Cardiac Electrical Stimulation

1 Cardiac Electrical Stimulation



Pathologic cardiac conditions such as asystole, bradycardia, conduction block, and dyssynchronous contraction may lead to significant morbidity and mortality. One of the most influential and effective medical device therapies developed in the last century is the cardiac pacemaker and implantable cardioverter-defibrillator (ICD). The lives of millions of patients have been improved and extended through the use of pacing techniques that regulate heart rate and improve cardiac output.


Direct electrical stimulation of excitable cardiac tissue causes a change in the transmembrane potential of resting cardiac cells. If the transmembrane potential is raised to a certain level, an action potential is initiated in the cells and spreads to adjacent working myocardial cells. The action potential induces mechanical contraction of the cardiac cells that spreads throughout the heart as a self-propagating wavefront.


This chapter reviews the fundamental concepts of artificial electrical cardiac stimulation, including the cellular aspects of myocardial stimulation, the influence of external current on cardiac tissue, waveform and electrode considerations, clinical applications and considerations, and ongoing research regarding cardiac stimulation.



image Concepts Related to Electrical Stimulation of the Heart



Static Electric Charge And Electric Fields


Physical objects may acquire an electric charge when they have a net excess or deficit of electrons relative to the number of protons. When the number of electrons exceeds the number of protons, the object is said to have a negative charge, whereas a net deficit of electrons results in the object acquiring a positive charge. An electrically charged object is surrounded by an electric field such that charged objects act at a distance on other objects having similar or opposite charge. The strength of the electric field is related to the magnitude of its charge. A gravitational field also acts at a distance, but an electric field has an important difference, polarity. Thus, electric fields have directionality with the convention that field lines are drawn away from positively charged and toward negatively charged objects. The electric fields surrounding charged objects interact with each other such that the presence of two electrically charged objects results in a force that either attracts the two objects (if they have opposite charges) or repels the two objects (if they have the same qualitative charge). This attractive or repulsive force acts at a distance, not requiring that the charged objects be in contact, and the attractive or repulsive force (F) is given by Coulomb’s law:



image



where Q1 and Q2 are the magnitude of the charges (measured in coulombs), r is the distance separating the charged objects, and k is a constant equal to image with ε being the permittivity of free space. According to Coulomb’s law, the force attracting or repelling charged objects increases with the magnitude of charge and decreases with the square of the distance that separates them.





Electric Circuit


An electric circuit is an electric charge–conducting pathway that ends at its beginning. For electric circuits involved in myocardial stimulation by pacemakers or ICDs, the voltage difference in the circuit is provided by a battery. The difference in voltage generated by the battery results in a flow of charge (current) from the pulse generator through the conductors in the leads, electrodes, extracellular electrolytes, cell membranes with highly regulated transmission of charged ions in both directions through the membrane, and intracellular ions and charged molecules. As current flows through the complete electric circuit, the net voltage change must be zero (Kirchoff’s voltage law). Thus, the electromotive force (voltage) generated by the battery (an increase in potential energy) must be completely dissipated (a decrease in potential energy) as current flows through all the elements of the circuit to end at the battery.


Electric circuits in the clinical practice of pacing have multiple elements, including the pulse generator battery, the lead conductor(s), the electrodes, the myocardium, and blood within the great veins and cardiac chambers. All these elements introduce opposition to the flow of current.





Electrode Polarity


All defibrillator and pacemaker electric circuits have both a positively charged electrode (the anode) and a negatively charged electrode (the cathode). The negatively charged cathode is typically the tip electrode on a pacing lead. Electrons from the pulse generator flow through the cathode-tissue interface and return to the anode, which may be located on a pacing lead or the pulse generator casing.


