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Monday, July 29, 2019

EKG: Cardiac Electrical Conduction System


EKG: Cardiac Electrical Conduction System


Focus topic: Cardiac Electrical Conduction System
As discussed in the prior chapter, the heart works as a pump to circulate blood throughout the body. However, without the specialized cells that make up the heart, it would not be able to function. The heart is able to maintain this pumping function through electrical impulses that spread from one cell to the next. This electrical system is also known as the cardiac conduction system. The electrical impulses of the heart are independent from the nervous system. This means that the heart can beat independently even if ties to the nervous system have been severed.

Cardiac Electrical Conduction System: Cardiac Conduction

Focus topic: Cardiac Electrical Conduction System

Cardiac Electrical Conduction System: Cardiac Cells

Focus topic: Cardiac Electrical Conduction System
The heart’s cardiac cells (myocytes) have one of two functions: mechanical (contractile) or electrical (pacemaker). The mechanical cells, also called myocardial cells, create the contractile strength of the myocardium. These contractile cells form the muscular layers of the atrial and ventricular walls and rely on the pacemaker cells to generate the impulse to contract.
Pacemaker cells, also called conducting cells or automatic cells, spontaneously produce and conduct electrical impulses without stimulation by a nerve. Cardiac cells have four characteristics that make them conducive to generating and transmitting electrical impulses (The Characteristics of Heart Cells).

Cardiac Electrical Conduction System: The Characteristics of Heart Cells

Focus topic: Cardiac Electrical Conduction System

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Clinical Alert
Medications such as digitalis, dopamine, and epinephrine can improve the heart’s ability to contract. This would be considered to be aninotropic response. (Chronotropic responses have to do with heart rate.)

Cardiac Electrical Conduction System: Action and Resting Potential

Focus topic: Cardiac Electrical Conduction System
Ions move across cell membranes causing a slight difference in the concentration of charged particles. This imbalance in the charged particles creates energy and makes the cells excitable. The inside of cardiac cells has a negative charge due to the larger amount of negatively charged molecules in the cell. When this intracellular negativity exists, the cell is considered to be polarized or in a resting state (resting potential). The difference in the electrical charges across the cell membrane is the membrane potential. The movement of the electrolytes across the cell membrane requires energy in the form of ATP (adenosine triphosphate) to create a flow of current that is then expressed in volts. This voltage produces wave-forms and spikes on the EKG recording. When the cell is in the resting state (polarized), it produces a straight line on the EKG known as the isoelectric line.
Action potential describes the electrolyte exchanges that occur across the cardiac cell membranes during depolarization. This action potential occurs in five stages and these stages are described in Phases of the Cardiac Action Potential.

Cardiac Electrical Conduction System: Phases of the Cardiac Action Potential

Focus topic: Cardiac Electrical Conduction System

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Depolarization occurs when cardiac cells are electrically activated by the passage of ions (such as sodium and potassium) across the cell membrane. Sodium moves rapidly into the cardiac cells while calcium moves more slowly. At this point, potassium leaves the cell. This movement causes the inside of the cell to become more positive that translates to a spike (waveform) on the EKG. Depolarization is a very active process and must occur before the heart can contract and pump blood.
Clinical Alert
Some medications impact the movement of calcium in the channels. One of these categories of drugs is known as calcium channel blockers and would slow the heart rate due to a decrease in conductivity. They also have other properties such as opening up (vasodilation) the coronary arteries and decreasing contractility. Examples of this type of medication would be diltiazem (Cardizem) and verapamil (Calan). Other medications that can be used to treat irregular heart rhythms disrupt the sodium (Na+) and potassium (K+) channels. When any of these medications are utilized, particular EKG changes can be noted for each type.
An impulse begins in the pacemaker cells of the sinoatrial (SA) node of the heart and moves through each heart cell until all the cells have been depolarized. This chain reaction is a wave of depolarization. The impulse spreads from the pacemaker cells to the myocardial cells that contract when stimulated. A P wave is recorded on the EKG when the atria are stimulated, representing atrial depolarization. A QRS complex represents ventricular depolarization and is recorded on the EKG when the ventricles are stimulated. The cardiac cycle and the EKGdisplays the cardiac cycle and its relationship to the depolarization and repolarization that is occurring within the heart.
Conversely, the resting potential occurs when a fully depolarized cell returns to its resting state and restores its electrical charges to normal in a process called repolarization. The electrical charges in depolarization reverse and return to normal leaving negatively charged particles inside the cell. Repolarization moves from the epicardium to the endocardium and occurs rapidly at first, then plateaus, and surges again until the resting state is achieved. This ventricular repolarization presents as an ST segment and T wave on the EKG.
It is important to remember that while these electrical events are taking place, they correlate with the mechanical action of the heart muscle as well. During the depolarization of the atria (the P wave), atrial systole is occurring.

