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
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