Resistance is opposition to the flow of electric current, and one aspect of cardiac pacing that often confuses the beginner is that resistance or impedance is not static during a pacemaker spike; the resistance increases with time. The rise of polarization resistance with time occurs because electricity is being conducted through a wire, through an electrolyte solution (i.e., the body), and then back through another wire. The tip of a bipolar wire is negatively charged and the ring or band electrode is positively charged. Positive ions in the electrolyte solution begin to travel toward the negatively charged metal surface and negative ions in the solution travel toward the positive electrode. This polarization effect resists the flow of electricity through the circuit. The longer the current is turned on, the greater is the extent of polarization (as shown in Fig. 3-1); hence, the polarization resistance increases with time. When the electricity is turned off, these polarized ions are apart for a brief period, and as they “re-mix” a transient current called after-potential is created—in effect, the polarization has created a brief “battery” with separation of the positive and negative ions.

The polarization effect becomes less prominent as the area of exposed metal surface increases. This inverse relationship explains why the impedance in a unipolar pacemaker is lower than in a bipolar pacemaker. Although in the unipolar system, electricity must travel a longer distance through the
body, the electrolyte solution that comprises intracellular and extracellular fluid is a good conductor of electricity and is not a major source of resistance. The distal electrode tips are small in both the unipolar and bipolar systems; however, the large exposed metal plate of the unipolar anode (as opposed to the small surface area of the bipolar band anode) causes the polarization resistance to be lower in the unipolar system.

As discussed in Chapter 2, the smaller the electrode tip, the more concentrated the electric charge. With more concentrated charge, the heart muscle can be stimulated with lower energy levels. If this were the only consideration, the smallest tip would be the best; in fact, electrode tips are now much smaller than they were initially. If an electrode tip is made too small, however, the increase in polarization resistance becomes a problem, especially with sensing. In sensing, the electricity generated by the heart itself must travel through the pacemaker circuit to be sensed; because the signal generated is a fairly weak one, its further attenuation by polarization resistance may result in nonsensing. Present-day pacemaker electrodes have exposed metal pacing
tips that are as small as practical to permit maximum charge concentration without causing sensing problems.

A subtle feature of pacemaker technology, related to polarization resistance, is that there is a recharge. This allows a reversal of charge between the anode and cathode just after the pacemaker spike. This is a very low voltage charge for about 10 msec; it is done by “shorting out” the anode and cathode, and it is not energy dependent. A recharge basically leads to a very low voltage for about 10 msec (much longer than the pacer spike itself, but at a much lower voltage) (Fig. 3-2). This reduces electrolytic coating of the exposed metal in the pacemaker leads. Recharging is essentially invisible to the patient and the physician following the patient and is mentioned only as an example of one of the complex problems that pacemaker engineers face in designing a functional device capable of years of metal–living tissue interaction.

Most permanent pacemakers are constant-voltage pacemakers because the voltage remains fairly constant throughout the pacemaker spike. In practice, no permanent pacemaker can maintain an exactly even voltage throughout the pacing spike. Voltage at the leading edge of the spike is higher than at the trailing edge because the capacitor in the pacemaker is necessarily small and cannot store a charge large enough to maintain a strictly constant voltage (this drop in voltage is referred to as tilt and is of greater clinical importance in implantable cardioverter defibrillators). Because of this limitation, the more accurate term constant-voltage capacitor-coupled pacemaker is sometimes used. Figure 3-3 compares a constant-voltage spike with a constant-voltage capacitor-coupled spike and Figure 3-4 describes a single constant-voltage capacitor-coupled pacemaker spike in terms of typical charges in voltage, impedance, and current.

Another type of pacemaker is the constant-current pacemaker, which is designed to provide a constant current despite variation in impedance. As impedance rises, voltage is increased to maintain a constant current. (Again recall that according to Ohm’s Law, V = iR.) The increase in voltage to
maintain a constant current in the face of rising impedance is limited by the voltage capacity of the pacemaker. For example, a pacemaker programmed to deliver a current of 10 mA with a 5 V battery could maintain a constant current of 5 mA only if the impedance of the system remained below 500 ω. Once impedance rises above 500 ω, the battery simply generates its maximum power of 5 V and acts like a constant-voltage pacemaker. Because of
this limitation, the more accurate term constant-current voltage-limited pacemaker is used often (Fig. 3-5).

Constant-current pacing in the past was used in some permanent pacemakers and it is used in almost all external pacemakers for temporary pacing. The power source of most external pacemakers supplies approximately 15 V; so a true constant current can be maintained in the face of considerable increases in impedance. Batteries in these temporary pacemakers are fairly large and have a short life span compared with those in permanent pacemakers, but neither size nor life span is a major consideration in temporary pacemakers.

The threshold in cardiac pacing is the minimal electric stimulus required to cause cardiac muscle contraction. Having made that general statement, an accurate description of threshold is more complex.