The term “supraventricular arrhythmia” refers to a diverse group of abnormal rhythms ranging from chronic atrial fibrillation to paroxysmal sinus tachycardia due to reentry within the sinus node. Supraventricular tachycardia on the other hand can be broadly defined as any tachycardia requiring the atrium or the atrioventricular (AV) node, either in whole or in part, for its perpetuation. The atrial arrhythmias vary considerably in their rate and regularity, their clinical manifestations and the setting in which they occur. These rhythms are characteristically abrupt in onset and termination and are often seen in patients who does not have evidence of organic heart disease. Although these disturbances in rhythm are generally benign, in patients with organic heart disease a rapid supraventricular rhythm may produce significant hemodynamic complications. In some patients with pre-excitation syndromes and antegrade conduction down an accessory pathway, there is a risk of sudden death.
According to recent AHA statement, the term supraventricular tachycardia includes atrioventricular nodal reciprocating tachycardia (AVNRT), atrioventricular reciprocating tachycardia (AVRT), and atrial tachycardia (AT). In this section we would be discussing the details of ECG recognition of AVNRT and AVRT. Atrial tachycardia has been discussed earlier. This section would deal with AVRT and AVNRT.All narrow QRS (duration<120msec) tachycardias are invariably supraventricular tachycardias.
According to recent AHA statement, the term supraventricular tachycardia includes atrioventricular nodal reciprocating tachycardia (AVNRT), atrioventricular reciprocating tachycardia (AVRT), and atrial tachycardia (AT). In this section we would be discussing the details of ECG recognition of AVNRT and AVRT. Atrial tachycardia has been discussed earlier. This section would deal with AVRT and AVNRT.All narrow QRS (duration<120msec) tachycardias are invariably supraventricular tachycardias.
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| Differential Diagnosis of Narrow QRS Tachycardia |
1) Atrial fibrillation;
2) Multifocal atrial tachycrdia; and
3) Paroxysmal atrial tachycardia or atrial flutter with varying block.
All regular narrow QRS tachycardias are broadly classified into two groups based on the relationship of RP and PR interval as short and long RP tachycardias. Short RP tachycardias include AVNRT, AVRT and atrialtachycardias. If no P-waves or evidence of atrial activity is apparent and the RR interval is regular, then AVNRT is most commonly the mechanism. P-wave activity in AVNRT may be only partially hidden within the QRS complex and may deform the QRS to give a pseudo–R-wave in lead V1 and/or a pseudo–S wave in inferior leads. If a P-wave is present in the ST-segment and separated from the QRS by 70 ms, then AVRT is most likely.
The long RP tachycardias are typical AVNRT, permanent form of junctional reciprocating tachycardia (PJRT) and atrial tachycardias. Responses of narrow QRS-complex tachycardias to adenosine or carotid massage may aid in the differential diagnosis . A 12-lead ECG recording is desirable during use of adenosine or carotid massage. If P waves are not visible, then the use of esophageal pill electrodes can also be helpful. Very rarely what appears to be a narrow RQS tachycardia is a fascicular VT or high septal origin VT.
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| Mechanisms of AVNRT |
The cartoon above shows the mechanism of typical AVNRT.
Slow-fast form: In this common form of AV nodal reentry tachycardia, a reentrant circuit is composed of a slow pathway with a short refractory period (RP) and a fast pathway with a long RP. A premature beat is required to initiate tachycardia, and the tachycardia uses the slow pathway for antegrade conduction and the fast pathway for retrograde conduction.
Fast-slow form: In this unusual form of AV nodal reentrant tachycardia, sometimes referred to as “incessant tachycardia”, the slow pathway has a long RP and the fast pathway has a short RP. A premature beat is not necessary to initiate tachycardia; a normally timed sinus beat may initiate it.
Summary of ECG criteria
• 150 to 250 beats/minute.
• QRS: normal duration unless bundle branch block is present.
• P-waves: When P-waves are identifiable, the P-wave morphology is often different from
sinus P-wave morphology, and the P-wave may precede, coincide with or follow the QRS complex.
