More recently, irrigated ablation has been investigated as a new technique for active cooling during the ablation procedure. Active cooling lowers the temperature of the ablation electrode and adjacent tissue to reduce the incidence of coagulum formation. It helps to achieve a more controllable ablation and allows for higher power delivery, resulting in larger lesions [4]. With active cooling, the basis for irrigated RF ablation, room temperature irrigation fluid is infused through an irrigation catheter designed specifically to cool the ablation electrode. Whereas passive cooling is pulsatile, and dependent upon cardiac output, active cooling can override the inconsistent and unknown characteristics of passive cooling.

With the irrigation catheter used in RDN procedures, irrigation fluid flows through the catheter and exits through small holes (irrigation ducts) around the electrode. The fluid is in direct contact with the electrode, blood and the surrounding tissue surface, and irrigation facilitates cooling of the electrode-tissue interface. When there is adequate irrigation flow rate, the temperature of this interface is lower, thus reducing the risk of thrombus or char formation [4]. Irrigated RF ablation creates renal artery lesions based on the same principles as conventional RF ablation. During lesion formation, the hottest tissue temperature is generated by resistive heating just below the surface of the tissue. Conduction transfers heat from this zone to the adjacent tissue and ablation electrode, allowing the lesion to form and grow. Increasing the power causes the tissue temperature to rise, resulting in a larger lesion [4]. Saline irrigation during power delivery cools both the electrodes and the surface of the tissue, dissipating some of the excess heat [4].

During RF ablation, even with irrigation, not all the power is delivered to the tissue. A significant percentage is always lost into the blood and saline. The distribution of power along one of two pathways – blood and saline surrounding the electrode, or the tissue in contact with the electrode – depends on their respective impedances and the electrode-to-tissue contact. Optimal contact reduces power loss into the blood and increases lesion size [4].

Active cooling of the ablation electrode and the adjacent tissue influences lesion shape and vessel wall viability. With conventional RF ablation, the largest lesion diameter is close to the surface of the tissue. Resistive heating in the tissue is only cooled by the blood – therefore, stagnant blood can prevent optimal cooling. The resultant lesion damages a very wide section of the vessel media layer [4]. With irrigated catheter tip ablation, the surface adjacent to the electrode is cooled not only by blood, but also by irrigation solution (which has better cooling properties than blood). Thus, the surface diameter of the lesion is smaller. Increased energy delivery to tissue can be achieved by active cooling. Experimental studies have shown that active cooling results in higher tissue temperature resulting in larger and deeper lesions compared to conventional RF ablation [12, 19, 20].

There are substantial histopathological differences between irrigated and non-irrigated RF ablation. These differences refer to parameters of efficacy such as injury of peri-arterial nerves and parameters of safety such as arterial and peri-arterial tissue damage. Semi-quantitative ordinal grading schemes are useful, when changes in the nerve, renal artery, and peri-arterial soft tissue are evaluated following denervation [21].

Nerve damage can be semi-quantitated using an ordinal grading system of 0–4: 0 = none, 1 = minimal, 2 = mild, 3 = moderate, and 4 = severe [21]. It is important to mention that tissue damage may affect the peri-neuronal and/or endoneuronal portions of the renal nerves. In the assessment of peri-neuronal injury, inflammation and fibrosis are recognized as important signs of peri-neuronal injury. While acute-phase nerve injury may not necessarily be accompanied by peri-neuronal inflammation or fibrosis, chronic-phase nerve injury usually exhibits peri-neuronal fibrosis. Vacuolisation and digestion chambers are unique findings after endoneuronal injury. Vacuolisation is defined by the presence of vacuolated areas, which contain loose connective tissue with rare areas of homogenous eosinophilic staining of cell cytoplasm along with compressed and deformed nuclei (pyknotic nuclei) [22]. Digestion chambers are characterized by variable amounts of aggregated myelin (eosinophilic hyaline globules) and vacuolated spaces with occasional cells interspersed [22]. Also, depending upon the RF energy used there may be necrosis of the nerves bundles as well as calcification. Since minimal or mild injury can be seen even in untreated animals, moderate and severe injury is considered as definite injury caused by thermal damage or toxins. Additional important parameters of renal peri-arterial nerve ablation procedures refer to the assessment of circumferential dimension and depth penetration of radiofrequency energy. These parameters are relevant for the examination of nerve injury and peri-arterial tissue damage.

Immunohistochemical stains may help to distinguish the morphological or functional presence of neuronal markers relevant to efferent sympathetic or afferent sensory activity. It is important to mention that most markers applied to date are not specific for sympathetic nerves, neither are they specific as markers of unidirectional neuronal signal transduction (afferent vs. efferent). Stains against tyrosine hydroxylase (TH), which is the enzyme for converting tyrosine to DOPA (dihydroxyphenylalanin), are used for the confirmation of norepinephrine synthesis [23]. Sensory nerve (afferent) as well as sympathetic nerve (efferent) fibers play a crucial role for the overall renal sympathetic nerve activity [24]. Immunostaining against calcitonin gene-related peptide (CGRP), which is a neurotransmitter in sensory nerves, can be used as a marker of afferent fibers [25]. The intensity and distribution of staining can be semi-quantified using a scoring system of 0–3; 0 = no reaction, 1 = very weak and/or patchy reaction, 2 = weak reaction, 3 = strong reaction [21].

To assess treatment reactions of the vascular and peri-vascular tissue including adjacent organs (kidney, lymph nodes, ureters, and renal veins), ordinal data can be collected for multiple parameters including endothelial loss, arterial and venous medial injury (depth and circumference), inflammation, degenerative changes and necrosis. Endothelial cells remain an important luminal barrier against activation of coagulation pathways and adhesion of thrombocytes. Endothelial loss can be observed in the treated vessel but also in adjacent non-treated vessels within the latitude of radiofrequency energy. Inflammation can be a sign of reversible or irreversible tissue damage and needs to be judged in association with the presence of degenerative changes or necrosis. These parameters can be semi-quantified using a scoring system of 0–4: 0 = none; 1 = minimal, 2 = mild; 3 = moderate; and 4 = marked [21]. In addition, distances from affected nerves to the intimal surface of the closest arterial segment through which RF energy was delivered can be measured with digital morphometry from histological slides stained with haematoxylin and Eosin.

In our laboratory, we compared the histopathology following irrigated, cooled-tip ablation and non-irrigated ablation in the swine model. In this regard, the overall nerve damage was comparable between irrigated and non-irrigated ablation. Furthermore, immunoreactivity to tyrosine hydoxylase, an efferent neuronal marker was also similar between irrigated and non-irrigated ablation groups. On the other hand, renal arterial media damage was significantly less in the irrigated ablation group as compared to non-irrigated ablation group (Fig. 18.2).

However, an important remaining concern with irrigated RDN relates to the question whether the effective temperature registered at the catheter tip truly reflects intramural tissue condition or rather an inconclusive product of heat dissemination secondary to radiofrequency application and active cooling through irrigation. To answer this question, it is extremely important to simultaneously monitor acute and chronic effects within the arterial and peri-arterial tissue. In this regard, our experience with irrigated vs non-irrigated catheter ablation of renal peri-arterial nerves suggests that the latter may achieve similar nerve injury without jeopardizing safety by excess tissue damage. Preservation of peri-arterial tissue is another important factor contributing to long-term efficacy and safety in peri-renal nerve ablation procedures and our data are in line with a favourable effect towards peri-arterial tissue protection in irrigated ablation.