Similar findings of undertreatment were documented by Hwang et al who analyzed trends in antihypertensive medication use among US patients with resistant hypertension from 2008 to 2014. Using a large, national claims database, which contains medical and prescription claims data that is representative of patients covered by employer-based insurance programs, the authors found that among patients with resistant hypertension (ie, prescribed ≥4 antihypertensive agents), the use of chlorthalidone and spironolactone, the recommended diuretics for treatment of resistant hypertension, remains extremely low. In spite of recognized superiority over hydrochlorothiazide and recommendations to use it preferentially for treatment of resistant hypertension, chlorthalidone use had increased only by 2.6% between 2008 and 2014 in patients with resistant hypertension, such that, by the end of 2014, 92.9% of patients were still receiving hydrochlorothiazide versus the 6.4% receiving chlorthalidone. Likewise, in spite of a large body of literature clearly establishing the benefit of spironolactone for treatment of resistant hypertension, use of spironolactone remained low, being prescribed to only ≈10% of patients. Because it is well recognized that optimization of diuretic treatment is essential for the effective treatment of resistant hypertension, these findings highlight the need for better education of practitioners on how to optimize antihypertensive regimens, including specifically, use of chlorthalidone and spironolactone.

Renal Nerve Denervation (RND)

Accessory renal arteries

Substantial antihypertensive benefit of RND for treatment of resistant hypertension has been observed in randomized, but unblinded clinical trials. In contrast, more rigorous blinded trials have not confirmed benefit of RND compared with sham procedure. In the last 2-3 years, many experimental and clinical studies have been done exploring potentially mediating factors related to the BP or lack of BP response with RND. One important factor may be the presence of accessory renal arteries, which, because of small diameters, may have precluded effective denervation. Such an effect was supported by de Jong et al who found that in patients treated with RND, increases in BP induced by renal nerve stimulation were substantially blunted after denervation of the main renal arteries. In contrast, renal nerve stimulation–induced BP elevations remained intact if accessory renal arteries had not been denervated. These findings suggest that the presence of accessory renal arteries that cannot be denervated may serve as important residual sources of sympathetic output.

Reinnervation

Another potential factor related to long-term antihypertensive effectiveness of RND may be the degree of reinnervation of renal nerves postprocedure. In that regard, Booth et al used functional, anatomic, and biochemical assessments to investigate the degree of reinnervation of the kidneys in normotensive sheep after RND. Immediately after denervation, renal sympathetic nerve activity was absent, and responses to renal nerve stimulation were blunted. However, at 11-month postprocedure, renal sympathetic nerve activity and responses to electric stimulation had returned to normal. This functional recovery was corroborated by histological and biochemical findings consistent with regrowth of ablated nerves.

Patients with long-standing and often severe hypertension typical of resistant hypertension likely have some alteration in renal nerve or renal function that could mediate or even blunt the response to RND. If true, one might anticipate a more favorable response to RND in patients with less severe hypertension, that is, mild resistant hypertension. To test for such a difference, Desch et al evaluated the benefit of RND in 71 patients with mild resistant hypertension defined as daytime systolic BP of 135 to 149 mm Hg or daytime diastolic BP of 90 to 94 mm Hg as measured by ambulatory BP monitoring. The study was done as a blinded, randomized comparison to sham procedure. Interestingly, there was no significant reduction in 24-hour systolic BP compared with sham RND at 6-month follow-up as analyzed per intention-to-treat, although there was a small, but significant difference in favor of RND when analyzed per protocol. These findings are informative in suggesting that the benefit of RND likely varies between patient subgroups, particularly in regards to duration or severity of the underlying hypertension.

Relatedly, evaluation of 731 patients with resistant hypertension referred to 11 different European hypertension specialty centers for consideration of RND for treatment of resistant hypertension found that only 40% of patients were eligible for RND. The main reasons of being ineligible were normalization of BP after adjustment of the antihypertensive regimen, recognition of secondary hypertension, and unsuitable renal artery anatomy. These findings, combined with recent study findings discussed earlier, emphasize that even among patients with presumed true resistant hypertension, a large proportion will not benefit from RND from simply being ineligible, based on currently applied guidelines, or because of limited or no BP reduction. The findings, however, also highlight the need to better identify which patients are likely to benefit from the intervention; to what extent eligibility criteria can be modified to potentially broaden the clinical role of RND for treatment of resistant or even nonresistant hypertension; and to identify and overcome current technological limitations that may blunt the effectiveness of RND.

Baroreflex Activation Therapy

Two recent clinical studies provided important insight into the potential benefit of baroreflex activation therapy for treatment of resistant hypertension based on unilateral as opposed to bilateral carotid sinus stimulation. In the first study, Heusser et al found that acute stimulation of the carotid sinus via a unilaterally implanted electrode acutely lowered both systolic and diastolic BP in 18 patients with uncontrolled resistant hypertension. There was not a significant change in muscle sympathetic nerve activity, but overall, reductions in diastolic BP did correlate with reductions in muscle sympathetic nerve activity. There was a wide range in the magnitude of BP and muscle sympathetic nerve activity reduction after acute carotid sinus stimulation, and about two thirds of the subjects experienced stimulation-related adverse effects, necessitating reduction in the stimulation intensity.

