Cardiac Pacing and Defibrillation

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Chapter 19 Cardiac Pacing and Defibrillation

PACEMAKERS

Battery-operated, implantable pacing devices were first introduced in 1958, just 4 years after the invention of the transistor. The complexity, calculation, and data storage abilities of these devices have grown in a manner similar to that seen within the computer industry. The natural progression of pacemaker developments led to the invention of the implanted cardioverter-defibrillator (ICD) around 1980. As this technology has advanced, the divisions between these devices have become less clear. For example, every ICD currently implanted has anti-bradycardia pacing capability, and patients, news media, and even physicians often misidentify an implanted defibrillator as a pacemaker. The consequence of mistaking an ICD for a conventional pacemaker can lead to patient harm, either due to electromagnetic interference (EMI) issues resulting in inappropriate ICD therapy or the unintentional disabling of ICD therapies in some ICDs that can be permanently disabled by magnet placement. Figure 19-1 shows a three-lead defibrillation system and identifies the right ventricular shock coil, which differentiates an ICD system from a conventional pacemaking system. The complexity of cardiac pulse generators, as well as the multitude of programmable parameters, limits the number of sweeping generalizations that can be made about the perioperative care of the patient with an implanted pulse generator. Population aging, continued enhancements in implantable technology, and new indications for implantation will lead to growing numbers of patients with these devices. Both the American College of Cardiology (ACC) and the North American Society for Pacing and Electrophysiology-The Heart Rhythm Society (HRS-NASPE)* have taken note of these issues, and guidelines have been published regarding the care of the perioperative patient with such a device.1 The American Society of Anesthesiology (ASA) has issued a practice advisory.2

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Figure 19-1 A defibrillator system with biventricular (BiV) anti-bradycardia pacemaker capability. Note that three leads are placed: a conventional, bipolar lead to the right atrium, a tripolar lead to the right ventricle (RV), and a unipolar lead to the coronary sinus (CS). This system is designed to provide “resynchronization biventricular-pacing therapy” in the setting of a dilated cardiomyopathy with a prolonged QRS (and frequently with a prolonged PR interval as well). The bipolar lead in the right atrium performs both sensing and pacing function. In the RV, the tip electrode functions as the cathode for pacing and sensing functions. The presence of a “shock” conductor (termed shock coil) on the RV lead in the RV distinguishes a defibrillation system from a conventional pacemaking system. In this particular patient, the RV shock coil also functions as the pacing and sensing anode (this is called an integrated bipolar defibrillator lead; true bipolar leads have a ring electrode between the tip electrode and the shock coil). The lead in the CS depolarizes the left ventricle, and the typical current pathway includes the anode in the right ventricle. Because of the typically wide QRS complex in a left bundle-branch pattern, failure to capture the left ventricle can lead to ventricular oversensing (and inappropriate anti-tachycardia therapy) in an implanted cardioverter-defibrillator (ICD) system. Many defibrillation systems also have a shock coil in the superior vena cava, which is electrically identical to the defibrillator case (called the “can”). When the defibrillation circuit includes the ICD case, it is called “active can configuration.” Incidental findings on this chest radiograph include the presence of sternal wires from prior sternotomy and the lung carcinoma seen in the right upper lobe.

Pacemaker Overview

Pacemaker manufacturers (more than 26 companies) have produced over 2000 models to date. More than 220,000 adults and children in the United States undergo new pacemaker placement each year, and nearly 3 million patients have pacemakers today. Many factors can lead to confusion regarding the behavior of a device and the perioperative care of a patient with a device.3 An understanding of pulse generators and their likely idiosyncrasies in the operating or procedure room is needed. Whether the patient with a pacemaker is at increased perioperative risk remains unknown, but reports suggest that these patients deserve extra perioperative attention. No discussion of pacemakers can take place without an understanding of the generic pacemaker code, which has been published by the HRS-NASPE and The British Pacing and Electrophysiology Group (BPEG). This code, initially published in 1983, was revised in February 2002. Shown in Table 19-1, the code (NBG) describes the basic behavior of the pacing device.4

Pacemaker Indications

Indications for permanent pacing are shown in Box 19-1. Devices have also been approved by the U.S. Food and Drug Administration (FDA) for three-chamber pacing (right atrium, both ventricles) to treat dilated cardiomyopathy (DCM) (also called biventricular pacing [Bi-V] or cardiac resynchronization therapy [CRT]).5 Also, specially programmed devices are used to treat hypertrophic cardiomyopathy (HCM) in both adults and children. Bi-V and HCM indications require careful attention to pacemaker programming, because effective pacing in these patients often requires a pacing rate greater than native sinus or junctional escape rate (often accomplished with drugs) and an atrioventricular (AV) delay shorter than the native PR interval so that the ventricle is paced 100% of the time. Inhibition or loss of pacing (e.g., from native conduction, atrial irregularity, ventricular irregularity, development of junctional rhythm, or EMI) can lead to deteriorating hemodynamics in these patients. Bi-V pacing can lengthen the QT interval in some patients, producing torsades de pointes.

Pacemaker Magnets

Despite often-repeated folklore, most pacemaker manufacturers warn that magnets were never intended to treat pacemaker emergencies or prevent EMI effects. Rather, magnet-activated switches were incorporated to produce pacing behavior that demonstrates remaining battery life and, sometimes, pacing threshold safety factors. Some pacemakers also demonstrate the detection of a problem during a telephone check, which should result in a call from the telephone center to the patient’s pacemaker physician.

