Atrial Fibrillation

Atrial Fibrillation



Basic Information image



Definition


Supraventricular arrhythmia characterized by very rapid and chaotic, self-sustaining atrial electrical activity with an irregularly irregular ventricular response.



Synonym(s)


Auricular fibrillation



Epidemiology



Species, Age, Sex




• Large breeds are more susceptible than small breeds.


• Rarely occurs in ponies unless severe cardiac pathology is present.



Risk Factors




• Underlying cardiovascular pathology leading to:


• Atrial dilation: Chronic valvular regurgitation (eg, mitral regurgitation), congenital heart disease (eg, ventricular septal defect)

• Structural lesions of the atrial myocardium (eg, fibrosis caused by heart failure or pressure overload)

• Frequent occurrence of atrial premature beats (eg, caused by electrolyte disturbances, structural lesions, viral disease, myocardial stretch [exercise])


• Electrolyte disturbances


• High vagal tone (leads to dispersion in refractoriness of the atrial myocardium)



Associated Conditions and Disorders




• Epistaxis during vigorous exercise


• Occasionally associated with ventricular ectopy



Clinical Presentation



Disease Forms/Subtypes




• Regarding etiology:


• Primary or “lone” atrial fibrillation (AF): No underlying cardiac pathology. This form is much more common in horses than in other species.

• Secondary AF: Underlying cardiac or noncardiac disease.

• Regarding duration:


• Persistent (sustained) AF: Can only convert to sinus rhythm with treatment (most frequently in horses).

• Paroxysmal AF (terminates spontaneously). Occasionally seen in racehorses (Thoroughbreds, Standardbreds) during (sub)maximal exercise. Within minutes, hours, or days, spontaneous conversion to sinus rhythm occurs.


History, Chief Complaint




• Obvious signs of performance loss in horses working at maximal speed: Pulling up during race with signs of respiratory distress, ataxia, or even collapse


• Less pronounced signs of decreased performance in jumping and dressage horses or horses working at lower levels


• May be an incidental finding in pleasure horses, breeding horses, or horses at rest


• Epistaxis during exercise


• Signs of heart failure in horses with severe underlying cardiac disease and secondary AF



Physical Exam Findings




• Irregular pulse with pulse deficits


• Auscultation


• Irregularly irregular rhythm

• No atrial sound; loud first heart sound

• Normal heart rate at rest or tachycardia in case of cardiac failure

• Signs of underlying cardiac disease


• Cardiac murmur

• Signs of heart failure (edema, jugular pulsation, tachypnea, weight loss)


Etiology and Pathophysiology




• Mechanism


• During AF, depolarization waves spread continuously and in a rapid and chaotic manner over the atrial myocardium (usually 350–450/min). These depolarization waves originate from reentry or from rapidly firing foci and result in a continuous source of electrical activity independent of the sinoatrial node.

• During AF, a (reversible) loss of atrial contractile function occurs.

• At rest and when no severe underlying cardiac disease is present, high vagal tone causes the atrioventricular (AV) node to block most of the electrical pulses, thereby maintaining a normal heart rate. Conduction toward the ventricle occurs randomly resulting in an irregularly irregular cardiac rhythm.

• Decreases in vagal tone (stress, exercise, heart failure, colic) results in an exacerbated increase in heart rate.

• Etiology


• To be sustained, AF depends on a trigger to start the arrhythmia (atrial premature beat or rapidly firing focus) and a substrate to maintain it (atrial myocardium). Factors in favor of AF are:

image A high trigger burden, that is, a high number of atrial premature beats: induced by stretch (eg, high atrial pressure during exercise), myocardial lesions, or electrolyte disturbances.

image A suitable substrate, that is, the atrial myocardium: large atrial size, high vagal tone, short refractoriness of the myocardium and structural lesions.

• In general, once initiated, AF rapidly (hours to days) becomes permanent, that is, it will never convert to sinus rhythm without therapy. In particular, during the first weeks to months the arrhythmia becomes increasingly stable.

• Occasionally paroxysmal AF occurs during exercise, most commonly in racehorses. A potential cause is a high trigger burden during (sub) maximal exercise, while the myocardium is not suited to maintain the arrhythmia for a prolonged period (due to its size, electrophysiologic properties). Spontaneous conversion generally occurs within the first 72 hours.

• Impact:


• In horses with lone AF:

image At rest: no impairment of cardiac function (long diastolic time renders ventricular filling sufficient).

image During exercise: impaired cardiac function occurs because of impaired ventricular filling as a result of (1) an absent atrial contractile function and (2) a disproportionate increase in heart rate. Some horses may present with epistaxis because of effects on pulmonary vascular pressures.

• In horses with severe underlying cardiac disease: exacerbation of clinical signs because of a further decrease in cardiac function.


Diagnosis image



Differential Diagnosis




• Second-degree AV block: regularly irregular on auscultation with presence of the atrial sound (S4).


• Frequently and irregularly occurring atrial and/or ventricular premature beats.



Initial Database




• AF may be strongly suspected on auscultation: an irregularly irregular rhythm, a loud first heart sound, and absence of the atrial sound.


• Final diagnosis is made with an electrocardiogram (ECG) by the typical characteristics (Figures 1 and 2):


• No P waves

• Presence of f (fibrillation) waves. Changes between coarse (clearly undulating baseline) and fine (minor deflections of the baseline) fibrillation waves frequently occur on the same ECG.

• Irregular RR interval (less obvious at high heart rates)

• QRS morphology is normal but often shows minor alterations because of superposition with the f waves. The T wave may change after short RR intervals.

• Assess for signs of underlying cardiac disease such as a cardiac murmur, ventral edema, jugular pulsations, and tachycardia.


image

FIGURE 1 Atrial fibrillation (AF). During AF, P waves are absent and replaced by (coarse and fine) fibrillation waves (f waves). QRS complexes have a normal morphology, but RR intervals are irregularly irregular.

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Atrial Fibrillation
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