The terminology used for electrode polarity may be confusing as applied to lead electrodes and the electrodes of a battery. The electrode in a battery at which oxidation occurs (e.g., oxidation of lithium to yield Li+ plus an electron, e) is the battery anode. The battery anode, by continuing oxidation, furnishes electrons to the circuit external to it. Therefore, in contradistinction to the terminology used for pacing leads, the terminal of the battery where electrons are provided to the circuit is the battery anode. From the battery anode, electrons flow through the circuitry and eventually enter the pacemaker lead electrode that is in contact with the myocardium. This electrode, receiving electrons from the pulse generator and furnishing electrons to the tissue, is the lead cathode. The return electrode located in the heart or on the pulse generator casing is the lead anode. It collects electrons from the tissue and returns them through the pulse generator circuitry to the positive electrode of the battery, the battery cathode, where reduction occurs (e.g., I2 + 2e yields 2I). The consistency in the terminology is that, when oxidation occurs, it occurs at an anode, and in the circuitry, an anode connects to a cathode that subsequently connects to another anode, and so on.




Capacitance


As mentioned earlier, current can be carried in different ways. Electrolyte is used as a generic term for the electrically conductive extracellular and intracellular fluids near pacemaker or defibrillator electrodes and elsewhere in the heart and blood vessels. The electrolyte conducts ions but not electrons. The difference between the conduction of electric currents by electrolytic fluids and the flow of electrons over metal wires is crucial to pacing. Because a negatively charged pacing electrode in contact with the endocardium is surrounded by blood and interstitial fluid, positively charged ions move toward that electrode during a pacing stimulus. This results in the phenomenon of polarization, which develops rapidly and dissipates slowly after the end of the stimulus. The opposite effect occurs on the positively charged anode. This effect of ions moving to oppose the flow of electric current has the effect of a capacitor in the circuit.


A capacitor is an object that stores energy in an electric field by holding positive charges apart from closely approximated negative charges. A capacitor requires a material or space between the layers of negative and positive charges that is normally nonconducting (the dielectric). A cell membrane, although leaky, acts as a capacitor by separating the negatively charged inside of the cell from the more positively charged outside. Cell membranes have very high capacitance per unit area of cell membrane. The interface between a pacing electrode and the charged electrolytes that surround the electrode at its surface in the myocardial tissue acts, in part, as a capacitor. The terms Helmholtz capacitor and Helmholtz capacitance are used in this chapter for capacitor-like effects that occur at pacemaker and defibrillator electrode-electrolyte interfaces.


Capacitance (C) is the term that specifies, for a given voltage applied across a capacitor, how much electrical charge (Q) can be stored by the capacitor. If V represents a steady voltage applied across the capacitor, then Q = CV. (If E is used instead of V as the symbol for electric potential, the relationship may be expressed as Q = CE.) The unit for capacitance is the farad. One farad is the capacitance of a capacitor that, on being charged to 1 volt, will have stored 1 coulomb of charge. Again, a coulomb is the amount of charge delivered by 1 ampere flowing for 1 second. Coulombs delivered can be expressed as follows:



image



in which Qt is the total charge delivered between time 0 and time t, and it is the instantaneous current at each time segment between time 0 and time t. The integral image is the net area under the instantaneous current-versus-time plot.




Reactance (Capacitance and Inductance)


For reactive elements connected in series, net reactance is the scalar sum of inductive reactance (positive in the mathematical complex plane) and capacitive reactance (negative in complex plane). Pure reactance values depend on the rates of change of current and voltage, whereas pure resistance values do not. Phenomena of this type are caused by differences in the timing of the peaks (phase angles) of the voltages or currents in the various reactive components. These reactive effects can be important in biventricular pacing threshold measurements (see later discussion). The component of reactance that is most relevant to both pacing electrodes and cell membranes is capacitance, with inductance being much less important. For example, the cardiac action potential spreading throughout the heart generates a changing magnetic field that transiently stores a very small amount of energy. However, the changing magnetic field generated by spread of the action potential is so small that it is not clinically significant except in the research setting.3


For circuits with resistance R, capacitance C, and inductance L in series, where qt represents the charge accumulated across the capacitance at any time t, and where the current through these combined elements at time t is it, the voltage Vt at time t is described by the following equation:



image



(remembering that image and that the voltage across a capacitor at time t is image).


