This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles.
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This is a free website for Nuclear Medicine Technologists and Students who wish to broaden their understanding of Nuclear Cardiology Practices and Principles. |
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LESSON 4bEKG SLIDESHOWThe Normal Electrocardiogram is characterized by the wave form shown below. Each of the individual segments of the wave is designated by a letter (or letters), and each represents a distinct phase of the cardiac cycle. The electrodes placed on the skin are positive terminals attached to lead wires. When a wave of depolarization advances toward a positive electrode, this produces a positive (upward) deflection on EKG. The advancing wave of positive charge in depolarizationcreates a positive deflection on EKG when this wave is moving toward a positive skin sensor.
< >Normal ECG Shows Pattern of P, Q, R, S, and T Waves in Cardiac Cycle
P wave. The P wave represents the electrical activity associated with the impulse generation at the SA node and its subsequent spread through the atria. When the P wave is of normal size and shape, it is assumed that the impulse began in the SA node. When P waves are absent or of abnormal size or shape, it is likely that the impulse originated outside the SA node. PR interval. The period from the start of the P wave to the beginning of the QRS complex is called the PR interval. During this interval, which normally does not exceed 0.20 second, the impulse traverses the atria and the AV node. QRS complex. The QRS complex represents depolarization of the ventricular muscle, and consists of an initial downward deflection (Q wave), a large upward deflection (R wave), and a second downward deflection (S wave). Together these waves reflect the time necessary for the impulse to spread through the bundle of His and its branches to complete ventricular excitation. The duration of the QRS complex is normally less than 0.12 second. If the duration is increased, it is an indication of bundle branch block, a condition in which the ventricles are stimulated in a delayed, abnormal manner. ST segment. The ST segment represents the period between the completion of depolarization and the beginning of repolarization of the ventricular musculature. The ST segment may be elevated or depressed by either ischemia or infarction. T wave. The T wave represents the normal repolarization of ventricular myocardium that occurs after ventricular contraction. Tissue injury or ischemia may cause abnormal repolarization and inversion of the T wave. The EKG records the electrical impulses that stimulate the heart to contract and gives us a valuable permanent record of electrical cardiac activity during resting and recovery periods. The electrocardiogram is inscribed on a paper strip ruled in one-mm squares. The electrocardiograph plots millivolts ( ↕ vertical axis ) over seconds ( ↔ horizontal axis ). Each vertical space represents a voltage change of 0.1 (one-tenth) mv; the voltage between two of the heavy horizontal lines is 0.5 (one-half) mv. Each small square represents 0.04 (four-hundredths) of a second: the time interval between the heavy lines is 0.2 (two-tenths) seconds in duration. By measuring along the horizontal axis, we can determine the duration of any part of the cardiac cycle. depolarization repolarization.
Resting heart cells "depolarize" when electrically stimulated and the negatively charged interiors of the myocardial cells become positively charged as the cells are stimulated to contract. A progressive wave of stimulation (depolarization) passes through the heart causing contraction of the myocardium. The heart remains physically quiet during repolarization, a strictly electrical phenomenon. Electrical activity is picked up by external skin sensors and recorded as an EKG. Upward spikes are called "positive" deflections; downward spikes are called "negative" deflections. As the positive wave of depolarization within the heart cells moves toward a positive (skin) electrode, there is a positive (upward) deflection recorded on EKG. One cardiac cycle (heartbeat) is represented by the P wave, QRS complex, and the T wave. This cycle is repeated continuously. Physiologically, a cardiac cycle represents atrial systole, ventricular systole (contraction), and the resting stage between beats. Each lead records cardiac electrical activity from a different angle. The electrode pairs are different for each lead, so the tracing changes slightly in each lead as we change the angle from which we monitor cardiac activity. The SA node begins the electrical impulse which spreads in wave fashion, stimulating both atria. The electrical stimulus originating from the SA node proceeds away from the node concentrically in all directions. This electrical impulse spreads across the atria, the atria contract, and a P wave is recorded on the EKG. The impulse then reaches the AV node, where there is a 1/10 second pause, allowing blood to travel through the atrioventricular valves and enter the ventricles. After the pause, the AV node is stimulated, initiating an electrical impulse that starts down the AV bundle into the Bundle Branches. The AV Bundle (Bundle of His) extends down from the AV node and divides into the Right and Left Bundle Branches within the interventricular septum. The QRS complex