WE HEAR IT everywhere we practice: cardiovascular disease is on the rise. The population of patients we serve is also aging. By 2025, 20% of Americans will be over the age of 65.1 As the population ages, so does the incidence of cardiovascular disease and its inherent complications. For patients at risk for a potentially lethal ventricular dysrhythmia such as ventricular tachycardia or ventricular fibrillation (VF), an implantable cardioverter defibrillator (ICD) is the only way to treat these dysrhythmias and reduce the risk of death. ICDs have been shown to decrease mortality in cardiac patients by more than 50%, and are considered superior to conventional antiarrhythmic medical therapy for secondary and primary prevention of sudden cardiac death (SCD).2,3 Because of the rising prevalence of both coronary artery disease and associated ventricular dysrhythmias, the prevalence of ICDs has also risen.4 Knowing how ICDs work and how to troubleshoot them is crucial. This article will review ICDs and focus on how to handle ICD lead fracture, a complication with serious adverse effects. For a review of ICD basics, see About ICDs.
Maintaining an ICD
Over the years, ICDs have evolved into sophisticated devices that can sense, evaluate, treat, and monitor all types of cardiac dysrhythmias, and also can perform all of the functions of a conventional pacemaker. Because of their complexity and ability to defibrillate the heart, ICDs require frequent monitoring by the practitioner to help detect complications.3 Unlike traditional pacemakers, which are interrogated in the office once a year and by telephone every 3 to 6 months, ICDs need to be interrogated in the office every 3 to 6 months to ensure they're functioning properly.5 The time interval between follow-up will also depend on health insurance, but long intervals between follow-up visits could jeopardize the patient's health.3
What's a lead fracture?
A lead fracture, or a break in the integrity of one of the ICD's pacer leads used in the ICD system, is considered the major complication of ICD therapy.6 A lead fracture often causes inappropriate electrical shocks (defibrillation when no true dysrhythmia is present), improper dysrhythmia detection, or ineffective electrical shocks when true dysrhythmias are present.7 These outcomes can cause decreased quality of life and increased patient mortality.
A common cause of lead fractures is mechanical stress.8 This can be caused by improper positioning of the lead-for example, if the lead doesn't have enough slack inside the constantly moving heart, each heartbeat puts mechanical stress on the lead, making it susceptible to fracture over time. Direct stress, such as from compression, friction, kinking, and stretching, also can cause fracture.8 If the lead travels through the costoclavicular space, for example, fracture can occur between the first rib and the clavicle due to compressive forces by other body structures in this area.8
Other areas prone to fracture are near the anchoring sleeve (where the lead enters the venous system) and at the site of the fixation suture.6,8 The longer the lead is in a patient, the more likely it is to fracture. Also, older model leads, not developed as well as the leads of today, may be more likely to fracture. Finally, manufacturing errors or misjudgments can cause leads to fracture sooner than expected. Certain types of leads have been recalled over the years due to their higher fracture likelihood.
How to detect a lead fracture
A lead fracture in an ICD system is a serious problem. Fortunately, there are a number of clues that can alert you to trouble ahead of time. Start by interrogating the ICD, using the special computer made by the ICD manufacturer that can communicate directly with the ICD. Interrogation lets you gather all of the important data in the ICD.3
* Variable or high impedance is often one of the first detectable signs of a lead fracture.7,9,10 Impedance (also called resistance) is opposition to the flow of an electrical current, and is measured in ohms. Impedance keeps the electrical signal on the proper path down an electrical wire, and should be fairly uniform at all times. If the ICD detects a large increase in a lead's impedance, or extremely high impedance, something (such as an impending lead fracture) may be starting to block the electrical signal down the lead. This is why pacing and shock impedance testing is routinely performed during outpatient ICD visits.11 If this change in impedance can be detected before an actual fracture occurs, inappropriate shocks can be avoided.
* Noise in one of the ICD leads is the most obvious clue to a lead fracture. The partial lead fracture lets outside vibrations and movements enter the wire itself, which shows up on the electrogram as small, tight waveforms. In most cases, the ICD is unable to detect this as a mechanical problem and sees it as a form of inappropriate dysrhythmia.12 The noise is seen as uniform or multifocal waveforms, often very close together. Because of the small intervals (measured in milliseconds) between the waveforms, the ICD often interprets this noise as VF, which may cause one or more inappropriate shocks.11,12 This is not only uncomfortable for the patient, but, over time, can cause SCD, especially if the inappropriate shock comes during a period of ventricular vulnerability (the T wave) and triggers a fatal dysrhythmia.11
* Changes in the ICD's ability to sense R waves is another indication of possible lead problems.10 The ICD measures the height (or amplitude) of the R wave, and this sensed R wave helps the device analyze the patient's underlying rhythm and knows when it needs to act on a rhythm change. If a lead is starting to fracture, the ICD may not be able to measure the wave properly, causing a variable amplitude that can confuse the ICD and its treatment protocols. An ICD that can't sense the patient's underlying rhythm won't know when to deliver a shock appropriately.
Because these measurements are so important to ensure proper ICD function, some clinicians feel that a follow-up in the office once every 3 months isn't enough for patients with ICDs. Only about 35% of ICD failures are caught in the office before something happens. One method of interrogation is via a home monitoring device. All of the current ICD manufactures have home monitoring devices that use a landline phone to transmit measurements from the pacemaker to the clinician's office to be analyzed. The clinician also can look at the stored electrograms to monitor for dysrhythmias, and determine whether the ICD was able to fix any dysrhythmias that occurred.3,4
|Figure. About ICDs|
Daily measurements are also taken by the ICD itself, without the patient having to do anything or even be aware of them, for another layer of security. These measurements include battery status, pacing (amplitude), and high-voltage impedance.11 Most ICDs also have an audible alarm that can sound for conditions such as a very low battery, large variation in the R wave amplitude, or too much variability in impedance measurements. This alarm can be heard during the night by the patient, and provides another warning that something may be wrong. Patients are taught to come into the clinic if they hear this alarm.
As more and more high-risk patients receive ICDs, knowing how to analyze the data is even more important. Following these simple steps can help you get through any device interrogation and help you provide better care for patients with ICDs.