These equations indicate that, for an instantaneous current it, the instantaneous voltage across the series circuit is the sum of the effects at that instant in time of the resistance, capacitance, and inductance of the circuit. Note especially that the instantaneous effects are highly related to the net amount of charge, image, that has accumulated in the capacitor from time 0 to the instantaneous time t. The equations show that the capacitance effect on the voltage decreases as the capacitance increases. This has clinical relevance in that, for example, the polarization voltage that interferes with autosensing pulse generators decreases as the electrode capacitance increases.



image Cellular Aspects of Myocardial Stimulation




Cell Membrane Characteristics


Cell membrane characteristics are major determinants of tissue excitability. The membrane of the cardiac myocyte is composed principally of phospholipids, cholesterol, and proteins.4 The membrane phospholipids have a charged polar headgroup and two long hydrocarbon chains arranged as shown in Figure 1-1. The cell membrane comprises two layers of phospholipids with their hydrophobic aliphatic chains oriented toward the central portion of the bilayer membrane and their polar headgroup regions toward the outside boundaries of the membrane. Because the membrane is composed of two layers of phospholipids, the polar regions of the phospholipid molecules interface with the aqueous environments inside and outside the cell. The lipid-soluble hydrocarbon chains are forced away from the aqueous phase to form a nonpolar interior.




Determinants of the Resting Transmembrane Potential


Relatively large gradients of individual ion concentrations exist across the cardiac cell membrane.5 The gradient of sodium ions (Na+) across the membrane is approximately 145 millimoles per liter (mmol/L) outside to 10 mmol/L inside. In contrast, the potassium ion (K+) concentration outside the cell is approximately 4.5 mmol/L, whereas the inside concentration is 140 mmol/L. In the absence of a cell membrane, both Na+ and K+ would rapidly move in a direction determined by the concentration gradient. The diffusion force tending to move K+ out of and Na+ into the cell is proportional to the concentration gradients of those ions. The potential energy attributable to the diffusion force (PEd) tending to move K+ out of the cell is given by the following equation:



[1-1] image



where R is the gas constant, T is the absolute temperature, ln is the natural logarithm operator, [K+]i is the concentration of potassium ion inside the cell, and [K+]o is the concentration of potassium ion outside the cell. If the ratio of [K+]i to [K+]o is large, the potential energy across the membrane is large. For each ion species, the difference in concentration between the inside and the outside of the cell results in that ion’s contribution to the difference in electric potential across the cell membrane.


In resting cardiac cells, the intracellular cytoplasm has a measured potential of about −90 millivolts (mV), relative to the extracellular fluid. This electric force tends to move positively charged ions such as K+ and Na+ to the inside of the cell and negatively charged ions such as chloride (Cl) to the outside of the cell in proportion to the potential gradient. The potential energy attributable to the electric force (PEe) tending to move K+ into the cell is expressed as follows:



[1-2] image



where z here is the valence (the number of positive or negative electrical charges) of the ion, F is the Faraday constant (96,500 coulombs/equivalent), and Vm is the transmembrane potential difference (transmembrane voltage, measured in millivolts). During equilibrium, the total of the potential energies from diffusion and electric forces is zero, and no net ionic movement occurs. Therefore, the sum of Equation 1-1 and Equation 1-2 may be set to zero. This yields the Nernst equation, which describes (in measurable electrical units) the potential that must exist for a single ionic species, here K+, to be in equilibrium across the membrane of a resting cardiac cell:



[1-3] image



or, in log base 10 terms,



image



Using known values for extracellular K+, Vm(K+) = −90 mV. When Equation 1-3 is solved using Na+ concentrations, a Vm(Na+) of +50 mV is obtained. Therefore, it is the equilibrium potential for potassium ion (not sodium ion) that is the major factor responsible for the resting transmembrane potential. This suggests that the resting membrane is more permeable to K+ than to Na+.


To calculate the transmembrane potential when multiple ionic species exist in different concentrations across the membrane, the Goldman constant field equation (modified by Hodgkin and Katz6) is used:



[1-4] image



where PK+, PNa+, and PCl− are the cell membrane permeabilities for the respective ions. At physiologic concentrations, this equation yields a transmembrane potential of −90 mV (the equilibrium potential for K+). Equation 1-4 describes how resting potentials vary as sodium and potassium ion concentrations are changed. Because there is a passive leak of charged ions through the membrane, the resting potential would not exist at the level of −90 mV unless it were actively maintained. This is accomplished by two active transport mechanisms that exchange Na+ ions for K+ and calcium (Ca2+) ions.


