Maintaining serum potassium between 4.5 mEq/L and 5 mEq/L shortens QT and may reduce the chance of recurrent torsades de pointes.S10.7-6,S10.7-7 5. In large clinical trials, sodium channel blockers increasedmortalityamongpatientsconvalescingfrom but similar trends were also seen with ear- lier trials of mexiletineS10.7-21 Based on CAST, flecainide is contraindicated in patients with ischemia, prior MI, and is avoided in patients with other structural heart diseases.S10.7-20 Sodium channel blockers increase defibrillation energy requirement and pacing thresholdsS10.7-8,S10.7-9 ; as a consequence, patients may require repro- gramming or revision of pacing or ICD systems or changes in their medication regimens (although modern pacing systems that provide automatic pacing threshold testing and adjustment of pac- ing output have mitigated the risk of loss of cap- ture). Sodium channel blockers can convert AF to slow atrial flutter, which can show 1:1 atrioven- tricular conduction with wide QRS complexes that can be confused with VT.S10.7-23 Sodiumchannelblockers,likeprocainamideand flecainide, can occasionally precipitate the typical Brugada syndrome ECG.S10.7-24,S10.7-25 This has been reported not only with antiarrhythmic medications but also with tricyclic antidepressantsS10.7-26 and Whether this represents unmasking individuals with clinically unapparent Brugada syndrome (see Section 7.9.1.3) or one end of a broad spectrum of responses to sodium channel blockers is unknown. In the setting of sodium-channel blocker tox- icity, limited animal data suggest that adminis- tration of sodium, as sodium chloride or sodium bicarbonate, may improve conduction slowing or suppress frequent or cardioversion-resistant Successful treatment with beta block- and intravenous fat emulsion and/or extracorporeal membrane oxygenation has also 6. QT-prolonging medications (www.crediblemeds. are not used in patients with congenital or acquired long QT syndrome unless there is no suitable alternative or the benefit greatly exceeds the risk. 