Cardiac Pacing and Defibrillation

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Chapter 19 Cardiac Pacing and Defibrillation

Marc A. Rozner, MD, Mark Trankina, MD

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

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

Table 19-1 NASPE/BPEG Revised (2002) Generic Paemaker Code (NBG)

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.

BOX 19-1 Pacemaker Indications

• Symptomatic sinus node disease

• Symptomatic atrioventricular node disease

• Long QT syndrome

• Hypertrophic obstructive cardiomyopathy ^*

• Dilated cardiomyopathy ^*

^* See text and Pacemaker Programming for special precautions.

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.

BOX 19-2 Pacemaker Magnet Behavior

• No apparent rhythm or rate change

• No magnet sensor (some pre-1985 Cordis, Telectronics models)

• Magnet mode disabled (possible with CPI, Pacesetter, Telectronics, Vitatron models)

• EGM mode enabled (CPI, others)

• Program rate pacing in already paced patient (many CPI, Intermedics, Telectronics, Vitatron, others)

• Improper monitor settings (pace filter on)

• Brief (10 to 100 beats) asynchronous pacing, then return to program values (most Intermedics, some Biotronik models)

• Continuous or transient loss of pacing

• Discharged battery (some pre-1990 devices)

• Pacer enters diagnostic “Threshold Test Mode” (some Siemens, Intermedics, Medtronic, others)

• Asynchronous pacing without rate responsiveness using parameters possibly not in patient’s best interest

^*See text and Pacemaker Programming for special precautions.

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.

BOX 19-4 Pacemaker Reprogramming Probably Needed

• Any rate-responsive device: see text (problems are well known, problems have been misinterpreted with potential for patient injury, and the U.S. FDA has issued an alert regarding devices with minute ventilation sensors).

• Special pacing indication (hypertrophic obstructive cardiomyopathy, dilated cardiomyopathy, pediatric patient)

• Pacemaker-dependent patient

• Major procedure in the chest or abdomen

• Special procedures (see Box 19-5)

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.

Intraoperative (or Procedure) Management

No special monitoring or anesthetic technique is required for the patient with a pacemaker. However, ECG monitoring of the patient must include the ability to detect pacemaker discharges. Often, noise filtering on the ECG monitor must be changed to permit demonstration of the pacemaker pulse, and devices such as a nerve stimulator can interfere with detection and display of the pacemaker pulses.

In addition, patient monitoring must include the ability to ensure that myocardial electrical activity is converted to mechanical systoles. Mechanical systoles are best evaluated by pulse oximetry, plethysmography, or arterial waveform display. Some patients might need an increased pacing rate during the perioperative period to meet an increased oxygen demand. A pulmonary artery catheter, an esophageal Doppler monitor, or a transesophageal echocardiogram can be used to evaluate pacing frequency and its relationship to cardiac output. In addition to blood pressure and systemic vascular resistance, the monitoring of acid-base status might be needed to determine adequacy of cardiac output.

With respect to anesthetic technique, no studies have championed one over another. Nevertheless, a number of reports of prolongation of the QT interval with the use of isoflurane, desflurane, or sevoflurane have been published, whereas halothane appears to reduce this interval.⁸ No interactions have been reported for enflurane.

Monopolar “Bovie” electrocautery (ESU) use remains the principal intraoperative issue for the patient with a pacemaker. Between 1984 and 1997, the U.S. FDA was notified of 456 adverse events with pulse generators, 255 from electrocautery, and a “significant number” of device failures.⁹ Monopolar ESU is more likely to cause problems than is bipolar ESU, and patients with unipolar electrode configuration are more sensitive to EMI than are those with bipolar configurations. Coagulation ESU will likely cause more problems than nonblended “cutting” ESU. Magnet placement during electrocautery might allow reprogramming of an older (pre-1990) generator; however, newer generators are relatively immune to such effects. In fact, most devices from CPI-Guidant as well as St. Jude cannot be reprogrammed in the presence of a magnet. Note, however, that strong EMI can produce an electrical reset or improper detection of battery depletion, which might change the programming mode, rate, or both. If monopolar electrocautery is to be used, then the current return pad should be placed to ensure that the electrocautery current path does not cross the pacemaking system. For cases such as head and neck surgery, the pad might be best placed on the shoulder contralateral to the implanted device. For breast and axillary cases, the pad might need to be placed on the ipsilateral arm with the wire prepped into the field by sterile plastic cover. Procedures with special pacing ramifications are shown in Box 19-5.

