Electrical Heart Function

Electrical heart function: introduction and anatomy

The heart can be viewed functionally as two pumps with the pulmonary and systemic circulations situated between the two pumps. The pulmonary circulation is the blood flow within the lungs that is involved in the exchange of gases between the blood and alveoli. The systemic circulation is comprised of all the blood vessels within and outside of organs excluding the lungs.

Structure of the heart

The right side of the heart comprises the right atrium and the right ventricle. The right atrium receives venous blood from the systemic circulation, and the right ventricle pumps it into the pulmonary circulation where oxygen and carbon dioxide are exchanged between the blood and alveolar gases. The left side of the heart comprises the left atrium and the left ventricle. The blood leaving the lungs enters the left atrium by way of the pulmonary veins. Blood then flows from the left atrium into the left ventricle. The left ventricle ejects the blood into the aorta, which then distributes the blood to all the organs via the arterial system. Within the organs, the vasculature branches into smaller and smaller vessels, eventually forming capillaries, which are the primary site of exchange. Blood flow from the capillaries enters veins, which return blood flow to the right atrium via large systemic veins (the superior and inferior vena cava).

First, the right and left sides of the heart, which are separated by the pulmonary and systemic circulations, are in series with each other. Therefore, all of the blood that is pumped from the right ventricle enters into the pulmonary circulation and then into the left side of the heart from where it is pumped into the systemic circulation before returning to the heart. This in-series relationship of the two sides of the heart and the pulmonary and systemic circulations requires that the output (volume of blood ejected per unit time) of each side of the heart closely matches the output of the other so that there are no major blood volume shifts between the pulmonary and systemic circulations. Second, most of the major organ systems of the body receive their blood from the aorta, and the blood leaving these organs enters into the venous system (superior and inferior vena cava) that returns the blood to the heart.

The heart as a pump

The heart acts as a pump that receives blood from venous blood vessels at a low pressure, imparts energy to the blood (raises it to a higher pressure) by contracting around the blood within the cardiac chambers, and then ejects the blood into the arterial blood vessels. The right atrium receives systemic venous blood (venous return) at very low pressures (near 0 mm Hg). This venous return then passes through the right atrium and fills the right ventricle; atrial contraction also contributes to the ventricular filling. Right ventricular contraction ejects blood from the right ventricle into the pulmonary artery. This generates a maximal pressure (systolic pressure) that ranges from 20 to 30 mm Hg within the pulmonary artery. As the blood passes through the pulmonary circulation, the blood pressure falls to about 10 mm Hg.

The left atrium receives the pulmonary venous blood, which then flows passively into the left ventricle; atrial contraction provides a small amount of additional filling of the left ventricle. As the left ventricle contracts and ejects blood into the systemic arterial system, a relatively high pressure is generated (100 to 140 mm Hg maximal or systolic pressure). Therefore, the left ventricle is a high-pressure pump, in contrast to the right ventricle, which is a low-pressure pump. The pumping activity of the heart is usually expressed in terms of its cardiac output, which is the amount of blood ejected with each contraction (i.e., stroke volume) multiplied by the heart rate.

Blood vessels constrict and dilate to regulate arterial blood pressure, alter blood flow within organs, regulate capillary blood pressure, and distribute blood volume within the body. In summary, arterial pressure is monitored by the body and ordinarily is maintained within narrow limits by negative feedback mechanisms that adjust cardiac function, systemic vascular resistance, and blood volume. This control is accomplished by changes in autonomic nerve activity to the heart and vasculature, as well as by changes in circulating hormones that influence cardiac, vascular, and renal function.

Electrical heart function: prof explanation

The heart is positioned more or less symmetrically to the chest, predominantly at the left side of the chest. It is not vertically orientated but it is more in a slanted (inclinato) direction. The apex of the heart is pointing downward, forward, and to the left. The right ventricle is actually more frontally positioned (anterior part).

In the ECG, the electrodes are positioned closer to the right ventricle than to the left ventricle, this giving rarely more information about the right ventricle. Most cardiac muscle is found in the free left ventricular wall and in the septum between the left and the right ventricle. This is the reason which the left ventricle predominates, for instance, the electrocardiogram.

Cardiac cell connections

We have to remember all the components: All cardiac cells are connected by intercalated disks consisting of desmosomes and gap junctions:

• Desmosomes form mechanical connections between cells.
• Gap junctions facilitate the exchange of ions between cells (facilitate electrical communication: ions can pass the gap junction from one cell to the other).

By this mechanism, spontaneous action potentials in a group of cells in an arbitrary site of the heart will be conducted over the entire heart, and cause a contraction of the entire heart muscle and the heart behaves as a functional syncytium. Single myocardial cell contract after each action potential. The behaviour of all the cardiac muscle cells together as a functional syncytium results in a complex but well-coordinated contraction of the heart.

The heart may never stop! This means stopping of the circulation and:

• 8 seconds of cardiac arrest: loss of consciousness (because the brain cannot function without conscious) without perfusion
• 6 minutes of cardiac arrest: irreparable damage of the brain.

