Research needs in the area of device-related treatments for hypertension

Testing individual baroreflex responses to hypoxia-induced peripheral chemoreflex stimulation

Introduction

Baroreflexes and peripheral chemoreflexes control efferent autonomic activity making these reflexes treatment targets for arterial hypertension. The literature on their interaction is controversial, with suggestions that their individual and collective influence on blood pressure and heart rate regulation is variable. Therefore, we applied a study design that allows the elucidation of individual baroreflex–chemoreflex interactions.

Methods

We studied nine healthy young men who breathed either normal air (normoxia) or an air–nitrogen–carbon dioxide mixture with decreased oxygen content (hypoxia) for 90 min, with randomization to condition, followed by a 30-min recovery period and then exposure to the other condition for 90 min. Multiple intravenous phenylephrine bolus doses were applied per condition to determine phenylephrine pressor sensitivity as an estimate of baroreflex blood pressure buffering and cardiovagal baroreflex sensitivity (BRS).

Results

Hypoxia reduced arterial oxygen saturation from 98.1 ± 0.4 to 81.0 ± 0.4% (p < 0.001), raised heart rate from 62.9 ± 2.1 to 76.0 ± 3.6 bpm (p < 0.001), but did not change systolic blood pressure (p = 0.182). Of the nine subjects, six had significantly lower BRS in hypoxia (p < 0.05), two showed a significantly decreased pressor response, and three showed a significantly increased pressor response to phenylephrine in hypoxia, likely through reduced baroreflex buffering (p < 0.05). On average, hypoxia decreased BRS by 6.4 ± 0.9 ms/mmHg (19.9 ± 2.0 vs. 14.12 ± 1.6 ms/mmHg; p < 0.001) but did not change the phenylephrine pressor response (p = 0.878).

Conclusion

We applied an approach to assess individual baroreflex–chemoreflex interactions in human subjects. A subgroup exhibited significant impairments in baroreflex blood pressure buffering and BRS with peripheral chemoreflex activation. The methodology may have utility in elucidating individual pathophysiology and in targeting treatments modulating baroreflex or chemoreflex function.

Pre-emptive medicine for hypertension and its prospects

Pre-emptive medicine is a novel medical concept proposed in Japan that aims to precisely predict the onset and progression of diseases and to provide therapeutic interventions during early stages, before symptoms appear. The concept of pre-emptive medicine considers the time-course of a disease in each individual and seeks medical interventions to prevent disease progression. Suitable and promising targets for pre-emptive medicine are non-communicable diseases, including hypertension. Recent advances in genomic analysis, information technology, and artificial intelligence should make this medical concept feasible in the near future. In this review, we focused on pre-emptive medicine for hypertension, referring to concrete plans for the future direction of this research. The ultimate goal of pre-emptive medicine is to completely prevent the onset and progression of hypertension by precisely predicting the elevation of blood pressure and performing interventions to avoid it. The diagnostic processes of hypertension, from the standpoint of pre-emptive medicine, should include the detection of abnormal blood pressure regulation as the earliest manifestation of the disease, the depiction of the present status of hypertension in an individual ("nowcasting"), and a prediction of the future trajectory of the disease ("forecasting"). Novel therapeutic strategies for hypertension, from the standpoint of pre-emptive medicine, should aim for the regression of hypertension through early treatments and the remission of hypertension through intermittent intensive therapies. An efficient modification of lifestyle and therapies, according to the progression of hypertension, should be required. If pre-emptive medicine for hypertension becomes established, it would greatly contribute to the extension of a healthy lifespan, which cannot yet be satisfactorily achieved.

Renal denervation has blood pressure-independent protective effects on kidney and heart in a rat model of chronic kidney disease

We elucidate the underlying mechanisms of bidirectional cardiorenal interaction, focusing on the sympathetic nerve driving disruption of the local renin-angiotensin system (RAS). A rat model of N(ω)-nitro-L-arginine methyl ester (L-NAME; a nitric oxide synthase inhibitor) administration was used to induce damage in the heart and kidney, similar to cardiorenal syndrome. L-NAME induced sympathetic nerve-RAS overactivity and cardiorenal injury accompanied by local RAS elevations. These were suppressed by bilateral renal denervation, but not by hydralazine treatment, despite the blood pressure being kept the same between the two groups. Although L-NAME induced angiotensinogen (AGT) protein augmentation in both organs, AGT mRNA decreased in the kidney and increased in the heart in a contradictory manner. Immunostaining for AGT suggested that renal denervation suppressed AGT onsite generation from activated resident macrophages of the heart and circulating AGT excretion from glomeruli of the kidney. We also examined rats treated with L-NAME plus unilateral denervation to confirm direct sympathetic regulation of intrarenal RAS. The levels of urinary AGT and renal angiotensin II content and the degrees of renal injury from denervated kidneys were less than those from contralateral innervated kidneys within the same rats. Thus, renal denervation has blood pressure-independent beneficial effects associated with local RAS inhibition.

