Alison Lovkay
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5. Be able to evaluate the different rhythms of a 3 lead and 12 lead EKG.
What are the cells of the heart?
There are three main cells in the heart; pacemaker cells, electrical conducting cells and myocardial cells. Pacemaker cells are the electrical power source. They depolarize over and over again. Each depolarization sends a wave that initiates one complete cycle of contraction and relaxation. The dominant pacemaker cells are located in the right atrium in the Sinus Atrial Node. These cells fire at a rate of 60-100 times a minute, which is a normal heart rate for an adult. The electrical conducting cells are the hard wiring of the heart. These cells carry the current to distant regions of the heart. Lastly, the myocardial cells are the contractile machinery of the heart. These cells are constantly contracting and relaxing, cause a flow of blood to the rest of the body. These cells contract and relax after being activated by the wave of depolarization. Then, calcium is released in the cell and it contracts, a process called excitation contraction coupling.
How does electricity in the heart work?
Cardiac cells in their resting state, are electrically polarized. This means that inside the cells, there is a negative charge, where the outside is positive. When the cells are depolarized, causing an action potential, they lose their negativity. This process is the fundamental electrical event that takes place in the heart. We use electrodes and monitors to see this event happen. Electrodes can detect these waves of depolarization, and this represents the flow of electricity. After depolarization, cardiac cells restore their resting polarity through repolarization. These events are seen by the different waves that come through on the EKG. For example, the P wave represents depolarization and the T wave represents repolarization.
The EKG:
EKG's are a good way to view the electrical current of the heart. By looking at the rate, rhythm and shape of the waves, you can get an idea about how the heart is working. The waves on an EKG reflect the electrical activity of the myocardial cells. Duration is measured in fractions of a second, amplitude is measured in millivolts and the configuration refers to the shape and appearance of the wave. EKG paper is set up so that the light lines circumscribe small squares of 1 x 1 mm and the dark lines circumscribe 5 x 5 mm. The horizontal axis measures time (each small square is 0.04 seconds and each large square is 0.2 seconds). The vertical axis measures voltage (one small square is 0.1 mV and one large square is 0.5mV). When the SA node initiates a contraction, an EKG does not pick it up. What is first seen is the atrial depolarization and contraction and is seen by looking at the P wave. The first half of the P wave is the right atrium while the second part reflects the left atrium. When the wave of depolarization spreads along the Bundle of His, bundle branches and Purkinje fibers, ventricular depolarization generates the QRS complex. The cells than repolarize, which is seen by looking at the T wave. There are also different intervals and segments you can look at to see how long each event takes place. The PR interval is the time from the start of atrial depolarization to the start of ventricular depolarization. The PR segment is then the time from the end of atrial depolarization to the start of ventricular depolarization. The ST segment records the time from the end of ventricular depolarization to the end of ventricular repolarization. The QT Interval is the start of ventricular depolarization to the end of ventricular repolarization. The QRS interval is again, the time of ventricular depolarization.
The 12 Lead EKG:
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6 Limb Leads: View the heart in a vertical plane called the frontal plane. View electrical forces moving up and down and left and right on the frontal plane.
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Lead I: Making left arm positive and right arm negative. Angle of orientation is 0. Positive deflection. Left lateral view.
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Lead II: Legs positive and right arm negative. Angle of orientation is 60. Has most positive P wave. Inferior view.
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Lead III: Legs positive and left arm negative. Angle of orientation is 120. Lies perpendicular to the atrial current. Frequently records a biphasic P wave. Inferior view.
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Lead AVL: Left arm positive and other limbs negative. Angle of orientation is -30. Positive deflection. Left lateral view.
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Lead AVR: Right arm positive and other limbs negative. Angle of orientation is -150. Sees electrical current as it’s moving away. Records a negative deflection. Most negative P wave.
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Lead AVF: Legs positive and other limbs negative. Angle of orientation is 90. Inferior view.
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6 Precordial Leads: Arranged across the chest in a horizontal plane. Records forces moving anteriorly and posteriorly.