AVRT
AV Reciprocating Tachycardia (Extranodal Bypass Pathway): This is the second most common form of PSVT and is seen in patients with WPW syndrome. The WPW ECG, shows a short PR,delta wave, and somewhat widened QRS.This type of PSVT can also occur in the absence of manifest WPW on a preceding ECG if the accessory pathway only allows conduction in the retrograde direction (i.e., concealed WPW).Like AVNRT, a PAC that finds the bypass track temporarily refractory usually initiates the onset of PSVT. The PAC conducts down the normal AV pathway to the ventricles, and reenters the atria retrogradely through the bypass track. In this type of PSVT retrograde P-waves appear shortly after the QRS in the ST-segment (i.e., RP’ < 1/2 RR interval). Rarely the antegrade limb for PSVT uses the bypass track and the retrograde limb uses the AV junction; the PSVT then resembles a wide QRS tachycardia and must be differentiated from ventricular tachycardia.
Preexcitation
This condition causes widening of QRS complex. QRS complex represents a fusion between two ventricular activation fronts.Early ventricular activation in region of the accessory AV pathway (Bundle of Kent) and ventricular activation through the normal AV junction, bundle branch system. ECG criteria include all of the following:
1) Short PR interval (<0.12s);
2) Initial slurring of QRS complex (delta wave) representing early ventricular activation through normal ventricular muscle in region of the accessory pathway;
3) Prolonged QRS duration (usually >0.10s); and
4) Secondary ST-T changes due to the altered ventricular activation sequence.
Slow-fast form: In this common form of AV nodal reentry tachycardia, a reentrant circuit is composed of a slow pathway with a short refractory period (RP) and a fast pathway with a long RP. A premature beat is required to initiate tachycardia, and the tachycardia uses the slow pathway for antegrade conduction and the fast pathway for retrograde conduction.
Fast-slow form: In this unusual form of AV nodal reentrant tachycardia, sometimes referred to as “incessant tachycardia”, the slow pathway has a long RP and the fast pathway has a short RP. A premature beat is not necessary to initiate tachycardia; a normally timed sinus beat may initiate it.
Summary of ECG criteria
• 150 to 250 beats/minute.
• QRS: normal duration unless bundle branch block is present.
• P-waves: When P-waves are identifiable, the P-wave morphology is often different from
sinus P-wave morphology, and the P-wave may precede, coincide with or follow the QRS complex.
AVRT
AV Reciprocating Tachycardia (Extranodal Bypass Pathway): This is the second most common form of PSVT and is seen in patients with WPW syndrome. The WPW ECG, shows a short PR,delta wave, and somewhat widened QRS.This type of PSVT can also occur in the absence of manifest WPW on a preceding ECG if the accessory pathway only allows conduction in the retrograde direction (i.e., concealed WPW).Like AVNRT, a PAC that finds the bypass track temporarily refractory usually initiates the onset of PSVT. The PAC conducts down the normal AV pathway to the ventricles, and reenters the atria retrogradely through the bypass track. In this type of PSVT retrograde P-waves appear shortly after the QRS in the ST-segment (i.e., RP’ < 1/2 RR interval). Rarely the antegrade limb for PSVT uses the bypass track and the retrograde limb uses the AV junction; the PSVT then resembles a wide QRS tachycardia and must be differentiated from ventricular tachycardia.
Preexcitation
This condition causes widening of QRS complex. QRS complex represents a fusion between two ventricular activation fronts.Early ventricular activation in region of the accessory AV pathway (Bundle of Kent) and ventricular activation through the normal AV junction, bundle branch system. ECG criteria include all of the following:
1) Short PR interval (<0.12s);
2) Initial slurring of QRS complex (delta wave) representing early ventricular activation through normal ventricular muscle in region of the accessory pathway;
3) Prolonged QRS duration (usually >0.10s); and
4) Secondary ST-T changes due to the altered ventricular activation sequence.
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| ECG Changes in WPW Syndrome |
The cartoon above shows the cardiac abnormality and consequent ECG changes in WPW syndrome. QRS morphology, including polarity of delta wave depends on the particular location of the accessory pathway as well as on the relative proportion of the QRS complex i.e. due to early ventricular activation (i.e., degree of fusion). Delta waves, if negative in polarity, may mimic infarct Q-waves and result in false positive diagnosis of myocardial infarction.
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