Chapter 33: Cardiac Pacing and Implanted Defibrillation

Cardiac pacing serves to maintain or restore myocardial depolarization and thus ensure adequate cardiac output. In the ED, pacing is performed therapeutically to correct an ongoing rhythm disturbance or in anticipation of the onset of a conduction problem with hemodynamic impact.¹

Indications for emergency pacing are listed in Table 33-1.

The indications for emergency cardiac pacing are found in chapter 18, "Cardiac Rhythm Disturbances."

All cardiac pacemakers deliver an electrical stimulus to the heart by electrodes that cause depolarization and subsequent cardiac contraction.² The modern pacemaker only stimulates the heart chamber if it does not recognize (sense) intrinsic electrical activity from that chamber after a selected time interval. Impulses are delivered to either the atria or ventricles, or to both.

Components of a cardiac pacemaker include:

• Pulse generator

• Electronic circuitry for sensing and pacing

• Lead system that connects the pulse generator to the electrode(s) and stimulates the myocardium

Relevant clinical details of these components are listed in Table 33-2.

Transcutaneous pacing is the emergency technique of choice because of its easy application. It uses externally applied electrodes to deliver an electric impulse directly across the intact chest wall to stimulate the myocardium. Transcutaneous pacers differ from standard pulse generators in several important ways. The pulse duration of the stimulating impulse is longer and the current output higher than in internal pacing. Muscle contraction (usually the chest wall or diaphragm) is notable during pacing, especially at high outputs, and may be painful. Severe twitching makes palpation of the radial, carotid, or femoral pulse difficult. Cardiac monitoring with standard ECG monitors is difficult due to interference from the large current outputs that create large-amplitude pacing spikes. Most of the newer transcutaneous pacing units have a monitor that filters pacing spikes, allowing simultaneous monitoring.

RISKS AND PRECAUTIONS

There is little risk of electrical injury to healthcare providers during transcutaneous pacing. The electrodes are insulated, and closed chest compressions can be done over pads while pacing. Inadvertent contact with the active pacing surface results only in a mild shock.

EQUIPMENT

Table 33-3 lists the equipment needed to perform transcutaneous pacing.

TECHNIQUE

If time and conditions allow, explain the procedure to the patient and administer IV sedation and analgesia.

Apply the external pacing pads as shown in Figure 33-1. The same pads and electrodes are used for pacing, cardioversion, and defibrillation in most of the newer defibrillator units. If separate defibrillator pads or paddles are used, place them at least 2 to 3 cm from the pacing pads.

In bradyasystolic arrest, turn the stimulating current to maximum output and then decrease the output after capture is achieved; after restoring pulses, you may titrate energy to a level just above loss of capture. In a patient with a hemodynamically compromising bradycardia outside of cardiac arrest, slowly increase the output from the minimum setting until capture is achieved—usually between 50 and 100 mA. Continue pacing at about 1.25 times the threshold of initial electrical capture.

Transcutaneous pacing may be fixed rate (asynchronous) or demand (synchronous). Asynchronous pacing delivers an electrical impulse at a regular interval without regard to intrinsic cardiac pacemaker activity. This creates the potential risk of precipitating a dysrhythmia if the pacing stimulus occurs during the vulnerable period of ventricular repolarization. While many state a preference for synchronous pacing, there are little outcome or safety data to support that preference.

OUTCOMES ASSESSMENT

Assess capture using the ECG on the filtered monitor of the pacing unit. Look for the presence of a consistent ST segment and T wave after each pacer spike. Palpate for carotid and femoral pulses. Bedside US can be used to assess external pacer capture. If these appear favorable, assess blood pressure by cuff or arterial catheter.

Failure to capture with transcutaneous pacing may be related to faulty electrical contact, electrode placement, patient size, or underlying pathology. Recheck the lead connections, skin–electrode contact, and electrode placement. On occasion, pneumothorax, pericardial effusion or tamponade, severe myocardial ischemia, and metabolic derangements limit capture. No clinical myocardial damage results from properly performed transcutaneous pacing. Unless an easily resolved trigger exists, arrange for temporary transvenous pacing as soon as possible since ongoing pacing is likely needed.

The indications for transvenous pacing are the same as for other methods of cardiac pacing.

