Modes of cardiac pacing: Nomenclature, Selection and Indications for permanent cardiac pacing

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General description of procedure, equipment, technique

The artificial pacemaker is a medical device that is surgically implanted, most commonly in the subcutaneous tissues overlying the prepectoral fascia in the upper chest. The pacemaker uses electrical impulses to stimulate myocardial contraction. The vast majority of pacemakers (>98%) are implanted due to a patient’s inability to maintain an adequate heart rate secondary to either symptomatic bradycardia or bradycardia due to block somewhere within the intrinsic electrical conducting system (sinoatrial node, atrioventricular junction, His-Purkinje system).

The pacemaker system is composed of a pulse generator and one or more leads that connect the generator to the heart. The primary components of the generator are a lithium-iodine battery (about 25% of the total volume), the hybrid circuit (consisting of a microprocessor and resistors, etc.), and a small computer controlling device, all of which are contained in a hermetically sealed titanium casing. There is also an outer, clear epoxy header into which the leads are placed and secured.

Communication with the pacemaker generator is through a specialized computer called a programmer. Typically, a wand is placed over the patient’s chest and bidirectional communication is enabled using a radiofrequency link. The programmer can be used to alter a large number of pacemaker settings and also download diagnostic data from the generator. Some newer pacemaker generators can send data wirelessly to a receiver across a room in the patient’s home; these data are then transmitted to a physician’s office via a phone line and ultimately the Internet. Each pacemaker manufacturer makes available programmers that function only with devices manufactured by that company.

Pacemaker batteries are designed to have predictable depletion over time, which can be monitored by their cell voltage and cell impedance. The life span of a particular generator in a particular patient is largely dependent on percent pacing, programmed voltage and pulse width, and electrical pacing impedances.

Typically, batteries last 5 to 10 years. When the generator has about 3 months of functional battery life remaining, interrogation with a programmer will signal an alert (“elective replacement indicator” or ERI) that a generator change should be performed soon. Pacemakers should be monitored with programmers or wireless communication every 3 to 6 months.

The other components of the pacemaker system are the leads. Leads are thin, flexible, insulated wires that conduct electrical impulses from the pacemaker generator to the heart and also relay electrical signals from the heart back to the generator. The primary portions of a lead are the lead body, containing one or two conductors encircled by insulation; the distal end, which contains one or two exposed electrodes in proximity to the myocardium, as well as a fixation with tines; and a connector pin at the proximal end of the lead, which is plugged into the header of the generator. The vast majority of pacemaker leads are inserted transvenously from the generator to the endocardium (“transvenous leads”). Less commonly, leads are attached directly to the epicardial surface of the heart with either a helical screw or a suture-on-plaque electrode via a thoracotomy (“epicardial leads”).

The two basic functions of the pacemaker system are pacing and sensing. Pacing refers to depolarization of the atria or ventricles, resulting from an impulse (typically 0.5 msec and 2 to 5 volts) delivered from the generator down a lead to the heart. Sensing refers to detection by the generator of intrinsic atrial or ventricular depolarization signals that are conducted up a lead. Sensed events are used by the pacemaker logic to appropriately time the pacing impulses.

Malfunction of pacemaker generators and pacemaker leads may occur. Failure is much more common in leads, the weakest link in the system. Pacemaker manufacturers publish at least twice-yearly product performance reports, which closely track the performance of their products. Occasional FDA recalls are issued for products with excessive failure rates or failures that pose a particular risk to the patient. It is important to note that an FDA recall does not necessarily, and typically does not, require explantation of the affected product.

Indications and patient selection

The American College of Cardiology, American Heart Association, and the Heart Rhythm Society have jointly published guidelines for the indications for pacemaker implantation that are widely accepted in the medical community. The following tables are structure on these guidelines. They are classified by the standard Class I, Class IIa, Class IIb, and Class III categories. It should be noted, however, that CMS has issued a separate National Coverage Decision (NCD), which diverges in many instances from the ACC/AHA/HRS guidelines. For each ACC/AHA/HRS recommendation in the following tables, the final column indicates whether that particular indication would be covered by the NCD.