One might ask: with all the potassium ions and other positively charged ions in the cell, and with a relatively small amount of negatively charged chloride ions in the cell, how can the interior of the cell be negative with respect to the outside? The answer is that an array of intracellular organic and inorganic anions inside the cell—molecules that do not cross the membrane—carry net negative charges sufficient to make the overall balance of charge negative.



image Ion Channels


Protein molecules embedded within the cell membrane have numerous functions, including those of being ion channels and signal transducers. The concept of ion channels was proposed in the 1950s by Hodgkin and Huxley.7 However, it was not until the introduction of the patch clamp technique by Neher and Sakmann in 1976 that the properties of these channels could be directly studied.8,9 There are two basic types of ion channels, distinguished by the factors that control opening and closing of the channel. Ion channels at muscle fiber end plates are chemically gated by specific transmitters. The opening of these channels is triggered by the binding of acetylcholine, and their closing is induced by its unbinding. In neuronal axons, conduction is mediated by faster, voltage-gated channels. These channels respond to differences in electric potential between the inside and outside of the cell, across the membrane. Voltage-gated channels for sodium, potassium, and calcium appear to operate in similar ways, sharing many of the same structural features. In addition, each type of channel can be subdivided into several subtypes with different conductance or gating properties (Fig. 1-2).




Voltage-Gated Channels


Voltage-gated channels open in response to an applied electric potential. The source of this voltage can be an action potential propagated from an adjacent cell or the electric field of an artificial pacemaker electrode. If depolarization of the membrane exceeds a threshold voltage, an action potential is triggered, resulting in a complex cascade of ionic currents flowing across the membrane into and out of the cell. As a result of this flow of charge across the membrane, the potential gradient across the membrane changes in a characteristic pattern of events that produce the cardiac action potential (Fig. 1-3).



Selective membrane-bound proteins (ion channels) determine the passive transmembrane flux of an individual ion species. The transmembrane currents determine or influence cellular polarization at rest, action potential depolarization and repolarization, conduction, excitation-contraction coupling, and myofibril contraction. The channels that regulate transmembrane conductance of Na+ and Ca2+ are voltage gated. The sodium channel is a large protein molecule composed of approximately 1830 amino acids.10 It contains four internally homologous repeating domains, believed to be arranged around a central water-filled pore lined with hydrophilic (“water-loving”) amino acids. It is estimated that there are 5 to 10 Na+ channels per square micrometer (µm2) of cell membrane. When an alteration changes the membrane potential to about −70 to −60 mV (the threshold potential), four to six positively charged amino acids move across the membrane in response to the change in electric field. This causes a change in the conformations of the channel proteins, resulting in opening of the channel. After a single Na+ channel changes to the open conformation, about 104 Na+ ions enter the cell. On depolarization of the membrane, the Na+ channels remain open for less than 1 millisecond (msec). After rapid depolarization of the membrane, the Na+ channel again changes to the closed conformation. In addition to the Na+ channel, specialized proteins are suspended in the cell membrane that have differential selectivity for K+, Ca2+, and Cl ions, with much different time constants for activation and inactivation.



Sinoatrial and Atrioventricular Nodal Cells


In contrast to Purkinje fibers and working myocardial cells, sinoatrial (SA) and atrioventricular (AV) nodal cells are characterized by no true resting potential. Following repolarization, the transmembrane potential of the nodal cells reach a minimum of approximately −60 mV. The SA and AV nodal cells have a slow, inward, depolarizing, combined Na+/K+ current known as the “funny” current (If). These If channels open and cause the transmembrane potential to rise slowly until Ca2+ channels are activated and an action potential is generated. The rate of this rise in transmembrane potential is affected by factors such as autonomic nervous stimulation and hormones.