BOX 19-5 Special Procedures in Patients with Implantable Generators

• Lithotripsy: acceptable with precautions to protect the generator and, possibly, programming out of an atrial pacing mode

• Transurethral resection (bladder, prostate) and uterine hysteroscopy: procedures using monopolar electrocautery that can be easily accomplished after device reprogramming

• Magnetic resonance imaging (MRI): absolutely contraindicated by most generator manufacturers, and deaths have been reported. However, a report suggests that appropriate patients can safely undergo MRI but not without appropriate precautions.

• Electroconvulsive therapy: requires asynchronous (nonsensing) mode

• Nerve stimulator testing/therapy: inappropriate detection of transcutaneous electrical nerve stimulation, neuromuscular, and chiropractic electrical muscle stimulation because ventricular tachycardia or fibrillation has been reported

The use of an ultrasonic cutting device, commonly called a “harmonic scalpel,” has been championed to prevent EMI while providing the surgeon with the ability to both cut and coagulate tissue. There are a number of case reports demonstrating successful surgery without EMI issues in these patients.

Magnetic resonance imaging (MRI) deserves special mention. In general, MRI has been contraindicated in pacemaker and ICD patients. However, reports suggests that MRI is probably safe for some patients with newer devices, as well as any patient who will be wide awake in the MRI tunnel, who is not dependent on his or her pacemaker for heart rhythm or survival, who will not need medication to undergo the MRI, and who can communicate regularly with the MRI care team. Nevertheless, not all MRI sequences and energy levels have been studied, and caution is advised.¹⁰

Pacemaker Failure

Pacemaker failure has three causes: (1) failure of capture, (2) lead failure, and (3) generator failure. Failure of capture owing to a defect at the level of the myocardium (i.e., the generator continues to fire but no myocardial depolarization takes place) remains the most difficult problem to treat. Myocardial changes that result in noncapture include myocardial ischemia/infarction, acid-base disturbance, electrolyte abnormalities, or abnormal levels of antiarrhythmic drug(s). Temporary pacing (transvenous, transcutaneous, transthoracic, or transesophageal) might inhibit pacemaker output at voltages that will not produce myocardial capture. Sympathomimetic drugs generally lower pacing threshold. Outright generator or lead failure is rare.

TEMPORARY PACEMAKERS

There are several techniques available to the anesthesiologist to establish reliable temporary pacing during the perioperative period or in the intensive care unit. Cardiovascular anesthesiologists are more likely than generalists to routinely use temporary transvenous or epicardial pacing in their practices. Temporary cardiac pacing can serve as definitive therapy for transient bradyarrhythmias or as a bridge to permanent generator placement.

The various forms of temporary pacing include many transvenous catheter systems, transcutaneous pads, transthoracic wires, and esophageal pacing techniques. This section reviews the indications for temporary cardiac pacing and discusses the techniques available to the anesthesiologist. Table 19-2 summarizes these techniques.

Table 19-2 Comparison of Temporary Pacing Techniques

Indications for Temporary Pacing

Temporary pacemakers are commonly used postoperatively after cardiac surgery, in the treatment of drug toxicity resulting in arrhythmias, with certain arrhythmias complicating myocardial infarction, and for intraoperative bradycardia due to β-blocker use. The placement of a temporary pacing system can assist in the hemodynamic management in the perioperative period. Abnormal electrolytes, preoperative β-blocker use, and many of the intraoperative drugs have the potential to aggravate bradycardia and bradycardia-dependent arrhythmias. Because drugs used to treat bradyarrhythmias have a number of important disadvantages compared with temporary pacing, hemodynamically unstable perioperative bradyarrhythmias should be considered an indication for temporary pacing (Table 19-3).If the patient already has epicardial wires or a pacing catheter or wires, or transesophageal pacing is feasible, pacing is preferred to pharmacologic therapy. However, transcutaneous and ventricular-only transvenous pacing, even if feasible, may exacerbate hemodynamic problems in patients with heart disease because these pacing modalities do not preserve AV synchrony (i.e., produces ventricular or global activation).

