1st, 2nd, and 3rd Degree Heart Block


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A heart block occurs when electrical signals cannot travel from the atria to the ventricles.  Heart blocks occur in 3 degrees of severity: 1st, 2nd, and 3rd.  

A catheter is placed in the bundle of His to determine the degree of heart block.

1st degree heart block has a pronounced P-R interval that is greater than 0.2 seconds.  All the electrical signals are coming through to the ventricle because QRS complexes follow all P waves.  However, there is a delay in the transmission of the signal.  This block is located above the bundle of His and has a characteristic where the signal from the atria to the bundle of His is prolonged.

2nd degree heart blocks occur when the electrical impulses from the atria are blocked at the atrioventricular node or junction.  Typically, the P-R interval lengthens until the signal is completely blocked at the AV node (the signal is depolarized).  The QRS complex is skipped until the next P wave.  2nd degree heart block has the unusual characteristic where QRS complexes do not follow all P waves.  Some P waves are independent of QRS complexes.  Another characteristic to look for when diagnosing 2nd degree heart block is to look for a ratio of 2 small integers of P waves to QRS complexes (usually 2 P waves for every QRS complex).  

Here's another example of a 2nd degree heart block:


The danger behind 2nd degree heart blocks is that the 2nd degree heart block might degrade to a 3rd degree heart block:

3rd degree heart blocks involve a complete blockage of the electrical impulse from the sinoatrial node at the atrioventricular junction, which results in no depolarization of the ventricles. This results in a lack of ventricular contraction. The harmful aspect behind this condition is that the atria and ventricles have to develop beating patterns independent of each other and could result in fibrillation. This condition usually results in slow ventricular rhythm, about 32 beats per minute, which results in inadequate circulation. Syncope (Stokes-Adams attacks) could arise due to insufficient cerebral blood flow.

Here's an example of 3rd degree heart block:

Yet another example (notice how it results in asystole):


Treatment of heart blocks:

For some cases of heart blocks, treatment is not necessary because the condition is usually temporary. However, cardioversion is used in emergency situations. Cardioversion involves applying an electrical shock to the chest wall to restore the heart to its normal rhythm. Another treatment is the use of a defibrillator to disrupt ventricular arrhythmia. The defibrillator effectively stops the heart in the hope that it will restart again with a normal beating pattern. Pacemakers are also used to treat some arrhythmias; an artificial pacemaker sets the electrical impulse for the heart and is usually placed on the right side of the heart. Finally, surgery is performed when all other treatments fail. The heart tissue that causes the arrythmia is removed so that normal heart functions might resume.

Sample Problems:

Which heart block does this ECG represent (1st, 2nd, or 3rd)?


Where does the signal from the atria become blocked during 3rd degree heart block?

Define the following condition from this description taken from an ECG:
	P-R interval of 0.28 seconds
	Normal QRS complexes
	A QRS complex follows every P wave.
Define the following condition from this description taken from an ECG:
	Every P wave does not initiate a QRS complex.
	There are two P waves for every QRS complex.

Define the following condition from this description taken from an ECG:
	P waves and QRS complexes behave independently from each other and result in asystole.



1. 3rd degree heart block
2. AV node
3. First Degree heart block
4. 2nd degree heart block
5. 3rd degree heart block


For more information on this topic, please refer to Berne & Levy , 43-49.

Also, check out the following links that may be helpful:

  1. http://www.thriveonline.com
  2. http://www.aptweb.org/ecg.htm
  3. http://www.mc.vanderbilt.edu/peds/pidl/cardio/
  4. http://www.medicalexchange.com/cardiology/
  5. http://www.hrt.org

And, as always, if you have questions, or if you can suggest a helpful link, please e-mail me.


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