Pacemaking and conduction

Essential for electrical heart activity is pacemaking (excitation is a spontaneous electrical activity starting somewhere and conduction through gap junctions). All cardiac cells are connected electrically and can conduct electrically (electrical activity). There are specific conductive pathway in the heart called the conduction system. Part of the conduction system is in the right atria and ventricle. In the normal functioning heart, the electrical impulse is generated by pacemaking itself in the sinoatrial node that is in the right atria. The electrical impulse is then conducted, through the right atria, to the atrioventricular node, then to the right bundle branch.

Opened right ventricle

• Pulmonary semilunar valve that leads to the pulmonary artery
• Supraventricular crest – Demarcates separation of the RV (right ventricle) inflow (from the RA to the RV) and RV outflow trajectories
• Tricuspid valve – Connection between right ventricle and right atrium
• Moderator band – First described by Leonardo da Vinci, carries part of the right bundle branch to the anterior papillary muscle (coordinated contraction of papillary muscles and other muscles). Likely prevents overdistension of the right ventricle when high pressure develops
• Anterior papillary muscle – Prevents "bulging out" of the tricuspid valve during systole

Opened right atrium

• Superior vena cava
• Tricuspid valve to the right ventricle
• Sinoatrial node
• Atrioventricular node by which the electrical impulse is conducted to the RV, and there is no specific conducting pathway in the RA
• Inferior cava

Embryonal development of the conduction system (this part of the embryo is not very important, do not enter in the detail) – Many problems of the conduction system connected with arrhythmias can be understood from the embryonic stage. We can see the microscopic figure of 6 weeks of human embryo: His bundle and right bundle branch. The embryonic heart starts as a simple tube. For the development of the conduction system, the ring concept was adopted. This latter describes how constricting band-like segments separate parts of the folding embryonic tube represent and form the future conduction system.

Early stage of embryonic tube

• Embryonic tube with constricting band-like segments that form transitional zones between cardiac segments.

t = truncus arteriosus
b = bulbus cordis
v = primitive ventricle
a = primitive atrium
sv = sinus venosus

When the embryonic tube is folding, this constricted band folds too. Part of these rings seem to come together in the atrioventricular junction.

• san= senoatrial node
• avn= atrioventricular node
• lbb= left bundle branch
• rbb= right bundle branch

Adult congenital heart disease

Several abnormalities in electrical cardiac functioning are caused by abnormal development of the conduction system in the embryonic phase. E.g., the AV-nodal range, where 3 bands meet, is extremely complex and can give rise to multiple forms of arrhythmias. Not seldomly in adults, and often AV-nodal arrhythmias require intervention by a cardiologist (electrophysiologist), by destroying part of the conduction system. In the figure, we can see one small meeting of three constricted bands reaching in the atrioventricular node.

Another structure that is essential for correct electrical functioning of the heart is the fibrous skeleton. This latter forms an electrical insulation between the atrial and the ventricular muscle. With a normally developed conduction system, the only electrical connection between atria and ventricles is the AV-nodal pathway. In the figure, we can see in yellow the conduction system pathway. Fibrous skeleton: electrical insulation between atria and ventricles and structure that holds the valves.

Conduction system: view with opened right atrium and right ventricle

• T = tricuspid valve
• AVN = atrioventricular node
• M = mitral valve
• H = His bundle
• P = pulmonary valve
• LBB = left bundle branch
• A = aortic valve
• RBB = right bundle branch

Aortic valve and conduction problems

Aortic valve surgery can damage the left bundle branch! A damaged left bundle branch can cause conduction problems in the left ventricle. Left atria has no specific structure related to the conduction system. In the left ventricle, His Bundle exits the fibrous skeleton.

AV node and fossa ovalis

The fossa ovalis is a depression in the right atrium of the heart, at the level of the interatrial septum, the wall between the right and left atrium. It is very close to the AV node. It is a remnant of the open foramen ovale, between the right and left atria, during foetal development. An open foramen ovale during adulthood ("patent foramen ovale") can give rise to problems (tricuspid valve stenosis, aneurism, brain/heart infarction). For the electrical functioning of the heart, fossa ovalis is important because it is the preferential site of transseptal puncture, when necessary for treatment (e.g. atrial fibrillation, an arrhythmia especially in the old patient).

Action potential

Cardiac cells, like all living cells in the body, have an electrical potential across the cell membrane. This potential can be measured by inserting a microelectrode into the cell and measuring the electrical potential in millivolts (mV) inside the cell relative to the outside of the cell. If measurements are taken with a resting ventricular myocyte, a membrane potential of about −90 mV will be recorded. This resting membrane potential (Em) is determined by the concentrations of positively and negatively charged ions across the cell membrane, the relative permeability of the cell membrane to these ions, and the ionic pumps that transport ions across the cell membrane.

