EKG Flashcards
What is an EKG
-An ECG (also called an EKG) is an indirect measurement of electrical activity within the heart. A recording of the electrical currents within the heart is obtained by placing electrodes containing a conductive media to each extremity and numerous locations on the chest wall to create a 12 lead ECG. Each specific). The purpose of using 12 leads is to obtain 12 different views of the electrical activity in the heart and therefore a more complete picture.
3 lead or 5 lead system
-Current standard of practice in most hospitals call for Pt risk for cardiac events of dysrhythmias to be placed on continuous ECG monitoring using the 3 lead or 5 lead system. These systems use only 3 or 5 leads, placed on the PTs chest, which is less cumbersome and allows for more PT mobility than the 10 leads placed on the chest and extremities for 12 lead ECG
-Holter monitor uses a 3 or a 5 lead setup
-Although these modified systems do not provide the overview that a 12 lead ECG does, they do allow for the recognition of gross abnormalities in the electrical conduction of the heart. Identifications of a rhythm abnormalities on a 3 lead or 5 lead tracing often indicates the need to obtain a more detailed 12 lead view if the heart
Value of EEG/EKG
-The ECG provides valuable info about the cardiac status of a PT presenting with signs and symptoms suggestive of heart disease. For example if the PT presents with dyspnea and chest discomfort, an ECG can aid in the diagnosis of an ischemic cardiac event
When should an ECG/EKG be Obtained
-Because an ECG is noninvasive and does not present a risk to the PT, it is reasonable to obtain an ECG when the PT has signs and symptoms suggestive of an acute or chronic cardiac disorder such as myocardial infarction or congestive heart failure
-Of course the process of obtaining the ECG should never delay the initiation of critically needed care such as oxygen therapy, airway placement, or cardiopulmonary resuscitation (CPR) -An ECG is often used as a screening tool to determine the PTs health status before major surgery. An ECG is especially helpful in this situation if the PT is older or has a history of heart disease. If an abnormality is identified, it may need to be treated before the operation is performed
Anatomy and physiology of the Heart and Electrical Activity
-Before discussing the interpretation of the ECGs, it is important to revir=ew the cardiac anatomy and physiology related to electrical activity within the heart. The heart is made up of four chambers: two upper chambers called atria and two lower chambers called ventricles. The heart typically is described as having two sides, the right and left. The right atrium receives deoxygenated blood from the vena cavae and directs the blood into the right ventricle.
-Right ventricular contraction ejects blood into the pulmonary artery, which carries blood to the lungs for oxygenation. The oxygenated blood returns to the left atrium of the heart via the pulmonary vein, where it is directed into the left ventricle. Left ventricle contraction ejects blood into the aorta, which branches off into the systemic circulation. Since the left side of the heart pumps blood throughout the entire body, it normally has a significantly larger muscle mass than the right side
Types of Heart Cells
-Pacemaker cells: specialized cells that have a high degree of automaticity and provide the electrical power for the heart
-Conducting cells: Cells that conduct the electrical impulse throughout the heart -Myocardial cells: Cells that contract in response to electrical stimuli and pump the blood
Electrical Activity of Heart
-Myocardial contractions occur because of electrical stimulation. For blood to move effectively it must be coordinated, and the electrical conduction system which is made up of pacemaker and conducting cells are responsible for that coordination
Causes and Manifestations of Dysrhythmias
-Disturbances in cardiac conditions are called dysrhythmias. Dysrhythmias can occur even in healthy hearts. Often, minor dysrhythmias produce no symptoms and resolve without any treatment. More serious dysrhtymias indicate significant acute or chronic heart disease
-When serious dysrhythmias occur, medication or electrical therapy often is required to increase or decrease the ventricular rate or to suppress an irritable area within the myocardium. Occasionally surgical intervention or thrombolytic therapy is needed to prevent the progression of injury or infarct, thereby salvaging viable tissue
-the drug adenosine is given to reset the heart reset the heart rate
-The application and improve delivery of oxygen often is a key factor in reducing or eliminating cardiac irritability. Causes of dysrhythmias include the following
-Hypoxia: Hypoxia results from inadequate delivery of oxygen to mycorardum and may be referred to as ischemia. Inadequate delivery may be caused by reducing martial oxygen levels, reduced hemoglobin levels, reduced arterial oxygen levels, reduced hemoglobin levels, reduced perfusion levels, or a combo of such factors
-Ischemia can lead to myocardial injury and infarction. Myocardial cells deprived of oxygen do not conduct nor contract well
-Sympathetic stimulation: Physical or emotional stress from fear or anxiety and conditions, such as hyperthyroidism and CHF, can elicit dysrthmias.Sypathietic stimulation can also result in cardiac ischemia caused by an increased workload on the myocardium without concurrent increase blood flow such as on the case of diseased coronary arteries
Causes and Manifestations of Dysrhythmias
-Drugs:
-Drugs: many prescribed medications taken in non therapeutic range or in the presence of inadequate biotransformation or clearance may produce dysrhythmias. Illegal use of sympathomimetic agents, such as cocaine or meth (Ritalin), may cause myocardial irritability and even infarction.
