Noradrenaline spill over uses radiotracer technology, is one of the most reliable to study whole body and regional noradrenaline kinetics in humans [56]. Radiolabelled norepinephrine (most commonly [3]) is infused intravenously and regional samples of blood are obtained from veins draining from specific organs. The variable measured is the release of noradrenaline into plasma. The difference in arteriovenous noradrenaline as measured by isotope dilution provides an estimate of regional/organ specific SNA [57, 58]. A limitation of this method is that it measures regional SNA at a single time point and does not allow continuous tracing. This method has proven very powerful, and has been used to document alterations in sympathetic innervation of the heart, the kidneys and skeletal muscle. Recent evidence suggests that at a high rate of nerve discharges there is no longer a linear correlation with the rate of noradrenaline spillover since noradrenaline released from the nerve terminals eventually reaches a plateau [59].

In 1968 Hagbarth and Valibo were the first to report on efficacy of microneurography to measure efferent multi-fiber sympathetic nerve activity (MSNA) [60]. Direct recordings (commonly from the peroneal nerve) reveal burst of nerve activity, synchronous with the heart beat. Observations show while repeat measurements within one subject are highly consistent, there is an up to tenfold variation of SNA from normotensive subject to the next [61–63]. This between individual variability appears to be due physiological variability, rather than variations in recording techniques. Recent refinements have led to single-fiber sympathetic nerve recording in humans [64, 65]. The results from single-fiber recording are supportive of recordings from the whole nerve [66, 67]. Similar to multi-fiber recordings, single-fiber firing is low in humans at baseline, however physiological stimuli and cardiovascular diseases increases single unit discharge rates [68, 69]. MSNA is commonly suggested to be a surrogate of global SNA. While MSNA correlates with noradrenaline spillover, it measures only the patterns of MSNA of the sampled nerve. As discussed above regional differences in sympathetic tone do not necessarily allow conclusions from one organ region to another. There is some evidence that in patients with HF MSNA correlates with mortality [70]. MSNA recording, while a very valuable tool, requires a dedicated laboratory, specialized equipment and skilled personal. For these reasons, it is not used in the clinical setting.

Another commonly employed method is power spectral analysis of heart rate and blood pressure variability. This identifies oscillations in heart rate and blood pressure that are modulated by inputs from the renin-angiotensin system, sympathetic and parasympathetic neurons and by locally released vasoactive factors such as nitric oxide. Obviously, assessment of heart rate variability requires continuous ECG or pulse monitoring, and assessment of blood pressure generally requires constant measurements using arterial cannulation. Fourier analysis and similar techniques allow one to differentiate between the influence of the sympathetic and parasympathetic impact on the heart rate. The analysis is based on the fact that the SNS and parasympathetic nervous system (PNS) operate in different frequency bands. Sympathetic outflow modulates low frequency oscillations, while parasympathetic tone affects both low and high oscillations of heart rate. Thus the ratio of low to high frequency heart rate variability reflects sympathetic cardiovascular modulation. Another approach is to measure baroreflex and chemoreflex sensitivity [71–73]. Patients with HF have increased heart rate and decreased heart rate variability, both of which are associated with increased mortality [74–76]. A major limitation of this method is that no individual heart rate spectrum is selective to the SNA. Factors like age, gender, respiration and baroreflex signals interfere with use of heart rate variability [75, 77]. Similar to the heart rate variability, variability in blood pressure is increased in conditions with increased sympathetic tone and correlates with a heightened MSNA [78]. Absolute values of low frequency blood pressure oscillations provide indirect assessment of sympathetic outflow. Moreover, increased blood pressure variability is associated with higher rates of cardiovascular mortality. However high frequency fluctuations depend on mechanical impact of respiration, whereas low frequency spectral powers are largely due to the complex interaction between vasomotor tone, vascular resistance, humeral and neural factors. Similar to the heart rate variability the use of blood pressure spectral powers are limited by its dependence on various factors, lowering its specificity thus limiting its use in clinical practice [71, 79].