represents the electrical impulse as it travels from the AV node to the Purkinje fibers and into the myocardial cells causing simultaneous contraction of the ventricles. The Q wave, when present, is the first downward stroke of the QRS complex and it is followed by the upward R wave. The upward R wave is followed by a downward S wave. This total QRS complex represents the electrical activity of ventricular contraction. The upward deflection is always called an R wave. There is a pause after the QRS complex, then a T wave appears. This pause is the ST segment, a flat piece of baseline between the QRS complex and the T wave. Repolarization restores the negative charge to the interiors of myocardial cells. The T wave represents repolarization of the ventricles so they may be stimulated again. Identification of Arrhythmias: The ECG Slide Show The normal electrocardiogram consists of a repetitive series of P, Q, R, S, and T waves which conform to established standards for size and shape and occur 60 to 100 times per minute. The term arrhythmia is used to describe any abnormality in the heartbeat, including disturbances in rate, rhythm, or conduction. Arrhythmias may be described as benign or lethal depending on the rhythm, the patient's underlying condition, and the patient's clinical response to the arrhythmia. For example, sinus tachycardia in a healthy person is usually of no clinical importance. Sinus tachycardia in a patient with a damaged heart, however, can result in life-threatening decreases in cardiac output. Arrhythmias may occur constantly or intermittently. They can occur because of myocardial ischemia, drug toxicity, electrolyte imbalance, or overactivity of the sympathetic or parasympathetic nervous systems. Treatment of the arrhythmia depends primarily on the patient's clinical response to the disturbance rather than the presence of the arrhythmia itself. In this slide show, you will see how abnormal ECG patterns differ from normal tracings. Furthermore, the etiology of the abnormalities will be presented. A firm understanding of ECG interpretation will enhance your ability to monitor a stress ECG study, identify arrhythmias prior to a radionuclide ventricular function study, and perform any nuclear cardiology procedure in the acute care or outpatient setting. Arrhythmias can be identified on the ECG by following these steps:
Since several types of arrhythmias are characterized only by rate changes, these calculations are extremely important.
Normal sinus rhythm
Rate: 60-100 beats per minute. Rhythm: Regular. P wave: Normal contour and precedes each QRS complex. PR interval: Normal and constant (0.12-0.20 sec.). QRS complex: Normal and constant (0.06-0.10 sec.). The impulse originates in the SA node and travels through the AV node and ventricles in a normal fashion. By following these steps, arrhythmias can be categorized according to their origin and mechanism. Let's evaluate the ECG strips below. Sinus bradycardia
Rate: Below 60 beats per minute. Rhythm: Regular. P wave: Normal contour and precedes each QRS complex. PR interval: Normal and constant. QRS complex: Normal and constant. Interpretation: The impulse originates in the SA node and follows the normal conduction pathways. Since the rate is below 60 beats per minute, this is a bradyarrhythmia. No medical treatment is necessary unless the rate is extremely slow and the patient becomes symptomatic (e.g., dizziness, hypotension, fainting). If treatment is necessary to increase heart rate, atropine is the drug of choice. Sinus tachycardia
Rate: Above 100 beats per minute. Rhythm: Regular. P wave: Present and precedes each QRS complex. In very rapid rates, the P wave may be difficult to identify if it is buried within the preceding T wave. PR interval: Normal and constant. QRS complex: Normal duration. Interpretation: The impulse originates in the SA node and follows normal conduction pathways. Since the rate is above 100 beats per minute, this is a tachyarrhythmia. In most individuals, stress, excitement, or exercise increases heart rate. Problems arise when this increased rate places a burden on a damaged heart. The goal of treatment is to lower the rate using drug therapy. Sinus arrhythmia
Rate: May vary, but usually slower than normal. Rhythm: Irregular. P wave: Normal contour and precedes each QRS complex. PR interval: Normal. Interpretation: This rhythm is benign and requires no medical treatment. Phasic increases and decreases in heart rate are usually associated with inspiration and expiration, and are most noticeable during sleep. Premature atrial contraction (PAC)
Rate: Normal, but may vary. Rhythm: Regular, with the exception of PAC. P wave: Normal P wave precedes each QRS complex, but the P wave associated with the premature beat may have a different configuration since it occurs outside the SA node. It doesn't follow the normal atrial conduction pathways and may be hidden completely. PR interval: PR interval associated with the early beat is usually longer than that of the sinus beat. Atrial fibrillation