Ion concentrations and membrane potential

• Of the many different ions present inside and outside of cells, the concentrations of Na⁺, K⁺, and Ca²⁺ are most important in determining the membrane potential across the cell membrane. Of the three ions, K⁺ is the most important in determining the resting membrane potential. In a cardiac cell, the concentration of K⁺ is high inside and low outside the cell. The opposite situation is found for Na⁺ and Ca²⁺; their chemical gradients favor an inward diffusion. The concentration differences across the cell membrane for these and other ions are determined by the activity of energy-dependent ionic pumps and the presence of impermeable, negatively charged proteins within the cell that affect the passive distribution of cations and anions.
• Cell in which K⁺ is the only ion across the membrane other than the large, impermeable, negatively charged proteins on the inside of the cell. In this cell, K⁺ diffuses down its chemical gradient and out of the cell because its concentration is much higher inside than outside the cell. As K⁺ diffuses out of the cell, it leaves behind negatively charged proteins, thereby creating a separation of charge and a potential difference across the membrane (negative inside the cell relative to outside). The membrane potential that is necessary to oppose the outward movement of K⁺ down its concentration gradient is termed the equilibrium potential for K⁺ (E; Nernst potential). The equilibrium potential is the potential difference across the membrane required to maintain the concentration gradient across the membrane.

The Em for a ventricular myocyte is about -90 mV, which is near the equilibrium potential for K⁺. Sodium ions also play a major role in determining the membrane potential. Because the Na⁺ concentration is higher outside the cell, this ion would diffuse down its chemical gradient into the cell. To prevent this inward flux of Na⁺, a large positive charge is needed inside the cell (relative to the outside) to balance out the chemical diffusion forces. This potential is called the equilibrium potential for Na⁺ (ENa). The calculated equilibrium potential for sodium indicates that to balance the inward diffusion of Na⁺ at these intracellular and extracellular concentrations, the cell interior has to be +52 mV to prevent Na⁺ from diffusing into the cell. The same reasoning can be applied to Ca²⁺ as just described for Na⁺. Its calculated E_Ca is +134 mV and net electrochemical force acting on Ca²⁺ is −224 mV. Therefore, like Na⁺, there is a very large net electrochemical force working to drive Ca²⁺ into the resting cell; however, in the resting cell, little Ca²⁺ leaks into the cell because of low membrane permeability to Ca²⁺ at rest.

The Em in a resting, non-pacemaker cell is very near E_K, and quite distant from E_Na and E_Ca. This occurs because the membrane is much more permeable to K⁺ in the resting state than to Na⁺ or Ca²⁺. If the membrane has a relatively higher permeability to one ion over the others, that ion will have a greater influence in determining the membrane potential. Membrane permeability for an ion determines the movement of an ion being driven by a net electrochemical force. Because this ion movement represents an electrical current, it is common to speak in terms of ion conductance (g), which is defined as the ion current divided by the net voltage (net electrochemical force) acting on the ion. Membrane permeability and ion conductance are related in that an increase in membrane permeability for an ion results in an increase in electrical conductance for an ion. Em is the sum of the individual equilibrium potentials for K⁺, Na⁺ and Ca²⁺, with each multiplied by the membrane conductance for that particular ion relative to the sum of all ion conductance.

In a cardiac cell, the individual ion concentration gradients change very little, even when Na⁺ enters, and K⁺ leaves the cell during depolarization. Therefore, changes in Em primarily result from changes in ionic conductance. The maintenance of these concentration gradients requires the expenditure of energy (adenosine triphosphate [ATP] hydrolysis) coupled with ionic pumps. Consider the concentration gradients for Na⁺ and K⁺. Na⁺ constantly leaks into the resting cell, and K⁺ leaks out. Moreover, whenever an action potential is generated, additional Na⁺ enters the cell, and additional K⁺ leaves. Although the number of ions moving across the sarcolemma membrane in a single action potential is small relative to the total number of ions, many action potentials can lead to a significant change in the extracellular and intracellular concentration of these ions. To prevent this change from happening (i.e., to maintain the concentration gradients for Na⁺ and K⁺), an energy (ATP)-dependent pump system (Na⁺/K⁺-adenosine triphosphatase [ATPase]), located on the sarcolemma, pumps Na⁺ out and K⁺ into the cell. Besides maintaining the Na⁺ and K⁺ concentration gradients, it is important to note that the Na⁺/K⁺-ATPase pump is electrogenic because it extrudes three Na⁺ for every two K⁺ entering the cell. By pumping more positive charges out of the cell than into it, the pump creates a negative potential within the cell. This electrogenic potential may be up to −10 mV, depending on the activity of the pump. Inhibition of this pump, therefore, causes depolarization resulting from changes in Na⁺ and K⁺ concentration gradients and from the loss of an electrogenic component of the membrane potential. In addition, increases in intracellular Na⁺ or extracellular K⁺ stimulate the activity of the electrogenic Na⁺/K⁺-ATPase pump and produce hyperpolarizing currents.

Because Ca²⁺ enters the cell, especially during action potentials, it is necessary to have a mechanism to maintain its concentration gradient. Two primary mechanisms remove calcium from cells. The first involves an ATP-dependent Ca²⁺ pump that actively pumps calcium out of the cell and generates a small negative electrogenic potential. The second mechanism is the sodium–calcium exchanger, through which Na⁺ and Ca²⁺ are transported in opposite directions. Ion channels have both open and closed states. Ions pass...