Symptomatic third-degree heart block with AV dissociation requires which immediate intervention?

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When the heart block is complete (third-degree) and AV dissociation is present, the ventricles aren’t being driven fast enough to maintain adequate blood flow, so the patient becomes symptomatic from bradycardia or hypotension. The most immediate and reliable way to correct this is pacing to actively drive the ventricles and restore a sufficient heart rate and perfusion. Temporary pacing methods—transcutaneous in a rapid, emergent setting or transvenous for longer control—are used until a stable rhythm is achieved or a permanent pacemaker is placed. Oxygen helps with any hypoxia but does not fix the slow rhythm. Atropine may help if the block is at the AV node, but it is often ineffective in complete block below the AV node. Defibrillation is not appropriate for bradyarrhythmias. So pacing best addresses the fundamental problem here.