Table 19-3 Temporary Pacing Indications

Patient Condition	Event Requiring Temporary Pacing
AMI	Symptomatic bradycardia, medically refractory New bundle-branch block with transient complete heart block Complete heart block Postoperative complete heart block Symptomatic congenital heart block Mobitz II with AMI New bifascicular block Bilateral bundle-branch block and first-degree AV block Symptomatic alternating Wenckebach block Symptomatic alternating bundle-branch block

Tachycardia treatment or prevention

Bradycardia-dependent VT

Torsades de pointes

Long QT syndrome

Treatment of recurrent SVT or VT

Prophylactic

Pulmonary artery catheter placement with left bundle-branch block (controversial)

New AV block or bundle-branch block in acute endocarditis

Cardioversion with sick sinus syndrome

Post–defibrillation bradycardia

Counteract perioperative pharmacologic treatment causing hemodynamically significant bradycardia

AF prophylaxis post–cardiac surgery

Post–orthotopic heart transplantation

AF = atrial fibrillation; AMI = acute myocardial infarction; AV = atrioventricular; SVT = supraventricular tachycardia; VT = ventricular tachycardia.

Nearly every indication for a permanent pacemaker is an indication for temporary pacing in patients without a pacemaker who, due to circumstances (e.g., emergency surgery, critical illness), cannot have elective permanent pacemaker implantation. Temporary pacing may also be needed before implantation of a permanent pacemaker to stabilize patients with hemodynamically significant bradycardia.

Temporary pacing is also indicated if a patient with a myocardial infarction complicated by second- or third-degree heart block is scheduled for emergency surgery. Bifascicular block in an asymptomatic patient is not reason enough for temporary pacing preoperatively. The development of a new bifascicular block immediately postoperatively, however, suggests perioperative myocardial ischemia or infarction, and temporary pacing might be required. Surgical resection of neck and carotid sinus tumors may give rise to bradyarrhythmias requiring temporary cardiac pacing during surgical manipulation. Neurosurgical procedures involving the brainstem may also be associated with significant bradycardia.

Relative contraindications to transvenous ventricular pacing include digitalis toxicity with ventricular tachycardia (VT), tricuspid valve prostheses, or the presence of a coagulopathy. Pacing in the setting of severe hypothermia might induce ventricular fibrillation (VF) or alter the normal compensatory physiologic mechanisms to the hypothermia. Atrial fibrillation, multifocal atrial tachycardia, and significant AV conduction system disease are relative contraindications to transvenous atrial pacing.

Transvenous Temporary Pacing

Transvenous cardiac pacing provides the most reliable means of temporary pacing. Temporary transvenous pacing is dependable and well tolerated by patients. With a device that can provide both atrial and ventricular pacing, transvenous pacing can maintain AV synchrony and improve cardiac output. Disadvantages include the need for practitioner experience, time to appropriately place the wire(s) to provide capture, the potential complications of catheter placement and manipulation, and the need for fluoroscopy in many cases.

Rapid catheter position is most easily obtained by using the right internal jugular vein, even without fluoroscopy. The left subclavian vein is also easily used in emergent situations. Other sites are often impassable without fluoroscopy. In addition, brachial and femoral routes can increase the frequency of lead dislodgment during motion of the extremities.

Once central access is obtained, the lead is guided into position using hemodynamic data (not possible with the simple bipolar lead) or by fluoroscopic guidance. ECG guidance is less desirable. The right atrial appendage and right ventricular apex provide the most stable catheter positions. Techniques for placement into these positions are part of cardiology training and likely are foreign to most anesthesiologists. When fluoroscopy is unavailable or in emergency situations, a flow-directed catheter can be attempted using ECG and pressure guidance. Once the right ventricle is entered, the balloon is deflated, if used, and the catheter gently advanced until electrical capture is noted. Flow-directed catheters and a right internal jugular approach afford the shortest insertion times. The reported incidence of successful capture in urgent situations without fluoroscopy ranges from 30% to 90%.

Once catheters are positioned, pacing is initiated using the distal electrode as the cathode and the proximal electrode as the anode. Ideally, the capture thresholds should be less than 1 mA and generator output should be maintained at three times threshold as a safety margin. In dual-chamber pacing, AV delays of between 100 and 200 ms are used. Many patients are sensitive to this parameter. Cardiac output optimization with echocardiography and/or mixed venous oxygen saturation can be used to maximize hemodynamics by adjusting AV delay.¹¹ AV sequential pacing is clearly beneficial in many patients, but it should be remembered that emergency pacing starts with ventricular capture alone. There is a potential risk of interference of external pacemaker generators by walkie-talkies and digital cellular phones. Clinicians should also be aware of all complications related to transvenous lead placement.

