Sinus Bradycardia

Every normal heartbeat starts in the sinus node, a small group of cells in the wall of the right atrium that works as the heart's natural pacemaker. In sinus bradycardia, the signal still comes from the right place and travels normally to the ventricles, but the rate is slower than 60 beats per minute. The rhythm is regular. This is what sets it apart from atrioventricular block, in which the sinus node works normally but its signal is blocked along the way.

Often a Sign of a Fit Heart

Sinus bradycardia is common in healthy people. Regular endurance training, such as long-distance running, cycling, or swimming, makes the heart pump more blood with each beat, so its resting rate drops. In trained athletes, a resting heart rate below 50 beats per minute is not unusual. The heart rate also slows naturally during sleep. In these situations, the heart rate rises normally during activity and there are no symptoms, so no treatment is needed. How the heart adjusts in athletes is covered on the Athlete's Heart page.

When the Sinus Node Has a Problem

Sinus bradycardia needs attention if it comes with tiredness, dizziness, shortness of breath, or fainting, or if your heart rate does not rise when you are active. The latter is called chronotropic incompetence. Both can signal sinus node dysfunction, which is often related to aging of the heart's electrical tissue. In some people, a slow heart rate alternates with a fast rhythm such as atrial fibrillation, covered on the Tachycardia-Bradycardia Syndrome page.

Causes Outside the Sinus Node

The sinus node can also be slowed by outside factors. Examples include beta blockers or certain calcium channel blockers, an underactive thyroid gland, a very low body temperature, pauses in breathing during sleep, increased pressure inside the head, and stimulation of the vagus nerve during vomiting or straining. This type of bradycardia usually improves once the cause is treated.

An overview of slow heart rates from all sources, including when a pacemaker is needed, is covered on the Bradycardia page. This page focuses on slow heart rates that come from the sinus node and how to tell normal ones apart from those that need treatment.

The main question for your doctor is whether your slow heart rate is normal or a sign of a sinus node problem. For this reason, the doctor asks about your exercise habits, symptoms, sleep patterns, and all the medications you take.

An ECG confirms that the rhythm comes from the sinus node and that there is no conduction block. A Holter monitor shows how slow the heart rate gets during the day and night, and whether there are long pauses when symptoms occur. A treadmill test assesses whether the heart rate can rise during activity, which is an important way to tell a fit heart apart from a weak sinus node. Thyroid function and electrolyte tests look for causes that can be treated. In athletes without symptoms whose heart rate is above 30 beats per minute, this finding is generally considered normal and does not need further testing.

Sinus bradycardia without symptoms in fit people is generally nothing to worry about. Make an appointment with your doctor if a slow heart rate comes with unusual tiredness, dizziness, shortness of breath during light activity, or difficulty concentrating, or if symptoms appear after starting a new medication. Athletes should get checked if their performance drops for no reason or they feel dizzy during training.

Go to the emergency room right away if:

• You faint or nearly faint
• A slow heart rate comes with chest pain or severe shortness of breath
• You suddenly become confused or are difficult to wake up