Gather all equipment needed, including that to insert a central venous catheter (see chapter 31, "Vascular Access.") Resuscitation equipment and drugs, and the pacing needs listed in Table 33-4.

TECHNIQUE

You should know the equipment and practiced or done the procedure before starting. If conditions permit, explain the procedure to the patient and obtain informed consent. Next, identify the access site and approach and position the patient. The primary sites of catheter insertion in the ED are the right internal jugular vein (preferred) and the left subclavian vein. The right internal jugular vein allows a relatively straight line of access through the superior vena cava and right atrium into the right ventricle.

The steps for transvenous pacemaker insertion are listed in Table 33-5.

The most commonly encountered difficulties with transvenous pacing are securing venous access and obtaining proper placement of the stimulating electrode in the right ventricle, both of which can be time-consuming. Placement of the catheter tip into the apex of the right ventricle is the key to successful transvenous pacing.

When patients have decreased or no forward blood flow, positioning of the pacer tip within the right ventricle is difficult. Balloon-tipped catheters do not aid during low- or no-flow states. In an emergency, connect the pacemaker electrodes to the power source and advance the catheter until the tip encounters the endocardium of the right ventricle and creates capture.

Set the initial rate between 80 to 100 beats/min, using the asynchronous mode (sensitivity off) initially in patients requiring emergency pacing for hemodynamically unstable bradycardias. Follow the ECG to determine the presence or absence of capture (Figure 33-3). Adjust subsequent rate and sensitivity settings as clinically indicated by the response and underlying rhythm disturbance.

Transvenous pacing is best used in urgent rather than emergent situations (with transcutaneous serving as the bridge), allowing fluoroscopy.

COMPLICATIONS

Complications include perforation of the myocardium, cardiac arrhythmias, air embolism, failure of the circuit, catheter dislodgement, and delayed infection, in addition to the complications possible with central venous access (see chapter 31).

Permanent pacing is not performed in the ED.¹ Permanent pacemaker pulse generators (battery) are placed subcutaneously on the side of the patient's nondominant hand (thus, usually in the left prepectoral region below the left clavicle). The endocardial transvenous leads are positioned in the right ventricle at the apex, septum, or right ventricular outflow tract, and in the case of a dual-chamber device, also in the right atrium. A subclavian or cephalic vein approach is common. An epicardial lead may be implanted during concomitant open heart surgery.

Pacemaker leads are either bipolar or unipolar in configuration; unipolar leads are prone to oversensing myopotential and electromagnetic interference. The disadvantage of the bipolar configuration is that it is larger and more prone to lead fractures.

PACEMAKER NOMENCLATURE

A five-letter code is used to describe the features of the pacemaker³ (Table 33-6). The first three code letters are used most commonly. The first letter refers to the chamber or chambers in which the pacing occurs: A = atrium, V = ventricle, and D = dual chamber or both A and V. The second letter refers to the chamber or chambers in which sensing occurs. The letters are the same as those for the first letter code. "I" indicates that a sense event inhibits the output pulse and causes the pacemaker to recycle the timing cycles, "T" means that an output pulse is triggered in response to a sensed event. "D" means that both "T" and "I" response can occur. The most common pacemakers are VVI and DDD. The fourth letter is used to indicate the presence or absence of an adaptive-rate mechanism (rate modulation). Pacemakers are then indicated as VVIR or DDDR. The fifth letter is used to indicate whether multisite pacing is present and is rarely used. See Figures 33-4 and 33-5 for examples of ECGs with pacemaker spikes.

RESUSCITATION IN PATIENTS WITH PERMANENT PACEMAKER

If a patient who has a permanent pacemaker requires countershock, place the pads or paddles at least 8 cm from the pulse generator. The current normal positioning of the paddles—below the right clavicle and apex of the heart—is safe because the majority of pulse generators are below the left clavicle. Alternatively, place adhesive electrodes in an anteroposterior configuration. After countershock, interrogate the pacemaker to ensure that it is still functioning normally.

Potential problems after defibrillation include:

• Pacemaker inhibition due to reversion to noise mode

• Deletion (reprogramming)

• Circuit damage

• Myocardial damage adjacent to the lead tip caused by current transmission via the electrode to the myocardial interface

Another reason that immediate return of pacing (capture) fails after defibrillation is that global myocardial ischemia increases pacing threshold. If this occurs, try transcutaneous pacing at higher delivered energy settings.

