PART 5
ELECTRODIAGNOSTIC TESTING

5.1 Electrocardiographic findings

Note: Changes in ECG measurements are relatively insensitive indicators of chamber size.

5.1.1 Alterations in P wave

Tall P wave (P pulmonale)

• Right atrial enlargement, e.g.
  • Chronic respiratory disease*
  • Dilated cardiomyopathy*
  • Tricuspid regurgitation*

Wide P wave (P mitrale)

• Left atrial enlargement*, e.g.
  • Dilated cardiomyopathy*
  • Mitral regurgitation*

Variable height of P wave (wandering pacemaker)

• Increased vagal tone*

Absent P wave

Atrial fibrillation*

• Acute atrial stretch
  • Volume overload
• Atrial pathology
• Excessive vagal stimulation
• Large atria*

Persistent atrial standstill

• Artefact
• Atrial pathology
• Hyperkalaemia

Sinus arrest/sino-atrial block

• Normal in brachycephalics
• Drugs, e.g.
  • Beta blockers
  • Calcium channel blockers
  • Digitalis glycosides
• Atrial disease, e.g.
  • Cardiomyopathy*
  • Dilatation*
  • Fibrosis
  • Hypertrophy
  • Necrosis
• Electrolyte imbalances*
• Increased vagal tone
  • Chronic respiratory disease*
  • Gastrointestinal disease*
• Sick sinus syndrome
• Stenosis of bundle of His

5.1.2 Alterations in QRS complex

Tall R waves

• Left ventricular enlargement, e.g.
  • Cardiomyopathy*
  • Hyperthyroidism* (C)
  • Mitral regurgitation*

Small R waves

• Acute haemorrhage
• Pericardial effusion

Wide QRS

Supraventricular

• Left bundle branch block
  • Cardiomyopathy*
  • Subaortic stenosis*
  • Drugs/toxins, e.g.
    • Doxorubicin
    • Tricyclic antidepressants
• Right bundle branch block
  • Occasionally seen in normal animals
  • Cardiac neoplasia
  • Heartworm disease
  • Inherited
  • Post cardiac arrest
  • Ventricular septal defect
• Left ventricular hypertrophy*
• Microscopic intramural myocardial infarction
• Quinidine toxicity
• Severe ischaemia

Ventricular

• Accelerated idioventricular rhythm*
• Ventricular ectopy*
• Ventricular escape complexes
• Ventricular premature complexes*
• Ventricular tachycardia*

Deep S waves

• Right ventricular enlargement, e.g.
  • Pulmonary hypertension
  • Pulmonic stenosis
  • Reverse-shunting patent ductus arteriosus
  • Tricuspid regurgitation

Electrical alternans

• Pericardial effusion

Slurred upstroke

• Ventricular pre-excitation/Wolff–Parkinson–White syndrome
  • Acquired heart defects, e.g.
  • Feline hypertrophic cardiomyopathy
  • Congenital
  • Idiopathic

5.1.3 Alterations in P–R relationship

Prolonged P–R interval (first-degree atrioventricular block)

• Occasionally seen in normal animals*
• Age-related degeneration of atrioventricular conduction system
• Drugs/toxins
  • Beta blockers
  • Calcium channel blockers
  • Cardiac glycosides
  • Quinidine
  • Tricyclic antidepressants
  • Vitamin D rodenticides
• Feline dilated cardiomyopathy (C)
• Heart disease*
• Hyperkalaemia q.v.
• Hypokalaemia* q.v.
• Increased vagal tone*

Short P–R interval

• Ventricular pre-excitation/Wolff–Parkinson–White syndrome
  • Acquired heart defects, e.g.
  • Feline hypertrophic cardiomyopathy
  • Congenital
  • Idiopathic

Intermittent failure of atrioventricular conduction (second-degree atrioventricular block)

• May be seen in normal animals
• Juvenile puppies at rest
• Physiological when seen associated with supraventricular tachycardia
• Drugs, e.g.
  • Alpha-2 agonists
  • Atropine
  • Beta blockers
  • Calcium channel blockers
  • Cardiac glycosides
• Electrolyte imbalances* q.v., e.g.
  • Hyperkalaemia q.v.
• Hyperthyroidism* (C)
• Increased vagal tone, e.g.
  • Chronic respiratory disease* q.v.
  • Gastrointestinal disease* q.v.
• Microscopic idiopathic fibrosis
• Myocardial diseases
• Stenosis of bundle of His

Complete atrioventricular block (third-degree atrioventricular block)