Rate: The atrial rate is usually above 350 beats per minute, and the ventricular rate is usually between 60 and 160, but may vary. Rhythm: The RR distances are unequal. Therefore, an irregular ventricular rhythm is present. P wave: Since no P waves can be identified, the sinus node is not the pacemaker. The existence of an ectopic pacemaker can be inferred. PR interval: The PR interval cannot be measured since there are no P waves. QRS complex: The QRS complex is 0.10 second and within normal limits. Therefore we may assume that conduction through the ventricles is normal and that the ectopic pacemaker is above the AV node. Interpretation: By definition, this is considered a supraventricular arrhythmia. By combining all this information we can reason that the abnormality originates in the atrium, but not in the SA node. Because the ventricular beat is irregular, we may assume that the impulse does not arrive at the ventricles in a rhythmic fashion. The mechanism for this particular abnormality is fibrillation. The ECG interpretation is atrial (site) fibrillation (mechanism). Using this approach to ECG analysis, let's now look at some specific arrhythmias. When interpreting rhythm strips, remember to use all of these steps, as they are equally important. Atrial flutter
Rate: The atrial rate is in the range of 250–350 beats per minute, with the ventricular rate depending on the degree of AV block. Rhythm: The atrial rhythm is regular, and the ventricular rate may be either regular or irregular. P wave: P waves are replaced by "f" (fibrillation) waves and identified by undulations of varying contour, amplitude, and spacing. PR interval: Unable to measure. QRS complex: Usually Normal. Interpretation: When impulses are discharging in the atria at such rapid rates, the normally slower rate of conduction through the AV node acts as a filter to block some of the impulses from reaching the ventricles. When the ventricles respond only to every other atrial impulse, it is defined as atrial flutter with 2:1 AV block. Ventricular response to every third impulse would result in a 3:1 AV block. In the latter case, the atrial rate would be 300 and the ventricular rate would be 100. The patient may be asymptomatic or may describe a fluttering sensation or palpitation. Treatment is the same as that for atrial fibrillation, with synchronous DC conversion employed when the patient is compromised hemodynamically. Signs of poor cardiac output include oliguria, hypotension, diaphoresis, and change in mental status. Supraventricular Tachycardia
Rate: 150–250 beats per minute.
Rhythm: Regular, except at onset and termination.
P wave: Difficult to identify, since it is often superimposed on the preceding T wave or hidden within the QRS complex.
PR interval: Usually constant if the P waves can be identified.
QRS complex: May be normal or widened if an aberrant conduction is present. Interpretation:
Supraventricular tachycardia is the term used to describe any tachycardia that occurs above the ventricles. This includes paroxysmal atrial tachycardia (PAT) and paroxysmal junctional tachycardia (PJT). Because the rapid rates can obscure the P waves, making it impossible to determine the origin of the impulse, the rhythm is termed SVT. Both PAT and PJT are characterized by rapid, regular, atrial or junctional tachycardias of sudden onset and termination, with rates of 150–250 beats per minute. Clinical presentation, ECG characteristics and treatment for both conditions are virtually the same. The rhythm usually begins with a premature atrial or junctional beat.
This arrhythmia can occur at any age and may or may not be associated with coronary artery disease. It can occur as a result of stress or excessive caffeine or nicotine intake. The patient may be asymptomatic, or may complain of angina-like pain, palpitations, or anxiety, depending on the duration and rate of the rhythm. Because of the rapid ventricular rate, SVT may cause reduced cardiac output, decreased cerebral perfusion, or syncope. First-degree AV Block
Rate: Normal.
Rhythm: Regular.
P wave: Normal contour and precedes each QRS complex
PR interval: Prolonged (greater than 0.20 second).
QRS complex: Normal duration. Interpretation:
The impulse originates in the SA node but is abnormally delayed as it travels through the AV node or bundle of His. Once the impulse passes the AV junctional region, it propagates normally through the ventricles. The ECG abnormality is demonstrated by the prolonged PR interval. This delay may be due to ischemia of the AV node or may be drug induced (e.g., lidocaine, digitalis, quinidine, or procainamide). If the arrhythmia is drug-induced, the physician should be notified and the patient's drug regimen reassessed. Second-degree AV Block (Wenckebach Mobitz Type I)
Rate: The atrial rate is normal. However, the ventricular rate is slower than the atrial rate because each atrial impulse does not always trigger a ventricular response.
Rhythm: The atrial rate is regular, but the ventricular rate is irregular.
P wave: Normal, but may be hidden in the preceding T wave as the PR interval lengthens.
PR interval: Progressively lengthens until a nonconducted P wave occurs.
QRS complex: Normal in duration. Interpretation:
In this arrhythmia, impulses from the SA node are progressively delayed as they travel through the AV node or bundle of His until one impulse is completely blocked. There is a P wave, but no subsequent QRS complex. The cycle is then repeated. This is a more advanced degree of block that is sometimes seen in acute inferior MI due to ischemia of the AV node. The physician should be notified. Second-degree AV Block (Mobitz Type II)
Rate: The atrial rate is normal, but the ventricular rate is slower.
