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Questions and Answers
If the wavefront of depolarization moves perpendicular to the recording lead axis, what will the recorded ECG deflection be?
If the wavefront of depolarization moves perpendicular to the recording lead axis, what will the recorded ECG deflection be?
Which of the following statements about the direction of the depolarization wavefront is TRUE?
Which of the following statements about the direction of the depolarization wavefront is TRUE?
In the normal activation sequence of the heart, which structure is responsible for the initial depolarization of the atria?
In the normal activation sequence of the heart, which structure is responsible for the initial depolarization of the atria?
The His-Purkinje system is responsible for the rapid depolarization of which part of the heart?
The His-Purkinje system is responsible for the rapid depolarization of which part of the heart?
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Which of the following would NOT be a factor influencing the magnitude of the ECG deflection?
Which of the following would NOT be a factor influencing the magnitude of the ECG deflection?
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What causes a wave of depolarization to spread through a myocyte?
What causes a wave of depolarization to spread through a myocyte?
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What happens to the electrical potential of a myocyte during repolarization?
What happens to the electrical potential of a myocyte during repolarization?
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Why is it important for cells to repolarize after depolarization?
Why is it important for cells to repolarize after depolarization?
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What is the relationship between the direction of the wave of depolarization and the deflection on the ECG?
What is the relationship between the direction of the wave of depolarization and the deflection on the ECG?
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What is the significance of the electrical field created by a wave of depolarization?
What is the significance of the electrical field created by a wave of depolarization?
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Why is the study of the electrical activity of the heart important?
Why is the study of the electrical activity of the heart important?
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Which of the following is a direct result of myocyte depolarization?
Which of the following is a direct result of myocyte depolarization?
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What is the purpose of the electrocardiogram (ECG)?
What is the purpose of the electrocardiogram (ECG)?
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What does the y-axis of the electrocardiogram (ECG) represent?
What does the y-axis of the electrocardiogram (ECG) represent?
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What do upward deflections on an ECG indicate?
What do upward deflections on an ECG indicate?
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Which component of the cardiac conduction system is responsible for the fastest conduction velocity?
Which component of the cardiac conduction system is responsible for the fastest conduction velocity?
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What is the primary function of the electrocardiogram (ECG)?
What is the primary function of the electrocardiogram (ECG)?
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Which wave on the ECG primarily represents ventricular repolarization?
Which wave on the ECG primarily represents ventricular repolarization?
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When recording a 6-lead ECG in a dog, which lead system is primarily used?
When recording a 6-lead ECG in a dog, which lead system is primarily used?
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Which of the following statements is true regarding ECG interpretation?
Which of the following statements is true regarding ECG interpretation?
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What does the P wave on the ECG primarily represent?
What does the P wave on the ECG primarily represent?
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What does a bifid (notched) P wave indicate in horses?
What does a bifid (notched) P wave indicate in horses?
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In a base-apex lead for a healthy horse, where is the positive electrode placed?
In a base-apex lead for a healthy horse, where is the positive electrode placed?
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What do negative QRS complexes indicate in cattle?
What do negative QRS complexes indicate in cattle?
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Which of the following parameters is not typically analyzed in veterinary medicine?
Which of the following parameters is not typically analyzed in veterinary medicine?
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In the context of electrocardiograms, what do the blue areas in the figure represent?
In the context of electrocardiograms, what do the blue areas in the figure represent?
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What is a common characteristic of the P, QRS, and T waves in a normal sinus rhythm?
What is a common characteristic of the P, QRS, and T waves in a normal sinus rhythm?
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What does the direction of the pink arrow in the figure indicate?
What does the direction of the pink arrow in the figure indicate?
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What should be expected in a healthy cow's electrocardiogram?
What should be expected in a healthy cow's electrocardiogram?
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What is the normal pacemaker rate for the SA node?
What is the normal pacemaker rate for the SA node?
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What is the conduction velocity of the AV node?
What is the conduction velocity of the AV node?
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What describes the direction of depolarization wave during the P wave?
What describes the direction of depolarization wave during the P wave?
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Which electrode is positioned over the left apex beat in a base-to-apex lead system?
Which electrode is positioned over the left apex beat in a base-to-apex lead system?
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Which of the following best describes the normal PR interval for dogs?
Which of the following best describes the normal PR interval for dogs?
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What happens during the ST segment of an ECG?
What happens during the ST segment of an ECG?
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What is the normal duration of the QRS complex in cats?
What is the normal duration of the QRS complex in cats?
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What is the consequence of hyperkalemia on the atrial internodal tracts?
What is the consequence of hyperkalemia on the atrial internodal tracts?
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Which of the following structures is responsible for the fastest conduction velocity?
Which of the following structures is responsible for the fastest conduction velocity?
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What does the T wave represent in an ECG?
What does the T wave represent in an ECG?
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Which leads are most commonly used in clinical settings?
Which leads are most commonly used in clinical settings?
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What describes the bundle of His and bundle branches?
What describes the bundle of His and bundle branches?
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What causes the bifid P wave typically seen in horses?
What causes the bifid P wave typically seen in horses?
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Study Notes
Electrocardiography Principles
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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
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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
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Label the x and y axes of the ECG
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Describe cell surface events as upwards deflections on ECG
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List the normal cardiac activation sequence from memory.
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For each component of the cardiac conduction system:
- Describe relative conduction velocity
- Determine if normal pacemaker activity is expected
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Describe methods for recording a 6-lead ECG in a dog and a base-apex ECG in a horse.
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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
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Sinoatrial (SA) node (pacemaker): 60-250 impulses/minute
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Atrial muscle: Impulse conduction is slower
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Atrioventricular (AV) node: 40-60 impulses/minute
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His-Purkinje system: Impulses are conducted very rapidly. (20-40 impulses/minute)
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Bundle of His
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Left bundle branch
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Right bundle branch
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Purkinje fibers
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Ventricular muscle: Impulse conduction is fast.
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Conducting tissue in the heart that transmit electrical impulses.
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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
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P wave = cell-by-cell atrial depolarization in Lead II, Positive in lead II, frequently bifid (M-shaped) in horses.
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PR (PQ) interval = depolarization in atria, AV node, and bundle of His; approximates signal transmission through the AV node.
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Normal duration < 0.13 seconds (dog) or < 0.09 seconds (cat)
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QRS complex = ventricular depolarization.
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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.
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ST segment = isoelectric (flat) line connecting the S and T waves. All ventricular cells have finished depolarizing, and no current is flowing.
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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.
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Description
Test your knowledge on the principles of electrocardiography with this quiz. You will explore the basics of ECG including its recording methods, limitations, and how to interpret key components. Perfect for students studying veterinary medicine and animal physiology!
