Assessment of Implantable Devices

Characteristics of AICDs

The basic components of an AICD, including sensing electrodes, defibrillation electrodes, and a pulse generator ( Fig. 13.2 ), can be seen on a chest radiograph. Transvenous electrodes have obviated the previous need for surgical placement. They are inserted into the pectoralis muscle. Many transvenous systems consist of a single lead containing a distal sensing electrode and one or more defibrillation electrodes in the right atrium and ventricle. Leads are inserted through the subclavian, axillary, or cephalic vein into the right ventricular apex. The left side is preferred because of a smoother venous route to the heart and a more favorable shocking vector. In an effort to improve the efficiency of defibrillation, an additional defibrillation coil may be used. Various placements of AICDs are demonstrated in Fig. 13.3 .

The pulse generator is a sealed titanium casing that encloses a lithium–silver–vanadium oxide battery. It has voltage converters and resistors, capacitors to store charge, microprocessors and integrated circuits to control analysis of the rhythm and delivery of therapy, memory chips to store electrographic data, and a telemetry module. Although a pacemaker can draw the voltage required for function from its component battery, the energy needed for defibrillation requires a battery that is prohibitively large. To circumvent this problem, an AICD contains a capacitor that maximizes the voltage required by transferring energy from the battery before discharge. To achieve the energy required, AICDs use capacitors that are charged over a period of 3 to 10 seconds by the battery and then release this energy rapidly for defibrillation. The maximal output is 30 J in most units and 45 J in higher-energy units. This energy is high enough that a discharge is very obvious and often distressing to the patient.

Most AICDs use a system in which the pulse generator is part of the shocking circuit, often described as a “can” technology, and most of them have a dual-coil lead with a proximal coil in the superior vena cava and a distal coil in the right ventricle. Current flows in a three-dimensional configuration from the distal coil to both the proximal coil and the generator. This dispersion of the electrical field increases the likelihood of depolarizing the entire myocardium at once, thereby leading to successful defibrillation.

AICDs may have the same programming capabilities as pacemakers and can be single chambered, dual chambered, or used with the aforementioned CRT. Single-chamber devices have only a right ventricular lead. They often have difficulty identifying atrial arrhythmias, which can result in inappropriate defibrillation of atrial tachycardias. Dual-chamber AICDs have right atrial and right ventricular leads and improved ability to discriminate rhythms. In most studies, dual devices have been found to offer improved discrimination between ventricular and supraventricular arrhythmias, thus decreasing inappropriate shocks as a result of rapid supraventricular rhythms or physiologic sinus tachycardia. Of AICDs implanted annually 27% are single chamber, 32% dual chamber, and 41% are CRT systems. In patients requiring both AICD and pacemaker functions, both these devices are placed together. The advent of technology has allowed placement of a single device that can perform both pacemaker and defibrillator functions.

AICDs use a combination of antitachycardia pacing, low-energy cardioversion, defibrillation, and bradycardiac pacing in a combination also known as tiered therapy . They are programmed with specific algorithms that identify and treat specific rhythms. Ventricular arrhythmias may initially be converted (or undergo attempts at conversion) with antitachycardiac pacing as opposed to immediate defibrillation. This overdrive pacing may terminate the rhythm without the need for electrical defibrillation in up to 90% of events. It is most successful for terminating monomorphic ventricular tachycardia with a rate of less than 200 beats/min. Overdrive pacing is better tolerated by patients than cardioversion and reduces the risk for inducing atrial fibrillation. These events may be silent, not felt by the patient, and discovered only by interrogating the device.

If unsuccessful, the next intervention may be low-energy cardioversion (<5 J). The device may be programmed to very low levels of electricity that are better tolerated by the patient. This works best for ventricular rates higher than 150 and lower than 240 beats/min. This may be followed by a high-energy defibrillation. Traditionally, the energy level of the first shock is set at least 10 J above the threshold of the last defibrillation measured. If the first shock fails a backup shock may be required, but this may induce or aggravate ventricular arrhythmias (see the later section on Pacemaker-Mediated Tachycardia ). Unlike the proarrhythmic effects of medication, these arrhythmias are almost never fatal, although they may be associated with increased morbidity. Currently used biphasic waveforms have improved defibrillation thresholds.

