This is an excerpt from ACSM's Exercise Management for Persons With Chronic Diseases and Disabilities-4th Edition.
A variety of factors contribute to optimal cardiac functioning, including atrioventricular (AV) synchronization and the chronotropic and inotropic responses to neurohormonal stimuli. Alterations in the normal sequence of atrial and ventricular filling and contraction can result in deterioration of hemodynamics and subsequent symptoms at rest, during exercise, or both. In persons who have light-headedness, syncopal spells, shortness of breath, and more rarely chest pain or other cardiovascular symptoms owing to these problems, a permanent pacemaker
- improves symptoms,
- enhances exercise performance, and
- improves quality of life.
According to guidelines developed jointly by the American College of Cardiology (ACC), the American Heart Association (AHA), and the Heart Rhythm Society (HRS), class I indications for a permanent pacemaker include the following:
- Sinus node dysfunction
- Third-degree block and advanced second-degree AV block
- Hypersensitive carotid sinus syndrome
- Symptomatic bradycardia
- Sustained pause-dependent ventricular tachycardia
- Left ventricular systolic dysfunction and New York Heart Association (NYHA) functional class III or ambulatory class IV
A typical pacemaker system consists of two basic components: a pulse generator and either one or two pacing wires. In a traditional pacemaker the pacing wires are insulated and are implanted transvenously into the right atrium, right ventricle, or both. With a biventricular pacemaker, a lead is also placed in the left ventricle. The leads are connected to the pulse generator, which is typically implanted subcutaneously just below the clavicle. The two main functions of the leads are sensing and pacing. Sensing involves detecting electrical signals (i.e., P-waves and R-waves) from the heart. When these signals are not sensed at the proper timing, the pacemaker generator fires an impulse that causes the atria or ventricles (or both) to contract. Optimally, the pacing system uses an atrial and ventricular lead to maintain AV synchrony, which serves to optimize cardiac output at rest and during exercise.
Pacemakers are described by a standardized code. The first letter represents the chamber paced; the second is the chamber sensed; and the third denotes the response to a sensed event. The fourth position is used to indicate that the pacemaker has rate-response capabilities. For example, VVIR is the abbreviation used when the ventricle (V) is the chamber being paced and sensed. When the pacemaker senses a normal ventricular contraction, the pacemaker is inhibited (I). The "R" indicates that the pulse generator is rate-responsive during exercise. The response by the pacemaker is to either trigger or inhibit a pacing stimulus, depending on the absence or presence, respectively, of atrial or ventricular conduction, separately or in combination, relative to the range of heart rates that are programmed into the pacer.
A commonly used mode of pacing is the DDDR, which has dual-chamber (i.e., atrium and ventricle) pacing and inhibiting and has rate-response capability. The DDDR pacemaker is widely regarded as the optimal pacing mode in individuals who have normal sinoatrial (SA) node function, because it provides AV synchrony and uses the client's own sinus rhythm to guide ventricular stimulation. This results in a heart rate and cardiac output that, for the rest of the circulatory system, is very nearly normal.
An implantable cardioverter-defibrillator (ICD) is another electronic device that can be permanently implanted in individuals who either have a history of, or are at increased risk for, a life-threatening ventricular dysrhythmia. These devices impressively reduce mortality in persons with cardiomyopathy. Cardioverter-defibrillators can be just an ICD or can be a model that functions as both an ICD and a permanent pacemaker; like pacemakers, they are usually implanted subcutaneously just below the clavicle.
Implantable cardioverter-defibrillators electrically terminate life-threatening ventricular tachyarrhythmias. They consist of two basic parts: the lead system and the ICD itself. Implantable cardioverter-defibrillators have lead systems that are placed transvenously, typically by way of the subclavian vein. The ICD leads pick up the electrical rhythm of the heart and transmit this to the pulse generator, which senses the rhythm. Implantable cardioverter-defibrillators can detect atrial and ventricular arrhythmias, can provide antitachycardia pacing and defibrillation, and can be programmed with multiple protocols and the ability to record an electrocardiogram. Ventricular tachycardia (VT) and ventricular fibrillation (VF) are recognized by their rapid rates. If either VT or VF is sensed, the pulse generator delivers defibrillation or a synchronized cardioversion to terminate the rhythm. Other accelerated rhythms will initiate a pacing therapy intended to restore the rate within the preprogrammed limits.
According to guidelines developed jointly by the ACC, AHA, and HRS, class I indications for an ICD include the following:
- Survivor of cardiac arrest due to ventricular fibrillation (VF) or sustained ventricular tachycardia (VT)
- Structural heart disease with VT
- History of syncope of undetermined origin with clinically relevant VF or sustained VT induced during an electrophysiology study
- Left ventricular dysfunction due to myocardial infarction (post ≥40 days) with an ejection fraction (EF) ≤35% and NYHA class II or III or an EF ≤30% and NYHA class I
- Nonischemic dilated cardiomyopathy with an EF ≤35% and NYHA class II or III
- Nonsustained VT due to myocardial infarction with an EF ≤40% and VF or sustained VT induced during electrophysiology study
Combination Pacemaker - Defibrillator Devices
Some implantable devices are capable of providing both pacing and defibrillation. Recent evolution in terminology of pacemakers and defibrillators is toward calling them cardiac resynchronization therapy (CRT), with subclasses that provide pacing alone (CRT-P) and those that provide both pacing and defibrillation (CRT-D).