Arrhythmias

An arrhythmia or dysrhythmia is a deviation from the regular rhythm. In dogs this may be normal or abnormal and may result from abnormal cardiac impulse formation, conduction, rate or regularity.

                                                     Regularity

Regularity refers to the predictability of an arrhythmia. Some arrhythmias occur in a predictable fashion and are said to be regularly irregular. These rhythms may be normal (e. g. sinus arrhythmia) or pathological. In others the onset of the next beat is completely unpredictable and the rhythm is said to be irregularly irregular (e. g. atrial fibrillation). Irregularly irregular rhythms are pathological in origin.

                                  Classification of Arrhythmias

Tilley and Goodwin (2001) classify arrhythmias according to:

Origin

Supraventricular arrhythmias arise from the atria or AV node whereas ventricular arrhythmias arise from the ventricles.

Rate

Arrhythmias with slow rates are bradyarrhythmias while those with fast rates are tachyarrhythmias.

Regularity

Fibrillation is a rapid, irregular, chaotic rhythm while tachycardia is a rapid but regular rhythm.

                             Normal Sinus Impulse Formation

Sinus Arrhythmia

Sinus arrhythmia is a regularly irregular sinus rhythm which is a normal finding in most dogs (especially brachycephalic breeds). Sinus arrhythmia is characterized by slight variations in the S1-S1 interval. These variations are related to changes in vagal tone to the heart and are often associated with inspiration (negative pressure created in the thorax) or use of sedative or anesthetic drugs. You can demonstrate sinus arrhythmia by palpating the radial artery on your wrist. Once you feel your pulse take a big deep breath and you should feel your pulse quicken and then slow down as you exhale.

                                Altered Sinus Impulse Formation

Sinus Bradycardia (Slow Heart Rate)

Sinus bradycardia has a regular rhythm and may result from systemic disease (renal failure), toxicities, increased vagal tone, elevated intracranial pressure or compression of the eyeball, hypothermia, hypothyroidism or drugs (tranquilizers, propranolol, morphine, various anesthetics) (Fox, 1988). Sinus bradycardia is diagnosed when the heart rate is less than 65 beats / minute and an ECG shows sinus rhythm.

Sinus Tachycardia (Fast Heart Rate)

Sinus tachycardia; often caused by stress; is the most common arrhythmia observed in dogs and has a regular rhythm. Sinus tachycardia may result if there is increased metabolism and oxygen demand or increased requirement for cardiac output (pain, fright, excitement), pathology (fever, shock, anemia, hypoxia, hyperthyroidism) or pharmacological agents (atropine, epinephrine, ketamine) (Fox, 1988). Sinus tachycardia is diagnosed when the heart rate is more than 160 beats / minute for most dogs (> 180 bpm for small / toy breeds or > 220 bpm in puppies) and an ECG shows sinus rhythm (Fox, 1988).

                     Altered Supraventricular Impulse Formation

Atrial Fibrillation

Atrial fibrillation is a common pathological arrhythmia in dogs. Auscultable characteristics of atrial fibrillation include a completely unpredictable rhythm, sometimes called a " jungle-drums" rhythm. Listen for long diastolic pauses between some beats and very short intervals between others. Sometimes the beats are so close together that S2 is not generated and two S1 sounds follow each other. The other hallmark of atrial fibrillation is a pulse deficit. Sometimes this can be detected because there is a large disparity between the heart rate and the pulse rate. If the heart beat is slow it is more reliably detected by simultaneous auscultation and palpation of the pulse. Normally every S1 heart sound is followed by a pulse wave. Abscence of a wave is called a pulse deficit.

The most common causes of atrial fibrillation are chronic atrioventricular valvular insufficiency in small breeds, dilated cardiomyopathy in large breeds, and congenital heart defects. Less common causes include heartworm disease, cardiac trauma, digitalis toxicity and severe metabolic disorders (Fox, 1988). Auscultable or palpable characteristics of atrial fibrillation include inconsistently filled femoral pulses, detection of an S1 without an S2 and a pulse deficit.

                                  Disrupted Impulse Conduction

Second Degree Atrioventricular (AV) Block

Second degree AV block may be of two types: Mobitz I, usually type A or Mobitz II, usually type B. The two types of second degree AV block are best distinguished by ECG. Mobitz I is a normal finding in dogs, especially in young animals and disappears with exercise. Mobitz II is pathological in origin and will not disappear with exercise. Both types of second degree AV block are manifested by a dropped beat detectable during auscultation. By exercising and immediately ausculting the dog, you can determine if the AV block is a Mobitz I (the dropped beats have disappeared) or Mobitz II (the dropped beats are still auscultable). Second degree AV blocks can be associated with sinus arrhythmia, increased vagal tone, supraventricular tachycardia, electrolyte imbalances or drugs (digitalis, intravenous atropine, xylazine) (Fox, 1988).