SNS hyperactivity suppresses the arterial baroreceptor reflex. Consequently the carotid baroreflex is blunted in animals and humans with cardiovascular disease like HTN and HF [80, 81]. At the same time resolution of those morbidities improves the baroreflex sensitivity (BRS). BRS is defined as the change in interbeat interval (IBI) in milliseconds per unit change in BP. This is most often quantified as a slope or gain function [82]. Initially the BRS was assessed with vasoconstrictive agents that increase BP and reflexly decrease heart rate affecting the IBI. New are non-invasive methods to measure the BRS. Here the spontaneous heart rate variability and BP variability are obtained continuously using noninvasive arterial pressure monitoring devices. A major limitation of this method includes that BRS assessment requires a sinus rhythm since small amounts of noise easily disrupts the analysis.

Preclinical animal and surgical human studies have outlined the significance of the peripheral chemoreflex in mediating sympathetic hyperactivity, heightened RSNA, and baroreflex inhibition [18, 19, 83–85]. Peripheral chemosensitivity is clinically assessed by measuring the ventilatory response and changes in heart rate or blood pressure in response to either inhibition or stimulation of the peripheral chemoreceptor by manipulating inhaled gas mixtures or applying chemoreceptor stimulating or inhibiting drugs [86–88]. Increased sensitivity of the chemoreflex reflects dysfunction of autonomic cardiovascular control and is linked to development and/or progression of diseases like HF and HTN. Again, give some examples and discuss pluses and minuses.

The ability to identify patients with elevated renal sympathetic efferent or afferent nerve activity may help identify optimal candidates for denervation. To guarantee success of RDN, ideal inclusion criteria are: sympathetically mediated disease with heightened sympathetic tone, originating from the renal afferent nerves. Current trials have focused on the broad spectrum of treatment resistant hypertensive patients. Present patient selection focuses to identify truly resistant hypertensive patients. Inclusion criteria for the Symplicity trials are a baseline systolic blood pressure of 160 mmHg or more (≥150 mmHg for patients with type 2 diabetes), despite taking three or more antihypertensive drugs (including a diuretic). However since RDN is a rapidly evolving field newer trials started applying the novel technology in patients with moderate resistant HTN [89].

So far there are at least two screening tests that predict the outcome RDN in patients with resistant HTN. A screening tool that is currently applied by most centers includes the use of ambulatory BP monitoring (24–72 h) to identify patients with truly resistant HTN. This technique also allows to exclude patients with pseudo-resistant HTN [90]. Not surprisingly higher baseline BP predicted more pronounced BP reduction following the procedure. More interestingly impaired cardiac baroreflex sensitivity identified patients with resistant HTN who respond to RDN [91]. In a study by Zuern et al. baroreceptor sensitivity was identified to be the strongest predictor of a patient’s BP response to RDN. Similarly other preprocedural measures of SNA could provide important insight into effects of RDN on patients and increase rate of procedural success. Central sympatholytic agents like clonidine could additionally be used to distinguish responders from non-responders. This is supported by the idea that RDN is more likely to be effective in patients with treatment resistant HTN with a significant component of sympathetic hyperactivation. A marked reduction in blood pressure following the use of a sympatholytic agent is more likely to result successful response to RDN [92].

More efforts are required to preselect patients for RDN in order to maximize response to the novel technology. A selection trying to quantify activity of the SNS rather than disease activity as shown by Zuern et al. opens new ways for effective and targeted screening. Further development of screening tools requires the understanding of how to measure response and how to define success of RDN. Since RDN proves itself to be successful for sympathetically mediated diseases other than HTN, novel tools need to be developed to identify responders for a broad variety of diseases including DM, HF etc..

In humans there are no reports of direct intraprocedural measures of technical success, neither on direct assessment of renal nerve activity during or after the intervention. A more viable and accessible option to measure activity of renal nerves may arise with the use of biomarkers of renal specific nerve activity. Since many of these biomarkers are modifiable they provide targeted measures of risk reduction following therapeutic interventions. Table 4.1 shows a list of potential biomarkers that can be used to confirm technical success.