In these nodal structures, depolarization is primarily mediated by inward Ca2+ conductance through specialized Ca2+ channels. There are two types of Ca2+ channels in the mammalian heart. The L-type channels are the major voltage-gated pathway for entry of Ca2+ into the myocyte, and they are heavily modulated by catecholamines.11 The T-type channels contribute to spontaneous depolarization of the cell associated with automaticity (pacemaker currents). The pore of the Ca2+ channel has a functional diameter of about 0.6 nanometers (nm), larger than that of the Na+ channels (0.3-0.5 nm).12 The selectivity for Ca2+ is high, up to 10,000-fold greater than that for Na+ or K+. The key elements are high-affinity binding sites for Ca2+, positioned along a single file pore. “Elution” of a Ca2+ ion occurs when another Ca2+ ion enters and is selectively bound.


In normal situations, the SA node has the most pronounced automaticity and highest spontaneous firing rate in the heart. If the SA node is not functioning properly, however, or if block develops between the SA node and the AV node, the AV node will display pacemaker activity at a slightly slower rate than the SA node. In the absence of a pacemaker signal from the AV node, Purkinje fibers will activate at an even slower rate than either the SA or the AV node.



Maintenance of Resting Membrane Potential


The resting membrane potential is maintained by the pumping of Na+ ions out of the cell and K+ ions into the cell. The Na+,K+—adenosine triphosphatase (ATPase) pump moves three Na+ ions out of the cell in exchange for two K+ ions moved into the cell.1315 The basic unit of the Na+,K+-ATPase protein (pump) consists of one alpha (α-) and one beta (β-) subunit. The α-subunit is large (1016 amino acids) and spans the entire membrane, whereas the β-subunit is a smaller glycoprotein. There appear to be about 1000 pump sites/µm2 of cardiac cell membrane. The fully activated pump cycles about 50 to 70 times per second (interval of 15-20 msec/cycle). Similarly, the Na+-Ca2+ pump moves three Na+ ions out of the cell in exchange for one Ca2+ ion.16,17 Therefore, both transport mechanisms result in the net movement of one positive charge out of the cell, polarizing the membrane and maintaining a negatively charged interior. The function of both exchange mechanisms depends on the expenditure of energy in the form of high-energy phosphates and is susceptible to interruptions in aerobic cellular metabolism (e.g., during ischemia).



The Cardiac Action Potential


When the voltage gradient across the membrane of a myocyte decreases so that the inside of the cell becomes less negatively charged with respect to the outside of the cell, a critical transmembrane voltage difference is reached, the threshold voltage. At threshold, the cell membrane suddenly undergoes a further depolarization that is out of proportion to the intensity of the applied stimulus. This abrupt change in the potential across the membrane is the start of a cascade of inward and outward currents that together are known as an action potential.18


The cardiac action potential is an enormously complex event and consists of five phases19 (see Fig. 1-3): phase 0, the upstroke phase of rapid depolarization; phase 1, the overshoot phase of initial rapid repolarization; phase 2, the plateau phase; phase 3, the rapid repolarization phase; and phase 4, characterized by a slow, spontaneous depolarization of the membrane in cells with spontaneous pacemaker activity, until the threshold potential is again reached and a new action potential is generated.




Phase 1: Initial Repolarization


After voltage-dependent activation of the Na+ current in phase 0, the membrane potential rapidly changes from negative to positive. The increased conductance of Na+ is rapidly followed by voltage-dependent inactivation. Phase 1 is characterized by the transient outward K+ current (IKto). The outward movement of K+ is a major contributor to the various repolarization phases. It is complex and has a number of discrete pathways.20,21 Most K+ currents demonstrate rectification, that is, decreased K+ conductance with depolarization. The K+ currents include the instantaneous inward rectifier K+ current, the outward (delayed) rectifier K+ current, the transient outward currents, and ATP-, Na+-, and acetylcholine-regulated K+ currents. The initial repolarization, however, is mainly the result of activation of a transient outward K+ current and inactivation of the fast inward Na+ current.22 The transient outward K+ current has two components, one voltage gated and the other activated by a local rise in Ca2+.



