ECG Practice Exam Answers (some at least) Scott Ewing, D.O. November 3, 2005

Normal ECG

Normal Intervals

Left Bundle Branch Block

Right Bundle Branch Block

QT Interval Heart rate dependent QTc  0.44-0.46 sec QTc = QT /  RR in seconds Causes long QTc Hereditary Drugs (many antiarrhythmics, tricyclics, phenothiazines, and others) Electrolyte abnormalities ( K+, Ca++, Mg++) CNS disease (especially subarrachnoid hemorrhage, stroke, trauma) Hereditary LQTS (e.g., Romano-Ward Syndrome) Coronary Heart Disease (some post-MI patients)

QT interval duration is greater than 50% of the RR interval, a good indication that it is prolonged Increased risk for malignant ventricular arrhythmias, syncope, and sudden death

Normal frontal plane axis -30º - 100º

Axis #1 No isoelectric lead in this ECG, the two closest leads are I and aVL If I isoelectric, the axis would be +90º If aVL isoelectric, the axis would be +60º Compromise is +75º (The two closest leads are always 30 degrees apart)

Axis #2 Lead I is isoelectric Perpendiculars to lead I are +90 and -90º Leads II, III, aVF are positive Therefore, the axis must be +90º

Axis #3 No isoelectric lead in this ECG, the two closest leads are II and aVF If aVF isoelectric, the axis would be 0º If II isoelectric, the axis would be -30º Compromise is -15º (The two closest leads are always 30 degrees apart)

Axis #4 Lead aVF is isoelectric Perpendiculars to lead aVF are 0 and +180º Leads I, II, aVL are positive Therefore, the axis must be 0º

Axis #5 (Bonus)

Axis #5 (Bonus) Lead I is negative, which usually means RAD Lead II is the isoelectric lead, which almost always means -30 degrees But in this example the axis is 180 degrees away from –30 (look at I, III and aVL) Unusual right axis deviation +150º

Northwest axis (no man's land) emphysema hyperkalaemia lead transposition artificial cardiac pacing ventricular tachycardia

Right Axis Deviation normal finding in children and tall thin adults right ventricular hypertrophy chronic lung disease even without pulmonary hypertension anterolateral myocardial infarction left posterior hemiblock pulmonary embolus Wolff-Parkinson-White syndrome - left sided accessory pathway atrial septal defect ventricular septal defect

Left Axis Deviation left anterior hemiblock Q waves of inferior myocardial infarction artificial cardiac pacing emphysema hyperkalaemia Wolff-Parkinson-White syndrome - right sided accessory pathway tricuspid atresia ostium primum ASD injection of contrast into left coronary artery