• Idiopathic
• Bacterial endocarditis
• Congenital heart defects, e.g.
  • Aortic stenosis
  • Ventricular septal defect
• Hyperkalaemia
• Isolated congenital atrioventricular block
• Myocardial diseases including infiltrative disorders
• Myocardial infarction
• Myocarditis
• Severe drug intoxication, e.g.
  • Beta blockers
  • Calcium channel blockers
  • Cardiac glycosides

5.1.4 Alterations in S–T segment

S–T segment depression/slur

• Acute myocardial infarction
• Cardiac trauma
• Digitalis toxicity
• Electrolyte disturbances* q.v.
• Myocardial ischaemia

S–T segment elevation

• Myocardial hypoxia
• Myocardial infarction
• Myocardial neoplasia
• Pericarditis

Secondary changes to S–T segment following QRS abnormalities

• Bundle branch block
• Ventricular hypertrophy
• Ventricular premature complexes*

Pseudo-depression of S–T segment (prominent atrial repolarisation wave)

• Pathological atrial changes
• Tachycardia q.v.

5.1.5 Alterations in Q–T interval

Prolonged Q–T interval

• Central nervous system disease q.v.
• Drugs/toxins
  • Amiodarone
  • Ethylene glycol
  • Quinidine
  • Tick paralysis
  • Tricyclic antidepressants
• Exercise*
• Hypocalcaemia q.v.
• Hypokalaemia* q.v.
• Hypothermia* q.v.

Shortened Q–T interval

• Hypercalcaemia q.v.
• Hyperkalaemia q.v.
• Drugs/toxins
  • Cardiac glycosides

5.1.6 Alterations in T wave

Tall T waves

• Anaesthetic complications
• Bradycardia q.v.
• Heart failure*
• Hyperkalaemia q.v.
• Hyperventilation during heat stroke
• Left bundle branch block
• Myocardial hypoxia
• Myocardial infarction
• Right bundle branch block

Small T waves

• Hypokalaemia* q.v.

T wave alternans

• Hypocalcaemia q.v.
• Increased circulating catecholamines
• Increased sympathetic tone

5.1.7 Alterations in baseline

• Atrial fibrillation
• Atrial flutter
• Movement artefact*
• Ventricular fibrillation
• Ventricular flutter

5.1.8 Rhythm alterations

Atrial fibrillation

• Anaesthesia
• Gastrointestinal disease*
• Hypoadrenocorticism (D)
• Hypothyroidism* (D)
• Primary/‘lone'
• Rapid, large-volume pericardiocentesis
• Severe atrial enlargement, e.g.
  • Dilated cardiomyopathy*
  • Mitral regurgitation*
  • Patent ductus arteriosus
• Volume overload

Atrial flutter

• Cardiomyopathy
• Iatrogenic
  • Cardiac catheterisation
• Severe atrial enlargement, e.g.
  • Dilated cardiomyopathy*
  • Mitral regurgitation*
  • Patent ductus arteriosus
• Drugs
  • Quinidine

Atrioventricular block q.v.

Parasystole

• Atrial
• Ventricular

Persistent atrial standstill

• Artefact
• Atrial pathology
• Hyperkalaemia

Sinus block/arrest

• Atrial disease, e.g.
  • Cardiomyopathy*
  • Dilatation*
  • Fibrosis
  • Hypertrophy
  • Necrosis
• Electrolyte imbalances* q.v.
• Increased vagal tone
  • Chronic respiratory disease*
  • Gastrointestinal disease*
• Sick sinus syndrome
• Stenosis of bundle of His
• Drugs, e.g.
  • Beta blockers
  • Calcium channel blockers
  • Digitalis glycosides

Supraventricular premature complexes/supraventricular tachycardia (sinus, atrial or junctional tachycardia)

May be normal

Structural cardiac disease, e.g.

• Atrial enlargement*
• Myocardial disease

Systemic disease, e.g.

