The waiting period following cavotricuspid isthmus ablation: Opportunity for watchful observation or a waste of time?
The atrial myocardium between the tricuspid valve annulus and inferior vena cava, known as the cavotricuspid isthmus (CTI), is the critical isthmus for maintenance of typical atrial flutter. Catheter ablation of the CTI offers the potential for curative treatment with low risk of complications and thus is considered a class I indication per the 2015 ACC/AHA/HRS guidelines for the management of supraventricular tachycardia.2 In general, linear ablation of the CTI is straightforward even in the absence of electroanatomic mapping (EAM) or intracardiac echocardiographic imaging. However, the anatomy is highly complex and heterogeneous with different lengths, widths, and thickness as well as pouches and ridges that complicate effective and durable ablation of the CTI.3 A large meta‐analysis of studies between 1988 and 2008 comprising over 10,000 cases of atrial flutter utilizing rapidly evolving ablation technologies and clinical endpoints found the acute success rate of atrial flutter ablation was 91.1% with a recurrence rate of 10.9%.4
Taking the anatomic and electrophysiologic substrate of CTI‐dependent atrial flutter into account, bidirectional block across the CTI is the near universal clinical endpoint for ablation of typical atrial flutter and is associated with a 60% reduction in the risk of recurrence.4 Recovery of CTI conduction is associated with clinical recurrence of atrial flutter and is thought to be due to suboptimal lesion contiguity and depth leaving damaged but viable myocardium that demonstrates acute functional bidirectional block or delay that recovers with time. There are several strategies employed to minimize short‐ and long‐term recovery of CTI conduction; these include a postablation waiting period, pharmacologic modulation (most commonly with isoproterenol),6 EAM,8 steerable sheaths, and open irrigated ablation catheters incorporating contact force sensing.10
A common approach by many electrophysiology laboratories to ensure persistent bidirectional block of the CTI (irrespective of EAM use and type of ablation catheter) is the use of a postablation waiting period, with or without subsequent infusion of isoproterenol. CTI conduction recovers acutely after ablation in 6% to 15% of cases, with the majority of recurrences occurring in the first 10 to 15 minutes.13 However, recovery within the first 24 hours has also been reported.15 In the presence of isoproterenol infusion, persistent bidirectional block across the CTI and noninducibility of atrial flutter is considered a reliable endpoint; a clinical recurrence of atrial flutter was observed in only 4.5% of these patients.6 Incomplete radiofrequency (RF) ablation is thought to worsen nonuniform anisotropic conduction and slow conduction velocity, leading to the misdiagnosis of CTI block during atrial flutter ablation.6 Isoproterenol reduces intercellular resistance and increases the rate of rise of phase 0 of the action potential, thereby increasing conduction velocity, and thus may unmask functional CTI block due to slow intratrial conduction.6 Although there is no consensus, waiting periods of 20 to 30 minutes are common; however, despite the potential benefits, isoproterenol is likely underutilized due to cost, time constraints, and limited outcomes data.
The challenge of ensuring durable ablation has been an area of intense focus in the catheter ablation of atrial fibrillation as recovery of pulmonary vein (PV) conduction is the most common reason for recurrence.