PACEMAKER COMPLICATIONS

Complications for which a patient may seek care in the ED in the early period after pacemaker insertion are listed in Table 33-7. Two are discussed in detail.

Pacemaker Syndrome

Atrioventricular synchrony and the presence of ventriculoatrial conduction are most common in the setting of VVI pacing but may occur with the DDI mode. With VVI pacing, the ventricle is electrically stimulated, resulting in ventricular contraction. If sinus node functions are intact, the atria can be depolarized by a sinus impulse and contract when the tricuspid and mitral valves are closed. This results in an increase in jugular and pulmonary venous pressures and may produce symptoms of congestive heart failure. Atrial distention can result in reflex vasodepressor effects mediated by the central nervous system. If the contribution of atrial contraction to late diastolic ventricular filling is important in maintaining an adequate cardiac output, orthostatic hypotension may occur. DDI pacing in a patient with atrioventricular block can result in pacemaker syndrome if the sinus node discharge rate exceeds the programmed rate of the pacemaker.

In most instances, symptoms are mild and patients adapt to them. In about one third of these patients, symptoms are severe. Treatment usually requires upgrading a VVI pacemaker to a dual-chamber pacemaker or lowering the pacing rate of the VVI unit, so that ventricular pacing does not occur but provided intrinsic conduction occurs. If symptoms occur in a patient paced in the DDI mode, optimizing the timing of atrial and ventricular pacing is usually required. Consult with a cardiologist.

PACEMAKER MALFUNCTION

Pacemaker malfunction is divided into four areas: failure to sense, failure to pace, failure to capture, or overpacing or pacemaker-associated tachycardia.⁴

Failure to Sense (Undersensing)

Undersensing occurs when the pacemaker cannot adequately detect the intrinsic electrical cardiac activity. This results from lead placement in an area of the heart with poor or variable conductivity, or if a lead is loose, dislodged, or physically broken. Sensing failure can occur if the programmed sensing threshold is set too high. With undersensing, if the pacer is set in an inhibit mode, it will fire at a set rate that is not coordinated with the patients underlying cardiac cycle (Figure 33-6A).

Failure to Pace (Oversensing)

Oversensing occurs when the pacemaker experiences interference from electrical signals within the body (i.e., skeletal or smooth muscle myopotentials) that are not related to the normal cardiac cycle. Sources of interference may include skeletal muscle, the diaphragm, nerve stimulators, broken pacer leads, and uncommonly, coarse atrial fibrillation. When electrical activity is oversensed, the pacemaker erroneously inhibits the pulse generator and the patient may develop bradycardic rhythms (Figure 33-6B).

Failure to Capture

Failure to capture occurs when the pulse generator fires but the current delivered to the endocardium is too low to initiate depolarization and wavefront propagation. Dislodged leads, poor cardiac conductivity due to myocardial disease (e.g., ischemia, acidosis, fibrosis), and programming problems are the most common reasons for this malfunction. Failure to capture can be intermittent (Figure 33-6C).

PACEMAKER-ASSOCIATED TACHYCARDIA

Occasionally the patient with an implanted device may present with a rapid paced rhythm. This may be due to:

• Rapid atrial arrhythmia triggering an upper rate response in a patient with complete heart block

• Pacemaker-mediated tachycardia

• Runaway pacemaker

Unless preprogrammed to mode switch, pacemakers detect rapid atrial rhythms and track them, resulting in pacing at the upper rate limit. Pacemaker-mediated tachycardia can occur in a dual-chamber pacemaker when a premature ventricular ectopic beat triggers a retrogradely conducted atrial depolarization outside the atrial refractory period, which is then sensed by the pacemaker, initiating ventricular pacing. If this continues, it can cause an endless loop tachycardia. Rarely the pacemaker may malfunction with acceleration of pacing (runaway pacemaker). This occur when the pulse generator discharges at a rapid rate above its preset upper limit and is most commonly associated with a battery failure or damage from external interference. Placing a magnet over the pacemaker may help in the pacemaker-mediated tachycardia or runaway pacemaker. However, interrogation of the device is usually required, and the device must be replaced if programming is not successful.

