10. Electrocardiography, Cardiac arrhythmias in dogs and cats. Flashcards
Electrocardiography?
Electrocardiography (ECG)
Types of recording: Intracardial or Epicardial
Intracardiac tracings show the normal intervals between:
§ Initiation of atrial depolarisation [A]
His bundle activation [H]
§ Ventricular depolarisation [V]
§ AH + HV = PR interval
WAVE SEGMENTS
INDICATIONS
§ Arrhythmia
§ Bradycardia
§ Tachycardia
§ Monitoring during anaesthesia
LIMITATIONS
Temporal → Can be solved by using a Holter monitor
Spatial
§ The heart is not a single dipole vector
§ Distortion by extracardiac effects
Technique?
Arrhythmias?
Arrhythmias
Any cardiac rhythm falling outside the sinus rhythm
Atrial
High HR
Sinus Tachycardia
Atrial Fibrillation
Atrial Flutter
Paroxysmal Supraventricular Tachycardia
Wolff-Parkinson-White Syndrome
± HR Premature Atrial Contraction
Low HR
Sinus Bradycardia
Sinoatrial (SA) Block
Atrioventricular (AV) Block
Sick Sinus Syndrome
P-Wave Atrial depolarisation
PR-Segment Impulse through AV node & bundle of His
Q-Wave Septal depolarisation
R-Wave LV depolarisation
S-Wave RV depolarisation
ST-Segment Interval of ventricular systole
T-Wave Ventricular repolarisation
QT-Segment Ventricular depolarisation &
repolarisation
L
Ventricular
High HR Ventricular Tachycardia
Ventricular Fibrillation
± HR Premature Ventricular Contraction
Low HR Intraventricular Block
Two types of arrhythmias
§ Impulse formative disorders
§ Impulse conductive disorders
Causes of arrhythmia?
Causes of Arrhythmia
§ Structural heart disease: Cardiac remodelling;
Neurohormonal changes; Inflammatory mediators; Free
radicals; Hypoxia
§ Systemic disease: Hypoxia; Vegetative tone; Temperature;
Ions; Drugs; Toxicosis
Primary arrhythmias
§ Boxer; Bulldog; Cat: Arrhythmogenic RV
cardiomyopathy (ARVC)
§ Mini Schnauzer; White Westie: Sick sinus syndrome
§ Cocker spaniel: AV block
§ Labrador; Boxer: AV accessory pathways – SVT
§ Doberman DCM: Ventricular arrhythmias
§ German Shepherd: Juvenile VT
§ Springer Spaniel: Silent atrium
Impulse formative arrhythmias?
Impulse Formative Arrhythmias
§ Normal/high heart rate
§ Abnormal automacity
§ Triggered activity
§ Re-entry
Disorder types (categorised by origin)
Normotop (originates from the sinus node)
§ Sinus tachycardia
§ Sinus bradycardia
§ Sinus arrest
§ Sick sinus syndrome
Heterotop/ectopic (originates from outside sinus node)
Supraventricular
Atrial extrasystole
Atrial tachycardia
Atrial fibrillation; Flutter
Junctional extrasystole
Junctional tachycardia
Ventricular
Ventricular extrasystole
Ventricular tachycardia
Ventricular fibrillation; Flutter
Sinus arrest: Long pause following a normal complex; Due to high
parasympathetic tone
Supraventricular arrhythmias?
SUPRAVENTRICULAR ARRHYTHMIAS
Atrial extrasystole/Atrial Premature Complexes (APCs)
§ Impulse from atrial tissue, not the SA node → Ectopic beat
§ Premature P-Wave (submerged/superimposed in T-wave)
§ Tx: Unnecessary
Atrial tachycardia
§ Differentiate from sinus tachycardia
§ Usually indicated by APCs; Usually secondary to atrial
enlargement
Atrial fibrillation (AF)
§ Predisposed: Dogs > Cats; Irish wolf hound (lone AF)
§ Secondary to Atrial enlargement
§ Like atrial tachycardia but rapid, irregular and chaotic;
Irregular ventricular response
§ Ø P-waves → Many F-waves (fibrillation)
Primary AF: Ø Underlying cardiac diseases involved; Idiopathic
Secondary AF: Severe cardiac disease e.g. CHF
Paroxysmal AF: Periodic & recurring AF for a short time
Persistent AF: AF for > 48 hrs; Only responds to treatment
Permanent AF: Ongoing AF; Ø Treatment
Atrial flutter
§ Often a precursor to AF
§ Premature electrical impulses rising in the atria → ↑ HR
§ Ø P-Waves → Many F-Waves (larger than in AF)
§ “Saw-toothed” appearance of F-Waves
Junctional extrasystole/AV junctional premature complexes
§ Abnormal impulse formation near the AV junction
§ Early P-Waves (often negative)
§ Digitoxin
Junctional tachycardia
§ Abnormal impulse formation near the AV junction
§ ↑ HR; Regular rhythm; Absent/negative/buried P-Wave
Ventricular Arrhythmias?
