Stimulation and Excitation of Cardiac Tissues

The Lateral Membrane

Cardiac myocytes are elongated rod-shaped cells that are electrically coupled to adjacent myocytes, predominantly along their long axis. The lateral membrane of the cardiac myocyte is composed of a phospholipid bilayer, in which the polar, phosphate heads of the phospholipids are oriented toward the periphery of the membrane (both inside and outside of the cell), whereas the hydrophobic aliphatic lipid chains are oriented toward each other in the center of the membrane. The polar regions of the bilipid membrane interface with the aqueous environment inside and outside of the cell ( Fig. 3-2 ). The lateral membrane is therefore relatively impermeable to the passage of charged ions. Cholesterol and membrane-spanning ion channel proteins (see below) are also incorporated into the lateral membrane.

There are two major structures that can be identified in the lateral membrane, the costamere and the t-tubule system. As cardiac myocytes undergo rhythmic contraction with each heartbeat, the structural integrity of the myocardium depends on linking of myocytes to an extracellular matrix. This linking to the extracellular matrix is achieved by the costamere ( Fig. 3-3 ). Costameres are structures oriented transverse to the long axis of the lateral membrane. They are anchored to the Z lines (see Fig. 3-3 ) of the sarcomere to form physical links between the Z lines, the sarcolemma, and the extracellular matrix. Costameres both sense and transmit external mechanical forces. Two distinct protein complexes are important for joining the costamere to the extracellular matrix: integrins and the dystrophin-glycoprotein complex. Integrins ( Fig. 3-4 ) are heterodimers with both alpha and beta chains. The alpha and beta chains include a long extracellular domain that binds to laminin in the extracellular space. Each integrin chain spans the lateral membrane with a short cytoplasmic tail that links to the actin cytoskeleton. Integrins function both as strong adhesion proteins but also transduce mechanical forces to the nucleus. Dystrophic-glycoprotein complexes (DGC) (see Fig. 3-3 ) are large proteins that also have extracellular, transmembrane, and intracellular components. The extracellular portion of the DGC binds to laminin, whereas the intracytoplasmic portion binds to actin. Thus both integrins and DGC provide structural integrity at the level of the costamere during physical stresses.

t-Tubules and Sarcoplasmic Reticulum

t-Tubules are narrow, tubular invaginations of the lateral membrane that interface tightly with the terminal cisternae of the SR to form a dyadic cleft ( Fig. 3-5 ). It is at the level of the dyadic cleft between the t-tubule and the SR that the calcium communication that drives electrical-mechanical coupling occurs. Although the membrane of the t-tubule and the closely apposed SR membrane are not physically in contact, calcium released in this cleft is responsible for the coupling of electrical excitation with mechanical contraction (EC coupling). EC coupling begins with the opening of L-type calcium channels located within the t-tubules in response to membrane depolarization. The calcium released by the L-type calcium channels diffuses into the dyadic cleft and activates the release of calcium from ryanodine receptors (RyRs) on the SR membrane. The regulation of calcium release from RyRs is tightly regulated by the proteins calsequestrin, junctin, and triadin. Atrial myocytes have fewer t-tubules than ventricular myocytes whereas sinoatrial (SA) nodal, atrioventricular (AV) nodal, and His-Purkinje cells have almost none.

Intercalated Disc

The ability of an electrical stimulus to propagate in a syncytium-like fashion through the myocardium is due to extensive interconnection between myocytes primarily at the level of the ID. Although action potentials propagate along the lateral membrane to the IDs at the ends of the cell, it is through gap junctions (GJs) in the IDs where propagation primarily occurs from one cell to another. Electrical coupling is far less supported along their transverse axis. Thus propagation through a three-dimensional sheet of myocardium is approximately three times faster along the long axis of fiber orientation than in the transverse axis, a property known as anisotropic conduction. In addition, the safety factor for conduction along the long axis of fibers is higher than transverse to the direction that the fibers are oriented.

