The whole process normally takes a fraction of a second, with all cells contracting nearly simultaneously. A The cardiac conduction system can malfunction in several ways. Or, impulses could be conducted too quickly or too slowly through the heart. All these abnormalities of both impulse formation and impulse conduction can occur. Some arrhythmias need no treatment, while others need immediate treatment.
Good treatments are available for most arrhythmias.
A The SA node can set a pace for the heart without any input from the rest of the body. But, under normal conditions, the SA node receives and heeds chemical signals from the body via hormones from the blood and chemical neurotransmitters from nerve endings. The SA node would set a pace of about 100 beats a minute without any outside input. With normal input from the nervous system and other feedback systems, the SA node's pacing rate is usually between 60 and 100 beats a minute. It can be much higher under special conditions, such as strenuous exercise.