1 EKG Interpretation and Axis Determination.ppt

EKG Interpretation: The Normal Electrocardiogram and Axis Interpretation R. Ward, PhD, CRNA, FAANA Spring 2024 Objectives: 1. Draw a normal electrocardiogram (EKG) and label the waves and intervals. 2. Understand the waves of the EKG and what each wave represents. 3. Understand the time and voltage...

EKG Interpretation: The Normal Electrocardiogram and Axis Interpretation R. Ward, PhD, CRNA, FAANA Spring 2024 Objectives: 1. Draw a normal electrocardiogram (EKG) and label the waves and intervals. 2. Understand the waves of the EKG and what each wave represents. 3. Understand the time and voltage calibration of the EKG. 4. Describe the leads of the EKG: standard limb leads, augmented limb leads, precordial (chest) leads. 5. Understand the steps for 12-lead EKG interpretation. 6. Understand the steps for QRS axis determination. 7. Describe the steps to determining axis deviation: distinguish between normal, left axis deviation, right axis deviation, and indeterminate axis. 8. Understand the etiologies for left and right axis deviations. 2 Resources: Hall, J. (2016). Textbook of Medical Physiology, 13th edition. Philadelphia, PA: Elsevier Publishing. Healio (2020). Determining Axis. Retrieved from: https://www.healio.com/cardiology/learn-the-heart/ecg-re view/ecg-interpretation-tutorial/determining-axis Klabunde, (2020). The mean electrical axis. https://www.cvphysiology.com/Arrhythmias/A016 Dubin, D. (2000). Rapid Interpretation of EKG’s, 6th edition. Fort Myers, FL: COVER Publishing Company. 3 Characteristics of the Normal EKG 4 Depolarization vs. Repolarization A: first half of muscle fiber has depolarized: red (+) charges inside / red (-) outside recording negative to positive B: completely depolarized: red (-) outside recording has returned to baseline because both electrodes are in areas of equal negativity C: first half repolarized: black (+) charges outside / black (-) inside recording positive to negative D: completely repolarized all black (+) charges outside recording returned to baseline because both electrodes are in areas of equal positivity 5 Similarity to Vectors Insert fig of positive and neg deflectionsDarlene, p.4 6 BASIC PRINCIPLES OF ELECTROCARDIOGRAPHY 7 EKG Voltage and Time Calibration 8 9 Complexes and Intervals on the ECG P wave Represents atrial depolarization. First part represents right atrial depolarization. Second part represents left atrial depolarization. Usually upright, > Travel through Bundle of His Normally 0.10 to 0.20 seconds. Values less than 0.10 seconds may be associated with WPW syndrome. Values greater than.20 sec are associated with conduction defects (AV block). Measured from the onset of the P wave to the onset of the QRS complex. 11 12 QRS Complex Indicative of ventricular depolarization. Impulse travels down the Bundle of His, bundle branches, Purkinje fibers, and into ventricular muscle. QT Interval Contraction of ventricle from beginning of Q wave to end of T wave. Ordinarily 0.35 seconds in duration 13 Q wave First negative deflection from the isoelectric line Less than 0.04 sec in duration Less than 1/3 height of the R wave. Q waves that are deeper or wider than normal are referred to as pathological Q waves. R wave First positive deflection from the isoelectric line. A second R wave is designated as R’. S wave First negative deflection after the first positive deflection. There cannot be an S wave without an R wave preceding 14 it. 15 ST Segment Indicates the beginning of repolarization. Begins at the end of the QRS complex and ends with the beginning of the T wave. Deviations of the ST segment >1mm from the isoelectric line may indicate myocardial injury. 16 T Wave Represents ventricular repolarization. Atrial repolarization is not seen. In most leads, T wave is upright and asymmetrical. U Wave Small positive wave that follows the T wave. Commonly observed in children. Often seen best in V3. May be associated with hypokalemia, hypercalcemia, quinidine, digitalis, epinephrine, intracranial hemorrhage or papillary muscle dysfunction. Negative U waves may be associated with coronary artery disease. 17 Q-T Interval Measured from the beginning of the Q wave to the end of the T wave. Normal interval varies with heart rate. The Q-T interval corrected for heart rate is termed QTc (corrected QT interval). QTc = QT interval + the square root of the R-R interval. In general, the QT interval should be less than ½ the preceding R-R interval (at HR 65-90): ~ 0.35 sec Prolonged QT intervals indicate a delay in repolarization and may be associated with reentrant dysrrhythmias and Torsades de Pointes. J Point The J point is the junction at which the S wave meets the ST segment. 18 19 12-Lead EKG A picture of the heart taken from 12 different, standardized views: 6 limb leads / 6 precordial leads 20 Frontal leads – detect impulses moving right, left, up, or down 21 22 23 Precordial leads – detect impulses moving anterior, posterior, right, or left 24 25 Normal QRS Complexes in the Precordial Leads 26 Precordial Leads 27 28 Placement of Positive ECG Electrodes ICS = intercostal space SB = sternal border MCL = midclavicular line AAL = anterior axillary line MAL = mid-axillary line 29 Steps to Interpretation of Twelve Lead ECGs 1. Check standardization (10 mm in height = 1 mv) 2. Calculate heart rate. 3. Measure intervals: PR, QRS, QT 4. Calculate mean QRS axis. 5. Interpret underlying rhythm as well as any arrthythmias. 