Causes and Manifestations of Dysrhythmias
-Electrolyte imbalances:
-Electrolyte imbalances: Electrical activity in the heart results from the exchange of electrolytes within the cardiac tissue. As a result, abnormal serum concentrations of electrolytes, such as potassium, magnesium, and calcium, can cause dysrhythmias.
Causes and Manifestations of Dysrhythmias
-Rate:
Rhymes that are too slow or too fast result in inadequate cardiac output. Cardiac output is a product of strike volume and cardiac rate. Stroke volume is the volume of blood pumped by one ventricle during one beat. Therefore, if the heart rate is too slow and the stroke volume is not increased proportionally, the cardiac output will be reduced. On the other hand, if the heart rate is too fast, the ventricles do not have enough time to fill with blood and stroke volume may be significantly reduced, resulting in poor cardiac output.
Causes and Manifestations of Dysrhythmias
-Stretch:
-Stretch: Atrial or ventricular hypertrophy can produce dysrhythmias. Hypertrophy may be a result of a genetic disorder or a consequence of increased workload on the myocardium (e.g. Chronic, uncontrolled high blood pressure)
depolarization
-The spread of electrical stimuli throughout the heart causes depolarization of the myocardial cells. Depolarization occurs when a polarized cell is stimulated.Polarized cells carry an electrical charge on their surface, the inside of the cell is more negatively charged than the outside of the cell. The sudden loss of negative charge within the cell is called depolarization, which is a result of potassium moving out of the cell and sodium moving into the cell
repolarization
-The return of the negative electrical charge is called repolarization (fig 11-4) and is a result of potassium moving back into the cell and sodium moving out of the cell. This produces waves of electrical activity that travel back and forth across the heart
waves of electrical activity
-These waves of electrical activity are represented by waves detected by the ECG electrodes. The magnitude or amplitude of each wave is determined by voltage generated by depolarization of a particular portion of the heart.
Basic ECG Waves
P wave
-Depolarization of the atria creates the initial wave of electrical activity detected on the ECG tracing, known as the P wave. Because the atrai usually are small, the atria generate less voltage than the ventricles and resulting P wave is small
-Repolarization of the atria is not seen on the ECG because it usually is obscured by the simultaneous depolarization of the ventricles -These waves of electrical activity are represented by waves detected by the ECG electrodes. The magnitude or amplitude of each wave is determined by voltage generated by depolarization of a particular portion of the heart.
Basic ECG Waves
QRS complex
-Depolarization of the ventricles is represented by the QRS complex. Because the ventricular muscle mass is larger than the atria and produces more voltage during depolarization, the QRS complex is normally taller than the P wave in most cases (see fig 11-5) Ventricular repolarization is seen as the T wave. The T wave is normally upright and rounded.
Basic ECG Waves
U wave
-Just after the T wave but before the next P wave, a small deflection known as the U wave is sometimes seen . the U wave is thought to represent the final [phase of ventricular repolarization. In most cases the U wave is not seen. The clinical significance of its presence or absence is not know
Basic ECG Waves
-QRS complexes usually consist of several distinct waves,
-QRS complexes usually consist of several distinct waves, each of which has a letter assigned to it as a label. This labeling system is needed because the precise configuration of the QRS complex can vary from one lead to the next and from one patient to the next. Top establish a standardized labeling system, several guidelines have been developed. If the first deflection of the QRS complex is downward (neg) it is labeled a Q wave. The initial upward (positive) deflection is called an R wave.
S wave of QRS
-The first negative deflection following an R wave is called an S wave (Figure 11-6) If the QRS complex has a second positive deflection, it is labeled R (R prime), and if a second S wave is also present it is called S (S prime). A negative deflection can be called a Q wave only if it is the first wave of the complex. In clinical practice, each ventricular depolarization complex is called a QRS complex whether it has all three waves or not.
