Determine the following:
P waves: _______________________________________
PR interval: ____________________________________
QRS complex: __________________________________
What's your interpretation?
(Answers on next page)
Name that strip: Answers
Rhythm: Regular (atrial and ventricular)
Rate: Atrial: 94 beats/minute; Ventricular: 44 beats/minute
P waves: Sinus (have no relationship to QRS complexes)
PR interval: varies
QRS complex: 0.14 to 0.16 second
Rhythm interpretation: Third-degree AV block
Comment: ST segment elevation is present
With third-degree (complete atrioventricular [AV]) block, the atria and ventricles beat independently of each other, and no relationship exists between atrial activity and ventricular activity (AV dissociation). The atria are usually paced by the sinus node at its inherent rate of 60 to 100 beats/minute, and the ventricles are either paced by a pacemaker in the AV junction at a rate of 40 to 60 beats/minute or in the ventricles at a rate of 30 to 40 beats/minute.
The P waves occur at regular intervals across the rhythm strip, but have no relationship to the QRS complexes, and therefore will be seen marching across the rhythm strip hiding inside QRS complexes or in the ST segments or T waves. The “hidden” P waves can be found by measuring the regularity of the atrial rhythm (the P-P interval). The PR intervals vary greatly. Both the atrial rhythm and the ventricular rhythm are usually regular.
The duration of the QRS complex and the ventricular rate reflect the location of the block. If the block is at the level of the AV node or bundle of His, the QRS complex is usually narrow and the ventricular rate between 40 and 60 beats/minute. If the block is at the level of the bundle branches, the QRS complex will generally be wide and the ventricular rate will be much slower (40 beats/minute or slower). Complete AV block with wide QRS complexes is generally less stable than complete AV block with narrow QRS complexes.
Complete AV block associated with an inferior wall myocardial infarction (MI) is usually a result of a block at the level of the AV node or bundle of His. The rhythm is usually stable, and the ventricles are paced by a junctional pacemaker with narrow QRS complexes and a ventricular rate of 40 to 60 beats/minute. Complete AV block associated with an inferior wall MI often resolves on its own.
Complete AV block associated with an anterior wall MI is usually a result of a block within the bundle branches. The rhythm is usually unstable, and the ventricles are paced by a ventricular pacemaker with wide QRS complexes and a ventricular rate of 40 beats/minute or slower. Complete AV block associated with an anterior MI often does not resolve on its own and may require permanent pacing.
Causes of complete AV block include coronary artery disease, MI, congenital heart disease, cardiac surgery, and digitalis toxicity, and in older adults with chronic degenerative changes in their conduction system. It has also been reported with Lyme disease. The patient's response to complete AV block is usually related to the ventricular rate and individual tolerance. If the rhythm is of gradual onset and the heart has time to compensate for the slow ventricular rate, the patient may be relatively asymptomatic with minor symptoms such as weakness, fatigue, dizziness, or exercise intolerance. This is often seen in older adults with degenerative changes in their conduction system over time. If the rhythm occurs suddenly with an abrupt decrease in ventricular rate, the patient is usually symptomatic (hypotension, dyspnea, heart failure, chest pain, or syncope). This often occurs following acute MI.
Regardless of its cause, complete AV block is a serious and potentially life-threatening dysrhythmia. Complete AV block can quickly progress to ventricular standstill with little or no warning. Treatment usually involves pacemaker therapy. I.V. atropine, transcutaneous pacing, I.V. dopamine infusion, or I.V. epinephrine infusion as per the American Heart Association Advanced Cardiovascular Life Support guidelines can be used for treatment of symptomatic complete AV block until transvenous pacing can be initiated. Unresolved complete AV block will require a permanent pacemaker.
Complete AV block: Identifying ECG features
Rhythm: Regular (atrial and ventricular)
Rate: Atrial—that of the underlying sinus rhythm. Ventricular—40 to 60 beats/minutes if paced by AV junction; 30 to 40 beats/minute (or less) if paced by the ventricles; will be less than the atrial rate.
P waves: Sinus P waves with no constant relationship to the QRS complex; P waves can be found hidden in QRS complexes, ST segments, and T waves
PR interval: Not consistent (varies)
QRS complex: Normal if block is located at the level of AV node or bundle of His; wide if block is located at the level of bundle branches