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Lead V1: Placed in the fourth intercostal space to the right of the sternum. Perpendicular to direction of current flow and records biphasic wave, just like lead III. Anterior view.
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Lead V2: Placed in the fourth intercostal space to the left of the sternum. Anterior view.
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Lead V3: Placed between V2 and V4. Anterior view.
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Lead V4: Placed in the fifth intercostal space in the mid-Clavicular line. Anterior view.
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Lead V5: Placed between V4 and V6. Positive deflection. Left lateral view.
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Lead V6: Placed in the fifth intercostal space in the midaxillary line. Left lateral view.
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Waves:
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P Wave: Small and usually positive in left lateral and inferior leads. Often biphasic in leads III and V1. Most positive in lead II and most negative in lead AVR.
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QRS Complex: Tall R waves are usually seen in left lateral and inferior leads. R wave progression refers to sequential leads from V1 to V5. Small initial Q wave can be seen in left lateral leads and sometimes in inferior leads.
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T wave: Usually positive in leads with tall R leads.
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Arrhythmias:
An arrhythmia refers to any disturbance in the rate, regularity, site of origin, or conduction of the cardiac electrical impulse. Reasons for arrhythmias include hypoxia (deprivation of oxygen), ischemia (such as myocardial infarctions), sympathetic stimulation, drugs, electrolyte disturbances, bradycardia, and stretch.
5 Basic Arrhythmias:
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Electrical activity follows usual conduction pathways but is too fast, too slow or irregular. These are arrhythmias of sinus origin.
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Electrical activity originates from a focus other than the sinus node. These are called ectopic rhythms.
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Electrical activity is trapped within an electrical racetrack whose shape and boundaries are determined by various anatomic or electrical myocardial features. These are called reentrant arrhythmias.
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The electrical activity originates in the sinus node and follows usual pathways but encounters unexpected blocks and delays.
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The electrical activity follows accessory conduction pathways that bypass the normal ones, providing an electrical shortcut. These are termed pre-excitation syndromes.
What are the 4 questions you should ask when looking at an EKG?
1. Are normal P waves present? P waves represent atrial depolarization so a presence of P wave simply means that the origin of the rhythm is in the atria. If no P waves are present, than the rhythm is originated in the AV node or in the ventricles. Retrograde activation, or current flowing backwards, can be seen by having a P wave on an abnormal axis.
2. Are the QRS complexes narrow or wide? A normal QRS should be narrow and it means that the ventricular depolarization is occurring along the normal pathway. It also means that it originates above the AV node. A wide complex is therefore abnormal and means that the depolarization is happening within the ventricles. This concludes that conduction is not following the right pathway.
3. What is the relationship between the P waves and QRS complexes? The P wave must precede the QRS complex in order to be normal. If this is not happening, it shows that the atria and ventricles are depolarizing and contracting independent of each other. This is a situation called AV dissociation.
4. Is the rhythm regular or irregular? Check for different arrhythmias.
Atrial Fibrillation:
Atrial Fibrillation occurs when the SA node is not firing correctly and causes over 500 impulses/min to be sent to the AV node. The AV node only allows occasional impulses to pass through, causing a fast and irregular rate. This rate is usually seen between 120 and 180 beats per minute. When looking at the EKG, you can see a few noticable differences. There will be no true P waves and the baseline will appear flat or slightly undulated. A-fib is most commonly caused by longstanding hypertension. it could also be caused by mitral valve disease, coronary artery disease, hyperthyroidism and pericarditis.
Premature Ventricular Fibrillation:
PVC's are the most common ventricular arrhythmias. They occur when the heartbeat is initiated by purkinje fibers in the ventricles rather than by the SA node. A PVC can be a sign of decreased oxygenation but most aren't something to worry about. When looking at the EKG, you will notice a difference in the QRS complex. The QRS complex will appear wide and bizarre because the depolarization is not following the normal pathway. A lot of the time, you will also notice that there is no P wave. After a PVC occurs, there is often a prolonged pause before the next beat appears. There are a few reasons when you should worry about a PVC. These include frequent PVC's, runs of consecutive PVC's (especially three or more in a row), and any PVC occuring at the beginning of a myocardial infarction.