This tiered approach obviates the need for unnecessary energy requirements. The devices also have antibradycardiac pacing that allows these patients to have one device instead of separate units. Additional complications associated with AICDs that have antibradycardiac pacing algorithms include a tendency toward oversensing, increased current drain, potential detection problems, and an increased incidence of hardware and software design problems. At the time of insertion, the amount of energy required for various AICD functions, such as the defibrillation threshold, is determined for any given patient, and output and sensing functions can be adjusted by reprogramming as needed.

In 2012 the Food and Drug Administration approved the use of a subcutaneous defibrillator. This consists of a subcutaneous lead with a shock coil flanked by two sensing electrodes; this coil is tunneled along the parasternal margin from the pulse generator that is implanted in a subcutaneous pocket. It is placed by anatomic landmarks and as such no fluoroscopy is required. It delivers an 80-J biphasic shock across the precordium. Fig. 13.4 is a radiograph of a patient with a subcutaneous defibrillator.

The subcutaneous implantable device may be an alternative to patients who have difficult vascular access or who are at risk for transvenous complications. These could include the pediatric populations, patients with congenital disease, complicated vascular anatomy, patients at high risk for infection, or patients on dialysis. The notable limitations of the subcutaneous internal defibrillator when compared with conventional devices is the inability to provide long-term bradycardia pacing or antitachycardiac pacing. Patients that need resynchronization therapy are also unsuitable candidates.

Left Ventricular Assist Devices

As the management of heart failure continues to evolve, emergency physicians may encounter patients who have surgically implanted ventricular assist devices (VADs). VADs are implanted pumps used to assist a failing ventricle. Although they have been used in the right ventricle and for biventricular support, they are primarily used as a left ventricular assist device or LVAD. The first-generation of these devices utilized a pulsatile flow pump. The second and third devices provide continuous flow through the use of axial flow or centrifugal pumps. These devices are smaller and more durable, with higher energy efficiency than the first generation models, which are no longer in use.

Initially used as a bridge to heart transplantation, they are now also being used as a bridge to recovery of heart function (such as in patients with myocarditis) and for permanent (destination) therapy. The LVAD consists of an inflow cannula that is placed in the apical part of the ventricle and allows blood to flow to a pump. The pump is attached to the outflow cannula, which is placed in the ascending aorta. The device is connected through a percutaneous lead or driveline to batteries located outside the body. Figs. 13.5 and 13.6 demonstrate the components of a left ventricular device.

History

Although patients who go to the ED because of implantable pacemaker–related issues may have one or more of several complaints, those with AICD-related issues are generally seen because their device has discharged. They will often describe a sensation of being kicked or punched in the chest, and the sensation is not subtle. In fact, some patients live in fear of the shock after having experienced it previously, and this is one reason for removal of the device. Ask the patient about the number of discharges and associated symptoms, including chest pain, shortness of breath, lightheadedness, palpitations, syncope, extremity edema (raising concern for congestive heart failure or lower extremity deep vein thrombosis), or dyspnea on exertion. In addition, elicit general symptoms such as fever, chills, nausea, or vomiting, which could be indicative of infection. Inquire about medication history. Ask about the specific implanted device that they possess. Most pacemaker and AICD patients should have an identification card on their person that will identify the manufacturer, model number, lead system, and a 24-hour emergency contact number. Sophisticated information and the prior electrical events and settings of the device can be ascertained in the ED by simply placing an external interrogating device over the unit.

When a patient with an LVAD arrives in the ED, the provider should identify who the patient's cardiologist and cardiothoracic surgeon are. In most instances contact should be made with these providers and the patient may need to be transferred to a center with expertise in these devices.

Physical Examination

First, assess for airway patency, adequate ventilation, and cardiovascular status. The patient's mental status may also be an important clue to the severity of the symptoms. Perform an appropriate physical examination with emphasis on examining the heart and lung to seek murmurs, pericardial friction rubs, and evidence of pulmonary effusion or other abnormalities. Inspect the pacemaker or AICD site for erythema or edema. Palpate the skin for evidence of obvious lead abnormalities. Examine the extremities for evidence of edema or erythema.

There are several unique characteristics to consider in patients with ventricular assist devices. On physical exam, the provider should listen to the chest to determine if an audible “hum” is present. Patients lack a palpable pulse given the device's continuous flow and because their cardiac function at baseline is poor. The blood pressure should be obtained by using a Doppler on either the radial or brachial artery. Inflate the cuff until you can't hear flow. The cuff pressure should then be released, listening for the presence of flow. There is some degree of controversy about whether this is mean arterial pressure (MAP) or systolic pressure, however. When in doubt, an arterial line would be the most accurate method to identify MAP. MAP is usually kept between 70 to 80. Pulse oximetry is also unreliable and a true measurement should be obtained with an arterial blood gas or arterial line.