Placement of a magnet over a generator might produce no change in pacing because NOT ALL PACEMAKERS SWITCH TO A CONTINUOUS ASYNCHRONOUS MODE WHEN A MAGNET IS PLACED. Also, not all models from a given company behave the same way. Although most pacemakers have “high-rate” (80 to 100 beats per minute) asynchronous pacing with a magnet some still switch to asynchronous pacing at program rate, and some will respond with a brief (10 to 64 beats) asynchronous pacing event before reverting to original programmed behavior. Possible effects of magnet placement are shown in Box 19-2. In some devices, magnet behavior can be altered via programming. Also, any pacemaker from CPI-Guidant ignores magnet placement after any electrical reset, which is a possibility in the presence of strong EMI. For all generators, calling the manufacturer remains the most reliable method for determining magnet response and using this response to predict remaining battery life. For generators with programmable magnet behavior (Biotronik, CPI-Guidant, Pacesetter, and St. Jude Medical), only an interrogation with a programmer can reveal current settings. Most manufacturers publish a reference guide, although not all of these guides list all magnet idiosyncrasies. A telephone call can also alert the clinician to any recalls or alerts, which are not uncommon with these devices.

PREANESTHETIC EVALUATION AND PACEMAKER REPROGRAMMING

Preoperative management of the patient with a pacemaker includes evaluation and optimization of coexisting disease(s). No special laboratory tests or radiographs (chest radiographs are remarkably insensitive for determination of lead problems) are needed for the patient with a pacemaker. Such testing should be dictated by the patient’s underlying disease(s), medication(s), and planned intervention. For programmable devices, interrogation with a programmer remains the only reliable method for evaluating lead performance and obtaining current program information. A chest radiograph might be useful to document the position of the coronary sinus (CS) lead in a patient with a Bi-V pacemaker or defibrillator, especially if central venous catheter placement is planned, because spontaneous CS lead dislodgment was found in more than 11% of patients in early studies. A chest radiograph is certainly indicated for the patient with a device problem discovered during his or her pacemaker evaluation.

The prudent anesthesiologist reviews the patient’s pacemaker history and follow-up schedule. Under the name NASPE, the HRS has published a consensus statement suggesting that pacemakers should be routinely evaluated with telephone checks for battery condition at least every 3 months. NASPE also recommends a comprehensive evaluation (interrogation) at least once per year. There are additional checks for devices implanted less than 6 or more than 48 (dual-chamber) or 72 (single-chamber) months. Rozner and associates reported a 2-year retrospective review of follow-up intervals in patients who presented for an anesthetic, and they found that more than 32% of 172 patients presenting for an anesthetic at their hospital did not meet the HRS-NASPE guideline for comprehensive evaluation.6 They also reported that 5% of the patients presented for their anesthetic with a pacemaker in need of replacement for battery depletion and that nearly 10% of patients had less-than-optimal pacing settings. Note that a recent, preoperative interrogation is now part of the ASA Pacemaker Advisory and the ACC guidelines.1,2

Important features of the preanesthetic device evaluation are shown in Box 19-3. Determining dependency on the pacemaker function might require temporary reprogramming to a VVI mode with a low rate. In patients from countries where pacemakers might be reused, battery performance might not be related to length of implantation in the current patient. It should also be noted that in a registry of 345 pacemaker generator failures, 7% of failures were not related to battery depletion.7

Appropriate reprogramming (Box 19-4) might be the safest way to avoid intraoperative problems, especially if monopolar “Bovie” electrocautery will be used. For lithotripsy, consideration should be given to programming the pacing function out of an atrial-paced mode, because some lithotriptors are designed to fire on the R wave and the atrial pacing stimulus could be misinterpreted as the contraction of the ventricle. All of the manufacturers stand ready to assist with this task. Reprogramming a pacemaker to asynchronous pacing at a rate greater than the patient’s underlying rate usually ensures that no oversensing or undersensing during EMI will take place, thus protecting the patient. Reprogramming a device will not protect it from internal damage or reset caused by EMI.

Experts do not agree on the appropriate reprogramming for the pacemaker-dependent patient. Setting a device to asynchronous mode to prevent inappropriate oversensing and ventricular output suppression can cause the pacemaker to ignore premature atrial or ventricular systoles, which could have the potential to create a malignant rhythm in the patient with significant structural compromise of the myocardium. Reviews demonstrate inappropriate R-on-T pacing with the development of a malignant ventricular rhythm.

In general, rate responsiveness and other “enhancements” (hysteresis, sleep rate, AV search, etc.) should be disabled by programming. Note that for many CPI devices, the Guidant Corporation recommends increasing the pacing voltage to “5 volts or higher” in any case in which the monopolar electrosurgical unit (ESU) will be used. Rozner and associates reported increases in both atrial and ventricular thresholds in 6 of 141 consecutive operations involving pacemaker cases in which the monopolar ESU was used, large volume and blood shifts were observed, or both.6 Although many of the operations were thoracic explorations, no pacing threshold changes were noted for these cases. No cardiopulmonary bypass cases were included in this cohort. Special attention must be given to any device with a minute ventilation (bioimpedance) sensor, because inappropriate tachycardia has been observed secondary to mechanical ventilation, monopolar “Bovie” ESU, and connection to an ECG monitor with respiratory rate monitoring.