Three types of adhesion structures are present in IDs. These are the fascia adherens junctions, desmosomes, and GJs. The fascia adherens junctions and desmosomes are composed of adhesion molecules that cross the sarcolemma of the ID to bind proteins from adjacent cells in the extracellular space. At the cytoplasmic side of the ID, these proteins are bound to the cytoskeleton. Thus fascia adherens junctions and desmosomes anchor the ID of one myocyte to the ID of another myocyte and function to sense mechanical stresses along the longitudinal axis of the myocardium. The transmembrane protein N-cadherin is connected to intracytoplasmic cytoskeletal protein actin by linker proteins such as vinculin and β-catenin in fascia adherens junctions. In desmosomes, the cadherin proteins desmoglein and desmocollin from adjacent cells attach in the intercellular space. At the cytoplasmic side of the desmosome, the proteins plakoglobin, plakophilin, and desmoplakin connect to the cadherin proteins and anchor these structures to the actin cytoskeleton via the linker protein desmin.

The third adhesion structure of the ID, and the one that is most important for cell-to-cell transmission of action potentials, is the GJ. GJs allow the diffusion of molecules up to approximately 1000 Daltons between cells, including ions, metabolites, and second messengers. GJs are highly plastic and undergo remodeling in response to hypertrophy, pulsatile mechanical stress, dilation, and chemical mediators. GJs are formed by two connexons, each serving as half of a channel (hemichannel) that connects one cell to its neighbor. Each connexon is formed by six connexins that span the lipid bilayer with a pore in the center. Each of the connexin proteins has four membrane-spanning domains with both intracellular N- and C-termini. There are 21 different human genes that encode connexins with each isoform having different conduction properties. In addition, connexons can be formed by six of the same connexin proteins (homomeric) or by combinations of different connexins (heterotypic). The human heart expresses Cx40, Cx43, and Cx45 as the dominant isoforms, with Cx43 being the main connexin in the working ventricular myocardium and a combination of all three connexins being expressed in the atrium. Cx45 is expressed in the SA and AV nodes whereas Cx40 and Cx45 are important in the His-Purkinje system.

Connexins Cx40 and Cx43 are highly selective for cations with permeability for both Na ⁺ and K ⁺ . The major intracellular ion carrying intercellular current via GJs is K ⁺ . GJs are in mainly the open state though lower levels of conductivity occur with depolarization of the cell membrane. GJs demonstrate voltage gating by both the transjunctional voltage (Vj) or transmembrane voltage (Vm) gradients. Cx45, found in Purkinje fibers, demonstrates significant voltage gating, whereas Cx43, found in ventricular myocardium, shows less voltage gating. GJs also demonstrate chemical gating with decreased conductance at lower pH and higher intracellular Ca ²⁺ concentrations. GJ conductance is also modulated by phosphorylation. Protein kinases tend to enhance GJ conductance, whereas phosphatases tend to reduce conductance. In addition, several forms of cardiac disease have been reported to reduce GJ density or cause heterogeneous expression of GJs resulting in slow conduction and increasing the likelihood of unidirectional conduction block. Pharmacologic agents that increase GJ conductance have not become commercially available. The peptides rotigaptide and GAP-134 are among the compounds that have been shown to increase GJ conductance.

Atrial Versus Ventricular Action Potentials

Compared with the typical spike and dome shape of the ventricular action potential, atrial action potentials typically have a more triangular shape ( Fig. 3-11 ). The resting membrane potential (phase 4) in the atrium is typically −65 to −80 mV compared with −80 to −90 mV in the ventricle. The lower degree of resting membrane polarization in the atrium is related to significantly lower density of the background inward rectifier, I _K1 . In addition, the upstroke velocity of phase 0 is lower in the atrium (150-300 V/S) than the ventricle (300-400 V/S). The lower upstroke velocity in the atrium is likely related to the relatively depolarized resting potential as compared with the ventricle which reduces I _Na . In contrast, inward Ca ²⁺ current is increased in the atria as compared with the ventricles. The L-type Ca ²⁺ current and Ca ²⁺ -induced Ca ²⁺ release are responsible for the plateau phase of the atrial action potential (AP), whereas the T-type Ca ²⁺ current is not present in atrial myocytes. Phase 1 of the atrial AP is caused by increased fast component of the transient outward K ⁺ current I _to,f . The slow component of the transient outward current (I _to,S ) is absent in the atrium. A major difference between the atria and ventricles is the outward ultrarapid K ⁺ current (I _KUR ), which is important in the atria but absent in the ventricle. In addition, the delayed rectifier currents I _Kr and I _Ks are significantly reduced in the atria, likely related to the triangular, more negative plateau phase. The main route of Na ⁺ efflux from atrial myocytes is the Na ⁺ /K ⁺ exchange pump, whereas the Na ⁺ /Ca ²⁺ exchange pump is responsible for Ca ²⁺ extrusion.

