ECG Interpretation Quiz

Electrocardiography

• Electrocardiography is the study of the recording and analysis of the electrical activities of the heart.
• The heart generates electrical impulses that are propagated as waves of depolarization and repolarization.
• These electrical impulses are recordable on a graph for visual analysis and interpretation.

ECG Paper

• The graph paper consists of horizontal and vertical lines at 1mm apart, forming small (1mm) and large (5mm) squares.
• Time measurement is done horizontally, with 1mm representing 0.04s (40ms).
• Voltage measurement is done vertically, with 1mm representing 0.1mV.

Standardization

• Paper speed: 25mm/s.
• Calibration signal or amplitude indicator: 10mm/mV.

ECG Denotations

• P wave: atrial depolarization.
• QRS complex: ventricular depolarization.
• Q wave: first negative deflection in ventricular depolarization.
• R wave: first positive deflection in ventricular depolarization.
• S wave: second negative deflection in ventricular depolarization or the first negative deflection after R wave.
• T wave: ventricular repolarization.

Intervals in ECG

• PR interval: period from the onset of atrial depolarization to the beginning of ventricular depolarization.
• QT interval: period from the onset of ventricular depolarization to the end of ventricular repolarization.
• RR interval: period from the peak of R wave to the next.

Other Denotations

• PR segment: isoelectric line from the end of atrial depolarization to the beginning of ventricular depolarization.
• ST segment: isoelectric line from the end of ventricular depolarization to the beginning of ventricular repolarization.
• J-Junction: the junction between the end of ventricular depolarization and the beginning of ST segment.
• U wave: any wave between T and P waves.

U Wave

• An inconstant wave.
• Exact mechanism of its genesis is still controversial.
• Suggested mechanisms include:
  • Repolarization of papillary muscle and neighboring structures (old concept).
  • Repolarization of mid-myocardial cells (M-cells).
  • Afterpotentials.

Standard 12-Lead ECG

• Recorded by the aid of 10 electrodes arranged on specific locations on the body surface.
• One electrode is attached to each limb to produce 6 limb leads.
• 6 electrodes are attached to the chest to produce 6 chest or precordial leads.

Classifications of Standard 12 Leads

• Types of leads: bipolar limb leads, augmented limb leads, and chest or precordial leads.
• Bipolar limb leads: I, II, III.
• Augmented limb leads: aVF, aVL, aVR.
• Chest or precordial leads: V1, V2, V3, V4, V5, V6.

Chest Electrode Placement

• V1: 4th intercostal space, right sternal edge.
• V2: 4th intercostal space, left sternal edge.
• V3: midway between V2 and V4.
• V4: 5th intercostal space, left midclavicular line.
• V5: 5th intercostal space, left anterior axillary line.
• V6: 5th intercostal space, left mid-axillary line.

Classification Based on Planes of Location

• Frontal plane leads: I, II, III, aVL, aVF, aVR.
• Horizontal/transverse plane leads: V1, V2, V3, V4, V5, V6.

Lead Classification Based on Heart Regions

• Left lateral leads: I, aVL, V5, V6.
• Inferior leads: II, III, aVF.
• Septal leads: V3, V4.
• Anterior leads: V1, V2, V3, V4, V5, V6.
• Right ventricular cavity leads: aVR, V1, V2.

QRS Axis

• QRS axis is the average direction of the ventricular depolarization (QRS) current.
• Normal axis points mostly downward and to the left because the more muscular left ventricle generates a stronger depolarizing current than the less muscular right ventricle.

Classification of QRS Axis

• Normal axis: 0 to +90 degrees.
• Left axis deviation: 0 to -90 degrees.
• Right axis deviation: +90 to +180 degrees.
• Extreme axis: -90 to 180 degrees.

Causes of Axis Deviation

• Left axis deviation: left anterior fascicular block, left ventricular hypertrophy, inferior myocardial infarction, Wolff-Parkinson-White syndrome, artificial cardiac pacing, mechanical shift (e.g. pregnancy), and congenital heart disease (e.g. atrial septal defect).
• Right axis deviation: normal variation in children and thin adults, left posterior fascicular block, left lateral wall infarction, right ventricular hypertrophy, dextrocardia, pulmonary embolism, and congenital heart disease (e.g. atrial septal defect).
• Extreme axis deviation: emphysema, artificial cardiac pacing, and lead misplacement.

QRS Axis Estimation

• Step I: Locate the class of the QRS axis by inspecting leads I and aVF.
• Step II: Estimate the QRS axis to the nearest 30 degrees by inspecting the limb leads, finding the equiphasic QRS complex, and identifying the angles of orientation of the lead perpendicular to it.

Estimation of Heart Rate

• Heart rate can be estimated using the following formulae:
  1. HR = 300/x, where x is the number of large (5mm) squares within the RR interval.
  2. HR = 1500/y, where y is the number of small (1mm) squares within the RR interval.
  3. HR = 60/RR interval.
• Normal heart rate: 60-100 beats per minute.

Estimation of QTc

• QT interval: period from the onset of ventricular depolarization to the end of ventricular repolarization.
• QT interval varies with heart rate.
• QTc can be calculated using Bazzet's formula: QTc = QTm / RRint ½.
• Normal QTc: 0.36-0.44s for males.

ST Segment

• ST segment: isoelectric line between the end of ventricular depolarization and the beginning of ventricular repolarization.
• ST segment begins at the J junction and ends at the K junction.
• ST segment can be elevated or depressed in some cardiac disorders.

ST Segment Changes

• ST segment elevation: early repolarization, acute myocardial infarction, and acute pericarditis.
• ST segment depression: myocardial ischaemia (angina pectoris) and hypokalaemia.

14 Steps in ECG Interpretation

• 1. Standardization.
• 2. Rhythm.
• 3. Rate.
• 4. P wave.
• 5. PR interval.
• 6. Q wave.
• 7. QRS complex.
• 8. J-junction.
• 9. ST segment.
• 10. T wave.
• 11. QT interval.
• 12. Miscellaneous.
• 13. Summary.
• 14. Conclusion.