Cardiac Electrophysiology Consultants of South Texas, P.A.

Medical Center Tower I
7950 Floyd Curl Drive
Suite 803
San Antonio, TX 78229
tel: 210-615-9500
fax: 210-615-9600
email: office at cecst.com
 Specializing in the compassionate care of people who suffer from abnormalities of the electrical system of the heart Current Insurance Plans: We accept most major commercial insurance plans. Please call for details.
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General information about the heart for patients, their family members, and concerned laymen

Table of Contents
Atrial fibrillation
     This is a supraventricular rhythm resulting from multiple reentrant circuits within either the right or left atria, or both. It is characterized by an irregularly irregular ventricular rate that is usually rapid in young patients, but may be normal or even bradycardic in elderly patients or patients taking medications that can cause atrioventricular nodal blockade.

It can occur alone, but is usually associated with hypertensive cardiomyopathy, COPD, or congestive cardiomyopathy. The new onset of either rhythm is seen in about 5% of cases of acute myocardial infarction. The clinician may also want to check for congestive heart failure, since worsening CHF can present with these rhythms.

Digitalis toxicity is suggested by a regular ventricular response (Accelerated junctional rhythm) in combination with atrial fibrillation.

In the presence of an accessory atrioventricular pathway, atrial fibrillation can manifest as a rapid, irregularly irregular wide complex tachycardia that can resemble ventricular tachycardia closely. It should be suspected particularly in young patients with very rapid tachycardia that is well tolerated hemodynamically. Close examination of the ECG will reveal irregularly irregular RR intervals. In this case:

  • It is important to obtain a 12-lead ECG before cardioversion because the location of the pathway, and therefore the risks of a subsequent curative catheter-mediated radiofrequency ablation procedure, can be estimated fairly accurately from the 12-lead ECG.

  • DO NOT give digitalis or verapamil to try to slow ventricular response if an accessory pathway is suspected. These drugs can accelerate conduction over accessory pathways, resulting in even more rapid ventricular activation which can, in turn, induce ventricular fibrillation.
There are four issues related to care of patients with this rhythm:
  • control of the rate of the ventricular response,

  • conversion of the atrial rhythm to sinus rhythm,

  • maintenance of sinus rhythm following conversion, and

  • prevention of embolic stroke from thrombi that form in the fibrillating left atrium.
Consultation with an Internist or Cardiologist is recommended for advice on these issues.
Atrioventricular nodal block, first degree
     This refers to an excessively long PR interval only. All P waves are conducted through the atrioventricular node to the ventricle. By itself, it is a benign condition, but may result from disease in the atrioventricular node, high vagal tone, or medication that reduces conduction through the atrioventricular node.
Atrioventricular nodal block, second degree, Mobitz Type I (Wenckebach)
     This refers to a gradual prolongation of the PR interval, with occasional failure to conduct a P wave through the atrioventricular node to the ventricle. By itself, it is a benign condition, but may result from disease in the atrioventricular node, high vagal tone, or medication that reduces conduction through the atrioventricular node. It is commonly seen in atheletic young patients, particularly during sleep.

This is an abnormal rhythm that can result from digitalis toxicity, particularly when it occurs in combination with Atrial tachycardia.

It is distinguished from Second degree Atrioventricular nodal block, Mobitz type II by the fact that the PR interval of the P wave that follows the non-conducted P wave is at least 10 msec shorter than the PR interval of the P wave that precedes the non-conducted P wave. Typically, the QRS complex is unchanged from the patient's normal QRS morphology. By contrast, the PR interval does not change in Mobitz type II block. Mobitz type II block is dangerous because it can progress to complete heart block and death without warning.

Atrioventricular nodal block, second degree, Mobitz type II
     This refers to occasional failure to conduct a P wave through the atrioventricular node to the ventricle without a change in the PR interval after the nonconducted P wave compared with before the nonconducted P wave. This is a dangerous condition because it can progress to complete heart block and death without warning. Immediate consultation with a Cardiologist for placement of a temporary pacemaker is advisable. Placement of an external pacemaker may be lifesaving if a temporary pacemaker cannot be placed immediately.

This condition, while dangerous, is very unusual. The QRS complex is usually wide, due to extensive disease of the His-Purkinje system, although a narrow QRS complex does not exclude the diagnosis. The clinician should measure the change in PR interval carefully, as described for Second degree Atrioventricular nodal block, Mobitz type I.

Third degree (complete) heart block
     This rhythm is characterized by failure of conduction from the atria through the atrioventricular node to the ventricles. The atrial rhythm is independent of the ventricular rhythm, unless an accessory pathway that conducts antegrade is present. It is most easily distinguished from high-grade atrioventricular nodal block when the atrial and ventricular rhythms are regular but have different rates. Because of weak coupling between the chambers by the autonomic nervous system, these rates can be very close to each other and in fact can oscillate around each other.

Complete heart block is one of three forms of atrioventricular dissociation. The other two forms are:

Of these three forms, only Complete Heart Block results from antegrade conduction block from the atria to the ventricles.
2:1 Atrioventricular nodal block
     This rhythm is diagnosed when the entire rhythm strip shows only conduction of every other P wave to the ventricle. Because the record does not show two consecutive P waves that conduct to the ventricle, it is not possible to measure prolongation of the PR interval, so that it is not possible to distinguish between Mobitz type I and the dangerous Mobitz type II Second degree Atrioventricular nodal block. By convention, recordings obtained at other recent times are used to make this distinction. This degree of block is usually considered pathological when the atrial rate is less than 150 beats per minute or so, because the normal AV node should be able to conduct 1:1 from the atrium to the ventricle at this rate.

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