Cardiac Electrical Conduction System: The cardiac cycle and the EKG

Focus topic: Cardiac Electrical Conduction System
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Ventricular depolarization (the QRS wave) causes ventricular systole. Atrial diastole or repolarization actually occurs at the same time that the ventricles begin their depolarization process however the strength of its signal is not as strong as the ventricular signal and therefore is “lost” within the QRS waves. The progression of the depolarization through the ventricles begins at the Q wave and continues through to the beginning of the T wave. The mass of the contraction essentially takes place at the end of the QRS during the plateau phase depicted on the EKG as the ST segment. Cardiac cells respond differently than skeletal muscle to stimuli. Skeletal muscles react with a short twinge type response, whereas, cardiac cells act with a longer, sustained type of contraction due to the necessity of pumping blood through the heart chambers. Repolarization of the ventricles, ventricular diastole, is identified on the EKG recording as the T wave.
Clinical Alert
When the Electrical Activity (depolarization) of the heart occurs, it does not necessarily mean that the mechanical portion of the heart’s contraction will occur as well. In order for the heart to function it must have both electrical and mechanical activity. If Electrical Activity occurs without the mechanics of a contraction, a condition known as PEA or pulseless Electrical Activity occurs. This is a serious situation which is not easily resolved. Electrical Activity is seen on the EKG; however, the patient does not have pulses or blood pressure.
The relationship of the EKG to Electrical Activity and contraction of the myocardium is found in Relationship of the EKG to electrical activity and contraction of the myocardium.

Cardiac Electrical Conduction System: Refractory Periods

Focus topic: Cardiac Electrical Conduction System
Another property of cardiac cells is called refractoriness, which is the ability to remain unresponsive to a stimulus or to reject an impulse. The refractory period where the heart recovers before responding to additional stimuli is longer than the heart’s actual contraction. The length of time for each of the refractory phases varies among individuals and is affected by medications, recreational drugs, electrolyte imbalance, disease, myocardial injury, and myocardial ischemia.
Figure Relationship of refractory periods and the EKG tracing depicts the timing of refractoriness.

Cardiac Electrical Conduction System: Relationship of the EKG to electrical activity and contraction of the myocardium

Focus topic: Cardiac Electrical Conduction System
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Refractoriness occurs in three periods:
  • Absolute refractory period: This period occurs when the cardiac cells cannot respond to stimuli. The myocardial cells do not contract and electrical impulses are not generated by the electrical conduction system despite the strength of the stimulus. This period is synonymous with depolarization and the beginning of repolarization and corresponds with the onset of the QRS complex to the peak of the T wave on the EKG. Phases 0, 1, 2, and part of phase 3 of the cardiac action potential are included in this period.

