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leads—and the right ventricle is best displayed in lead II. Most ECG systems do not employ three simultaneous leaddetection 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. artifact w avef orms Most pacing pulses have very fast rising edges. The rise time measured at the pacemaker output is generally about 100 nsec. When measured at the skin surface, the rise time will be slightly slower because of the inductance and capacitance of the pacing lead. Most pacing artifacts at the skin surface are on the order of 10 μsec or less. As complex devices with built-in protection, pacemakers can produce high-speed glitches that do not affect the heart but do affect pacemaker-detection circuits. Figure 2 shows an example of an ideal pacing artifact. The positive pulse has a fast rising edge. After the pulse reaches its maximum amplitude, a capacitive droop follows, and then the trailing edge occurs. The artifact next changes polarity for the recharge portion of the pacing pulse. The recharge pulse is required so that the heart tissue is left with a net-zero charge; with a monophasic pulse, ions would build up around the electrodes, creating a dc charge that could lead to necropsy of the heart tissue. Introducing cardiac-resynchronization devices adds another degree of complication in detecting and displaying pacing artifacts. These devices pace the patient in the right atrium and both ventricles. The pulses in the two ventricles can fall close together, overlap, or occur at exactly the same time; the left ventricle can even be paced before the right ventricle. Currently, most devices pace both ventricles at the same time, but studies have shown that adjusting the timing will benefit some patients by yielding a higher cardiac output. Detecting and displaying both pulses separately is not always possible, and many times the pulses will appear as a single pulse on the ECG electrodes. If both pulses were to occur at the same time with the leads oriented in opposite directions, the pulses could cancel each other out on the skin surface. The probability of such an occurrence is remote, but one can envision the appearance on the skin surface of two ventricle-pacing artifacts with opposite polarities. If the two pulses were offset by a small time interval, the resulting pulse shape might be very complex. Figure 3 shows scope traces of a cardiac-resynchronization device pacing in a saline tank. This is a standard test environment for pacemaker validation, designed to mimic the conductivity of the human body. The proximity of the scope probes to the pacing leads causes the amplitudes to be much larger than what would be expected on the skin surface, however, and the low impedance that the saline solution presents to the ECG electrodes results in much less noise than would normally be seen in a skin-surface measurement. The first, second, and third pulses shown in the figure (l to r) are the atrial, right-ventricle, and left-ventricle pulses, respectively. The leads were placed in the saline tank with vectors optimized to see the pulses clearly. The negativegoing pulse is the pace; the positivegoing pulse is the recharge. The amplitude of the atrial pulse is slightly larger pacin g artifacts ’ smal amplitude, narow width, and varyin g waveshapes make them dificult to detect. than the two other pulse amplitudes because the lead was in a slightly better vector than the ventricle leads; in actuality, all three pacing outputs in the resynchronization device were programmed to have the same amplitude and width. With real patients, the amplitudes and widths are often different for each pacemaker lead. artifact detec tion It is impossible to detect all pacing artifacts and reject all possible noise sources in a cost-effective manner. Among the challenges are the number of chambers that the pace detection must monitor, the interference signals encountered, and the wide variety of pacemakers in use. Solutions for detecting artifacts may range from hardware implementations to digital algorithms. The pacing leads for cardiac-resynchronization devices will not all have the same vector. The right-atrium lead usually aligns with lead II, but it can sometimes point straight out of the chest, so a Vx (precordial lead) vector may be needed to see it. The right-ventricle lead is usually placed at the apex of the right ventricle, so it usually aligns well with lead II. The left-ventricle pacing lead, threaded through the coronary sinus, is actually on the outside of the left ventricle. This lead usually aligns with lead II but may have a V-axis orientation. The pacing leads of implantable defibrillators and resynchronization devices are sometimes placed in areas of the heart that have not had an infarction. Placing them around infarcts is the main reason that this system uses three vectors and requires a high-performance pacing-artifact detection function. A major noise source is the H-field telemetry scheme used in most implantable heart devices. Other sources of noise are transthoracic-impedance measurements for respiration, electric cautery, and conducted noise from other medical devices connected to the patient. Complicating the problem of acquiring pacing artifacts, each pacemaker manufacturer uses a different telemetry scheme. In some cases, a single manufacturer may use different telemetry systems for different implantable-device models. Many implantable devices can communicate using both H-field telemetry and either ISM- or Medical Implant Communication Service (MICS)-band telemetry. The variability of H-field telemetry from one model to the next makes filter design difficult. ECG devices have to be Class CF—the most stringent classification—as there is direct conductive contact with the heart, whereas other medical devices may be built to less stringent Class B or BF requirements, and their higher leakage currents may interfere with the performance of ECG-acquisition devices. artifact-detec ting A FE The ADAS1000 (Figure 4) is a fivechannel analog front end designed to 30 EDN Europe | MARCH 2013 www.edn-europe.com


EDNE MARCH 2013
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