Essential Revision Notes for MRCP Third Edition - PasTest

Essential Revision Notes for MRCP• Triggered activity – shares features of bothmechanisms and is seen in both primaryarrhythmias and in drug toxicity1.5.2 Supraventricular tachycardiasThere are two major groups of re-entrant tachycardiasoften described as SVT:• AV nodal re-entry tachycardia (AVNRT):involves a re-entry circuit in and around the AVnode• AV re-entry tachycardia (AVRT): this involvesan accessory pathway between the atria andventricles some distance from the AV node (egWPW syndrome and related conditions)AV nodal re-entry tachycardiaDifferential conduction in tissue around the AVnode allows a micro re-entry circuit to be maintained,resulting in a regular tachycardia.Accessory pathwaysAn accessory pathway that connects the atrium andventricle mediates the tachycardia by enabling retrogradeconduction from ventricle to atrium. Moreseriously, the accessory pathway may predispose tounrestricted conduction of AF from atria to ventriclesas a result of anterograde conduction throughthe pathway. This may lead to ventricular fibrillation.WPW is said to be present when a delta wave(partial pathway-mediated pre-excitation) is presenton the resting ECG. Associations with WPWinclude: Ebstein’s anomaly (may have multiplepathways), HCM, mitral valve prolapse and thyrotoxicosis;it is more common in men.Some accessory pathways are not manifest by adelta wave on the resting ECG but are still able toparticipate in a tachycardia circuit.Atrial tachycardias, including flutter, AF, sinus tachycardiaand fascicular ventricular tachycardia may allbe mistaken for SVT.1.5.3 Atrial arrhythmiasAtrial £utterThe atrial rate is usually between 250 and 350beats/min and is often seen with a ventricular responseof 150 beats/min (2:1 block). The block mayvary between a 1:1 ratio and a 1:4 or even a 1:5ratio. Isolated atrial flutter (without atrial fibrillation)has a lower association with thromboembolism;however, recommendations for anticoagulation arethe same as for AF.• The ventricular response may be slowed byincreasing the vagal block of the AV node (egcarotid sinus massage) or by adenosine, which‘uncovers’ the flutter waves on ECG• This is the most likely arrhythmia to respond toDC cardioversion with low energies (eg 25 volts)• Amiodarone and sotalol may chemicallycardiovert, slow the ventricular response or actas prophylactic agents• Radiofrequency ablation is curative in up to95% of casesAtrial flutter is described as typical when associatedwith a sawtooth atrial pattern in the inferior leadsand positive flutter waves in V1. Atypical flutterstend to occur in congenital heart disease or aftersurgery or prior ablation.Atrial ¢brillation (AF)This arrhythmia is due to multiple wavelet propagationin different directions. The source of the arrhythmiamay be myocardial tissue in the openingsof the four pulmonary veins, which enter into theposterior aspect of the left atrium, and this is particularlythe case in younger patients with paroxysmalAF. AF may be paroxysmal, persistent (but‘cardiovertable’) or permanent, and in all threestates is a risk factor for strokes. Treatment is aimedat ventricular rate control, cardioversion, recurrenceprevention and anticoagulation. Catheter ablation isindicated in symptomatic individuals who are resistantto, or intolerant of, medical therapy.With AF a major decision is whether to control rate26