Impedance

Impedance is the sum of all factors that oppose the flow of current in an electric circuit. Impedance is not necessarily the same as resistance. The relationship between voltage (V), current (I), and resistance (R) in an electrical circuit is estimated by Ohm's law, V = IR. For circuits that follow Ohm's law, impedance and resistance are equal. If voltage is held constant, the current flow is inversely related to the resistance of the circuit (I = V7R). The leading-edge voltage of a constant-voltage pulse generator is fixed, and the lower the resistance, the greater the current flow. In contrast, the greater the resistance, the lower the current flow. Because implantable pulse generators are powered by lithium iodine batteries with a fixed amount of charge, pacing impedance is an important determinant of battery longevity.

The total pacing impedance is determined by factors that are related to the lead conductor (resistance), the resistance to current flow from the electrode to the myocardium (electrode resistance), and the accumulation of charges of opposite polarity in the myocardium at the electrode—tissue interface (polarization). Thus, the total pacing impedance (Ztotal) = Zc Ze Zp, where Zc is the conductor resistance, Ze is the electrode resistance, and Zp is the polarization impedance. The resistance to current flow provided by the lead conductor results in a voltage drop across the lead with a portion of the pacing pulse converted into heat. Thus, this component of the total pacing impedance is an inefficient use of electrical energy and does not contribute to myocardial stimulation. The ideal pacing lead would have a very low conductor resistance (Zc). In contrast, the ideal pacing lead would also have a relatively high electrode resistance (Ze) to minimize current flow and maximize battery life.65 The electrode resistance is largely a function of the electrode radius, with higher resistance provided by a smaller electrode. An electrode with a small radius minimizes current flow in an efficient manner. In addition to providing a greater electrode resistance, pacing electrodes with a small radius provide increased current density and lower stimulation thresholds. Because of these properties, newer pacing and implantable cardioverter-defibrillator (ICD) leads take advantage of smaller electrodes to increase electrode resistance, allowing the total pacing impedance to exceed 1000 ohms. Compared with a standard pacing lead with a total impedance of 500 ohms, a lead with 1000 ohms of impedance would decrease current drain of each pacing pulse by 50%, thereby prolonging the usable battery life of the implantable pulse generator. The routine use of these leads will allow implantable devices to become smaller while maintaining battery longevity. There is a practical lower limit to the size of the electrode, which is related to the likelihood of maintaining stable contact of the electrode with the endocardium throughout the cardiac and respiratory cycles. For example, pacing leads with a very small distal electrode may be associated with a relatively high pacing threshold in a small proportion of patients (<5%), probably as a result of radiographi-cally imperceptible "microdislodgement" of the electrode as it contacts the endocardium.

The third component of pacing impedance, polarization impedance, is an effect of electrical stimulation and is related to the movement of charged ions in the myocardium toward the cathode. When an electrical current is applied to the myocardium, the cathode attracts positively charged ions and repels negatively charged ions in the extracellular space. The cathode rapidly becomes surrounded by a layer of hydrated Na+ and H3O- ions. Farther away from the cathode, a second layer forms of negatively charged ions (Cl-, HPO42-, and OH-). Thus, the negatively charged cathode induces the accumulation of two layers of oppositely charged ions in the myocardium. Initially, the movement of charged ions results in the flow of current in the myocardium. As the cathode becomes surrounded by an inside layer of positive charges and an outside layer of negative charges, a functional capacitor develops that impedes the further movement of charge. The capacitive effect of polarization increases throughout the application of the pulse, peaking at the trailing edge and decaying exponentially following the pulse as charged layers dissolve into electrical neutrality (Fig. 2.10). Because polarization impedes the movement of charge in the myocardium, it is inefficient and results in an increased voltage requirement for stimulation. Thus, polarization impedance reduces the effectiveness of a pacing stimulus to stimulate the myocardium and wastes current. Polarization impedance is directly related to the duration of the pulse and can be minimized by the use of relatively short pulse durations. Polarization is inversely related to the surface area of the electrode. To minimize the effect of polarization (Zp) but maximize electrode resistance (Ze), the surface area of the electrode can be made large but the geometric radius small by the use of a porous coating on the electrode.66-68 Electrodes constructed with activated carbon,69,70 or coated with platinum black71 or iridium oxide, are effective in minimizing the wasteful effects of polarization and in diminishing afterpotentials, which can interfere with sensing.

The evolution of pacing impedance is usually characterized by a fall over the first 1 to 2 weeks following implantation.32 The chronic pacing impedance then rises to a stable value that is, on average, approximately 15% higher than that at implant. Serial measurements of pacing impedance are extremely valuable for the assessment of lead integrity; low impedance measurements usually reflect a failure of conductor insulation, and high values often suggest conductor fracture or a loose set-screw at the proximal connector. It should be emphasized that the method of measurement greatly influences the impedance value. For example, if the pacing impedance is measured at the leading edge of the pulse, the value reflects Zc and Ze but not Zp. In contrast, measurements near the midpoint of the pulse are a more accurate reflection of total pacing impedance. For clinical purposes, serial assessments of impedance should use a consistent method of measurement.

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