Rhythm: The atrial rhythm is regular, but the ventricular rhythm is irregular.
P wave: Normal, but not always followed by a QRS complex.
PR interval: May be normal or slightly prolonged but remains constant, except for nonconducted P waves.
QRS complex: Usually widened, since a bundle branch block is frequently present. Interpretation:
In Mobitz Type II, the atria are stimulated normally by the SA node. Not all of the impulses, however, are conducted through the AV node and bundle of His to stimulate the ventricles. The patient may be asymptomatic or may show signs of decompensation, depending on the ventricular response. If the patient is tolerating the rate well and not showing any clinical signs of decompensation, then the only acute treatment may consist of continued monitoring. Third-degree AV Block with Ventricular Pacemaker
Rate: The atrial rate is faster than the ventricular rate.
Rhythm: The atrial and ventricular rates are regular but independent of each other.
P wave: Normal contour, but no fixed relationship to the QRS complex.
PR interval: Constantly changing.
QRS complex: The configuration depends on the site of impulse formation. If the impulse arises high enough in the conduction system, the QRS is fairly normal. If it originates in the ventricles, however, it is wide and aberrant. Interpretation:
Despite normal impulse initiation in the SA node and depolarization of the atria, no impulses are transmitted to the ventricles. Because the ventricles are not receiving any stimulation, a subsidiary pacemaker assumes the role of impulse initiation for the ventricles. Therefore, the atria and the ventricles each have their own pacemaker with their own rates and are stimulated and contract independently of each other. Premature Ventricular Contraction
Rate: Usually normal.
Rhythm: Regular, except when interrupted by the premature beat.
P wave: Normal contour, and precedes each QRS complex except with the premature beat, when there is no P wave.
PR interval: Constantly changing.
QRS complex: Normal, except with the premature beat, when the QRS is wide and bizarre, with the T wave usually opposite in direction to the QRS complex. Interpretation:
PVCs occur because one or more ventricular foci discharge prematurely, causing a ventricular contraction. Because the impulse originates in the ventricle and does not follow the normal pathway of conduction, the resulting QRS complex is wide and bizarre. There is no P wave preceding the premature beat because there is no atrial stimulation.
Some PVCs are benign, resulting from cigarette smoking, caffeine intake, lack of sleep, or emotional stress. PVCs also may be caused by pathological conditions such as ischemia or hypokalemia. Because PVCs occur before the ventricle has filled completely, the stroke volume of the resulting contraction is less than normal. If PVCs are numerous, they may burden a damaged heart. The patient may complain of a thumping, fluttering, or skipping sensation in the chest or may be symptom-free.
PVCs are usually treated in acute MI patients if they occur more often than five per minute, if there is a run of two or more together, or if they are multiformed, as evidenced by PVCs having at least two different configurations. Multiform PVCs may indicate that the PVCs are coming from more than one irritable area in the ventricle. Treatment is also necessary if PVCs occur during the preceding T wave, which is the vulnerable period during which ventricular tachycardia is more likely to occur.
Ventricular Tachycardia
Rate: The ventricular rate is usually 120–200 beats per minute.
Rhythm: Regular or slightly irregular.
P wave: Usually obscured by QRS complexes, but may be inverted (opposite of sinus rhythm) and not associated with the ventricular rhythm.
PR interval: Not able to determine.
QRS complex: Widened beyond 0.12 second. It has a bizarre configuration, with the T wave usually opposite in direction to the QRS complex.
Interpretation:
Ventricular tachycardia is defined as a series of three or more consecutive ventricular complexes. It may last seconds, hours, or days. In most cases the rate is too fast to provide an adequate cardiac output. Mental confusion, convulsions, or unconsciousness will reflect a decreased blood flow and oxygen supply to the brain. Immediate treatment is necessary if any of these symptoms occur. DC cardioversion is the treatment of choice if the patient is hemodynamically unstable. Otherwise, intravenous lidocaine, procainamide, or bretylium can be used.
Ventricular Fibrillation
Rate: The ventricular rate is greater than 300 beats per minute.
Rhythm: Grossly irregular.
P wave: Unable to measure.
PR interval: Unable to measure.
QRS complex: Replaced by undulations of varying contour, amplitude, and spacing.
Interpretation: The lack of synchronous ventricular activity results in an ineffective ventricular contraction. Because the ventricles are merely quivering rather than contracting, there is no effective cardiac output. Thus, loss of consciousness, convulsions, and death occur rapidly unless treatment is instituted immediately. DC cardioversion with cardiopulmonary resuscitation (CPR) employed between countershocks to maintain circulation is the only effective treatment of ventricular fibrillation. |
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