Cardiac Arrhythmias in Animals
Cardiac arrhythmias in animals may be defined as deviations from the normal heart rate rhythm or rhythms originating from abnormal locations within the heart.
Etiology
- Heart muscle disease (such as dilated cardiomyopathy, hypertrophic cardiomyopathy), congenital heart defects (especially subaortic stenosis), severe valve leakage and enlargement of the cardiac chambers.
- Situations such as fear, excitement, exercise, anemia, or hyperthyroidism may cause arrhythmia.
- Genetic
Pathophysiology
Many times there is no observable anatomical pathology in the myocardium that correlates with the rhythm disturbance.
Alterations in normal rhythm result from either abnormal impulse formation or abnormal impulse conduction within cardiac muscle fibers.
Altered impulse formation may occur as a result of ischemia (decreased supply of oxygenated blood), hypocalcemia (low calcium levels), cardiomyopathy, hypercalcemia (high calcium levels), excess catecholamines, or reperfusion injury.
Conduction disturbances result when alternate pathways develop for depolarisation of cardiac muscle.
Arrhythmias affect the hemodynamics of the body. Cerebral blood flow is reduced as much as 8% to 12% by premature beats, 14% by supraventricular (originating above the ventricles) tachycardia (rapid rates), 23% by AF, and 40% to 75% by ventricular tachycardia (VT).
Types of Arrhythmias
Supraventricular arrhythmias may be atrial (P-wave positive but abnormal) or junctional (P-wave negative in lead II). This class of arrhythmias includes:
- Supraventricular tachycardia (SVT)
- Atrial premature contractions
- AF
In SVT (or sinus tachycardia), the heart rate typically exceeds 160 to 180 beats/min in the dog, whereas the P-QRS-T complexes remain normal.
The heart rate may be slowed by vagal stimulation. Situations such as fear, excitement, exercise, anemia, or hyperthyroidism may cause this arrhythmia. The ECG would display normal complexes with a higher-than-normal heart rate.
In atrial premature contractions, abnormal P waves occurring earlier than would normally be expected are seen on ECG. The P wave is usually followed by a normal QRS complex. These premature contractions may be associated with left atrial enlargement or atrial disease of any type (MMVD).
Clinical Signs
Animals are usually asymptomatic, but the veterinarian may palpate a pulse deficit and auscultate a variable heart sound.
Diagnosis
Many arrhythmias can be easily auscultated and confirmed by ECG.
Treatment
Treatment involves correcting the underlying cause when possible or controlling the arrhythmia when it is not possible to correct the underlying cause.