Phase 2: Plateau


The net current during the plateau phase is apparently small, although the individual currents (inward Na+ and Ca2+ and outward K+) are each about an order of magnitude larger.23 Among the inward currents are the slowly activating Na+ current, a Ca2+ current, and an Na+-Ca2+ exchange current. Outward currents include a slowly activating K+ current (IKs), a Cl current, a more rapidly activating K+ current (IKr), an ultra-rapidly activating K+ current (IKur), and the Na+-K+ electrogenic pump. During phase 2 of the action potential, the absolute refractory period, the cardiac cell cannot be excited by an electrical stimulus, regardless of its intensity.




Phase 4: Automaticity and the Conduction System


Automaticity is the property of certain cells by which they are able to initiate an action potential spontaneously. It has been known for centuries that the heart can exhibit spontaneous contraction even when completely denervated. Leonardo da Vinci observed that the heart could “move by itself.”24 William Harvey reported that pieces of the heart could “contract and relax” separately.25 Many cells within the specialized conduction system have the potential for automaticity.


Not all parts of the heart, however, possess this property. In fact, cells in different areas of the heart have different transmembrane potentials, thresholds, and action potentials. Fast responses are characteristic of ordinary working ventricular muscle cells and His-Purkinje fibers, with resting membrane potentials of −70 to −90 mV and rapid conduction velocities. Normal SA and AV nodal cells have slow responses, with resting potentials of −40 to −70 mV and slow conduction velocities. Cells, or group of cells, with the fastest rate of spontaneous membrane depolarization during phase 4 are the first to reach threshold potential and initiate a propagated impulse. Therefore, cells with the steepest slope in phase 4 become the heart’s natural pacemaker. Ordinary working myocardial cells usually are not automatic.


Normally, depolarization is initiated at the SA node.26,27 Figure 1-4 shows action potentials from different types of cardiac cells.28 Rather than maintaining a stable resting membrane potential, the repolarization of the action potential is followed by a slow depolarization from about −71 to −54 mV, the threshold required to initiate another action potential. This slow, spontaneous depolarization drives cardiac automaticity and is related to a specialized current (If). In the case of AV nodal cells, the fast upstroke is carried predominantly by an inward Ca2+ current. Repolarization is caused by delayed activation of the K+ current. The balance of inward and outward currents determines the net “pacemaker” current and is finely regulated by both adrenergic and cholinergic neurotransmitters. In the presence of AV block or abnormal SA nodal function, AV junctional cells in the region of the proximal penetrating bundle usually assume the role of pacemaker at rates slower than that of the sinus node. In the absence of disease in the AV junction, the escape rhythm occurs with a frequency that is about 67% of the sinus rate.29




image Artificial Electrical Stimulation of Cardiac Tissue


Artificial lipid membranes in their pure form are electrical insulators. The myocyte cell membrane (sarcolemma) is much more complicated. Specialized protein molecules in the membrane allow it to be conductive.30,31 These proteins, either singly or in certain groupings, form channels that open and close for transport of specific ions through the membrane in response to particular stimuli. The channel proteins are the end stages of processes that provide both active and passive transport of ions and molecules through the membrane.


When application of a pacemaker or defibrillator pulse produces a local electric field gradient, ion drift in the extracellular fluid at that site, as well as ion flow within the cell and within the membrane, are affected by the field. The field-induced ion drift cannot be uniform within and outside the cell because of the different drift properties of different ion types, different ion and protein concentrations within and without the cell, and the barrier impedance effect of connections between cells.


The effect of the stimulus is to change the transmembrane voltage of nearby myocytes sufficiently so that depolarization begins in and spreads from these myocyte membranes. Propagation of the stimulus to nearby myocytes occurs because the local transmembrane depolarization changes the voltage gradient across adjacent membranes sufficiently to trigger depolarization of those membranes. To cause an action potential to spread throughout the whole heart, an electrical pulse must stimulate a minimum of approximately 50 closely coupled cardiac cells.5 The result is a self-regenerating action potential that progresses in a wavelike, relatively slow manner beyond the local effect of the pacemaker stimulus. Away from immediate vicinity of the electrode, transmission of depolarization and its velocity depend in part on the resistance and capacitance properties of the membrane, on the opening and closing of ion channels, and on ion flows through the sarcolemma.


Jun 5, 2016 | Posted by in CARDIAC SURGERY | Comments Off on Cardiac Electrical Stimulation

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