Diagnostic Evaluation

buried within the QRS complex. If a P wave is visible, it usually manifests as a small deflection at the junction of the QRS complex and the ST segment (see Table 3). With orthodromic AVRT, ventricular activation must precede atrial activation, so the P wave may be visible further out on the ST segment. In antidromic AVRT, the QRS complexes are wide and bizarre—an exaggeration of the delta wave seen in sinus rhythm. Typical heart rates of AVNRT and AVRT overlap considerably, making this measure a poor method of discriminating between these diagnoses. QRS alternans is said to be more typical of AVRT than AVNRT, but this can also occur with AVNRT at extremely rapid rates. Patients with AVNRT frequently describe neck pounding during tachycardia, which has been called the "frog sign" (29).

The long RP tachycardias are frequently difficult to distinguish electrocardiographi-cally. With PJRT and atypical AVNRT, the atria are activated in retrograde fashion. Thus, the P waves must be inverted in the inferior leads. In atrial tachycardia, the P-wave axis and morphology depend on the site of the tachycardia focus. Thus, a tachycardia in which the P waves are upright in the inferior leads must be atrial (ectopic, sinus node reentrant, etc.) in origin. Conversely, a long RP tachycardia with inverted P waves can be atrial tachycardia, PJRT, or atypical AVNRT.

If the mechanism of tachycardia cannot be discerned from careful review of the ECG, the response to vagal maneuvers or adenosine infusion (see Fig. 15) can be very useful. Tachycardias that continue despite transient AV block are atrial in origin. Although most tachycardias that terminate with adenosine infusion are AVRT or AVNRT, atrial tachycardias are also occasionally adenosine-sensitive (30). Tachycardias that terminate with a nonconducted P wave are much more likely to be AVNRT or AVRT than atrial tachycardia.

Once PSVT is documented, little other diagnostic work-up is necessary in the absence of other cardiovascular disease. An echocardiogram is reasonable to rule out structural heart disease, particularly in patients with recurrent SVT. This is also useful in patients with WPW and apparent right-sided accessory pathways to evaluate for Ebstein's anomaly. Checking for hyperthyroidism is reasonable, although the yield is low. Exercise

Preexcited Atrial Fibrillation

Fig. 10. ECG in a patient with WPW who presented after a syncopal spell. AF is present, with extremely rapid pre-excited QRS complexes. Pre-excited R-R intervals as short as 160 ms are present, suggesting an accessory pathway with an extremely short anterograde refractory period and therefore an extremely high risk. Cardiac arrest occurred moments after this ECG was recorded. The patient was successfully resuscitated, and underwent successful catheter ablation of a left septal pathway the next day.

Fig. 10. ECG in a patient with WPW who presented after a syncopal spell. AF is present, with extremely rapid pre-excited QRS complexes. Pre-excited R-R intervals as short as 160 ms are present, suggesting an accessory pathway with an extremely short anterograde refractory period and therefore an extremely high risk. Cardiac arrest occurred moments after this ECG was recorded. The patient was successfully resuscitated, and underwent successful catheter ablation of a left septal pathway the next day.

testing and coronary angiography should not be performed routinely, but rather reserved for patients for whom there is a specific indication.

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