SA Nodal Impulse Generation

In the human heart, pacemaking normally occurs in specialized cells within the SA node that have limited contractile function. Secondary pacemaker cells with slower rates are found in the AV node and His-Purkinje system. The SA nodal cells are located within the crista terminalis with a higher mixture of atrial cells near the periphery of the node and higher density of specialized pacemaker cells in the interior of the node. The pacemaker cells have the highest frequency, due to the most rapid rise in membrane potential to threshold during diastolic depolarization in phase 4 ( Fig. 3-12 ). The amount of collagen in the SA node is higher than for normal atria and increases with age. Coupling within the SA node is relatively weak and electrotonic inhibition of SA automaticity by the atrial myocardium is observed. The rate of firing in the SA node is closely controlled by a variety of factors in response to demands such as neurotransmitters, circulating hormones, and stretch. The underlying mechanisms controlling automaticity include specialized ion channels in the membrane, as well as a Ca ²⁺ “clock” that determines Ca ²⁺ cycling ( Fig. 3-13 ). It has been demonstrated that the site of pacemaking activity may shift markedly from lower in the crista terminalis at slow rates to higher at faster rates.

The predominant pacemaker currents in the SA node flow through hyperpolarization-activated cyclic nucleotide-gated channels (HCN), L-type and T-type Ca ²⁺ channels (I _Ca,L and I _Ca,T ), and acetyl choline-activated channels. The SA node has reduced inward rectifier (I _K1 ) current density and is devoid of connexin Cx40 but has more expression of the low conductance Cx45. Cells at the periphery of the SA node express Cx43 allowing for conduction to the atrium. The gene HCN4 encodes membrane channels that carry the hyperpolarizing-activated, or “funny,” current (I _f ). The I _f current is a mixed cationic current carried by both Na ⁺ and K ⁺ ions. It is activated by membrane hyperpolarization with a threshold between −50 and −65 mV. These channels have a binding site for cAMP, resulting in acceleration of the rate of spontaneous diastolic membrane depolarization in SA nodal cells. In contrast, muscarinic agonists reduce the funny current thereby decreasing the heart rate. The upstroke of the SA node action poten­tial (phase 0) is predominantly determined by with a much lower upstroke velocity than in the ventricles. In addition, there is very little I _to within the SA node with repolarization determined by the delayed rectifiers (I _Kr and I _KS ). The plateau phase is markedly reduced with a characteristic AP morphology shown in Figure 3-11 .

The voltage-dependent calcium currents and carry the depolarizing current in the SA and AV nodes during phase 0. However, spontaneous diastolic release of cytosolic Ca ²⁺ in phase 4, which acts as a Ca ²⁺ clock, is essential for pacemaker automaticity. This spontaneous cytosolic Ca ²⁺ increase of the calcium clock peaks during late diastolic depolarization leads to opening of the membrane calcium currents. This spontaneous release of Ca ²⁺ is from the SR and depends on RyR activation. The NCX exchange pump is also important in diastolic depolarization because it is a net inward current. Although the relative importance of the transmembrane I _f current and spontaneous oscillations of the calcium clock has been debated, the available evidence suggests both are important for pacemaking activity. Both mechanisms are highly responsive to changes in beta adrenergic stimulation via G-protein (G _s ) activation and cAMP/phosphokinase A phosphorylation.

