*For rhythms that break or recur spontane- ously, synchronized cardioversion is not appropriate. IV indicates intravenous. 6. Manifest and Concealed Accessory Pathways Accessory pathways can be manifest or concealed; can con- duct in the anterograde direction, retrograde direction, or both; and can be associated with several different supraven- tricular arrhythmias. Some anterograde pathways may place patients at risk of SCD. Typically, pathways directly connect the atrium and ventricle, bypassing the normal conduction through the AV node and His Purkinje system. The pathways are considered manifest if they conduct in the anterograde direction, demonstrating pre-excitation with a delta wave on the ECG. Manifest pathways occur in 0.1% to 0.3% of the population and may conduct in both the anterograde and ret- rograde directions or, less commonly, only in the anterograde Concealed pathways conduct only in the retro- grade direction and therefore do not cause pre-excitation on the standard 12-lead ECG. The most common tachycardia associated with an accessory pathway is orthodromic AVRT, with a circuit that uses the AV node and His Purkinje system in the antero- grade direction, followed by conduction through the ventri- cle, retrograde conduction over the accessory pathway, and completion of the circuit by conduction through the atrium back into the AV node. Orthodromic AVRT accounts for approximately 90% to 95% of AVRT episodes in patients with a manifest accessory pathway. Pre-excited AVRT, including antidromic AVRT, accounts for 5% of the AVRT episodes in patients with a manifest pathway and involves conduction from the atrium to the ventricle via the acces- sory pathway, causing a pre-excited QRS complex. This is called antidromic AVRT tachycardia when the return reentrant conduction occurs retrogradely via the AV node. In rare cases of pre-excited AVRT, the return conduction occurs via a second accessory AV pathway. 