Novel strategies for treatment of resistant hypertension

Resistant hypertension, defined as blood pressure (BP) remaining above goal despite the use of 3 or more antihypertensive medications at maximally tolerated doses (one ideally being a diuretic) or BP that requires 4 or more agents to achieve control, occurs in a substantial proportion (>10%) of treated hypertensive patients. Refractory hypertension is a recently described subset of resistant hypertension that cannot be controlled with maximal medical therapy (⩾5 antihypertensive medications of different classes at maximal tolerated doses). Patients with resistant or refractory hypertension are at increased cardiovascular risk and comprise the target population for novel antihypertensive treatments. Device-based interventions, including carotid baroreceptor activation and renal denervation, reduce sympathetic nervous system activity and have effectively reduced BP in early clinical trials of resistant hypertension. Renal denervation interrupts afferent and efferent renal nerve signaling by delivering radiofrequency energy, other forms of energy, or norepinephrine-depleting pharmaceuticals through catheters in the renal arteries. Renal denervation has the advantage of not requiring general anesthesia, surgical intervention, or device implantation and has been evaluated extensively in observational proof-of-principle studies and larger randomized controlled trials. It has been shown to be safe and effective in reducing clinic BP, indices of sympathetic nervous system activity, and a variety of hypertension-related comorbidities. These include impaired glucose metabolism/insulin resistance, end-stage renal disease, obstructive sleep apnea, cardiac hypertrophy, heart failure, and cardiac arrhythmias. This article reviews the strengths, limitations, and future applications of novel device-based treatment, particularly renal denervation, for resistant hypertension and its comorbidities.

Impaired cardiac baroreflex sensitivity predicts response to renal sympathetic denervation in patients with resistant hypertension

OBJECTIVES

This study sought to evaluate cardiac baroreflex sensitivity (BRS) as a predictor of response to renal sympathetic denervation (RDN).

BACKGROUND

Catheter-based RDN is a novel treatment option for patients with resistant arterial hypertension. It is assumed that RDN reduces efferent renal and central sympathetic activity.

METHODS

Fifty patients (age 60.3 ± 13.8 years [mean ± SD mean systolic blood pressure (BP) on ambulatory blood pressure monitoring (ABPM) 157 ± 22 mm Hg, despite medication with 5.4 ± 1.4 antihypertensive drugs) underwent RDN. Prior to RDN, a 30-min recording of continuous arterial BP (Finapres; TNO-TPD Biomedical Instrumentation, Amsterdam, the Netherlands) and high-resolution electrocardiography (1.6 kHz in orthogonal XYZ leads) was performed in all patients under standardized conditions. Cardiac BRS was assessed by phase-rectified signal averaging (BRSPRSA) according to previously published technologies. Response to RDN was defined as a reduction of mean systolic BP on ABPM by 10 mm Hg or more at 6 months after RDN.

RESULTS

Six months after RDN, mean systolic BP on ABPM was significantly reduced from 157 ± 22 mm Hg to 149 ± 20 mm Hg (p = 0.003). Twenty-six of the 50 patients (52%) were classified as responders. BRSPRSA was significantly lower in responders than nonresponders (0.16 ± 0.75 ms/mm Hg vs. 1.54 ± 1.73 ms/mm Hg; p < 0.001). Receiver-operator characteristics analysis revealed an area under the curve for prediction of response to RDN by BRSPRSA of 81.2% (95% confidence interval: 70.0% to 90.1%; p < 0.001). On multivariable logistic regression analysis, reduced BRSPRSA was the strongest predictor of response to RDN, which was independent of all other variables tested.

CONCLUSIONS

Impaired cardiac BRS identifies patients with resistant hypertension who respond to RDN.