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).The following figure shows QRS complexes having an abnormal shape and prolonged duration because ventricular depolarization does not follow normal conduction pathways. Atrial rate and rhythm may be completely normal (green arrows indicate P waves, although in the last beat it is obscured by the QRS), but ventricular rate will be reduced the extent of which depends on the location of the site generating the ventricular rhythm. Ventricular rates typically range from 30 to 40 beats/min if the ventricular foci generating ventricular rhythm is below the bundle of His. There will be complete asynchrony between the P wave and QRS complexes. Therefore, QRS complexes will not be preceded by P waves. QRS complexes still occur (escape rhythm), but they originate from within the AV node, bundle of His, or other ventricular regions. Third-degree AV nodal block - conduction through the AV node is completely blocked so that no impulses can be transmitted from the atria to the ventricles. Also note that the QRS complexes appear normal in shape and duration because they are still being triggered by impulses from the atria passing through the AV node. In either type of second-degree block, the ventricular rate will be less than the normal sinus rhythm (indicated by green arrows in figures). This is an example of a 2:1 rhythm because there are two P waves for each 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 in the second tracing below. The fifth beat starts this cycle over again. In the fourth beat, the P wave is not followed by a QRS therefore, the ventricular beat is dropped. In the first tracing below, the PR interval for the first beat is 0.16 sec and increases to 0.24 sec by the third beat. In Mobitz I (also called "Wenckebach"), the PR interval gradually increases over several beats until it is sufficiently prolonged (that is, AV conduction is sufficiently impaired) that the impulse cannot pass into the ventricles (i.e., a QRS will not follow the P wave). There are two subtypes of second-degree AV blocks: Mobitz Type I and Mobitz Type II. This results in P waves that are not followed by QRS complexes. Second-degree AV nodal block - the conduction velocity is slowed to where some impulses from the atria cannot pass through the AV node. This type of block can be caused by enhanced vagal tone, digitalis, beta-blockers, calcium channel blockers, or ischemic damage to the nodal tissue. Rate is not altered by the prolonged PR interval because it is still being controlled by the SA node. In the tracing below, the PR interval is 0.24 sec. 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.įirst-degree AV nodal block - the conduction velocity is slowed so that the PR interval is increased to greater than 0.20 seconds.Junctional escape rhythm - SA node suppression can cause AV node-generated rhythm of 40-60 beats/min (not preceded by P wave).Atrial fibrillation - uncoordinated atrial depolarizations.Atrial flutter - sinus rate of 250-350 beats/min. This type of rhythm includes paroxysmal atrial tachycardia (PAT).
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