Atrioventricular Node Propagation

The AV node is composed of three types of cells: spindle-shaped cells within the compact AV node near its junction with the His bundle; transitional cells having features that are intermediate between atrial myocytes and spindle-shaped cells typical of the compact AV node; and left and right inferior extensions of the AV node that approach the compact AV node from the left and right atria ( Fig. 3-14 ). The unique aspect of AV nodal conduction is that there is significant delay in propagation of impulses from the atria to the His bundle, allowing time for the mechanical transport of blood through the AV valves. Transitional cells approach the compact AV node (AVN) from the limbus of the fossa ovalis, spreading across the tendon of Todaro. The inferior extension of the AVN approaches the compact AV node inferiorly from the left atrium and from fibers located parallel to the septal leaflet of the tricuspid valve. These inferior extensions have spontaneous depolarizations and pacemaker activity expressing I _f current, features not found in transitional cells. As compared with atrial myocytes, transitional cells of the AV node express connexin Cx43, whereas the compact AV nodal cells have minimal Cx43 expression. The action potential of transitional cells has high upstroke velocity typical of I _Na , whereas the compact AV nodal cells and inferior extension cells have a lower upstroke velocity related to .

The compact AVN expresses low conductance Cx30.2 and Cx45, whereas Cx43 is much less expressed in the AVN. As compared with the His bundle (which has large amounts of I _Na ), there is much less I _Na in the AVN. These differences in connexins and sodium channels explain the rapid conduction typical of the His bundle and slow conduction typical of the compact AVN. Based on characteristics of the action potential, AV nodal cells can be distinguished as atrionodal (AN), nodal (N), and nodo-His (NH) cells. Compared with atrial and AN cells, N cells in the compact AVN have a less negative resting potential, slower upstroke velocity, smaller amplitude, and pacemaker activity. NH cells demonstrate a more negative resting potential, higher upstroke velocity, and a more prolonged plateau phase of the action potential. The pacemaker activity of the AV junction occurs in the NH cells and His bundle where I _f and the calcium clock are more highly expressed ( Figs. 3-15 and 3-16 ).

Decremental conduction, whereby conduction delay is greater at faster pacing rates, is characteristic of AV nodal conduction. Decremental conduction can be explained by a gradual increase in the diastolic membrane potential with faster pacing rates leading to slowed conduction velocity as the relative refractory period is progressively encroached. Dual AV nodal pathways are common and have been demonstrated in both experimental animal models and in humans where AV nodal reentrant tachycardia is a common form of supraventricular tachycardia.

His-Purkinje System

The insulated His bundle, right and left bundle branches, and the branching Purkinje network are designed to ensure an effective sequence of ventricular contraction that proceeds rapidly and evenly from apex to base ( Fig. 3-17 ). In the human heart there are very few Purkinje fibers that penetrate from the endocardium to the epicardium. The Purkinje network includes free-running fibers known as false tendons and branching subendocardial fibers. The action potentials of the Purkinje network are insulated over large distances and activate the ventricular myocardium at distinct Purkinje-muscular junctions. Purkinje fibers are enriched by the high conductance connexin Cx40 and the I _Na channel, Nav1.5. Purkinje conduction is minimally responsive to autonomic stimulation. The AP duration of Purkinje cells is longer than for the ventricular myocardium, with a more rapid rate of rise and longer plateau. The plateau potential of Purkinje fibers is lower than for the ventricular myocardium with no significant difference in resting potential. The Purkinje-ventricular muscle junctions are formed by specialized cells with high resistance, which allows the localized stimulation of the ventricular myocardium while limiting retrograde conduction from the muscle to the Purkinje fibers. The Purkinje-ventricular muscle junctions demonstrate discontinuous conduction properties with significant conduction delay.

Purkinje cells demonstrate automaticity at a lower frequency than in the SA node and demonstrate diastolic I _f current. Similar to the SA node, Purkinje cells also demonstrate spontaneous intracellular Ca ²⁺ oscillations that may be the primary cause of pacemaker activity. Purkinje cells lack t-tubules but have junctional and nonjunctional types of SR. L-type calcium channels open in response to depolarization resulting in Ca ²⁺ -induced Ca ²⁺ release as calcium binds to RyRs, triggering release from the SR. Thus as compared with ventricular myocytes, Ca ²⁺ release in Purkinje cells relies less on voltage-gated Ca ²⁺ channels and more on Ca ²⁺ -induced Ca ²⁺ release from the SR.

Electric Fields and Charge

Physical objects 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, without requiring that the charged objects be in contact. The magnitude of the attractive or repulsive force (F) is given by Coulomb's law:

where Q ₁ and Q ₂ are the magnitude of the charges (measured in coulombs), r is the distance separating the charged objects, and k is a constant whose magnitude depends on the medium between the two charged objects. According to Coulomb's law, the force attracting or repelling charged objects increases with the magnitude of their charges and decreases with the square of the distance that separates them.