Pacing Pulmonary Artery Catheters

The pulmonary artery AV pacing thermodilution (TD) catheter allows for AV sequential pacing via electrodes attached to the outside of the catheter, as well as routine pulmonary artery (PA) catheter functions. Combination of the two functions into one catheter eliminates the need for separate insertion of temporary transvenous pacing electrodes. However, several potential disadvantages exist with this catheter, including (1) varying success in initiating and maintaining capture, (2) external electrode displacement from the catheter, and (3) relatively high cost compared to standard pacing PA catheters. The Paceport PA catheter provides ventricular pacing with a separate bipolar pacing lead (Chandler probe), which allows for more stable ventricular pacing as well as hemodynamic measurements. This catheter has been used for successful resuscitation after cardiac arrest during closed-chest cardiac massage when attempts to capture with transcutaneous and transvenous flow-directed bipolar pacing catheters had failed. However, this unit does not provide the potential advantages associated with atrial pacing capability. The newer pulmonary artery A-V Paceport adds a sixth lumen to the older Paceport to allow placement of an atrial J-wire, flexible-tip bipolar pacing lead. Both of these Paceport catheters are placed by transducing the right ventricular pressure port to ensure correct positioning of the port 1 to 2 cm distal to the tricuspid valve. This position usually guides the ventricular wire (Chandler probe) to the apex where adequate capture should occur with minimal current requirements. Although ventricular capture is easily obtained, atrial capture can be more difficult and less reliable.¹¹ This catheter has been used successfully after cardiac surgery. The atrial wire can be used to diagnose supraventricular tachycardia (SVT) by atrial electrograms and to overdrive atrial flutter and reentrant SVT.

Transcutaneous Pacing

Transcutaneous pacing is readily available and can be rapidly implemented in emergency situations. Capture rate is variable and the technique may cause pain in awake patients, but usually it is tolerated until temporary transvenous pacing can be instituted. It may be effective even when endocardial pacing fails. It is now considered by many to be the method of choice for prophylactic and emergent applications.

The large patches typically are placed anteriorly (negative electrode or cathode) over the palpable cardiac apex (or V₃ lead location) and posteriorly (positive electrode or anode) at the inferior aspect of the scapula. The anode has also been placed on the anterior right chest with success in healthy volunteers. The skin should be cleansed with alcohol (but not abraded) to reduce capture threshold and improve patient comfort. Abraded skin can cause more discomfort. Typical thresholds are 20 to 120 mA, but pacing may require up to 200 mA at long pulse durations of 20 to 40 ms.¹² Transcutaneous pacing appears to capture the right ventricle followed by near-simultaneous activation of the entire left ventricle. The hemodynamic response is similar to that of right ventricular endocardial pacing. Both methods can cause reductions in left ventricular systolic pressure, a decrease in stroke volume, and an increase in right-sided pressures due to AV dyssynchrony. Capture should be confirmed by palpation or display of a peripheral pulse. Maintenance current is set 5 to 10 mA above threshold as tolerated by the patient. Success rates appear to be highest when the system is used prophylactically or early after arrest—upward of 90%.

Coughing and discomfort from cutaneous stimulation are the most frequent problems. The technique poses no electrical threat to medical personnel, and complications are rare. There have been no reports of significant damage to myocardium, skeletal muscle, skin, or lungs in humans despite continuous pacing up to 108 hours and intermittent pacing up to 17 days. Several commercially available defibrillators include transcutaneous pacing generators as standard equipment.

Esophageal Pacing

The newest technique available to anesthesiologists is esophageal pacing, and it has been shown to be quite reliable.¹³ Significant bradycardia, secondary to underlying pathology or pharmacologic effects, can occur during anesthesia. The response to pharmacologic therapy for significant bradycardia with vagolytic drugs can be unpredictable and difficult to sustain accurately. Chronotropic drugs may have little effect and can lead to tachyarrhythmias and/or myocardial ischemia. Esophageal pacing is relatively noninvasive and well tolerated even in the majority of awake patients, and it appears to be devoid of serious complications. This modality is useful for heart rate support of cardiac output, for overdrive suppression of reentrant SVT, and for diagnostic atrial electrograms. Ventricular capture must be excluded before attempts at rapid atrial pacing for overdrive suppression to prevent potential VT or VF.

Problems with esophageal pacing include (1) the necessity for special generators that must provide 20 to 30 mA of current with wide pulse widths of 10 to 20 ms and (2) the ability to pace only the left atrium reliably and not the left ventricle, which can be a significant problem in emergency situations. By comparison, typical temporary generators designed for endocardial pacing have a maximum output of 20 mA with pulse width durations of only 1 to 2 ms.