EKG Paper
• The electrical activity of the heart is recorded on paper that has gridlike boxes with light and dark lines running horizontally and vertically (Figure 11-7). The light lines circumscribe small boxes (1 x 1 mm) and the dark lines circumscribe larger boxes (5 × 5 mm).
EKG Paper mV
-On The vertical axis, voltage, or amplitude, of the ECG waves is measured. The exact voltage of any ECG wave can be measured because the electrocardiogram is standardized so that 1 mV produces a deflection 10 mm in amplitude. Therefore, the standard for most ECG recordings is 1 mV= 10mm. Each small square represents 1 mm.
measure amplitude
-To measure the amplitude of a specific wave, the isoelectric baseline must be identified. This is the flat line seen just before the P wave or right after the T or U wave. Any movement of the ECG stylus above the line is considered positive, any downward movement is considered negative.To measure the degree of positive or negative amplitude of a specific wave, the isoelectric line is used as a reference point marking zero voltage.
-R waves are measured from isoelectric lines to the top of the R wave. Q and S waves are measured from the isoelectric line to the bottom of the wave. P waves can be either positive or negative and are also measured from the isoelectric line to the top (if positive) or bottom ( if negative) of the wave.
Normal values for P, QRS, T , and Intervals
PR interval
-The PR interval represents the time it takes for the electrical stimulus to spread through the atria and to pass through the AV junction. The normal PR interval is between .12 and .2 second (3 to 5 small boxed) If conduction of the impulse through the AV junction is abnormally delayed, the PR interval will exceed .2 second.
Interverval and Segment
-In addition to the amplitude of any wave, the duration of waves, intervals, and segments can be measured. Segment is a straight line between two waves. An interval encompasses at least one wave plus the connecting straight line.
-Interverval line + wave -Segment line between waves
Normal values for P, QRS, T , and Intervals
P wave measurement
-The normal P wave is less than 2.5 mm in height and not more than .10 second in length.The PR interval is important regarding conduction time. The interval is measured from the beginning of the P wave, where the P wave lifts off the isoelectric line, to the beginning of the QRS complex.
QRS interval
The duration of ventricular depolarization is determined by measuring the QRS interval. This interval is measured from the first wave of the QRS complex to the end of the last wave of the last wave of the QRS complex.
ST segment
Very important segment to evaluate is the ST segment. This segment is portion of the ECG cycle from the end of the QRS complex (even if no S wave is present) to the beginning of the T wave. It measures the time from the end of ventricular depolarization to the start of the ventricular repolarization
The normal ST segment is isoelectric ( No positive charge or negative voltage) or at least does not move more than 1 mm above or below baseline. Certain pathologic abnormalities such as myocardial ischemia or injury, cause the ST segment to be elevated or depressed. The duration of the ST segment is not as important as its configuration
QT interval
-The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The interval represents the time from the beginning of ventricular repolarization. The normal values for the QT interval depend on the heart rate. As the heart rate increases, the QT interval normally shortens, as the heart rate decreases, the QT interval increases.
-AS a general rule, the QT interval that exceeds one half of the RR interval is prolonged if the heart rate is 80 beats/ min or less. Common causes of abnormally prolonged QT interval include HYpokalemia (low potassium), hypocalcemia ( low calcium) and the side effects of certain medications such as quinidine.
Heart Rate
-If the heart rate is regular, one of the easiest ways of determining the heart rate is to count the number of larger (.2 second) boxes between 2 successive QRS complexes and divide this number into 300. For example, if there is one large box between successive R waves, then each R wave is separated by .2 seconds.
-Over the course of 1. Second there will be 5 QRS complexes and 300 QRS complexes in 60 seconds. Therefore the heart rate is 300 beats / min. Following this logic
-2 large boxes= rate of 150 beats/ min (300/2 * 150)
-3 Large boxes= rate of 100 beats/ min (300/3*100) -4 large boxes= rate of 75 beats/ min (300/4* 75) -5 large boxes= rate of 60 beats/ min (300/5*60) -6 large boxes= rate 50 beats/min (300/6*50) -If the heart rate is irregular, this method will not be accurate because the spacing between QRS complexes will vary from beat to beat
The most accurate way to figure heart rate is as follows.
-The most accurate way to figure heart rate is as follows. Count the number of small boxes (.04) from R wave to the other R wave and divide that into 1500. If a PT has 10 small boxes from one R to the other R, then it would be as follows, 1500/ 10=150 beats per minute.