Potential Difference

Electrical potential energy is possessed by a charged particle by virtue of the magnitude of the charge and the position of the particle in relation to other charged particles. That is, a charged object in space has potential energy because of the forces that other charges exert on that object. When two charged objects repel one another, energy is required to move them closer to each other, thereby increasing the potential energy. However, if the charged object is attracted to another charged object, potential energy will be converted into kinetic energy as the charged objects move closer together. Thus the force generated by the interaction of electrical charge is electromotive, tending to move charged objects either toward one another if they have opposite polarity or away from one another if they have the same polarity. Voltage is the potential energy per unit charge for a charged object in an electric field. Thus in order to move a charge from point A to point B, the work required (measured in joules ) is calculated by multiplying the charge, Q, by the voltage difference between points A and B, V _AB . Because voltage is potential energy per unit of charge, it is measured in terms of joules/coulomb, as follows:

When one refers to “voltage,” the reference is to a difference in potential between two points in space.

Electric Current

An electric current, I, is present when there is a movement of electric charge. Although electric charge may move through several mechanisms, for clinical purposes, electric charge is usually carried by the flow of electrons through a wire (e.g., pacemaker lead) or by the movement of ions in blood, interstitial fluid, across cell membranes, or within the cytoplasm of cells. By historical convention, current is considered to flow in the direction that positive charges would move. In reality, however, an electric current in a wire is carried by the movement of electrons that are negatively charged. For clarity in this chapter, current is stated in terms of electron or ion motion. Because electric current is the movement of charge, it is measured in terms of coulombs per second (I _t = dQ/dt), with 1 ampere of current equal to the movement of 1 coulomb/sec.

The electromotive force for cardiac pacemakers or implantable cardioverter-defibrillators (ICDs) is determined by the chemistry of its battery. For lithium-iodine batteries at beginning of life, the chemical reaction generates approximately 2.8 volts of electromotive force. The total amount of charge that is available in the battery is measured in terms of the amount of current that can be provided multiplied by the duration that the current can be sustained. For pacemaker or ICD batteries the amount of charge that can be stored in the battery is measured in terms of ampere-hours, with 1 A-hr equal to 3600 coulombs of charge.

Series Circuits

In a series circuit, or circuit module, the elements are connected so that current must flow sequentially through each element in the circuit. Thus the current flowing through all elements in series is the same, with the voltage difference decreasing sequentially as it passes through each element in the circuit. The total resistance (R _T ) of a circuit with two resistive elements is series (R ₁ and R ₂ ) is additive, such that (R _T = R ₁ + R ₂ ).

Parallel Circuits

In a parallel circuit, two or more elements are joined at each end to a common conductor (node in the circuit). Therefore the potential difference is the same before each element and after each element, and current will flow from one of the common conductors to the other through any or all of the elements. The quantity of current flow in each element is inversely related to the factors that oppose the flow of electric charge in that element. At any node (junction) in an electric circuit, the sum of currents flowing into that node is equal to the sum of currents flowing out of that node (Kirchoff's current law). Most biologic circuits are made of various combinations of series and parallel modules or subcircuits. For example, because of electrochemical effects, an electrode placed in the heart may act as a capacitor in parallel with a resistor, both in series with the lead joining the pulse generator to the electrode. The total resistance in a parallel circuit (R _T ) composed of two possible current pathways (R ₁ and R ₂ ) is less than the resistance in any of the individual elements:

Electrode Polarity in Batteries and Leads

All defibrillator and pacemaker electric circuits have both a positively charged electrode and a negatively charged electrode. The negatively charged electrode of a pacing or ICD lead (the cathode ) is typically the tip electrode. 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 seem 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 becomes positively charged (Li ⁺ ) while furnishing electrons to the circuit external to it. Therefore the terminal of the battery where electrons are provided to the circuit is the battery anode. From the battery anode, electrons are conducted through circuitry in the pulse generator and eventually enter the pacemaker lead, where they are conducted through the conductor to an 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 more proximally on a lead within 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., I ₂ + 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.