Tachy Brady Syndrome Anatomy
To better understand sick sinus syndrome, it helps to understand the anatomy of the heart.
The heart is a muscular pump that contains four chambers: right atrium, left atrium, right ventricle and left ventricle. The two small atria make up the top of the heart, and the two large ventricles make up the bottom of the heart. The right atrium pumps blood to the right ventricle, and the left atrium pumps blood to the left ventricle. A wall, called the septum, separates the right atrium and right ventricle, from the left atrium and left ventricle.
Blood flows through the heart in the following manner:
- The right atrium receives oxygen-poor blood from the body, and then pumps the blood through the tricuspid valve and into the right ventricle.
- The right ventricle pumps the blood through the pulmonic valve and to the lungs, where it picks up more oxygen.
- The left atrium receives the oxygen-rich blood from the lungs, and then pumps the blood through the mitral valve and into the left ventricle.
- The left ventricle pumps blood through the aortic valve and to the rest of the body.
- The blood supplies oxygen to the body and the cycle starts again.
- Normal circulation through the heart
- The human heart
- Cross-section of heart at the level of the atria
- Cross-section of heart at the level of the ventricles
- The heart sits inside the pericardium
- The heart valves
The coronary arteries supply oxygen to the heart muscle.
The heart has three main coronary arteries:
- Right coronary artery: supplies the right ventricle
- Left coronary artery: supplies the left ventricle
- Posterior circumflex artery: supplies the posterior aspect of both ventricles
- Coronary angiogram
- Front view of the heart and coronary arteries
- Back view of the heart and coronary arteries
Cardiac Conduction System
An electrical impulse stimulates the muscle fibers in the heart to contract. The impulse spreads through the heart in a very organized manner, so that the atria contract first, followed by the ventricles.
The electrical impulse proceeds in the following manner:
- The electrical impulse originates at the sinoatrial (SA) node, which is located in the wall of the right atrium.
- The SA node is the heart's natural pacemaker: it regulates the heart rate.
- The impulse proceeds through the atria, stimulating them to contract.
- After the atria are stimulated to contract, the atrioventricular (AV) node slows the electrical impulse before it proceeds to the ventricles. This pause allows the ventricles to fill with blood before they contract.
- The AV node is located between the atria and the ventricles.
- After the pause, the impulse then proceeds through the ventricles, stimulating them to contract.
- The cardiac conduction system
- EKG showing electrical activity of heart
- Adan V, Crown LA. Diagnosis and treatment of sick sinus syndrome. Am Fam Physician. 2003 Apr 15;67(8):1725-32. 
- Albertsen AE, Nielsen JC. Selecting the appropriate pacing mode for patients with sick sinus syndrome: evidence from randomized clinical trials. Card Electrophysiol Rev. 2003 Dec;7(4):406-10. 
- Seifer C, Kenny RA. The prevalence of falls in older persons paced for atrioventricular block and sick sinus syndrome. Am J Geriatr Cardiol. 2003 Sep-Oct;12(5):298-301.