The heart is the blood pump of the body. It pumps blood to all parts of the body through circulatory system. The blood carries food and oxygen to the cells and carries away carbon dioxide and waste materials. The heart is made up of four chambers which contract in a remarkable synchronization to pump the deoxygenated blood coming from body to the lungs where this blood picks up oxygen and other nutrients. The blood then comes back into the heart from where it is sent throughout the body. The beating of the heart is the sign of life. This beating of the heart is initiated and controlled by sinoatrial (SA) node which is also known as sinus node.
The sinoatrial node is the impulse-generating (pacemaker) tissue located in the right atrium of the heart, and thus the generator of normal sinus rhythm. Sinus arrest is a condition wherein the sinoatrial node of the heart transiently ceases to generate the electrical impulses that normally stimulate the myocardial tissues to contract and thus the heart to beat. Failure of sinoatrial node to generate an impulse at the expected time leads to sinoatrial arrest. If sinus arrest is persistent and is not due to a drug effect then it is an indication of sick sinus syndrome (SSS) — an impulse formation disorder within SA node.
Sinoatrial block is a disorder of the conduction of electrical impulse. In this condition, an impulse is formed in the sinus node but it fails to travel to the atria or it does reach the atria but with a delay. Impulse conduction failure does not disturbe impulse formation within SA node.
Sinoatrial block is classified into first, second and third degree on the basis of severity of blockage. Furthermore, second degree SA block is further divided into Mobitz type I (known as Wenckebach periodicity) and Mobitz type II.
Common symptoms associated with SA block include weakness, lethargy, fainting, pale gums and very slow heart rate (bradycardia). Often, affected dogs are asymptomatic (without clinical signs).
First degree sinoatrial block is characterized by slowed conduction of the electrical impulse.
Second degree sinoatrial block occurs when conduction failure is intermittent. This condition is further divided into Mobitz type I and Mobitz type II.
Mobitz type I is when speed of conduction slows gradually until one impulse is skipped.
Mobitz type II is when block is all, or none, until complete conduction failure takes place
Third degree sinoartial block is when complete failure to conduct the electrical impulse occurs.