ECG and Cardiology Basics Quiz

Electrocardiogram (ECG)

• William Einthoven, a Dutch physiologist, developed electrocardiography and was awarded the Nobel Prize in 1924
• ECG records the electrical activity of the heart
• ECG refers to extracellular recording of summed electrical events from all cardiac muscle fibers during each heartbeat
• The heart behaves like a dipole, with an excited region (depolarized segment) forming a negative pole and a non-excited region forming a positive pole.
• Electrocardiography is the recording of these potential fluctuations during the cardiac cycle.
• Electrocardiograph is the recording device.

Bipolar Recording

• Electrodes are placed on opposite sides of the heart.
• Electrical potentials generated by current are recorded
• The recording is known as an electrocardiogram (ECG)
• In bipolar recording, both electrodes are active
• One electrode is connected to the negative terminal of the ECG machine, and the other to the positive terminal

Unipolar Recording

• In unipolar recording, one electrode is active, the other is an indifferent electrode, at zero potential
• Two types of unipolar leads are used:
  • Unipolar chest leads (precordial leads)
  • Unipolar limb leads

Unipolar Chest Leads (Precordial Leads)

• V1: Right fourth intercostal space, near the sternum.
• V2: Left fourth intercostal space, near the sternum.
• V3: Halfway between V2 and V4.
• V4: Left fifth intercostal space at the midclavicular line.
• V5: Left fifth intercostal space at the anterior axillary line.
• V6: Left fifth intercostal space at the midaxillary line.

Augmented Limb Leads

• In these leads, one limb carries a positive electrode, while a central terminal represents the negative pole, which is at zero potential
• Lead aVR: Active electrode is from the right arm (RA) and indifferent electrode is from left arm (LA) and left leg (LF).
• Lead aVL: Active electrode is from LA and indifferent electrode is from RA and LF.
• Lead aVF: Active electrode is from LF and indifferent electrode is from RA and LA.

ECG Leads and Regions of Left Ventricle

• The table below shows the regions of the left ventricle represented by different ECG leads:

ECG Paper Calibration

• Vertically: 1 small box = 0.1 mV, 5 mm; 1 large box = 0.5 mV, 5 small boxes
• Horizontally: 1 small box = 0.04 sec, 1 mm; 1 large box = 0.20 sec, 5 small boxes

ECG Waves, Segments, and Intervals

• ECG consists of P wave, QRS complex, and T wave.
  • P wave: Atrial depolarization (0.12–0.20s)
  • QRS complex: Ventricular depolarization (0.06–0.10s)
  • T wave: Ventricular repolarization (0.12–0.20s)
  • P-R interval: Impulse conduction time through the atria and A-V node.
  • Q-T interval: Total time for ventricular depolarization and repolarization.

QRS Complex

• QRS complex is caused by ventricular depolarization
• QRS Complex is Normally less than 0.08s
• Q wave is 0.1–0.2 mV, R wave is 1.0 mV, and S wave is 0.4 mV

Normal U Wave

• Small rounded wave produced by slow and late repolarization of the intraventricular Purkinje system and the papillary muscle (after the main ventricular mass has been repolarized).
• It becomes prominent in hypokalemia.

P Wave

• ECG wave representing atrial depolarization.
• Normal duration is less than 0.11 seconds
• Normal amplitude is between 0.1 and 0.25 mV.

QRS Complex

• ECG wave representing ventricular depolarization; should not exceed 0.12 seconds.
• Indicates conduction through ventricles.

ST Segment

• ECG segment representing isoelectric period between ventricular depolarization and repolarization.
• Should be isoelectric or slightly elevated

T wave

• ECG wave representing ventricular repolarization
• Should be upright in most leads.

PR Interval

• Measured from the onset of the P wave to the onset of the QRS complex.
• Indicates the time taken for the impulse to travel from the atria to the ventricles, including the AV nodal delay.

QT Interval

• Measured from the beginning of the QRS complex to the end of the T wave.
• Indicates total time of ventricles' depolarization and repolarization.
• Shortens with higher heart rates, lengthens with lower heart rates.

ST Interval

• Begins after QRS complex and ends before T wave.
• Represents isoelectric period between end of ventricular depolarization and beginning of ventricular repolarization

Cardiac Rhythm/Normal Heart Rate

• Normal heart rate (HR) is 60–100 beats per minute
• Bradycardia: HR less than 60 beats per minute
• Tachycardia: HR greater than 100 beats per minute

Vectorcardiogram

• Point 5 is the zero reference point
• Negative end of all successive vectors

Bundle Branch Block

• Either the left or right ventricle may depolarize late, creating a "wide" or "notched" QRS complex

Ventricular Fibrillation

• The most serious cardiac arrhythmia due to the uncoordinated contraction of the ventricles which results in the absence of cardiac output.
• Can be caused by various conditions such as shock, ischemia in the heart, or ischemia of specialized conducting system.

Atrial Fibrillation

• Cells in the atria depolarize, repolarize, and excite again randomly, resulting in irregularly shaped heartbeats.
• Absence of clear P waves.
• Causes include enlarged atria, AV valve diseases.

Atrial Flutter

• Characterized by a regular, but rapid heart rate (220 - 350 beats per minute)
• Represented by a repeating saw-tooth pattern instead of P waves, indicating an ectopic focus or re-entry phenomenon.
• The ventricular rate is frequently reduced to the half or third/fourth of the atrial rate due to the AV node's blocking impulses.

AV Block

• Types:
  • First-degree AV block: Prolonged PR interval (0.2 - 0.3 seconds)
  • Second-degree AV block, Mobitz I: PR interval increases progressively until a P-wave is not conducted to the ventricles, resulting in a cycle-repeating pattern
  • Second-degree AV block, Mobitz II: PR interval is constant until a P-wave is not conducted to the ventricles
  • Third-degree AV block: Complete block between the atria and ventricles.

Myocardial Infarction (MI)

• Three stages: Ischemia, injury, infarction; stages of heart tissue damage due to a blockage of blood vessels
• Associated ECG changes include: ST segment elevation, Q waves, and T wave inversions, depending on the location of the infarction.

Hypertrophy

• When one ventricle hypertrophies, the axis of the heart shifts toward the hypertrophied ventricle.
• Hypertrophy takes longer for the depolarization wave to travel across the ventricles, resulting in more time-consuming heartbeats.

Ionic Changes

• Low sodium levels: low voltage ECG complexes

• Potassium changes:

  • Normal levels (4-5.5mEq/L): typical ECG pattern
  • High levels (hyperkalemia): tall, peaked T waves, wide QRS complex, and potentially lethal condition
  • Low levels (hypokalemia): prolonged PR interval, depressed ST segment, and late T-wave inversion

• Calcium changes (hypercalcemia): irregular heart rhythms, heart stops in systole.

  • Hypocalcemia: prolonged ST and QT intervals

Long QT Syndrome

• Genetic abnormality affecting K+ channels, leading to a prolonged QT interval, increasing vulnerability to ventricular arrhythmias, including torsades de pointes.

Test your knowledge on ECG and the interpretations related to various cardiac conditions. This quiz covers topics like QRS axis deviation, P wave alterations, T wave characteristics, and key figures in electrocardiography. Perfect for medical students and practitioners looking to refresh their understanding of ECG fundamentals.