The presence of pre-excitation during sinus rhythm recordings favours the former diagnosis. The P-waves are often difficult to see as they are buried in the ST segments and it can be difficult to differentiate antidromic AVRT from ventricular tachycardia (VT). Less commonly, antidromic tachycardia can occur where the AP conducts antegradely and the node provides the retrograde limb of the tachycardia as it re-enters the atrium from the ventricle via the His–Purkinje network the QRS complex therefore appears broad and fully pre-excited. 9 The finding of electrical alternans (alternating beat variation in QRS amplitude/axis) during tachycardia is highly suggestive of AVRT, although it may also be seen in AVNRT. 2 Retrograde P-waves are often visible after and separate from the QRS. Approximately 90% of AVRTs are orthodromic, in which the impulse travels from atrium to ventricle via the AV node and from ventricle to atrium via the AP. This gives the characteristic ECG findings of a short PR interval (<120 ms), slurred onset and broadening of the QRS (see Fig Fig2). In sinus rhythm with manifest ventricular pre-excitation, both the AV node and AP may conduct to the ventricle, resulting in conduction via the AP ‘pre-exciting’ the ventricle ahead of conduction via the AV node and His–Purkinje tissue. Concealed APs are clinically safe as the maximum rate of ventricular activation during AF remains restricted by the AV node, even in AF. 8 These extra-nodal connections may conduct exclusively from atrium to ventricle (manifest ventricular pre-excitation or Wolff–Parkinson–White pattern ECG), from ventricle to atrium (not visible on sinus rhythm ECG and referred to as ‘concealed’) or be capable of conduction in both directions. In AVRT, one critical limb of the circuit is the AV node, while the other consists of an embryological remnant connecting the atria and ventricle, the accessory pathway (AP). 4,7 Close inspection and comparison of the QRS complex in sinus rhythm with that during tachycardia can assist in identifying the P-wave (Fig (Fig2 2). 6 While a pseudo r’ wave is more sensitive and consequently considered more valuable, previous studies have described the presence of a pseudo S’ deflection to have a higher specificity and positive predictive value for AVNRT. When visible, they may be subtle, materialising as a pseudo r’ wave in lead V1 or a pseudo S’ deflection in the inferior leads. An atrial (or less commonly a ventricular) premature beat is required to initiate repetitive re-entry between the fast and slow pathways, manifest on the surface ECG as a narrow QRS tachycardia in the absence of bundle branch block.įor the most common form of AVNRT, P-waves are not easily seen on the surface ECG during tachycardia due to almost simultaneous activation of the atria and ventricles. 5 It is facilitated by the presence of two functionally distinct electrophysiological tracts of differing conduction velocities and refractory periods within the AV node: a fast pathway and a slow pathway. Management can range from conservative, if symptoms are rare and the patient is low risk, to catheter ablation which is curative in the majority of patients.Ītrioventricular nodal re-entrant tachycardiaĪVNRT is the most common SVT in the general population and accounts for over 60% of patients undergoing invasive cardiac electrophysiology study. Long-term treatment is dependent on several factors including frequency of symptoms, risk stratification, and patient preference. All patients treated for SVT should be referred for a heart rhythm specialist opinion. Alternative therapies include the use of beta-blockers and calcium channel blockers. Recent European Society of Cardiology guidelines continue to advocate the use of vagal manoeuvres and adenosine as first-line therapies in the acute diagnosis and management of SVT. In many cases, the underlying mechanism can be deduced from electrocardiography during tachycardia, comparing it with sinus rhythm, and assessing the onset and offset of tachycardia. The most common SVTs include atrioventricular nodal re-entrant tachycardia, atrioventricular re-entrant tachycardia and atrial tachycardia. Supraventricular tachycardia (SVT) is a common cause of hospital admissions and can cause significant patient discomfort and distress.
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