Irregular Heart Rhythms in Dogs

The heart pumps the blood into the body through coordinated contractions of its different parts. Sinoatrial node (SA node) is the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. When the conduction impulses from SA nodes do not reach the ventricles for some reason or when the rate of these impulses falls below the base rate determined by the ventricular pacemaker cells, the impulses are generated by lower heart region, resulting in idioventricular rhythm or ventricle escape complexes commonly known as irregular heartbeats. In other words, an inhibition or blockage of sinus conduction impulses to reach and stimulate ventricles causes automatic taking over of pace maker role by the lower part of the heart. This phenomena is termed as idioventricular rhythm. 

A decrese in the frequency of sinus node pacemaker impulses or its blockage to the ventricles results in taking over of pacemaker role by lower heart region, which results in ventricular escape complexes or an idioventricular rhythm. 

It is important to note that although pacemaker cells are located in the sinoatrial (SA) node, which is located in the wall of the right atrium, other cells in the heart can function as pacemakers, including atrioventricular node cells, atrioventricular bundle cells, Purkinje cells, and cells of the heart muscle itself, but these normally only kick in when the SA node isn't working. That is why the lower part of the heart is able to generate its own impulses when electrical impulses from SA node do not reach the ventricles.

The heart is made up of four chambers. Upper two chambers are called left and right atria and bottom two chambers are called left and right ventricles. Heart valves are present between right atrium and ventricle, left atrium and ventricle, from right ventricle to the main pulmonary (lung) artery and from left ventricle to aorta (main artery of body).

To pump the blood to the body and lungs, the heart needs to work in a coordinated fashion. It has an electrical conduction system which generates electrical impulses that propogate throughout the heart to stimulate muscles which contract and push the blood into the arteries and out into the body. Two nodes of the heart play an important role in this conduction system. One is sinus node, or sinoatrial (SA) node, which is a clustered collection of similar cells located in the right atrium. The other is the atrioventricular (AV) node, located in right artium near ventricle. Normal control or pacemaker of the heart is sinoatrial (SA) node. It starts electrical impulse that causes contraction of atria so that blood is pumped into ventricles. The AV node receives the impulse and after a small delay, that allows atrium to eject blood into the ventricle, directs the impulse to the ventricles. The pulse moves into vetnricles causing contraction and pumping of blood into lungs (right ventricle) and the body (left ventricle).

The normal ECG is composed of a P wave, a QRS complex and a T wave. The P wave represents atrial depolarization and the QRS represents ventricular depolarization. The T wave reflects the phase of rapid repolarization of the ventricles.

In dogs with idioventricular rhythm, the P wave is either absent or hidden behind the QRS complex. The P waves in found to be occuring in wrong place. There is no connection between the P waves and the QRS complex on the ECG graph. Arrangement of the complex QRS is disoriented. It is very wide and  aligns with the complex of the premature ventricular system.

This condition only affects dogs with weak body mechanism or dogs with affected with some other disease. There is no sex, age or breed predisposition nor there is any hereditary basis. However, springer spaniels seem to get affected more than other breeds with a condition known as atrial standstill – an absence of electrical activity in the atria, which shuts down the heart mechanism and affects the blood flow. Pugs, Dalmatians, and the Schnauzers are the breeds known to suffer from conduction irregularities. 

While some dogs may remain asymptomatic, common signs that are evident in dogs with idioventricular rhythm include weakness, heart failure, lethargy, exercise intolerance and irregular fainting.

An idioventricular rhythm is not a primary disease, but a secondary result of a primary disease. The scape rhythm is a safety mechanism to maintain cardiac output.

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