Abnormal Rhythms - Definitions

General Terms:

• Normal sinus rhythm - heart rhythm controlled by sinus node at a rate of 60-100 beats/min; each P wave followed by QRS and each QRS preceded by a P wave.
• Bradycardia - a heart rate that is lower than normal.
• Tachycardia - a heart rate that is higher than normal.
• Paroxysmal - an arrhythmia that suddenly begins and ends.

Specific Arrhythmias:

• Sinus bradycardia - low sinus rate <60 beats/min.
• Sinus tachycardia - high sinus rate of 100-180 beats/min as occurs during exercise or other conditions that lead to increased SA nodal firing rate.
• Sick sinus syndrome - a disturbance of SA nodal function that results in a markedly variable rhythm (cycles of bradycardia and tachycardia).
• Atrial tachycardia - a series of 3 or more consecutive atrial premature beats occurring at a frequency >100/min; usually due to abnormal focus within the atria and paroxysmal in nature, therefore appearance of P wave is altered in different ECG leads.  This type of rhythm includes paroxysmal atrial tachycardia (PAT).
• Atrial flutter - sinus rate of 250-350 beats/min.
• Atrial fibrillation - uncoordinated atrial depolarizations.
• Junctional escape rhythm - SA node suppression can result in AV node-generated rhythm of 40-60 beats/min (not preceded by P wave).
• AV nodal blocks - a conduction block within the AV node (or occasionally in the bundle of His) that impairs impulse conduction from the atria to the ventricles.

First-degree AV nodal block - the conduction velocity is slowed so that the P-R interval is increased to greater than 0.2 seconds.  Can be caused by enhanced vagal tone, digitalis, beta-blockers, calcium channel blockers, or ischemic damage.

Second-degree AV nodal block - the conduction velocity is slowed to the point where some impulses from the atria cannot pass through the AV node.  This can result in P waves that are not followed by QRS complexes.  For example, 1 (as shown below) or 2  P waves may occur alone before one is followed by a QRS.  When the QRS follows the P wave, the P-R interval is increased.  In this type of block, the ventricular rhythm will be less than the sinus rhythm.

There are two subtypes of second-degree AV blocks: Mobitz I and Mobitz II. In Mobitz I (Wenkebach block), the P-R interval gradually increases over several beats until it is sufficiently prolonged (that is, AV conduction is sufficiently impaired) that the impulse fails to pass into the ventricles (i.e., a P wave will not be followed by a QRS). Mobitz II occurs is when the P-R interval is fixed in duration, but some P waves are not followed by a QRS (as illustrated below).

Third-decree AV nodal block - conduction through the AV node is completely blocked so that no impulses are able to be transmitted from the atria to the ventricles.  QRS complexes will still occur (escape rhythm), but they will originate from within the AV node, bundle of His, or other ventricular regions.  Therefore, QRS complexes will not be preceded by P waves.  Furthermore, there will be complete asynchrony between the P wave and QRS complexes.  Atrial rhythm may be completely normal, but ventricular rhythm will be greatly reduced depending upon the location of the site generating the ventricular impulse.  Ventricular rate typically range from 30 to 40 beats/min.

• Supraventricular tachycardia (SVT) - usually caused by reentry currents within the atria or between ventricles and atria producing high heart rates of 140-250; the QRS complex is usually normal width, unless there are also intraventricular conduction blocks (e.g., bundle branch block).
• Ventricular premature beats (VPBs) - caused by ectopic ventricular foci; characterized by widened QRS; often referred to as a premature ventricular complex, or PVC.
• Ventricular tachycardia (VT) - high ventricular rate caused by aberrant ventricular automaticity (ventricular foci) or by intraventricular reentry; can be sustained or non-sustained (paroxysmal); usually characterized by widened QRS (>0.14 sec); rates of 100 to 280 beats/min; life-threatening.
• Ventricular flutter - very rapid ventricular depolarizations >250/min; sine wave appearance; leads to fibrillation.
• Ventricular fibrillation - uncoordinated ventricular depolarizations; leads to death if not quickly converted to a normal rhythm or at least a rhythm compatible with life.

For information on the pharmacologic treatment of arrhythmias, go to: www.cvpharmacology.com/antiarrhy/antiarrhythmic.htm.

Revised 03/16/10