milliseconds, between two consecutive events in the same chamber without an intervening sensed event (A-A or V-V); in pacing, intervals are more useful than rate because pacemaker timing is based on intervals.
Intrinsic: Naturally occurring, native.
Microshocks: Low voltage electrical current from ungrounded equipment or static electricity that may pass through to the patient, with as little as 0.1 mA causing ventricular fibrillation.
Milliampere (mA): Unit of measure of the pulse generator's electrical current output.
Output: The energy from pulse generator supplied to the heart muscle sufficient to stimulate a contraction. It is determined by three components:
- Rate: the setting that provides a pacing rate (bpm) to the myocardium.
- Amount: level of energy delivered from the pulse generator to the heart to initiate depolarization and is measured in mA.
- Chamber: the atria, the ventricles, or both chambers can be paced. If both the atria and ventricles are paced, a separate output setting is required for each chamber.
Pacemaker spike: Vertical line on the ECG that indicates the pacemaker has discharged. Note: a pacing spike does not indicate perfusion, merely the delivery of eletrical current.
Pacing Wires: Atrial pacing wires are sutured to the right atrial appendage, or the body, of the right atrium. Ventricle pacing wires are placed on the anterior or diaphragmatic surface of the right ventricle.
Polarity: Having two oppositely charged poles, one positive and one negative. Polarity determines the direction in which current tends to flow. Unlike other temporary pacing leads, epicardial pacing wires are unipolar - having no designation of positive or negative because polarity is assigned once the lead is attached to the pacemaker generator. The negative electrode is in contact with the myocardium with the pulse generator acting as the positive pole.
Phrenic Nerve Stimulation: The proximity of the right and left branches of the phrenic nerves to the epicardium makes it possible for impulses from pacing leads to produce inadvertent phrenic nerve stimulation. The rate of diaphragmatic stimulation is typically much greater than that of normal respiration, contributing to symptoms including dyspnea, uncomfortable muscle twitching, hiccups, and general malaise.
Pulse generator: The power source that houses the battery and controls for regulating a pacemaker.
Sensing: Ability of a pacemaker to recognize and respond to an intrinsic electrical activity; the pacemaker’s response to sensed activity depends on its programmed mode and parameters. Occurs only in synchronous mode.
Stimulation Threshold: The minimum energy (output) from the pacemaker required to maintain consistent capture.
Synchronous (demand): Paces only when the heart fails to depolarize on its own; the pacemaker fires on demand only. With synchronous pacing, the pacemaker’s sensing circuit is capable of sensing intrinsic cardiac activity and then inhibiting pacer output when intrinsic activity is present.
Temporary Epicardial Pacing: Method of stimulating through the use of Teflon-coated unipolar, stainless steel wires that are inserted loosely to the epicardium after cardiac surgery. The epicardial wires may be attached to the right atrium for atrial pacing, the right ventricle for ventricular pacing or both for atrioventricular (AV) pacing. Temporary epicardial pacing may be especially helpful after valvular surgery where the incidence of heart block or arrhythmia is increased.
Transvenous Pacing: Type of temporary pacing where pacing wires are inserted into the veins via an introducer sheath and passed through the venous system to the heart.
Transthoracic Pacing: Technique of electrically stimulating the heart by use of a set of pads placed externally on the torso. ECG electrodes are placed on the patient to sense ventricular events (spontaneous or paced), and the pulse generator delivers a wave pulse when a predetermined escape interval has elapsed.
Ventricular pacing: Pacing system with a lead attached to the right ventricle.
Indications for Pacing
Pacing can sustain a heart rate to provide sufficient filling and emptying of the heart’s chambers and maintaining cardiac output when abnormalities occur in the conduction system.The following cardiac rhythms may require temporary pacing IF hemodynamically unstable:
- Asystole of greater than 3 seconds
- Sinus Arrest/Sinus Block
- Sinus bradycardia
- Second degree AV block, Type II
- Third degree AV block
- Junctional bradycardia
Physicians may use the pacemaker to overdrive tachyarrhythmias (supraventricular or ventricular) refractory to pharmacological therapy in an attempt to restore normal conduction.
Please note, however, this is not the purpose of pacing on our unit and is not a nursing responsibility.