PACEMAKER PROGRAMMING ERRORS

Pacemakers are computer-controlled devices with complex software that requires adjustment and maintenance. Reprogramming is accomplished noninvasively with devices that communicate to the pacemaker through radiofrequencies. Occasional settings can be inadvertently altered, and software, as with any computer, can fail. Software changes may occur in set rates, sensing thresholds, and current outputs.

Implantable cardioverter-defibrillators are the treatment of choice for sudden cardiac death, reducing mortality from approximately 30% to 45% per year to <2% per year.⁵ This remarkable efficacy, coupled with the failure (and potentially proarrhythmic effects) of pharmacologic therapy and the increasing sophistication and miniaturization of the devices, has led to an explosion in implantable cardioverter-defibrillator use. Expanding indications for implantable cardioverter-defibrillator implantation make it likely that emergency physicians will regularly see patients with these devices.

An implantable cardioverter-defibrillator consists of a pulse generator, a lead system with both sensing and shocking electrodes, circuitry to analyze the cardiac rhythm and trigger defibrillation, and a power supply.

In the past, these devices generally were placed by thoracotomy or sternotomy, and defibrillation occurred through electrodes positioned inside or outside the pericardium. Rate-sensing electrodes were placed epicardially or transvenously. Roughly one third of patients with second-generation implantable cardioverter-defibrillators received inappropriate shock(s) triggered by a supraventricular tachycardia during the working life of the device.

In newer implantable cardioverter-defibrillators, sensing-pacing-defibrillation electrodes are placed transvenously, and the device itself is generally implanted subcutaneously in the subpectoral region or in an abdominal pocket. Newer implantable cardioverter-defibrillators are better at discriminating supraventricular tachycardia and are capable of a variety of responses to ventricular tachycardia and fibrillation. Most are programmed to follow a tiered approach to ventricular dysrhythmias: antitachycardia pacing, low-energy cardioversion, and finally defibrillation. Depending on the frequency of discharge and whether the pacemaker function is used, the latest implantable cardioverter-defibrillators have a projected life span of approximately 6 to 9 years, depending on whether they are single- or dual-chamber devices.

Implantable cardioverter-defibrillators are remarkably effective in preventing sudden cardiac death. The most common cause of death in patients with implantable cardioverter-defibrillators is congestive heart failure, which should be managed in standard fashion in the ED.

ED EVALUATION

The most common reason an patient with an implantable cardioverter-defibrillator comes to the ED is evaluation after a delivered shock. Causes of inappropriate shock delivery are summarized in Table 33-8. Determine the number of shocks delivered, the activity of the patient at the time of shock, and any prodromal symptoms or postshock trauma. Ask about any recent changes in rhythm-directed drug therapy. Focus the physical examination on the vital signs, the cardiovascular status, the generator pocket, and evidence of incidental trauma. Place each patient on a continuous ECG monitor, and obtain a 12-lead ECG; any shock-related ST-segment elevations or depressions should resolve within 15 minutes; ongoing changes suggest new ischemia. Examine a chest radiograph for electrode migration, displacement, or fracture. Obtain rhythm-related drug levels and serum electrolyte levels.^6,7

If the patient is receiving repeated inappropriate shocks for a nonlethal rhythm, temporarily deactivate the implantable cardioverter-defibrillator by placing a magnet over the device. Defibrillation can be re-enabled by removing the magnet. All implantable cardioverter-defibrillators should be evaluated by a cardiologist after exposure to a magnet.

For a patient with an implantable cardioverter-defibrillator in cardiac arrest, follow all normal resuscitation measures. If defibrillation is necessary, do not place either the paddle or pad directly over the implantable cardioverter-defibrillator pulse generator. Perform CPR in the usual fashion. If the implantable cardioverter-defibrillator discharges during CPR, the CPR provider may perceive a small electrical shock, but the shock is neither uncomfortable nor dangerous.

DISPOSITION AND FOLLOW-UP

Admission criteria depend on the reason for shock delivery; consultation with the treating cardiologist is key. The ICD can be interrogated by external telemetry devices that are manufacturer and model specific. General admission recommendations include any sign of cardiovascular instability, a history of two or more shocks in a 1-week period, correctable causes of dysrhythmia, and any sign of infection or mechanical disruption of the implantable cardioverter-defibrillator or lead system.

Acknowledgment: The authors thank Edward S. Bessman, author of this chapter in the previous edition.