Veterinary Cardiology Quiz

Electrocardiography Principles

• Electrocardiography (ECG or EKG) records the heart's electrical activity from the body surface.
• ECG records extracellular signals resulting from depolarization/repolarization waves through cardiac myocytes.
• The graph depicts voltage (mV, y-axis) over time (sec, x-axis).
• Changes in voltage are recorded as waves/complexes, using letters (e.g., P, QRS, T).

Learning Objectives

• Students will be able to label the ECG's x- and y-axes.
• Students will be able to describe cell surface events during upward ECG deflections.
• Students will be able to list the normal cardiac activation sequence from memory.
• Students will be able to describe the conduction velocity for each component of the cardiac conduction system.
• Students will be able to determine if normal pacemaker activity is expected for each component.
• Students will be able to describe a 6-lead ECG recording method in a dog and a base-apex ECG in a horse.
• Students will be able to mark individual ECG waves and explain the correlated cellular events.

Physiology Review: Cardiac Electrophysiology

• At rest, cardiac myocytes are polarized (membrane negatively charged - inside vs. outside).
• Stimulation causes resting myocytes to depolarize (membrane polarity reverses).
• Depolarization spreads as a wave, stimulating adjacent cells.
• Extracellular currents associated with the depolarization wave are detected by the ECG.
• Cells must repolarize to allow the cycle to repeat.

ECG Electrodes

• ECG electrodes placed on either side of a depolarization wave allow measurement of the electrical field.
• By convention, if a wavefront of negative extracellular charges moves toward the positive electrode, an upward deflection is recorded on the ECG.

Effect of Depolarization Wavefront Orientation

• When a wavefront moves directly toward an electrode in parallel with the lead axis, the largest possible deflection is recorded.

Normal Cardiac Conduction System

• The sinoatrial (SA) node is the primary pacemaker, setting the heart rate at ~60-250 impulses/minute for most species
• The sinoatrial (SA) node initiates the electrical impulse at approximately 60-250 beats (impulses) per minute in most species.
• The atrioventricular (AV) node slows conduction to allow the atria to empty into the ventricles before ventricular contraction. 
• The His-Purkinje system rapidly distributes the depolarization wave through the ventricles.

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

• Patients are placed in right lateral recumbency.
• Limbs are positioned parallel to each other, perpendicular to the trunk.
• Electrodes are placed on the elbows (white/black) and stifles (red/green) to avoid contact with the trunk and each other.

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

• No universal lead system exists.
• Position electrodes over the right jugular furrow (white) and left apex beat (black).
• Lead axis is oriented cranio-caudally (head to tail). 

The Normal (Lead II) ECG Tracing

• Depolarization waves are shown as blue regions on ECG diagrams with corresponding arrows showing the direction of depolarization.

Electrical Correlates of ECG Waves/Intervals

• P wave: Atrial depolarization (positive in lead II); often bifid (M-shaped) in some species.
• PR interval (PQ interval): Atrial, AV node, and Bundle of His depolarization; approximately 0.13 seconds in dogs and 0.09 seconds in cats.
• QRS complex: Ventricular depolarization (typically tall, skinny and upright in lead II for small animals); typically under 0.06 seconds in dogs and 0.04 seconds in cats
• ST segment: All ventricular cells depolarized, no current flow (isoelectric line connecting the S and T waves).
• T wave: Ventricular repolarization (can be negative, positive, or biphasic).

Normal Base-Apex ECG in Horses/Ruminants

• Bifid P waves are common in horses.
• Negative QRS complexes are normal in horses/ruminants.

Sinus Arrhythmia

• The sinus rhythm is characterized by cyclic slowing and speeding, commonly linked with respiration.
• This is a normal finding in dogs and fit horses but is uncommon in clinical feline cases

Sinus Tachycardia

• Tachycardia is defined by a sinus rhythm with a rapid heart rate.
• Rate cutoffs for tachycardia diagnosis vary by species

Atrial Fibrillation (A-fib)

• Characterized by irregular, "sawtooth" baseline (no P waves).
• Supraventricular QR complexes are narrow and upright in lead II.
• Irregularly irregular rhythm (no pattern in R-R intervals).
• Rapid heart rate.

Atrioventricular (AV) Block

• Conduction between atria and ventricles inhibited at the AV node (resulting in a dissociation between P-wave and QRS complex activity)

1st-Degree AV Block

• Characterized by a lengthened PR interval (and no other clinical or rhythm disturbances).

2nd-Degree AV Block

• Types I and II are characterized by differing degrees of AV nodal block (Type I AV block demonstrates progressive prolongation of PR interval prior to the complete interruption from the AV node) with clinical significance (or lack thereof), depending on the degree.

3rd-Degree AV Block

• Complete dissociation of atrial and ventricular excitation (separate, unrelated rhythms).

Ventricular Premature Depolarizations/Complexes

• Premature ventricular complexes (VPCs) occur early during the cardiac cycle, before the next expected sinus beat.
• The QRS complexes are wide and bizarre and lack a related P wave
• Characterized by wide/bizarre QRS complexes lacking a related P wave and may be associated with several specific underlying conditions.
• "R-on-T phenomenon" occurs when a VPC lands on the T wave of the proceeding beat, increasing ventricular fibrillation risk.

Ventricular Tachycardia (V-tach)

• Characterized by four or more consecutive VPCs at a rate exceeding 160 beats per minute in dogs, 200 beats per minute in cats, or 40 beats per minute in horses.
• QRS complexes are wide and bizarre; no related P waves.
• The rhythm is usually regular (consistent interval between beats)
• Ventricular tachycardia is a potentially fatal and very dangerous rhythm.

Ventricular Flutter

• A rapid rhythm with characteristic "sine wave" morphology, lacking isoelectric periods between beats, originating from the ventricles.

Ventricular Fibrillation

• Extremely disorganized, chaotic ventricular activity, typically lethal if untreated.

Additional notes

• The provided study notes are based on the information you shared.
• The diagrams provide visual aids, but the content focuses on the actual text
• The data from many sources can be more complicated.

Test your knowledge on the treatment and management of ventricular premature complexes (VPCs) in veterinary medicine. This quiz covers ECG interpretation, rate control, and key characteristics associated with VPCs. Perfect for veterinary students and professionals alike.