Hypertension
Figure 1: Schematic Illustration of the Percutaneous Catheter-based Approach to Functionally Denervate the Human Kidney
Similar to a routine angiogram, access to the renal artery is obtained via a sheath in the femoral artery. Renal artery angiography is performed to assess anatomical eligibility for the procedure and to confirm absence of significant renal artery stenosis. The treatment catheter is then introduced into the renal artery and manipulated to ensure sufficient contact between the tip of the catheter and the vessel wall after being positioned in the distal aspect of the renal artery. The proximal end of the catheter is connected to a radiofrequency (RF) generator to apply a discrete RF ablation lasting two minutes. Up to six ablations are performed in each artery, separated both longitudinally and rotationally to achieve circumferential coverage of the renal artery. Catheter tip temperature and impedance are constantly monitored during ablation, and RF energy delivery is regulated according to a pre-determined algorithm.
Additional observations from these studies may shed some light on the potential mechanisms involved in the blood pressure reduction achieved by this approach. As discussed above, experimental data clearly indicate a role for afferent sensory nerves. Although afferent signalling cannot be measured directly in humans, the recent demonstration of a substantial and progressive reduction in central sympathetic outflow from baseline through to 12-month follow-up is perhaps indicative of alterations in afferent fibre signalling that may play an important role in the blood pressure effects associated with this procedure.27
Furthermore, it is also
and that cardiac baroreflex sensitivity was also improved after renal denervation (from 7.8 to 11.7msec/mmHg). In addition, cardiovascular imaging using magnetic resonance imaging revealed a substantial reduction of left ventricular mass from 184 to 169g (78.8 to 73.1g/m2) at 12-month follow-up compared with baseline.27
noteworthy that renal denervation decreased renin secretion by around 50%27
Devices Interfering with Baroreflex Sensitivity As discussed above, renal denervation improves baroreflex sensitivity, which to some extent may account for the favourable effects on blood pressure. Indeed, baroreflex mechanisms are also crucial contributors to blood pressure control via stretch-sensitive baroreceptors in the carotid artery and the aortic wall that are activated by increases in blood pressure and induce counter- regulatory adjustments in sympathetic and parasympathetic activity
20
to stabilise blood pressure. Hence, diminished baroreflex sensitivity in the scenario of hypertension is another obvious target for therapeutic intervention. Indeed, electric stimulation of baroreflex afferent nerves was the subject of various previous attempts to lower blood pressure in hypertensive patients via stimulation- induced central sympathoinhibition.28
Recently, a novel implantable device has been developed that appears to have overcome the technical problems of earlier approaches. The Rheos System (CVRx, Minneapolis, MN, US) is a chronically implanted carotid sinus baroreflex-activating system with a pulse generator and bilateral perivascular carotid sinus leads. Implantation is performed bilaterally with patients under narcotic anaesthesia (to preserve the reflex for assessment of optimal lead placement). Dose–response testing is assessed before discharge and at monthly intervals thereafter; the device is activated after a one- month recovery period. The device produces an electric field stimulation of the carotid sinus wall, which in dogs has been demonstrated to produce a sustained reduction in sympathetic nervous system activity and blood pressure.28
In this small series of 10 patients with resistant hypertension on a median of six antihypertensive medications, implantation of the device was successful without significant
This system has recently been tested in pre-clinical and clinical studies including patients with severe hypertension refractory to drug therapy.29
EUROPEAN CARDIOLOGY
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