VENTRICULAR ARRHYTHMIAS
Ventricular extrasystole/Ventricular premature complex (VPC)
§ Abnormal impulse formation distal to the AV junction
§ Wide QRS-Complex; Ø Preceding P-Wave; Deep Q-Wave
§ Occasional VPCs are considered normal
Ventricular tachycardia (VT)
§ Abnormal impulse conduction → Ectopic rhythm
§ Inadequate cardiac output; May lead to VF
§ Multiple QRS-complexes; Ø P-Waves (looks like VPCs)
Ventricular fibrillation (VF)
§ Requires immediate treatment → Electrical defibrillation
§ Irregular pattern of high & low amplitude waves
§ Often leads to cardiac arrest
Ventricular flutter (VFlut.)
§ May precede VF (VT → VFlut. → VF → Cardiac arrest)
§ Ø P-Wave; QRS is indistinguishable from T-Wave
§ High ventricular rate; Regular rhythm
§ Requires immediate treatment
Impulse conductive arrhythmias?
Impulse Conductive Arrhythmias
Where the impulses are generated but not properly conducted
Normal/low heart rate; Slow conduction; Unidirectional/bidirectional
block
Sinoatrial (SA) block
§ SA node impulses are blocked → Ø Cardiac tissue
conduction → Pause in the ECG
§ Can be clinically insignificant
Atrial standstill
§ SA node sends impulses → Ø Atrial contraction; Ø PWave
§ Predisposed: Springer spaniel
§ Cause: Hyperkalaemia (acute); Atrial fibrosis (chronic)
§ Antiarrhythmic drugs are contraindicated
AV Block
Causes: Toxicosis; ↑ Vagal tone; Hyperkalaemia;
Hypothyroidism; Inflammation; Neoplasia; Amyloidosis
I-Degree AV block
Impulse conduction delay in the AV node region
Prolonged but constant PR-interval
II-Degree AV block
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Impulse conduction delay/block in AV node region → Some P-Waves
are followed by QRS
Mobitz type-I: PR-intervals gradually lengthen
Mobitz type-II: PR-intervals are constant
2:1 AV block; High degree
III-Degree AV block
Impulse completely blocked
Ø Association between P-Wave & QRS-Segment → QRS is formed by
escape rhythm, not AV node
Bundle branch block: Defect in the bundle of His’ conduction to the right & left fascicles
Consequences of arrhythmia?
Consequences of Arrhythmia
§ Innocent → Ø Clinical complication (most common)
§ Weakness
§ Syncope
§ Sudden death
§ Heart failure
Treatment of arrhythmia?
Treatment of Arrhythmia
Don’t treat if there are no CSx & no severe haemodynamic changes;
Do search for the cause
Do not treat:
✗ Non-frequent atrial/ventricular extrasystole
✗ Slow idioventricular/junctional rhythm
✗ Lone AF
✗ I-degree AV block
✗ Mobitz-I AV block
General considerations
§ Performance of an ECG is essential
§ Exclude heart disease
§ Diagnostic workup (electrolytes)
§ Bradyarrhythmia → Atropine response test
§ Before antiarrhythmic treatment: Oxygen; Symptomatic tx
Treatment options?
TREATMENT OPTIONS
Physical manoeuvres → Vagus nerve stimulation
Artificial pacemaker
Electric cardioversion
Radiofrequency catheter ablation
Medical management
§ Class I (Na+-channel blockers): Lidocaine; Mexiletine
→ ↓ Phase 0 slope & Action potential peak
§ Class II (Beta-blockers): Atenolol; Propranolol
→ Block sympathetic activity; ↓ HR
§ Class III (K+-channel blockers): Amiodarone; Sotalol
→ Delay repolarisation, ↑ AP duration
§ Class IV (Ca+-channel blockers): Verapamil; Diltiazem
→ SA & AV nodes: ↓ HR & conduction
§ Class V (Unclassified): Digoxin; Adenosine;
Anticholinergic sympathomimetic drugs