• Drugs, e.g.
  • Digoxin
  • General anaesthesia
• Hyperthyroidism* (C)
• Inflammation*
• Neoplasia*
• Sepsis*

Ventricular premature complexes/ventricular tachycardia

Cardiac disease

• Cardiomyopathy, e.g. dilated cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy
• Congestive heart failure*
• Endocarditis, e.g.
  • Bacterial
• Inherited, e.g.
  • German Shepherd dogs
• Myocardial infarction
• Myocarditis, e.g.
  • Idiopathic
  • Traumatic
  • Viral
• Neoplasia
• Pericarditis

Extra-cardiac disease

• Anaemia* q.v.
• Autonomic imbalances*
• Coagulopathies q.v.
• Disseminated intravascular coagulation
• Drugs/toxins
  • Atropine
  • Anti-dysrhythmics, e.g.
    • Amiodarone
    • Digoxin
    • Lignocaine
    • Sotalol
  • Dobutamine
  • Dopamine
  • Glycopyrronium bromide
  • Halothane
  • Propantheline bromide
  • Theobromine
  • Tricyclic antidepressants
  • Xylazine
  • Vitamin D rodenticides
• Endocrinopathies*
• Gastric dilatation/volvulus*
• Hypoxia
• Nutritional deficiencies
• Pancreatitis*
• Sepsis*
• Uraemia* q.v.

Ventricular flutter/fibrillation

Ventricular asystole

• Electrolyte/acid–base disorders
• Severe sino-atrial block
• Terminal systemic disease
• Third-degree atrioventricular block

5.1.9 Alterations in rate

Tachycardia

Sinus tachycardia

• Physiological
  • Excitement*
  • Exercise*
  • Fear*
  • Pain*
• Drugs/toxins
  • Adder bites
  • Baclofen
  • Blue-green algae
  • Cannabis
  • Ethylene glycol
  • Glyphosate
  • Ibuprofen
  • Metaldehyde
  • Paracetamol
  • Paraquat
  • Petroleum distillates
  • Phenoxy acid herbicides
  • Pyrethrins/pyrethroids
  • Salbutamol
  • Selective serotonin reuptake inhibitors
  • Terfenadine
  • Theobromine
  • Tricyclic antidepressants
  • Vitamin D rodenticides
  • Heart failure*
  • Respiratory disease*
  • Shock*
• Pathological
  • Systemic disease
    • Anaemia* q.v.
    • Fever* q.v.
    • Hyperthyroidism* (C)
    • Hypoxia
    • Sepsis*

Other supraventricular tachycardia

• Atrial fibrillation
• Atrial flutter
• Ectopic atrial tachycardia
• Junctional tachycardia
  • Automatic junctional tachycardia
  • AV nodal re-entrant tachycardia
  • Bypass tract-mediated macro-re-entrant tachycardia
• Sinus nodal re-entrant tachycardia
• Ventricular pre-excitation/Wolff–Parkinson–White syndrome
• Ventricular tachycardia q.v.

Bradycardia

• Atrial standstill
  • Atrioventricular myopathy
  • Dilated cardiomyopathy*
  • Hyperkalaemia q.v.
• Heart block q.v.
• Sick sinus syndrome
• Sinus arrest

Sinus bradycardia

• Normal in athletic dogs, during rest/sleep
• Cardiac disease
  • End-stage heart failure*
  • Feline dilated cardiomyopathy (C)
• Drugs/toxins
  • Adder bites
• Anti-dysrhythmics
  • Beta blockers
  • Calcium channel blockers
  • Digoxin
  • Baclofen
  • Cannabis
  • Carbamate
  • Daffodil
  • Glyphosate
  • Ivermectin
  • Loperamide
  • Organophosphates
  • Paraquat
  • Phenoxy acid herbicides
  • Rhododendron
  • Theobromine
  • Vitamin D rodenticides
  • Yew
• Hypoglycaemia q.v.
• Hypothyroidism*
• Increased vagal tone, e.g.
  • Gastrointestinal disease* q.v.
  • Respiratory disease* q.v.
• Neurological disease, e.g.
  • Coma
• Severe systemic disease*

5.2 Electromyographic findings

5.2.1 Spontaneous activity

• Normal end-plate noise
• Electrode-insertion artefact
• Fibrillation potentials
  • Denervation
• Myotonic potentials (dive bomber sound)
  • Myotonia
• Pseudo-myotonic potentials
  • Polymyositis
  • Primary myopathies
  • Steroid myopathy

5.2.2 Evoked activity

Decreased muscle action potential

• Junctionopathies
  • Botulism
  • Tick paralysis
• Neuropathies
• Primary myopathies

Increased muscle action potential

• Aged animals
• Chronic neuropathies

Decremental decrease after repeated stimulation

• Myasthenia gravis
• Re-innervation

5.3 Nerve conduction velocity findings

5.3.1 Decreased velocity

• Demyelinating neuropathies
• Distal part of extremity
• Hypothermia of adjacent tissues*
• Protein malnutrition
• Very old/young animals*

5.3.2 Increased velocity

• Proximal part of extremity