Radiography

The imaging modality of choice, at least initially, in a patient with complaints related to an implanted pacemaker or AICD is the plain chest radiograph. If the patient's stability is in question, obtain a portable anteroposterior film. In addition to the standard cardiac, pulmonary, vascular, and skeletal evaluation, this study will probably confirm the location of the pulse generator case, as well as the current location of any leads. Survey the radiograph for evidence of lead fracture or displacement (see Fig. 13.1 E ). Compare with previous chest films. If the patient does not have a device identification card in immediate possession, take an overpenetrated radiograph to look for a radiopaque marker identifying the model type. Fig. 13.9 demonstrates a radiograph of a patient with an LVAD.

Electrocardiography

Perform an ECG to seek evidence of pacing, ectopy, and ST-segment or T-wave abnormalities. A comparison ECG may be helpful. If an AICD shock has been delivered, the shock itself can cause transient electrocardiographic changes, and waiting for several minutes to repeat the ECG may identify whether the changes are caused by the discharge or an ongoing disease process.

Cardiopulmonary Resuscitation, ACLS Interventions, and External Cardiac Defibrillation in Patients With Implanted Pacemakers or AICDs or LVADS

In general, ACLS interventions may be performed safely and effectively in patients with pacemakers and AICDs when indicated. Cardiopulmonary resuscitation (CPR) can usually be performed in standard fashion. If an AICD is present, rescuers may notice mild electrical shocks while performing CPR; these shocks are harmless to the rescuer. If the AICD shocks are impeding rescuer performance of CPR, or if supraventricular tachycardias are noted during resuscitation, disable the AICD by applying a magnet over the corner of the device from which the leads emerge. This location is generally found easily by palpation but may be located blindly by slowly relocating the magnet until AICD activity ceases.

External cardiac defibrillation may be performed safely in patients with pacemakers and AICDs with the standard expected efficacy; however, it is recommended that external paddles or defibrillator pads be placed at a location approximately 10 cm distant from the pulse generator if possible. A transcutaneous cardiac pacemaker may also be used in similar fashion with a recommendation that the pacing pads be placed in anatomically appropriate locations but preferably at a distance of 10 cm from the pulse generator.

Placing the external defibrillation or transcutaneous pacemaker pads in an anteroposterior configuration is advised because this configuration may circumvent energy shunting and shielding. Every attempt should be made to avoid application of the defibrillators directly over the device. Use of the lowest possible energy setting for cardioversion or defibrillation is recommended. If available, biphasic cardioverter defibrillators are further suggested. In the event of successful resuscitation and return of spontaneous circulation, the pacemaker or AICD should be interrogated expeditiously by a cardiologist or electrophysiologist to ensure that no damage was sustained as a result of the resuscitation effort.

Regarding pharmacologic adjuncts, amiodarone has been reported to be more effective for the treatment of potentially lethal arrhythmias in the setting of implanted devices. Antiarrhythmic medications may be required for a resistant malignant rhythm when the AICD is functioning properly but the arrhythmia persists ( Fig. 13.10 G ).

Several additional considerations are unique to the setting of ACLS in patients with a pacemaker or AICD. In cases of acute myocardial infarction involving areas of the myocardium in contact with the pacemaker leads, the implanted pacemaker may experience operative failure. Therefore, maintain a high level of suspicion for the potential requirement for supplemental transcutaneous or transvenous cardiac pacing.

LVAD patients that go into cardiac arrest offer some unique challenges. Ensure that the machine is connected, the battery charged, and the machine has an audible hum. The lack of peripheral pulses also make the use of ACLS problematic. In the case of arrest, follow the standard Advanced Cardiac Life Support algorithms. There is concern that standard CPR may dislodge the cannula or damage the unit's outflow conduit, but there have also been reports that this is not as harmful as previously thought. There was also a report of an emergency physician who replaced the driveline connections in a patient who had cut the wires resulting in subsequent resumption of the pump function.

In the postresuscitation setting involving a patient with an implanted pacemaker or AICD, it is important to maintain a higher index of suspicion for device lead fracture or disruption resulting from CPR. Finally, in the postresuscitation phase the clinician should closely watch for the development of pneumothorax, hemothorax, pericardial effusion, or other aforementioned pathophysiologic processes that could adversely affect the function of the implanted device.