Electrocardiography Principles Quiz

Electrocardiography Principles

• Electrocardiogram (ECG or EKG)

  • Records the heart's electrical activity from the body surface
  • Records extracellular signals generated by depolarization/repolarization waves
  • Presents voltage (mV) on the y-axis and time (seconds) on the x-axis

• ECG Does Not Record Mechanical Activity

  • ECG cannot measure if the heart is contracting or the strength of contractions.
  • ECG cannot assess the presence/absence of heart failure.
  • ECG records electrical activity, not mechanical activity

Learning Objectives

• Label the x and y axes of the ECG

• Describe cell surface events as upwards deflections on ECG

• List the normal cardiac activation sequence from memory.

• For each component of the cardiac conduction system:

  • Describe relative conduction velocity
  • Determine if normal pacemaker activity is expected

• Describe methods for recording a 6-lead ECG in a dog and a base-apex ECG in a horse.

• Label individual ECG waves and explain the corresponding cellular events.

Physiology Review: Basic Cardiac Electrophysiology

• Cardiac tissues are excitable.
• At rest, myocardial cells are polarized (negative charge inside, positive outside).
• Stimulation causes depolarization (reversal of charge).
• Depolarization spreads as a wave, generating extracellular currents detectable by the ECG.
• Cells must repolarize to enable further depolarization cycles.

Physiology Review: Normal Cardiac Conduction System

• Sinoatrial (SA) node (pacemaker): 60-250 impulses/minute

• Atrial muscle: Impulse conduction is slower

• Atrioventricular (AV) node: 40-60 impulses/minute

• His-Purkinje system: Impulses are conducted very rapidly. (20-40 impulses/minute)

• Bundle of His

• Left bundle branch

• Right bundle branch

• Purkinje fibers

• Ventricular muscle: Impulse conduction is fast.

• Conducting tissue in the heart that transmit electrical impulses.

• Function as electrical pathways enabling organized heart contraction.

Physiology Review: Basics of Electrocardiography

• Electrodes placed on either side of a depolarization wave allow measurement of electrical fields. By convention, a negative wave front toward the positive electrode results in a positive deflection on an ECG.
• Wavefront moving towards a positive electrode = positive deflection
• Wavefront moving away from a positive electrode= negative deflection

Physiology Review: Normal Cardiac Conduction System (Special Notes)

• Atrial internodal tracts are rapidly-conducting tissues connecting the SA and the AV nodes
• Atrial internodal tracts are relatively resistant to the effects of excessive potassium (hyperkalemia).
• Bundle of His/Bundle branches divides into right and left bundle branches conducting rapidly to terminal Purkinje fibers.
• Terminal Purkinje fibers conduct rapidly along the endocardium

Method for Recording a 6-Lead Surface ECG in Small Animals

• Patient in right lateral recumbency with limbs parallel and perpendicular to the trunk.
• A calm environment is optimal with non-conductive surface to reduce artifacts.
• Use white/black electrodes for forelimbs and red/green for hindlimbs.
• Avoid contact between electrodes and the trunk.
• Placement should follow "Black-and-white TV came before color; white on right and grass on the ground" rule
• Some machines require a grounding electrode.

Method for Recording a 6-Lead Surface ECG in Small Animals (Additional Details)

• Several standard leads are used.
• A lead is an electrode pair. One is positive and one is negative.
• Lead II is commonly used in the clinic.
• Lead II has a negative electrode on the right forelimb and a positive electrode on the left hindlimb.
• Lead II axis is oriented craniocaudal and right to left

Method for Recording a "Base-to-Apex" ECG in Horses and Ruminants

• No universal lead standard for large animals.
• Electrode placement for a "base-to-apex" lead commonly uses a single lead (RA to LA+).
• Electrode placement: right jugular furrow for white electrode, and left apex for black electrode (LA)
• Lead axis is oriented craniocaudal and right-to-left

The Normal (Lead II) ECG Tracing

• The ECG traces the heart's electrical activity over time
• Blue = Depolarized tissue
• Pink arrow = direction of depolarization wave

Electrical Correlates of ECG Waves/Intervals

• P wave = cell-by-cell atrial depolarization in Lead II, Positive in lead II, frequently bifid (M-shaped) in horses.

• PR (PQ) interval = depolarization in atria, AV node, and bundle of His; approximates signal transmission through the AV node.

• Normal duration < 0.13 seconds (dog) or < 0.09 seconds (cat)

• QRS complex = ventricular depolarization.

• Should be tall, skinny, and upright in lead II (small animals). Normal < 0.06 seconds (dog), < 0.04 seconds (cat). In horses/ruminants its typically negative deflection.

• ST segment = isoelectric (flat) line connecting the S and T waves. All ventricular cells have finished depolarizing, and no current is flowing.

• T wave = ventricular repolarization, a more complex process. T waves can be negative, positive, or biphasic. Not studied extensively in Veterinary medicine.

Normal Base-Apex ECG in Horses/Ruminants

• Bifid (notched) P waves are normal in horses
• Negative QRS complexes are normal in horses and cattle.