The acute findings described by Heusser et al were extended by Wallbach et al who reported, based on an unblinded, uncontrolled assessment, that unilateral carotid sinus stimulation significantly reduced 24-hour ambulatory systolic and diastolic BP in 65 patients with uncontrolled resistant hypertension 6 months after device implantation. The achieved reduction in BP allowed for a small decrease in the average number of prescribed antihypertensive medications (from 6.5 to 6.0). Combined, these 2 studies are important in demonstrating the efficacy of unilateral carotid sinus stimulation for treatment of resistant hypertension. Randomized controlled trials are now needed to confirm and quantify this benefit.

Use of Aldosterone Antagonists

A large number of studies have demonstrated the efficacy of spironolactone for treatment of resistant hypertension. These studies, however, were often limited in being single center studies of small cohorts or having been done in an unblinded and uncontrolled fashion. These limitations were overcome with publication of the PATHWAY-2 results. In a rigorous, blinded, placebo-controlled, crossover evaluation of a large cohort of patients with confirmed resistant hypertension, Williams et al reported that spironolactone, as a fourth antihypertensive agent, was superior to doxazosin and bisoprolol based on reduction of home systolic BP. The PATHWAY-2 findings are clinically important in that they firmly establish spironolactone as the most appropriate fourth agent for treatment of resistant hypertension. The preferential benefit of aldosterone blockade for treatment of resistant hypertension is consistent with a large body of literature demonstrating that resistant hypertension is commonly characterized by varying degrees of hyperaldosteronism and accompanying intravascular fluid retention.

In a randomized, open-label, multicenter evaluation, intensification of pharmacological therapy, including use of spironolactone, was compared with RND for treatment of resistant hypertension in the PRAGUE-15 Study. The cohort consisted of 106 patients with confirmed resistant hypertension based on documentation of medication adherence, exclusion of secondary causes of hypertension, and exclusion of white coat effects by 24-hour ambulatory BP monitoring. At 6- and 12-month follow-up, the 2 interventions had induced comparable reductions in 24-hour ambulatory systolic BP. 

In the DENERHTN trial (Renal Denervation for Hypertension), intensification of pharmacological therapy, including the addition of spironolactone, was compared, in open-label fashion, with RND in combination with intensification of pharmacological therapy in patients with confirmed resistant hypertension. After 6-month follow-up, both interventions had significantly reduced 24-hour systolic BP, but RND had provided an additional 5.9 mm Hg reduction compared with intensification of pharmacological therapy alone. The divergent results of the PRAGUE-15 Study and the DENERHTN trial need to be reconciled with further evaluations, but the findings of the 2 studies suggest that RND and intensified pharmacological treatment with use of spironolactone will be complimentary and that, on an individual patient basis, both will be important options for overcoming treatment resistance.

Refractory Hypertension

Historically, the terms resistant and refractory hypertension have been interchangeable to refer to patients with difficult-to-treat hypertension. Recently, refractory hypertension has been applied in reference to an extreme phenotype of antihypertensive failure. While the definition of refractory hypertension has been broadly based on failure to control BP with use of ≥5 antihypertensive agents of different classes, the most stringent definition has specifically required use of ≥5 agents, including a long-acting thiazide or thiazide-like diuretic, such as chlorthalidone, and an aldosterone antagonist, such as spironolactone. Based on the latter definition, refractory hypertension is rare, affecting only 5-10% of patients referred to a hypertension specialty clinic for uncontrolled resistant hypertension.

Early studies of the phenotype suggest that it is more common in Blacks, women, and in patients with CKD and diabetes mellitus.

Unlike resistant hypertension, in general, indices of intravascular fluid status do not suggest that refractory hypertension is characterized by persistent fluid retention.

Instead, indirect assessments of sympathetic tone suggest that refractory hypertension may be more neurogenic in pathogenesis, that is, secondary to heightened sympathetic to output.

While undertreatment, a common cause of pseudoresistance, is excluded by the definition of refractory hypertension, unknown is to what extent other pseudo-causes of treatment failure, such as poor medication adherence and white coat effect, contribute to apparent treatment failure.

Summary

Resistant hypertension remains a strong focus of both experimental and clinical research. Recent studies indicate that patients with resistant hypertension, particularly in the setting of CKD, have a worse prognosis than patients with more easily controlled hypertension. Apparent vs. true resistant hypertension is an important clinical distinction, with the former often being attributable to pseudocauses of treatment resistance.

Important study results indicate that poor adherence and undertreatment likely represent the 2 most common causes of lack of BP control versus true treatment resistance. From a treatment perspective, rigorous clinical trials have firmly established spironolactone as the most appropriate fourth agent for treating resistant hypertension.

In regards to device-based therapies, experimental and translational studies suggest that RND will not likely be a universal solution for resistant hypertension. Recent study findings indicate that the presence of accessory arteries that cannot be accessed by ablation catheters may preclude adequate RND, thereby, minimizing benefit, while reinnervation of kidneys may be an important mediator of duration of antihypertensive benefit.

Finally, initial studies of refractory hypertension, an extreme phenotype of antihypertensive failure, suggest that it is rare, but may represent a unique phenotype distinct from resistant hypertension in general by having a neurogenic pathogenesis as opposed to being volume dependent.