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 bradycardia refers to a slower 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 needs of the body. The body's nervous system stimulates the sinus node to fire more quickly during times of increased activity, for example during exercise. Likewise, the sinus node firing rate slows down during times of decreased metabolic rate, for example during sleep.
Sinus bradycardia simply refers to a slower than normal heart rate. In many instances, sinus bradycardia is an expected finding. For example, a typical heart rate of a normal adult is anywhere from about 60 to 90 beats per minute. However, most normal adults have periods during sleep when their heart rates may drop into the 40 range, or even occasionally slower. This is termed sinus bradycardia, but during sleep it is a completely normal and expected finding.
In many instances a child may be referred for evaluation by a pediatric cardiologist for slow heart rate to determine whether this is abnormal. Fortunately truly pathologic sinus bradycardia in children is fairly rare. In fact, a primary heart related cause of pathologic sinus bradycardia in children is extremely uncommon. It is most often found in children who have undergone open heart surgery, especially in those with surgery involving the tissue near the sinus node. If the tissue of the sinus node becomes damaged or irritated, it may fire more slowly than normal, resulting in sinus bradycardia. This can be seen in certain types of surgery involving the atrium, for example with the Mustard or Senning procedures for transposition of the great arteries, or the Fontan procedure for patients with hypoplastic left heart or other forms of single ventricles.
Rarely primary sinus bradycardia may be a manifestation of some type of infectious or inflammatory process in the heart. Likewise, sinus bradycardia may be caused by a diffuse electrical system disorder of the heart.
Most of the time, sinus bradycardia in children and teenagers is either physiologic, or secondary to some other process outside the heart. Physiologic sinus bradycardia is seen most commonly in a healthy, well conditioned athletes. Athletic training creates a more efficient heart which therefore does not need to beat as fast during times of rest. Many healthy athletic high school students may normally have resting heart rates in the 50 range, or sometimes even lower.
Secondary sinus bradycardia is caused by something outside of the heart. This is seen most commonly in infants and babies, especially premature babies. Problems such as gastroesophageal reflux, apnea, or immature breathing control can stimulate a large nerve called the vagus nerve. This can cause a reflex slowing of the heart rate. Usually these episodes are relatively short-lived.
Since physiologic sinus bradycardia is a normal finding, it does not require any treatment. Secondary bradycardia may necessitate treatment of the underlying cause. Primary sinus bradycardia requires treatment if the patient is symptomatic or there are other concerns. Usually this necessitate the placement of a permanent pacemaker.