A slow heart rate is a condition known as bradycardia. A normal heart rate falls in the range of 60 to 100 beats per minute. According to the Mayo Clinic, a person with a slow rate, or bradycardia, has a heart that beats fewer than 60 times per minute. For some people, there are no problems with the slow heart rate. But bradycardia can cause insufficient levels of blood to reach cells that need the oxygen found in blood.
The heart rate is controlled by electrical signals that are initiated by the sinus node in the upper-right chamber of the heart. After traveling across the upper chambers of the heart, causing them to contract, the electrical signals reach the atrioventricular node. The atrioventricular node sends electrical signals to the bundle of His and corresponding branches of cells specialized for carrying these electrical signals, causing the lower chambers to contract. Problems with the speeds of these electrical signals and interruptions of these signals can cause the heart to beat at a slower pace than normal.
If the sinus node is not functioning properly, the heart can beat at a slow rate. The sinus node can initiate the electrical signals at a slower rate or could pause periodically. These conditions would slow the heartbeat. Also, the electrical signal could be blocked by malfunction in the sinus node.
Problems with the bundle of His and the branches of specialized cells can cause failure of electrical to reach the lower chamber. When the electrical signals that pass through the heart's upper chambers signals don't reach the lower chambers correctly, a condition called a block occurs.
A first-degree block occurs when the signals from the upper chambers reach the lower chambers but the signals are slowed down.
In a second-degree block, some of the electrical signals from the upper chambers are dropped. The missing signals can cause the heart to beat at a slow rate.
None of the electrical signals from the upper chambers reach the lower chambers. Substitute signals are generated by the bundle of His and the specialized branches, but generation of the substitute signals causes a delay and a reduced heart rate.
A bundle-branch block occurs when the electrical signals are blocked somewhere along the branches of specialized cells from the bundle of His. A bundle-branch block can cause a slow heart rate.
Medications and Conditions
In addition to listing medications as something that can cause a slow heart rate, the Mayo Clinic lists other conditions that can result in bradycardia. These conditions include high blood pressure, birth defects, an electrolyte imbalance and an under-active thyroid. Heart tissue can be a cause of a slow heart rate. An infection of heart tissue, damage to heart tissue, and deterioration of heart tissue due to aging can all result in a slow heart rate. Inflammatory diseases can cause slow heat rates as well as sleep apnea. A build-up of iron in the organs and complications from heart surgery can also result in slow heart rates.