The pacing stimulus is delivered through a modified esophageal stethoscope. Pacing is initiated by connecting the system and placing the esophageal stethoscope to a depth of 30 to 40 cm from the teeth. Capture should be confirmed using the peripheral pulse (i.e., from the pulse oximeter plethysmogram or an invasive hemodynamic monitor), because the pacing stimulus often is large relative to the QRS and frequently fools the ECG counting algorithm on the monitor. Atrial capture is obtained in virtually all patients using outputs of 8 to 20 mA, and the output should be set to two to three times the threshold for capture. Thresholds are not influenced by weight, age, atrial size, or previous cardiac surgery. Because there is no sensing element involved, esophageal pacing is AOO mode pacing. In general, transesophageal pacing requires reprogramming of a permanent pacemaker or icd. It is contraindicated in these patients without expert assistance.

POSTANESTHESIA PACEMAKER EVALUATION

Any pacemaker that was reprogrammed for the perioperative period should be reset appropriately. For non-reprogrammed devices, most manufacturers recommend interrogation to ensure proper functioning and remaining battery life if any electrocautery was used. ACC guidelines recommend a postprocedure interrogation.

IMPLANTED CARDIOVERTER-DEFIBRILLATORS

The development of an implantable, battery-powered device able to deliver sufficient energy to terminate VT or VF has represented a major medical breakthrough for patients with a history of ventricular tachyarrhythmias. These devices prevent death in the setting of malignant ventricular tachyarrhythmias, and they clearly remain superior to antiarrhythmic drug therapy. Initially approved by the U.S. FDA in 1985, more than 80,000 devices will be placed this year, and industry sources report that more than 240,000 patients have these devices today.

A significant number of technologic advances have occurred since the first ICD was placed, including considerable miniaturization (pectoral pocket placement with transvenous leads is the norm) and battery improvements that permit permanent pacing with these devices. Thus, a pectoral ICD could easily be confused with a pacemaker. Like pacemakers, ICDs have a generic code to indicate lead placement and function, which is shown in Table 19-4.

Table 19-4 North American Society for Pacing and Electrophysiology/The British Pacing and Electrophysiology Group Generic Defibrillator Code (NBD)

Newer ICDs (since 1993) have many programmable features, but essentially they measure each cardiac RR interval and categorize the rate as normal, too fast (short RR interval), or too slow (long RR interval). When the device detects a sufficient number of short RR intervals within a period of time (all programmable), it begins an antitachycardia event. The internal computer decides to choose antitachycardia pacing (less energy use, better tolerated by patient) or shock, depending on the presentation and device programming. If shock is chosen, an internal capacitor is charged. Most newer devices are programmed to reconfirm VT or VF after charging in order to prevent inappropriate shock therapy. Typically, ICDs have six therapies available for each type of event (VT, fast VT, VF), and some of these therapies can be repeated before moving to the next higher energy sequence. Thus, ICDs can deliver 6 to 18 shocks per event. In an ICD with antitachycardia pacing, once a shock is delivered, no further antitachycardia pacing can take place.

Twenty to 40 percent of shocks are for rhythms other than VT or VF despite reconfirmation. SVT remains the most common cause of inappropriate shock therapy. Advances in ICDs to include dual-chamber detection might be able to lower the inappropriate shock rate, although it is still reported as high as 17%.¹⁴ Whether inappropriate shocks injure patients remains a subject of considerable debate, but a significant number of patients who receive an inappropriate shock demonstrate elevated troponin levels in the absence of an ischemic event.

Programmable features in current ICDs to differentiate VT from a tachycardia of supraventricular origin (SVT) include the following:

1. Onset criteria: in general, onset of VT is abrupt whereas onset of SVT has sequentially shortening RR intervals.

2. Stability criteria: in general, the RR interval of VT is relatively constant, whereas the RR interval of atrial fibrillation with rapid ventricular response is quite variable.

3. QRS width criteria: in general, the QRS width in SVT is narrow (<110 ms), whereas the QRS width in VT is wide (>120 ms).

4. “Intelligence” in dual-chamber devices attempts to associate atrial activity with ventricular activity.

