The Cardiac Conduction System
Electrical impulses are the stimulus that causes the heart to beat. Specialized muscle cells within the heart’s walls signal the heart to contract, or squeeze to push blood out. These signals normally follow a certain pathway, beginning at the sinoatrial (SA) node, and traveling down to the atrioventricular (AV) node, the bundle of His, through the bundle branches, and finally to the Purkinje fibers. When the signal has passed from the atria (top chambers) to the ventricles (bottom chambers), the ventricles in turn contract and push blood out of the heart. A block anywhere at the AV node or below is a third degree hear block.
Causes of heart block can either be congenital (present at birth) or acquired. Many cardiac medications such as beta blockers, calcium channel blockers, and antiarrhythmics can cause heart block in high doses. Certain types of myocardial infarcts, or heart attacks, coronary artery disease, and surgical injury can lead to third degree block. Lyme carditis caused by Lyme disease, acute rheumatic fever, and severely high potassium levels also cause heart block.
Signs and Symptoms
Depending on where the blockage is, the heart rate may be as low as in the 20’s. Patients will usually have severe fatigue, especially with exertion, shortness of breath, chest pain, lightheadedness or confusion. Blood pressure may become extremely low, and patient may sweat and become pale. A few patients may only notice that become tired with activity if the block is high enough and the heart rate is not excessively slow.
Third degree heart block is most commonly diagnosed by a 12-lead EKG. A full cardiac workup is normally performed, including blood work to check electrolyte levels, kidney function, and cardiac enzymes. In patients who may have been exposed to Lyme disease or digoxin, Lyme titers and a digoxin level should be performed. Expect serial cardiac enzymes to be drawn every six hours and repeat EKGs to be performed per hospital protocols. A chest x-ray is also evaluated.
Treatment of heart block includes administering oxygen and ensuring that an IV line is in place for infusion of fluids and medications. The patient should be on a continuous cardiac monitor so that heart rhythm, rate, and vital signs can be monitored continuously. A cardiologist should be consulted to manage patient’s care. Placement of a pacemaker should be anticipated for the patient who is showing severe signs and symptoms caused by the block. Use of the temporary pacemaker found on the crash cart may be used until a permanent pacemaker can be placed surgically.
If you are diagnosed with third degree heart block, you will most likely have a permanent pacemaker placed. Be sure to follow up with your cardiologist as directed, and to be aware of any signs of complications, such as shortness of breath, chest pain, fainting, or fatigue with exertion. If you are given prescription medication, be sure to take it as directed and know the possible side effects to look for. Many people continue to live long and healthy lives with pacemakers.
American Heart Association: Heart Block-https://www.americanheart.org/presenter.jhtml?identifier=4611
EKG Interpretation Made Incredibly Easy. Philadelphia: Lippincott Williams and Wilkins, 2008.
eMedicine: Heart Block, Third Degree-https://emedicine.medscape.com/article/758454-overview
Penn Medicine: What Makes Your Heart Beat?-https://www.pennmedicine.org/encyclopedia/em_DisplayAnimation.aspx?gcid=000001&ptid=57