The following tables do not include one category of indications, “Cardiac Resynchronization Therapy in Patients With Severe Systolic Heart Failure,” included in the ACC/AHA/HRS recommendations, as this subject is addressed in another chapter of Decision Support in Medicine. The ACC/AHA/HRS guidelines also contain guidelines for implantation of ICDs, which is also addressed in another chapter of Decision Support in Medicine.

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Figure 1.n

Modes and Codes

In order to standardize and facilitate the use and understanding of pacemakers, the North American Society for Pacing and Electrophysiology (NASPE) and British Pacing and Electrophysiology Group (NASPE/BPEG) groups devised the standard pacemaker codes, the NBG coding system. The most recent iteration of the NBG code was revised in 2002. The NBG code has five positions that denote pacemaker function; however, the last position is rarely used and will not be included in this discussion.

In the context of the NBG code, “sensing” refers to the detection, by the pulse generator, of spontaneous cardiac depolarizations. The effect of a “sensed” event is either a “triggered” pacing stimulus or an “inhibited” pacing stimulus.

(Figure 11)

Position I refers to the chambers that are paced. “A” refers to atrium, “V” to ventricle, and “D” or “dual” to both the atrium and the ventricle.

Position II refers to the chambers that are sensed. The letter codes are the same as above. In addition, the letter “O” refers to a mode in which there is no sensing, in other words, asynchronous pacing.

Position III refers to the pulse generator’s response to a sensed event. The letter “I” indicates that the pulse generator will inhibit a pacing stimulus in response to a sensed event. The letter “T” indicates that the pulse generator will trigger a pacing stimulus in response to a sensed event. The letter “D” is restricted to dual chamber systems and will both inhibit and trigger in response to sensed events. For example, a sensed event in the atrium will inhibit a pacing stimulus in the atrial channel, but triggers ventricular output. After the programmed AV delay, if the pulse generator senses a ventricular event, output will be inhibited in the ventricular channel. If the pulse generator does not sense a ventricular event, then a pacing stimulus will be triggered. The letter “O” indicates that there is no response to a sensed event and is most commonly used in the asynchronous pacing mode, as discussed below.

Position IV is unique in that it refers to the presence or absence of rate modulation or rate-adaptive pacing. If present, the letter “R” indicates that that the pulse generator incorporates a sensor, which uses a variable—such as mechanical vibration, minute ventilation, or acceleration—to adjust the programmed paced heart rate in response to a patient’s activity. If rate adaptive pacing is not used or is not available, rather than using the letter “O”, “R” is simply omitted.

Numerous factors must be considered when programming the pacing mode, such as the patient’s age, exercise capacity, chronotropic response, medical comorbidities, and the intrinsic cardiac rhythm. To detail each pacing mode would be beyond the scope of this article; however, the examples of more commonly used modes are detailed below.

VVI or VVIR: VVI(R) is one of the more commonly used pacing modes. VVI(R) is ventricular demand pacing. The ventricle is paced, sensed, and the pulse generator inhibits pacing output in response to a sensed ventricular event. This mode of pacing prevents ventricular bradycardia and is primarily indicated in patients with atrial fibrillation with a slow ventricular response. However, since the pulse generator only paces and senses in the ventricle, there is loss of AV synchrony, which can potentially lead to pacemaker syndrome. This mode of pacing is available in every pacemaker system that has a ventricular lead.

AAI or AAI(R): AAI(R) is atrial demand pacing. The atrium is paced, sensed, and the pulse generator inhibits pacing output in response to a sensed atrial event. This mode is used for patients purely with sinus node dysfunction, yet maintain AV nodal function. This mode is being used in isolation uncommonly as the subsequent development of AV nodal conduction disease would render the patient vulnerable to bradycardia. This mode of pacing can be used in patients with dual chamber systems to minimize ventricular pacing along with an algorithm that can switch between AAI and DDD mode depending on sensed AV nodal conduction.

