Cardiac Ablation and Renal Denervation Systems Have Distinct Purposes and Different Technical Requirements
Letter to the Editor
Ahmed et al. report the use of a cardiac electrophysiology ablation system for renal denervation in a single-center, single-armed, noncontrolled trial of 10 patients with severe refractory hypertension (1). Cardiac ablation and renal denervation systems are designed to comply with distinct and divergent technical requirements.
Radiofrequency cardiac ablation systems are required to produce deep, permanent, scarring lesions of the ventricular myocardium, necessitating high power levels and risking steam pops and the formation of char or coagulum. Irrigation is commonly used because it allows for higher power, deeper or larger lesions, and minimizes char formation.
In contrast, the renal artery constitutes a radically different physical environment. The target for ablation is not the whole thickness of the ventricular wall, but a delicate skein of nerve fibers in the arterial adventitia (2). Renal artery diameter is an order of magnitude smaller than any cardiac chamber, and blood flow is an order of magnitude faster, providing an intense, incessant, natural irrigation and enabling the most intense heating effect to be localized away from the lumen.
The extent of tissue damage during radiofrequency ablation is roughly proportional to both temperature and time. However, precise temperature control is much more difficult when applying much larger diameter irrigated electrophysiological electrodes. This creates the potential for both over- and underablation and the potential for collateral damage to vital tissues adjacent to the renal artery, a possibility wholly untested by Ahmed et al. (1). The Symplicity renal ablation system (Medtronic, Minneapolis, Minnesota) delivers a maximum power that is 10-fold lower than typical cardiac applications. Real-time digital control algorithms were designed in concert with the specific ablation catheter electrode that is considerably smaller in length and diameter than cardiac ablation electrodes. The proprietary control algorithm analyzes both temperature and impedance to adjust power and is able to cease all energy application within a thousandth of a second if feedback is not appropriate. Extensive animal testing and early human trials have prioritized caution and safety. Yet, it is not clear from the Ahmed report if appropriate animal testing was performed before human testing.
Immediate damage to the endothelium is inevitable during denervation, but peer-reviewed laboratory histological data show fully healed endothelium within days, rather than char (3). Moreover, follow-up angiography and renal imaging performed in 2 prospective clinical trials and several single-center investigations confirm the absence of stenosis and recovery of renal arterial function following renal denervation therapy (4,5). Even after clinical application in thousands of patients, the automated algorithm has shown a substantial track record of safety beyond 3 years, with so far 1 report of renal artery stenosis requiring intervention (6). This can indeed be improved upon, although claims of prevention of rare events demand large studies (7).
Concern for risk to patients is admirable, but it should be followed through logically. Applying nonpurpose-designed ablation systems to the renal artery might be better—or worse—than conventional care with the Symplicity system and would be wisely executed within the context of an appropriately designed randomized controlled trial designed to address an actual clinical need.
1. : "Renal sympathetic denervation using an irrigated radiofrequency ablation catheter for the management of drug-resistant hypertension". J Am Coll Cardiol Intv 2012; 5: 758.
2. : "Catheter-based renal sympathetic denervation for resistant hypertension: a multicentre safety and proof-of-principle cohort study". Lancet 2009; 373: 1275.
3. : "Catheter-based renal sympathetic denervation: chronic preclinical evidence for renal artery safety". Clin Res Cardiol 2011; 100: 1095.
4. : "Catheter-based renal sympathetic denervation for resistant hypertension: durability of blood pressure reduction out to 24 months". Hypertension 2011; 57: 911.
5. : "Renal sympathetic denervation in patients with treatment-resistant hypertension (The Symplicity HTN-2 Trial): a randomised controlled trial". Lancet 2010; 376: 1903.
6. : "Secondary rise in blood pressure after renal denervation". Lancet 2012; 380: 778.
7. : "Duration and magnitude of the effect of a single statin tablet in primary prevention of cardiovascular events". Int J Cardiol 2011; 149: 102.