Heart Disease of the Sinus Node in Dogs

The role of the heart is to pump oxygen-rich blood to every living cell in the body. In order to achieve its goal, it must continuously beat for an animal's entire lifespan. Because of its vital role, a non-beating heart always results in death. The heart is made up of four chambers. The upper two chambers are called atria (singular atrium) and lower two chambers are called ventricles. In order to beat, the heart needs electrical impulse that causes contraction in its different chambers in a synchronized manner. This electrical impulse is generated by sinoatrial (SA) node or SAN. Also known as sinus node, the SA node is a group of specialized cells present in the right atrium. The sinus node initiates electrical impulse that travels to the entire musculature of the heart through its conduction system.

Sick Sinus Syndrom (SSS) is an umbrella term that describes conditions that involve a disorder of electrical impulse formation within, and conduction out of, the sinus node. SSS causes abnormal heart rhythms (arrhythmias). This abnormal heart rhythm is caused by malfunciton of sinus node; the impulse generator or pacemaker of the heart. It is also a disorder of conduction of electrical impulse out of the sinus node. This condition usually accompanies depressed subsidiary pacemaker automaticity. Subsidiary pacemaker cells take over the control of the heart rhythm if the sinus node should become diseased.

Also known as sinus node dysfunction or bradycardia-tachycardia syndrome,this condition is a disorder that causes an erratic change in the heartbeat. Normally, a dog's heart rate is 40 beats per minue during sleep and 280 beats per minute when it is involved in strenous activities.Some dogs may experience heart rates as slow as 10 beats per minute even during physical exercise  while others may experience very fast heart rates.

Bradycardia-tachycardia is a varient of SSS in which the heart beats too slowly and then too quickly. In this condition, slow arrhythmias and fast arrhythmias alternate. 

Some dogs may remain asymptomatic (without any signs). Dogs that do show symptoms, they are commonly related to slow heart rate such as weakness, fainting, fatigue, collapse, seizure, and pauses in the heart rate. Other symptoms are abnormally fast and abnormally slow heart rates and rarely, sudden death.

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