DDD or DDD(R): DDD or DDD(R) is a dual chamber system. It possesses pacing and sensing capabilities in both the atrium and the ventricle, and it is the most commonly used pacing mode. This mode is most appropriate for patients with combined sinus node dysfunction and AV nodal dysfunction. It is also appropriate for patients with sinus node dysfunction and normal AV node conduction, normal sinus node function with AV nodal conduction abnormalities, and carotid hypersensitivity with symptomatic cardio-inhibitory response.

There are four different rhythms that can be observed in DDD(R) pacing mode:

• Normal sinus rhythm (NSR) with no pacing (A sense, V sense).
• Atrial pacing with ventricular sensing and a native QRS (A pace, V sense).
• Atrial sensing with a native P wave and ventricular pacing (A sense, V pace).
• Atrial pacing and ventricular pacing (A pace, V pace).

The use of DDD(R) pacing mode in conjunction with an algorithm that minimizes ventricular pacing is preferred.

Asynchronous modes, VOO or DOO: These are asynchronous pacing modes in which the pulse generator delivers a pacing stimulus at a fixed rate, without any sensing capabilities. Therefore, the pacemaker is not “in sync” with the patient’s native rhythm and continues to deliver a pacing stimulus regardless of what the native conduction is doing.

These modes are rarely used for extended periods of time. They are typically used when a pacemaker dependent patient is undergoing a surgical procedure that uses electrocautery that could be sensed by the pacemaker as native electrical conduction, which would inhibit pacemaker output and subsequently the patient could have profound bradycardia or even asystole. There is a small possibility that pacing in an asynchronous mode could induce a pacing stimulus in the vulnerable period (on the T wave), which could potentially induce a lethal ventricular tachyarrhythmia.

Contraindications

While there are no absolute contraindications to pacemaker implantation, there are several factors that must be considered prior to implantation and which may alter the timing or approach to implant. These include vascular access difficulties, concurrent infection, bleeding diatheses, or need for future MRI. If access cannot be obtained percutaneously due to vascular thrombosis or occlusion, a surgical epicardial approach may be considered or rarely venous angioplasty.

Pacemaker implantation is generally not performed when the patient has a concurrent infection, and is typically deferred until the active infection is appropriately treated. In some instances, active bleeding diatheses will preclude implantation until the bleeding is controlled.

Pacemakers, however, are often implanted in patients with therapeutic international normalized ratio (INR) and who are concurrently taking one or more antiplatelet medications. Thoughtful consideration should also be given to the patient who will require multiple MRIs in the future. MRIs are considered contraindicated in patients with implanted pacemakers, although there is reported experience that in selected cases off-label MRIs can be performed with reasonable safety. The FDA has recently approved a pacemaker generator and lead model with which the safety of some types of MRI studies have been formally documented, and on-label MRIs can be performed in patients whose pacing system is comprised of these components.

Details of how the procedure is performed

Procedures to implant transvenous pacemakers are considered low-risk. In adults, implantation is nearly always performed with intravenous conscious sedation and local anesthesia in an electrophysiology (EP) laboratory or an operating room. General anesthesia is often used in children and others thought to be poor candidates for conscious sedation.

In the vast majority of instances, a pacemaker can be implanted in the upper pectoral region via a transvenous approach. The patient is administered conscious sedation and perioperative antibiotics by the anesthesiologist or the nursing staff, who also monitor the patient’s heart rate, blood pressure, oxygen saturation, and general condition during the operation.

Subcutaneous lidocaine is administered and a 4 to 5 cm infraclavicular incision is created. Electrocautery dissection is carried down to the level of the prepectoral fascia and a pocket large enough to accommodate the pacemaker is also created.