Ventricular Tachycardia:
When you have three or more consecutive PVC's, it is called ventricular tachycardia. The rate is usually between 120 and 200 beats per minute. Ventricular tachycardia can be dangerous because they can easily go into cardiac arrest. Two different types of ventricular tachycardia are polymorphic and uniform. Polymorphic VT is commonly associated with acute coronary ischemia or infarction. Uniform VT is more seen is more seen with healed infarctions.
Ventricular Fibrillation:
Ventricular fibrillation is a more serious arrhythmia because it is seen solely in dying hearts. It is also the most encountered arrhythmia in adults that experience sudden death. When a person is in V-fib, their heart is generating no cardiac output. This means that resucitation and electrical defibrillation must be performed at once. In an EKG, you will see that there are no true QRS complexes.
Conduction Blocks:
A conduction block is any obstruction or delay of the normal pathways of electrical conduction. There are three types of conduction blocks; sinus node block, AV block and a bundle branch block. A sinus node block is when the sinus node is firing but wave of depolarization is block and not transmitted into atrial tissue. It causes a pause in the normal cardiac cycle.
AV Block:
An AV block is a block between the sinus node and the purkinje fibers. There are three degrees of an AV block. A first degree block is when the PR interval is longer than 0.2 seconds. Every atrial impulse does eventually make it through to activate the ventricles. The QRS complexes are still preceded by a single P wave. A second degree block is when not every atrial impulse passes onto the ventricles. The ratio of P waves to QRS complexes is greater than 1:1. There are two types of second degree blocks. The first is called Morbitz Type I and is the prolongation of the PR interval until a QRS is dropped. The Morbitz Type II is when the QRS complezes are dropped without prolongation of the PR interval. A third degree block is when there are no atrial impulses that make it through to activate the ventricles. The ventricles and atria are then contracting at different rates thus causing a complete heart block. This is an event called AV dissociation in which the independent pacemakers drive the atria and ventricles.
Right Bundle Branch Block:
A right bundle branch block is when conduction through the right bundle is obstructed. This causes ventricular depolarization to be delayed and it does not begin again until the left ventricle is almost fully depolarized. The There are a few differences that you can see on an EKG. The QRS complex widens beyond .12 seconds because of the delay. There will be RSR' in V1 and V2 that are described to look like rabbitt ears. There will also be ST segment depression and T wave inversion. Lastly, you can see reciprical changes in V5, V6, I and AVL.
Left Bundle Branch Block:
A left bundle branch block is when the left ventricular depolarization is delayed. The QRS complex widens beyond 0.12 seconds. You will also be able to see a notched R wave with prolonged upstroke in leads V5, V6, I and AVL. Similar to the RBBB, there will be ST segment depression and T wave inversion. You will also see reciprocal changes in V1 and V2.
Who gets Bundle Branch Blocks?
Right bundle branch blocks can be caused by diseases of the conducting system but can also occur in normal healthy hearts. Left BBB’s rarely occur in normal hearts. They usually reflect a significant underlying cardiac disease such as degenerative disease of the conduction system or ischemic coronary artery disease. Both can be fixed.
Myocardial Infarction:
There are three parts to a myocardial infarction. It starts with the T wave peaking followed by T wave inversion. The T waves become tall and narrow (this is when it peaks) and a few hours later, they invert. This change reflects myocardial ischemia, or the lack of blood flow to the myocardium. If the infarction does occur, the T wave will remain inverted for potentially years. The second part is ST segment elevation. The elevation signifies myocardial injury. This reflects cellular damage beyond ischemia. This is a reliable sign that a true infarction has occured. Most ST elevation returns to normal after a few hours. If it doesn't return to normal, it can indicate the formation of a ventricular aneurysm. The third part is the appearance of new Q waves. This indicates that irreversible myocardial cell death has occured. This is a diagnostic tool for myocardial infarctions. The Q waves appear within several hours of the onset of the infarction. The ST segment will return to baseline once the time Q waves have appeared. Once Q waves appear, they will persist for the lifetime of the patient.