6. Inspect for AV blocks. 7. Inspect for bundle branch blocks and hemiblocks. 8. Inspect for pre-excitation. 9. Inspect for enlargement and hypertrophy. 10.Inspect for coronary artery disease (ST segments, T waves, 30 Q waves). Determining Axis Axis of the EKG is the major direction of the overall electrical activity of the heart. It can be normal, leftward (left axis deviation (LAD), rightward (right axis deviation (RAD), or indeterminate (northwest axis). – Deviations give insight into underlying disease states and help with differential diagnoses QRS axis is the most important to determine 31 What is a Vector? General direction and magnitude of the movement of depolarization – Bigger vectors represent greater magnitude Depicted by an arrow The mean QRS vector is the sum of all smaller vectors of depolarization The origin of the mean QRS vector is the AV node Because the small vectors of the thicker LV are larger, the mean QRS vector points more left 32 Mean QRS Vector Described in degrees within a circle drawn over patient’s chest The limb leads are used to determine the axis of the mean QRS vector in the “frontal plane” Normally points downward and to patient’s left (between 0 and +90o) Depicted by green arrow at right To determine: – Find lead axis that has biphasic (equally positive and negative) QRS deflections – Find axis that is perpendicular (90 degrees) to biphasic lead and has a positive deflection – In this example, aVL is biphasic. The positive perpendicular axis to aVL is +60 – Therefore the mean electrical axis (vector) is ~ +45 and is normal 33 Normal Axis A normal axis is when the mean QRS vector points downward and is between 0 to +90 degrees The mean QRS vector therefore gives valuable information about: – The position of the heart, – Insight into ventricular hypertrophy, – Information concerning myocardial infarction 34 Displaced Mean QRS Vector If the heart is displaced, the mean QRS vector is also displaced in the same direction AV node is always the tail of the vector “Vertical heart” – Common in tall, slender individuals 35 Displaced Mean QRS Vector If the heart is displaced, the mean QRS vector is also displaced in the same direction AV node is always the tail of the vector “Horizontal heart” – Obese individuals, diaphragm is pushed up, pushing heart to left 36 Ventricular Hypertrophy Greater depolarization activity of hypertrophied side Displaces mean QRS vector toward hypertrophied side 37 Myocardial Infarction Necrotic area of heart has lost its blood supply and does not depolarize Unopposed vectors from other side draw mean QRS vector away from the infarct 38 Steps to determine QRS axis Focus on the QRS complexes in specific leads: I, II, and aVF The positive ends of these 3 leads fall within normal axis region If all 3 leads have positive QRS complexes, the axis is normal 39 Determining QRS axis Normal QRS is between -30 and +90 degrees Left axis deviation (LAD) falls between -30 and -90 degrees Right axis deviation is between +90 and +/-180 degrees Indeterminate axis is between +/180 and -90 degrees Simplest method: – Find the major direction of the QRS complex (positive or negative) in leads I and AVF 40 Example: Normal QRS axis QRS complex is upright (positive) in both lead I and lead aVF Vector is headed toward positive of Lead I, and positive of lead aVF Axis is normal - 41 Example: Left axis deviation (LAD) QRS is upright in Lead I (positive), and downward in lead aVF – Axis is between 0 and -90 degrees However LAD is defined as between -30 and -90, so this is not always technically LAD. In this scenario, the QRS could fall between 0 and -30, which is normal; therefore we look in Lead II as well If Lead II is downward (neg), then axis is toward -120 and LAD is present If Lead II is upward (pos) then axis is toward +60, and QRS would be normal - 42 Causes of LAD Normal variant (physiologic, often age-related) Conduction defects: left anterior fascicular block or LBBB Left ventricular hypertrophy Mechanical shift of heart in the chest (expiration, high diaphragm from pregnancy, ascites, abdominal tumors, obesity, lung disease, prior chest surgery, etc.) Inferior myocardial infarction Prexcitation syndromes: Wolff-Parksinon-White syndrome Ventricular arrhythmias (eg. ventricular tachycardia) Congenital heart disease (eg. atrial septal defect) Hyperkalemia 43 Example: Right axis deviation (RAD) QRS is downward (negative) in Lead I Upright (positive) in aVF - 44 Causes of RAD Normal variant (eg. children, young adults) Limb-lead reversal (left-and right-arm electrodes) Right ventricular overload syndromes (acute or chronic) Right ventricular hypertrophy Conduction defects: left posterior fascicular block, RBBB) Lateral wall MI Preexcitation syndromes (Wolff-Parkinson-White syndrome) Ventricular rhythms (ventricular tachycardia) Congenital heart disease (atrial septal defect) Dextrocardia – apex of heart on right side of body Left pneumothorax Mechanical shift (inspiration or emphysema) Conditions causing right ventricular strain (pulmonary embolism, pulmonary stenosis, pulmonary HTN, chronic lung disease, and corpulmonale) 45 Example: Indeterminate axis QRS is downward (neg) in Lead I and downward (neg) in Lead aVF. Referred to as “northwestern axis” Uncommon; usually from ventricular arrhythmias, but can be from paced rhythm, lead misplacement, or congenital heart diseases - 46 Summary Table for QRS Axis 47