5. Morphology waveform analysis with comparison to stored historic templates.

Note that once the RR interval becomes sufficiently short for VF detection, the ICD begins a shock sequence. As noted, once the device delivers any shock therapy, no further anti-tachycardia pacing takes place. An ICD with anti-bradycardia pacing capability will begin pacing when the RR interval is too long. In July 1997, the U.S. FDA– approved devices with sophisticated dual-chamber pacing modes and rate-responsive behavior for ICD patients who need permanent pacing (about 20% of ICD patients).

Indications

Initially, ICDs were placed for hemodynamically significant VT or VF. Newer indications associated with sudden death include long QT syndrome, Brugada syndrome (right bundle-branch block, ST-segment elevation in leads V₁ to V₃), and arrhythmogenic right ventricular dysplasia. Studies suggest that ICDs can be used for primary prevention of sudden death (i.e., before the first episode of VT or VF) in young patients with hypertrophic cardiomyopathy, and data from the second Multicenter Automatic Defibrillator Intervention Trial (MADIT II) suggest that any patient after myocardial infarction with an ejection fraction less than 30% should undergo prophylactic implantation of an ICD.¹⁵

Trials are under way for the cardiomyopathy patient with nonischemic cardiomyopathy as well. The Sudden Cardiac Death-Heart Failure Trial (SCD-HeFT) results and the previously published Defibrillators In Non-Ischemic Cardiomyopathy Treatment Evaluation (DEFINITE) study suggest that ICD placement will result in lower mortality in any patient with an ejection fraction less than 35% regardless of the cause of the cardiomyopathy. The DEFINITE results are important, because the patients in this study were randomized only after initiation of β-blockade and angiotensin-converting enzyme inhibitor therapy, which form the backbone of medical therapy for cardiomyopathy. The SCD-HeFT results showed ICDs reduced mortality by 23%. Based on these results, Medicare expanded its ICD coverage.^16,¹⁷

Box 19-6 reviews ICD indications.

BOX 19-6 Implanted Cardioverter-Defibrillator Indications

• Ventricular tachycardia

• Ventricular fibrillation

• Brugada syndrome (right bundle-branch block, ST-segment elevation V₁ to V₃)

• Arrhythmogenic right ventricular dysplasia

• Long QT syndrome

• Hypertrophic cardiomyopathy

• Prophylactic use after myocardial infarction, ejection fraction < 30%

Dilated Cardiomyopathy

With the advent of Bi-V pacing for the patient with a DCM and prolonged QRS interval, and the approval of ICDs with Bi-V capability, the presence of a defibrillator with Bi-V pacing will become more common. About 550,000 new diagnoses of congestive heart failure are made in the United States every year, and the prevalence of this disease includes nearly 5 million patients. Significant risk factors for the development of congestive heart failure include ischemic heart disease and/or hypertension. These data, combined with the results from SCD-HeFT and MADIT II trials (ICD is indicated in any patient with cardiomyopathy and ejection fraction < 30% to 35%), suggest that the number of patients eligible to receive a defibrillator to include Bi-V pacing will increase dramatically. Whether any country’s economy can absorb this economic burden remains to be seen. At present, Bi-V pacing without defibrillation capability improves functional status and quality of life and was shown to reduce mortality.^5,¹⁸

Implanted Cardioverter-Defibrillator Magnets

Like pacemakers, magnet behavior in ICDs can be altered by programming. Most devices suspend tachyarrhythmia detection (and therefore therapy) when a magnet is appropriately placed to activate the reed switch. Some devices from Angeion, CPI, Pacesetter, and St. Jude Medical can be programmed to ignore magnet placement. Some Guidant/CPI ICDs have been programmed to ignore magnet placement due to a faulty magnet switch. Anti-tachycardia therapy in some Guidant/CPI devices can be permanently disabled by magnet placement for 30 seconds. In general, magnets will not affect anti-bradycardia pacing mode or rate. Interrogating the device and calling the manufacturer remain the most reliable methods for determining magnet response.

Preanesthetic Evaluation and Implanted Cardioverter-Defibrillator Reprogramming

All ICDs should have their anti-tachycardia therapy disabled before the induction of anesthesia and commencement of the procedure.¹ Guidelines from HRS-NASPE and the ASA Advisory suggest that every patient with an ICD have an in-office comprehensive evaluation every 1 to 4 months. Devices with Bi-V pacing must have a sufficiently short AV delay for sensed events to ensure that all ventricular activity is paced. Failure of ventricular pacing (either right or left) owing to native AV conduction or threshold issues has been associated with inappropriate antitachycardia therapy (i.e., shock).