In today’s clinical world, technical success of RDN is defined solely by a reduction in blood pressure and quantified by the extent of BP reduction. It is important to point out that a reduction in sympathetic tone does not necessarily reflect itself in a reduction of BP. This is also true for various other surrogate markers that are clearly linked to the activity of the SNA. In the case of BP as a surrogate outcome, conceivable technical success of RDN does not have to be accompanied by immediate or any BP changes at all [93]. In parts this can be attributed to the complexity of pathophysiology of HTN and the variable role that SNA can play in it. The rate of ineffective reduction in BP following RDN ranges from 10 % to 50 % [94, 95]. Additionally, blood pressure cannot be used as a technical success measure in all populations, as it is not expected to fall following renal denervation in normotensive patients undergoing the procedure in hopes of reducing the renal contribution to elevated sympathetic tone, such as patients with HF, or sympathetically mediated tachyarrhythmias. Since technical success and BP reduction do not have sufficient overlap, other biomarkers and surrogate markers or a combination of both has to be applied to define technical success. Most effective would be a combined assessment of afferent and efferent nerve activity as outlined further below in this chapter.

Unlike in animals intraprocedural and postprocedural direct nerve recordings in humans are limited by the simple fact of inaccessibility of the renal nerves and invasiveness of the procedure. Chinushi et al. first recorded renal nerve activity in animals during RDN [96]. Renal nerve stimulation prior to the ablation increased systemic BP, serum catecholamine and heart rate variability. Renal nerve ablation attenuated BP and HR responses following nerve stimulation. Thus the attenuation or elimination of the different components of the response to renal nerve stimulation following RDN could be the evidence of successful damage to renal fibers and serve as technical success of the intervention. Translated to humans this could mean that technical success eventually could require demonstration of blunted efferent and afferent response to proper stimuli (e.g. exposure to chemosensitive agent fails to trigger afferent signals).

The initial safety and durability experience for therapeutic RDN document infrequent serious adverse events and durable treatment benefits in patients with elevated blood pressure and drug resistant HTN [128]. Besides a reduction in ambulatory and office blood pressure, animal and human studies provide evidence for a variety of other beneficial effects on cardiovascular physiology and concomitant diseases. Some of the effects are an improvement in reduced heart rate [129, 130], insulin resistance [131, 132], improved exercise tolerance [133] and renal function [134, 135], with attenuation of HF [136–138] as well as improved sleep apnea and cardiac arrhythmia [139–142]. Developing a vision of endpoints and surrogates of treatment efficacy becomes a critical next step in the development and application of RDN. Measuring the success of RDN requires careful differentiation of the technical success (biomarker) from the two measures of successful clinical outcome; surrogate endpoint and clinical endpoint.

Selection and validation of a biomarker and successive advancement to a surrogate endpoint are key processes in the development of new drug and device therapies as shown in Fig. 4.2. Streamlining the use of biomarkers over surrogate outcomes to clinical outcomes requires deep understanding of the pathophysiology of the underlying disease. Thoughtful selection, validation, and application of surrogate endpoints of clinical outcomes offers the potential to bring valuable interventions to clinical care expeditiously. Reduction in sample size and trial duration are important motivations for the use of surrogate endpoints that replace rare or distal endpoints by more frequent or proximate ones.

It is obvious that long term endpoints such as mortality are the ultimately desired endpoints of every clinical study. However no study has studied the effects of RDN on hard clinical outcomes. Prudent consideration of clinical surrogate standards and biomarkers of desired outcomes may accelerate appropriate applications of therapeutic RDN. It is unrealistic to demand mortality endpoints for all new promising therapies especially if the procedure like RDN continues to show low serious adverse events. Moreover, some clinical endpoints, such as blood pressure, as a surrogate for mortality and morbidity in HTN, have attained an indisputable status, and several biomarkers convey confidence about prognosis. Table 4.2 summarizes surrogate and clinical outcomes that have been used or can be used for past and future RDN studies.

In the following part of this chapter we would like to identify disease specific measures which can be used to assess the effect of RDN on a particular disease. We will focus on surrogate outcomes as well as biomarkers which could be used to describe qualitative and quantitative result of RDN. A variety of measures can be used to characterize disease activity however not all of them can necessarily be used to capture the immediate or short-term effects of RDN on a particular organ or organ system.