Detecting and distinguishing cardiac-pacing artifacts
When a heart patient with an implanted pacemaker undergoes electrocardiogram testing, the cardiologist must be able to detect the presence and effects of the pacemaker (see sidebar, “When the heart’s electrical subsystem malfunctions”). A simple implanted pacer’s activity is generally not perceptible on a normal ECG trace, because the very fast pulses—with typical widths of hundreds of microseconds—get filtered due to low-bandwidth display resolution (monitor/diagnostic 40-/150-Hz bandwidths). The pacer’s signal, however, can be inferred through the changed morphology of the ECG trace, which is representative of the heart’s own electrical activity as recorded at the skin surface via ECG leads.
It is important to be able to detect and identify pacing artifacts because they indicate the presence of the pacemaker and help in evaluating its interaction with the heart. But the artifacts’ small amplitude, narrow width, and varying waveshape make them difficult to detect, especially in the presence of electrical noise that can be many times their amplitude. At the same time, pacing therapy has become extremely advanced, with dozens of pacing modes available for single- to three-chamber pacing. Complicating the detection of pacing artifacts, pacemakers produce lead-integrity pulses, minute-ventilation (MV) pulses, telemetry signals, and other signals that can be incorrectly identified as pacing artifacts.
The use of real-time pacemaker telemetry has made the display of pacing artifacts on an ECG strip less important than it used to be. An individual skilled in pacing therapies can look at the strip and sometimes infer the type of pacing therapy being administered to the patient and determine whether the pacemaker is working properly.
In addition, all pertinent medical standards require the display of pacing artifacts, though they vary somewhat in their specific requirements for the height and width of the captured pacer signal. The applicable standards include Association for the Advancement of Medical Instrumentation (AAMI) specifications EC11:1991/(R)2001/(R)2007 and EC13:2002/(R)2007, as well as International Electrotechnical Commission specifications IEC 60601-1 ed. 3.0b:2005, IEC 60601-2-25 ed. 1.0b, IEC 60601-2-27 ed. 2.0:2005, and IEC 60601-2-51 ed. 1.0:2005.
How pacemakers pace
Implantable pacemakers (Figure 1) are typically lightweight and compact. They contain the circuitry necessary to monitor the heart’s electrical activity through implanted leads and to stimulate the heart muscle as necessary to ensure a regular heartbeat. Pacemakers must be low-power devices, as they operate with a small battery that typically has a 10-year lifespan. The National Academy of Engineering estimated in 2010 that more than 400,000 pacemakers are implanted in patients every year (Reference 1).
Figure 1 Pacemakers must be light, compact, low-power devices (Reference 2).
In unipolar pacing, the pacing leads consist of an electrode at the tip of a single pacing lead and the metal wall of the pacemaker housing itself. The pacing artifacts caused by this mode of pacing can be several hundred millivolts at the skin surface, with a width of up to 2 msec. Unipolar pacing is no longer commonly used, however.
In bipolar pacing, which today accounts for the bulk of pacing artifacts created, the heart is paced from the electrode at the tip of the pacing lead. The return electrode is a ring electrode located very close to the tip electrode. The artifacts that this type of lead produces are much smaller than those produced by unipolar pacing; pulses on the skin surface can be as small as a few hundred microvolts high and 25 μsec wide, with average artifacts measuring 1 mV high and 500 μsec wide. The amplitude of the artifact can be much smaller when the detection vector does not line up directly with the pacing lead vector.
Many pacemakers can be programmed for pulse widths as short as 25 μsec, but the short-pulse-width settings are typically used only in pacemaker threshold tests performed in an electrophysiology laboratory. Setting the lower limit to 100 μsec eliminates the problem of falsely detecting MV and lead-integrity (LV lead) pulses as valid pacing artifacts. These subthreshold pulses are usually programmed to be between 10 and 50 μsec.
Various types of pacemakers are available for pacing specific chambers of the heart. Single-chamber pacing delivers pacing therapy to either the right atrium or the right ventricle. Such a pacer can be either unipolar or bipolar. Dual-chamber pacing delivers pacing therapy to both the right atrium and the right ventricle. Biventricular pacing delivers pacing therapy to both the right ventricle and the left ventricle; in addition, the heart is usually paced in the right atrium.
The biventricular pacing mode can be difficult to display properly, for two main reasons. First, the two ventricle paces may occur at the same time, appearing as a single pulse at the skin surface. Second, the left-ventricle lead placement is generally not on the same vector as the right-ventricle lead and may actually be orthogonal to it. Usually, the right atrium is best displayed in lead aVF—one of the augmented limb leads—and the right ventricle is best displayed in lead II. Most ECG systems do not employ three simultaneous lead-detection circuits or algorithms, making the left ventricle the toughest lead to pick up. Thus, it is sometimes best detected in one of the V leads.