Cardiac arrhythmias

CARDIAC ARRHYTHMIAS

Principles

  • Diagnosis at slow heart rates can be done by auscultation; but electrocardiography is more accurate and the only valid method at high heart rates
  • Those which occur commonly are:
    • Horses: first degree heart block, second degree heart block, sinoatrial block, ventricular extraystoles, artial extrasystoles, artial fibrillation
  •  Arrhythmia in horses at rest which disappears with exercise thought to be of no significance
  • Transient arrhythmia common in newborn foals
  • Cardiac arrhythmia may be associated with gastrointestinal disease in cows and horses; resolves spontaneously when primary disease corrected
  • Most cases of arrhydthmia dealt with by treating the primary condition

Sinus tachycardia

  • Significance: Physiologically normal fast rate.
  • Occurs in Painful states, excitement, exercise, etc; disappears when stimulus removed.
  • Signs include Rate per minute >48 in horses,> 80 in cows.

Sinus bradycardia

  • Significance: A normal decreased rate
  • Occurs in
    • Highly trained horses
    • High blood pressure
    • Space occupying cranial lesions
    • Pituitary abcess
    • Hypothermia
    • Hypogycemia
    • Following xylazine administration
    • Cases of vagus indigestion, diaphragmatic hernia in cows, disappears with exercise or atropine administration
  • Signs include Heart rates down to 26/minute in horses, 48 beats/minute in cows.

Sinus arrhythmia

  • Significance: A physiologically normal arrhythmia.
  • Occurs in
    • Uncommon in large animals compared with dogs
    • Commonest in young, and tame sheep, goats
    • Occurs during hypercalcemia of early milk fever treatment in cows
  • Clinical signs include
    • Occurs at slow heart rate
    • Rate increases in inspiration , decreases in expiration; also unassociated with respiration in horses
    • Abolished by exercise or atropine
  • ECG Variations in P-P intervals, sometimes in P-R intervals; often associated with a wandering pacemaker.

Sinoartial block

  • Significance Considered physiologically normal unless it persists during and immediately after exercise.
  • Occurs in Fit horses at rest
  • Clinical signs include
    • Dropped hear beats, pulse waves with regular underlying rhythm
    • For one heart beat period complete absence heart sounds, jungular artrial wave, arterial pulse
  • ECG complete absence PQRST complexes for one heat

Atrioventricular block

  • First-degree atrioventricular block
    • Significance None
    • Occurs in Transistent episodes.
    • Clinical signs include Detectable only by electrocardiograph
    • ECG P-R interval prolonged

  • Second degree atrioventicular Block (parital heart block)
    • Significance
      • Can be a physiological variation when abolished by exercise or atropine.
      • Considered abnormal in horses when it persists during exercise, in other species always indicates myocardial disease
      • May progress to complete to third-degree block
    • Occurs in
      • Up to 20% light horses at rest, in quiet surroundings.
      • In some horses accompanies myocarditis and reduced performance
      • At fast heart rates occurs in horses with alkalosis in cases of duodenitis/proximal jcjunitis
      • In electrolyte disturbances in all species
      • Overdosing with calcium salts
      • Digitals poisoning
      • Cardiomyopathy, myocarditis in infectious or nutritional deficiency disease
    • Clinical signs include
      • Random or regular occurrence of absence of first and second hear sounds, arterial pulse
      • In horses fourth heart sound audible in block period
      • Jugular pulse detectable
      • Intensified first heart sound in first cardiac cycle after block
      • ECG P wave present but QRS and T waves absent in bloced heat
    • Treatment
      • Not usually necessary, but treat primary disease
      • Short-term relief with atropine injection

  • Third-degree atrrioventricular block (Complete heart block)
    • Significance
      • Almost always fatal
      • Not well defined clinically
    • Occurs
      • During anesthesia; possibly associated with arrhymogenic drugs
      • During hypxia, hypercarbia, acid-base inbalances
    • Clinical signs include
      • Bradycardia unresponsive to exercise or atropine
      • Rarely audible atrial tachycardia
      • Periodical artial cannon waves shoot up jugular
      • Variable intensity of first heart sound
      • Very poor exercise tolerance
      • General signs of heart failure
      • History of syncopal attacks
    • ECG QRS complexes completely dissociated from faster P waves
    • Treatment
      • Prognosis very poor
      • Correct acid-base balance
      • Corricosteriod and dextrose intravenously
      • Atropine (0.02 mg/kg) used but may not allevate; dopamine hydrochloride (3-5 µg/kg/minute) effective

Premature beats (extrasystoles)

  • These may be artrial ; junctional (arising in the artrioventricular mode or conducting tissue), or ventricular
  • Careful exercise usually increases severity and occurrence
  • All indicator myocardial disease except artial premature heats which occur commonly in gastrointestinal disease in cattle

Artial premature beats

  • Normal rhythm to gross, variable arrhythmia; usually periodic interruption of a normal rhythm by a dropped pulse or one markedly decreased in amplitude
  • Can progress to artial fibrillation
  • ECG P wave early and abnormal configuration

Junctional premature beats

  • Irregularity usually consisting of a premative hart heat followed by a longer than normal interval, a compensatory pause, then a regular rhytym
  • ECG P wave vector opposite to normal but P and QRS configuration normal

Ventricular premature beats

  • Arrhythmia similar to that in junctional premature beats
  • ECG Bizarre configuration of QRS complexes, T waves have greater duration and magnitude. Vector orientation, opposite of normal.

Arrhythmias with tachycardia

  • Includes paroxysmal and ventricular tachycardias and atrial and ventricular fibrillation.

Paroxysmal tachycardia

  • Occurs in spontaneously or following excitement.
  • Clinical signs include
    • Bouts of tachycardia with abrupt commencements and endings.
    • Rate during bout excessively high.

Ventricular tachycardia 

  • Significance
    • Indicates severe cardiac disease.
    • May progress to fatal ventricular fibrillation.
  • Clinical signs include
    • Either a regular or irregular rhythm with a very rapid rate.
    • The defect with regular rhythm is easily missed clinically.
    • The irregular rhythm is grossly abnormal and marked by pulse deficits and cannon waves in the jugular veins.
    • Signs of acute heart failure.
  • ECG Multiple, irregular extrasystoles with abnormal amplitude and duration of the QRS and T complexes. P waves may be missing or have no relationship to QRS and T complexes.
  • Treatment of horses
    • Standard remedy in horses is quinidine sulfate 20 mg/kg orally, then 10mg/kg every 8 hours.
    • Lignocaine intravenously used also but has disadvantages.
    • No recommended treatment for cows; quinidine has been used but therapeutic index narrow.
    • Excitement exacerbates the condition.

Ventricular fibrillation

  • Occurs in
    • Lightining stroke
    • Anesthetic overdue
    • Plant poisonings, e.g dimethly tryptaminesm cardiac glycosides
    • Terminal stages of most acquired cardiac disease
  • Clinical findings
    • Complete absence heart sounds, pulse
    • Not usually observed clinically because sudden death the usually outcome
  • Treatment Impractical
Last modified: Tuesday, 5 June 2012, 12:45 PM