-An increase or decrease in heart rate by more than 20% of the baseline value is significant charge and should be evaluated further. An abnormally slow heart rate may reduce cardiac performance to the point of compromised perfusion. Recall that cardiac output is a product of stroke volume and heart rate. A heart rate below 60 beats/min is referred to as an absolute bradycardia
Heart Rate Bradycardia
-However, Bradycardia that may require intervention is relative to the individual PT. For example, a well conditioned runner may present with a heart rate of 50 beats/min with no sign of inadequate cardiac output. On the other hand, a person with poor myocardial contractility that presents with a heart rate of 50 beats/ min is likely to show signs of compromised perfusion or even cardiogenic shock in severe cases.
Heart Rate
tachycardia
-At the other extreme, an increase in heart rate above 130 beats/min may compromise cardiac performance. This is because an abnormally rapid heart rate ( tachycardia) will increase myocardial oxygen demand, possibly to the point of inducing ischemia if the demand exceeds the supply.
-Additionally, significant tachycardia may further induce ischemia because it shottens the diastolic period, which is the period when most coronary perfusion occurs. In sinstance of extreme tachy where that heart rate exceeds 160 beats/min, the ventricular filling time may be so short that cardiac output decreases. Such PT should be monitored carefully and attending physician notified immediately
Lead Placement
-The 12 leads can be subdivided into two groups:
6 extremity (limb) leads and 6 chest leads. To obtain the six limb leads, two electrodes are placed on the patient’s wrists and two on the patient’s ankles. The ECG machine can vary the orientation of these four electrodes to one another to create the six limb leads. The chest leads are created by attaching six electrodes across the patient’s chest. The chest leads are discussed after the limb leads are reviewed.
ECG leads
-Because the heart is a three-dimensional organ, a more complete picture of the electrical activity in the heart will be obtained if it is viewed from several different angles.
The standard ECG uses 12 different leads to provide 12 different views from different angles of the heart. Interpretation of the 12 leads is a little more difficult, but the information obtained is more complete and abnormalities are not likely to be missed.
Limb Leads
-The six limb leads are called leads I, II, III, aVR, aVL, and aVF. Leads I, Il, and Ill are bipolar.
Each lead is created by comparing the difference in electrical voltage between two electrodes. For lead I, the ECG machine temporarily designates the electrode on the left arm as a positive lead and the electrode on the right arm as negative.
leads I, II, III
-The measured difference in voltage between these two leads results in lead I. For lead 11, the right arm electrode remains negative and the left leg electrode is positive. Lead Ill is created by making the left arm negative and the left leg positive.
aVR, aVL, and aVF
-The other three limb leads (aVR, aVL, and aVF) are called augmented leads because the ECG machine must amplify the tracings to get an adequate recording. The augmented leads are created by measuring the electrical voltage at one limb lead, with all other limb leads made negative.
- For the augmented leads, the ECG machine must augment the recorded voltages by about 50% to get an adequate recording. Lead aVR is created by making the right arm positive and all the others negative. Lead aVL calls for the left arm to be positive, and lead aVF is created by making the left leg positive.
frontal plane
-The six limb leads view the heart in a vertical plane called a frontal plane. Any electrical activity that is directed up, down, left, or right is recorded by the limb leads (Figure 11-10
221). The frontal plane can be envisioned as a giant circle that surrounds the patient and lies in the same plane as the patient. This circle can be marked off in 360 degrees, as shown in Figure 11-10.
angle of orientation for each of the bipolar limb
- The angle of orientation for each of the bipolar limb leads can be determined by drawing a line from the designated negative lead to the designated positive lead. For lead I, the angle of orientation is 0 degrees; for lead II, +60 degrees; and for lead III, +120 degrees.
angle of orientation for augmented leads
• For the augmented leads, the angle of orientation can be determined by drawing a line from the average of the other three limb leads to the one that is designated as the positive lead. The angle of orientation is -150 degrees for lead aVR, -30 degrees for lead aVL, and 90 degrees for lead aVF.
the limb leads consist of three bipolar leads and three unipolar leads
• In review, the limb leads consist of three bipolar leads and three unipolar leads. The three bipolar leads are called leads 1, 11, and III.
The three unipolar leads are called aVR, aVL, and aVF. The abbreviation a refers to augmented, V to voltage, and R, L, and F to right arm, left arm, and left leg (foot), respectively.
- The limb leads measure the electrical activity in the heart that occurs in the frontal plane, and each lead has its own specific view or angle of orientation to the heart (Table 11-2 222).