Venous access is obtained either with subclavian vein puncture, axillary vein puncture, or cephalic vein cutdown. Guided by fluoroscopy, leads are then advanced to the appropriate chambers and secured to the endocardium. Pacing impedance, sensing, and capture thresholds are measured; if not acceptable then repositioning of the lead is required.

Once a lead is placed, it is sutured to the prepectoral fascia near the venous entry site using a suture sleeve for stability. Each lead is then plugged into the appropriate port of the generator’s header and secured with set screws. Typically, the pocket is then irrigated using an antibiotic solution. The pocket is finally closed using two or three layers of suture.

The incision is then covered with an occlusive dressing. Device parameters are programmed at the end of the procedure prior to the patient departing from the lab. Following the procedure, a chest x-ray should be obtained to assess for lead position and any perioperative complications such as pneumothorax. The patient is typically observed in-hospital overnight. Device function is rechecked the morning after the procedure to ensure appropriate sensing, capture thresholds, and lead impedances.

Outcomes (applies only to therapeutic procedures)

In the vast majority of patients, pacemakers are implanted because of symptomatic bradycardia. Implantation of a pacemaker is very effective in alleviating the symptoms of bradycardia. Failure to respond to pacing therapy should raise doubt about the original diagnosis that symptoms were in fact secondary to bradycardia.

Alternative and/or additional procedures to consider

There are few alternatives to consider when it comes to pacing therapy. If pharmacologic therapy is a suspected cause of symptomatic bradycardia, consideration can be made to eliminate the medication unless it is thought to be essential for the patient. In the setting of tachy-brady syndrome ,where bradycardia is secondary to an antiarrhythmic drug, ablation could certainly be considered as an alternative to the antiarrhythmic drug, avoiding the need for pacemaker implantation. In patients with symptomatic bradycardia in the setting of neurocardiogenic syncope or carotid sinus hypersensitivity, lifestyle modification and avoidance of triggers may be successful strategies and avoid the need for pacemaker implantation.

Complications and their management

Procedures to implant transvenous pacemakers are considered low-risk. In adults implantation is nearly always performed with intravenous conscious sedation and local anesthesia. General anesthesia is often used in children and others thought to be poor candidates for conscious sedation.

Acute complications occur in approximately 2% to 4% of pacemaker implantations. Complications rates are generally higher in more elderly patients. The risk of devastating complication such as death, myocardial infarction, or stroke is extremely low. Roughly 85% of the acute complications of pacemaker implantation can be attributed to lead dislodgement, perforation, pneumothorax, pocket hematoma, or infection. With the exception of infection, these acute complications are generally recognized within 24 hours of implantation.

Lead dislodgement is classified as either macrodislodgement (observed radiographically) or microdislodgement (not observed radiographically). Microdislodgement is therefore detected by device failure to sense or failure to capture. In both scenarios, re-operation is typically required.

Perforation of a vein, atrium or ventricle may also occur. Perforation is most commonly encountered when manipulating a lead or fixing the screw of an active fixation lead. It is suspected based on radiographic appearance, when there is failure of the lead to pace or sense, or if there is stimulation of the diaphragm or other noncardiac muscle. Occasionally, pericardial effusion and cardiac tamponade ensue, requiring prompt diagnosis and percutaneous pericardiocentesis.

Pneumothorax, resulting from inadvertent needle puncture of the pleura during placement of the leads, complicates approximately 2% of pacemaker implant procedures. Risk for pneumothorax can be minimized by obtaining extrathoracic venous access via cephalic vein cutdown or needle puncture of the lateral axillary vein. However, because of operator familiarity and ease of use, blind subclavian venipuncture or venography-guided subclavian venipuncture is frequently used. A pneumothorax can often be managed conservatively with observation and follow-up chest x-ray or invasively with a chest tube if required.

