cuit usually travels down the slow pathway and up the fast pathway, but can be reversed or even involve multiple slow pathways. AVRT involves an accessory AV bypass pathway where the circuit typically conducts down the AV node and up the bypass pathway, producing a narrow complex tachycardia. In WPW pattern, the circuit also may conduct down the bypass pathway and up the AV node. Since the ventricular myocardium is activated by the bypass tract and not the His-Purkinje system, there is a resultant wide complex tachycardia. More commonly in AVRT there is a concealed bypass pathway that only conducts retrograde and, therefore, is not seen on ECG. Lastly, atrial tachycardia occurs from anabnormal focus within the atrium that activates faster than the sinus node. Atrial fibrillation and flutter are other types of SVT. Atrial fibrillation is the most common abnormal SVT and is defined by the rapid and irregular atrial electrical activation with the resultant dysfunctional atrial mechanical activation. Atrial flutter is characterised by a macro-reentrant circuit within the atrium and is more organised than atrial fibrillation. Prevalence of SVT The incidence of SVT is difficult to determine because of the various study populations and ascertainment methods. Many studies have included elderly patients, with coronary disease or heart failure, which are not relevant to a younger athletic population. SVT is rarely found on a screening ECG, as most young athletes are symptomatic with SVT. In a study of 32 652 Italian subjects, 29 (0.09%) had SVT and 5 (0.02%) had atrial fibrillation or flutter.50 Uncontrolled supraventricular arrhythmias led to disqualification in 73 of 42 386 subjects in the Veneto region of Italy from 1982 to 2004.59 In 32 561 screening ECGs, an ectopic atrial tachycardia was found in only 4 (0.01%) and atrial fibrillation in 2 (<0.01%).10 These studies suggest the presence of SVT to be quite rare on a screening ECG. Contribution as a cause of SCD SVT, atrial fibrillation and atrial flutter very rarely lead to SCD, but are more likely to lead to symptoms (ie, palpitations) that prevent intense physical activity due to uncontrolled ventricular rates or lack of adequate AV mechanical synchronisation. Another concern is whether SVT or atrial fibrillation/flutter is reflective of an underlying cardiomyopathy or channelopathy that would place the athlete at risk for SCD. These may include hypertrophic or other cardiomyopathies, BrS, myocarditis, short QTsyndrome or WPW.60 ECG findings in SVT SVT is usually narrow complex, but in the presence of a bundle branch block can be wide complex. It is often difficult to see P waves in AVNRT, since the atrium and ventricles activate near simultaneously, but it is sometimes possible to see inverted P waves in the inferior leads and a pseudo R0 in V1 suggesting right bundle branch block, which are not present when in sinus rhythm (figure 13). AVRT will also usually show retrograde P waves, but they do not necessarily have to be inverted in the inferior leads. Some patients with AVRT will have a WPW pattern on their baseline ECG. Atrial tachycardia demonstrates a regular atrial rhythm which is faster than 100 bpm with P waves all of the same morphology. The ECG in atrial fibrillation shows fibrillation waves instead of P waves (figure 14). These vary in size, morphology and frequency, but are usually low amplitude with changing shape and rate. The ventricular response to atrial fibrillation is irregular with varying QRS intervals. Atrial flutter, however, has regular atrial activity characterised by flutter waves. Counterclockwise typical atrial flutter, the most common type, shows negative, sawtooth flutter waves in leads II, III and aVF and a positive deflection in lead V1 (figure 15). The atrial activity in atrial flutter is almost always continuous with no isoelectric segment. Evaluation of SVT If paroxysmal SVT is seen, carotid sinus massage, Valsalva manoeuver or facial dunking in an ice bath should be completed while recording the ECG to determine if the rhythm terminates (suggestive of AVNRT or AVRT) or ventricular rate slows to reveal hidden P waves (suggestive of an atrial tachycardia or atrial flutter). Once the patient is no longer in SVT, a baseline ECG should be completed to assess for WPW, and if seen, an electrophysiologist should be consulted to discuss electrophysiology study and possible ablation. If the baseline ECG is normal, it is reasonable to obtain an echocardiogram to look for structural heart disease and consider an electrophysiology consultation to discuss possible ablation. If the rhythm is atrial fibrillation or atrial flutter, an echocardiogram and 24 h Holter monitor should be completed to look for structural heart disease and assess the heart rate throughout the day, including with exercise. If a Holter cannot be performed during exercise, treadmill testing should be completed to assess maximum heart rate while in atrial fibrillation/flutter. Thyroid, liver and renal laboratory testing also should be completed. nummer 5 | november 2013 | Sport & Geneeskunde 29 Pagina 28
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