The sinus node is the pacemaker of the heart. It is responsible for generating the electrical impulses that stimulate the heart muscle to contract. Sinus tachycardia refers to a faster than normal heart rate.
The sinus node is very carefully regulated by the brain. During normal activity, the firing rate of the sinus node is maintained according to the metabolic need to the body. The nervous system stimulates the sinus node to fire faster during times of increased activity, for example during exercise. Likewise, the sinus node fires more slowly during times of decreased metabolic rate, for example during sleep.
Sinus tachycardia refers to a faster than expected heart rate. In an otherwise healthy adult, sinus tachycardia usually means a heart rate over 100 beats per minute. Babies and children have faster resting heart rates than adults, so the criteria for sinus tachycardia is different. For a baby, sinus tachycardia is usually means a heart rate over 160-170 beats per minute. In a school age child, sinus tachycardia is usually considered a heart rate over 120 beats per minute.
Most of the time sinus tachycardia is not a problem but actually a normal physiologic response of the body. The brain is very good at controlling the heart rate based on the metabolic needs of the body. During exercise, for example, the metabolic requirements of the body and muscles increase dramatically. Therefore the brain signals the heart rate to increase in order to deliver more blood and oxygen to the body. Sinus tachycardia is an expected finding in this setting. Sinus tachycardia is also very common in other stress responses of the body. For example, pain, fever, or emotion cause the body to release adrenaline which increases the heart rate. Again, sinus tachycardia in this setting is expected and normal.
Occasionally a child may be noted to have a faster than expected heart rate, and is subsequently referred to a pediatric cardiologist for evaluation of sinus tachycardia. More commonly a child may be referred because they have the sensation of palpitations. In this setting, the child feels his or her heart beating rapidly. The pediatric cardiologist then works to determine whether these symptoms are due to an expected sinus tachycardia, or something more concerning such as supraventricular tachycardia.
Primary pathologic sinus tachycardia in children is rare. Almost always sinus tachycardia is caused by a secondary problem outside the heart. For example fever, infection, hypothyroidism, anemia, or any other problem that increases the metabolic rate can result in sinus tachycardia. Likewise, sinus tachycardia can be seen in any state that produces an increased amount of adrenaline, for example with anxiety or other emotional disorders.
Primary sinus tachycardia caused by a heart related problem is usually due to some type of infectious or inflammatory condition with the heart. Inflammation of the heart muscle (myocarditis) or of the lining of the heart (pericarditis) can produce sinus tachycardia.
Sinus tachycardia in and of itself usually does not require treatment. Again, usually it is a normal physiologic response to the body and therefore perfectly appropriate.