Hematoma can occur due to bleeding from inside the pocket or from back-bleeding around the venous entry site. A hematoma is more common in patients who are taking antiplatelet or anticoagulation medications. In general, however, most pacemaker implantations can be safely performed on patients who have a therapeutic INR. The hematoma is typically managed conservatively with manual compression, pressure dressings, and reversal of anticoagulation if indicated. Aspiration is generally not advised. Open evacuation of the hematoma is considered if there is continued bleeding, hematoma expansion, compromise of the suture line or skin integrity, or if it should become tense or painful. The greatest consequence of hematoma is predisposition to pocket infection.

Infection poses a low but worrisome risk. The risk is variously reported to be 2% to 8%; however, in our experience the risk is approximately 1%. Infection may present with only superficial skin involvement and respond well to antimicrobial therapy. More commonly, however, it will present as early or delayed pocket infection, endocarditis, or septicemia. When more than superficial infection is present, complete device explantation with lead extraction is generally required.

Extraction is followed by any necessary pocket debridement and appropriate antimicrobial therapy. Infection is more common when there is preexisting hardware in the pocket (e.g., during routing generator changes) and when hematoma complicates the procedure. Perioperative antibiotics have been shown to decrease risk of infection and should be used routinely.

Less common acute complications include air embolism, venous thromboembolism, subclavian arterial puncture or laceration, hemothorax, wound dehiscence, device migration, and skin erosion. Device-related complications, such as conductor fracture, lead insulation failure, lead failure, generator-lead connection problems, and premature battery failure or other generator malfunction. Patients should be made aware of the risk of complications from the procedure and the implanted device at the time of the original implant.

What’s the evidence?

Epstein, AE. “ACC/AHA/HRS 2008 Guidelines for device-based therapy of cardiac rhythm abnormalities”. JACC. vol. 51. 2008. pp. e1-62. (The guidelines are the standard reference for appropriateness of pacemaker implantation.)

“National Coverage Determination (NCD) for Cardiac Pacemakers”. (The NCD is the Medicare-based decision regarding appropriate indications for pacemaker implantation.)

Ellenbogen, K. Clinical Cardiac Pacing, Defibrillation, and Resynchronization Therapy. 2007. (This is a standard textbook for cardiac pacemakers, ICDs, and CRTs.)

Al-Amad, A, Ellenbogen, K. Pacemakers and Implantable Cardiac Defibrillators, An Expert’s Manual. 2010. (This is a standard textbook for cardiac pacemakers, ICDs, and CRTs.)

Pavia, S, Wilkoff, B. “The management of surgical complications of pacemakers and implantable cardioverter-defibrillators”. Curr Opin Cardiol. vol. 16. 2001. pp. 66-71. (This is a peer reviewed article describing surgical complications of pacemaker surgeries.)

Ramza, BM. ” Safety and effectiveness of placement of pacemaker and defibrillator leads in the axillary vein guided by contrast venography”. Am J Cardiol. vol. 80. 1997. pp. 892-896. (This is a peer reviewed article describing an alternative pacemaker implantation surgical technique.)

Hayes, D. “Modes of cardiac pacing: Nomenclature and selection”. Available at . (This is a peer reviewed online educational and reference tool.)

Saman Nazarian, MD. “Clinical utility and safety of a protocol for noncardiac and cardiac magnetic resonance imaging of patients with permanent pacemakers and implantable-cardioverter defibrillators at 1.5 tesla”. Circulation. vol. 114. 2006. pp. 1277-1284. (This is a peer reviewed article describing the safety of patients with pacemakers undergoing MRIs, which is often contraindicated.)

Bernstein, AD, Daubert, JC. ” The revised NASPE/BPEG generic code for antibradycardia, adaptive-rate, and multisite pacing”. Pacing Clin Electrophysiol. vol. 25. 2002. pp. 260(This is a peer reviewed article describing the basic codes and nomenclature for pacemakers.)

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