Cardiac Electrical Conduction System: Relationship of refractory periods and the EKG tracing

Focus topic: Cardiac Electrical Conduction System
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  • Relative refractory period: This period is also called the vulnerable period, a time when only a strong stimulus can cause depolarization. During this period, repolarization is almost complete and some cells may respond to stimuli, some cells may respond in an unusual fashion, and some cells may not respond at all. Cardiac cells are extremely vulnerable during this period and may respond in a disorganized manner resulting in a life-threatening dysrhythmia. The down-slope of the T wave, during the period of ventricular repolarization, relates to this refractory period.
  • Supernormal period: This period occurs at the very end of the T wave. This corresponds to the completion of Phase 3 and the beginning of Phase 4. Cardiac cells can respond to weaker stimuli at this time and is therefore, also a very vulnerable period of time for the development of dysrhythmias.
Clinical Alert
One situation that can occur during the final upswing of the T wave is known as commotio cordis. This most commonly occurs in young mid teenage boys who are hit in the chest at exactly the right time in the cardiac cycle. It can also occur in other individuals, as well, who are in some way administered a blow to the heart during the period of time of 10 to 30 ms before the peak of the T wave. This is in the final stages of depolarization and the beginning of repolarization. This causes instantaneous ventricular fibrillation and must be treated immediately with defibrillation.

Cardiac Electrical Conduction System: Cardiac Conduction System

Focus topic: Cardiac Electrical Conduction System
The cardiac conduction system is comprised of an electrical system of pathways among the sinus node (SA node), atrial tissue, the atrio-ventricular (AV) junction, the bundle of His, the right and left bundle branches, and the Purkinje fibers (Cardiac conduction system). When impulses originating in the SA node are conducted through this system in a normal fashion, the heart chambers will contract in the proper manner to produce each heartbeat.

Cardiac Electrical Conduction System: Cardiac conduction system

Focus topic: Cardiac Electrical Conduction System
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Cardiac Electrical Conduction System: Sinus Node

Focus topic: Cardiac Electrical Conduction System
The sinus node (also called the sinoatrial node or SA node) is located in the upper right corner of the right atrium where the right atrium and superior vena cava join. The SA node is 10 to 20 mm long and 2 to 3 mm wide in the adult and is made up of different types of cells, including pacemaker cells. The SA node is dense with sympathetic and parasympathetic nerve fibers and receives its blood supply from the SA node artery that runs lengthwise through the center of the node. The sinus node is referred to as the pacemaker because it reaches potential more quickly than the rest of the cardiac tissue and generates impulses 60 to 100 times per minute. This is the fastest of the intrinsic rates of pacemaker tissue found in the heart. However, other areas of the heart can assume the role of pacemaker if certain situations occur such as the SA node failing to stimulate the atria to contract, not firing at the correct rate, a block in the conduction of the impulse, or inability of the SA node to produce an adequate electrical impulse at all.
As an electrical impulse leaves the SA node, it spreads across the atrial muscle and produces contraction of the right atrium, travels through the interatrial septum through Bachman’s Bundle, and then enters the left atrium. This causes the right and left atria to contract at the same time. The ventricles do not contract because fibrous tissue separates the atrial and ventricular myocardium that only allows the atria to contract. This impulse doesn’t flow backward, but instead only in a forward motion, because the cardiac cells are unable to respond to a stimulus immediately after the process of depolarization. The contraction of the atria is the P wave. Since the right atrium is depolarized first, this is indicated by the upswing of the P wave and the left atrium is then the down-stroke of the P wave (Contraction of the right and left atria on EKG tracing).
The impulse travels from the SA node, to the AV node. This impulse reaches the atrioventricular junction through three passages known as the internodal pathways. This is a group of fibers that contain both functional myocardial cells and impulse transmission fibers. Each pathway has a particular name: Bachmann’s bundle (anterior tract), Wenckebach’s bundle (middle tract), and Thorel’s pathway (posterior tract). As described above, Bachmann’s bundle is also the transmission pathway for the left atrium.

Cardiac Electrical Conduction System: Contraction of the right and left atria on EKG tracing

Focus topic: Cardiac Electrical Conduction System
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