Friday, November 24, 2006

Device-based Hypertension Therapy?

All of us have had our blood pressure taken when we visit the doctor's office. Many of us have been told by a doctor that our blood pressure is too high and that we should "watch" it. Few of us, however, leave the doctor's office understanding how serious a health problem high blood pressure is or that it can lead to heart disease, stroke and kidney failure.

It is not that high blood pressure is especially difficult to control. Today's doctors can prescribe a wide range of good, affordable drugs that will lower your blood pressure and help you live a longer, healthier life. The problem is many of them simply don't. Some estimates have suggested that only 30-47% of patients with hypertension receive adequate therapy.

But occasionally, even with lots of medications and careful follow-up, there are a few patients in whom controlling blood pressure can be particularly difficult. For these patients there might be another option soon.

A new pacemaker-like device recently debuted at the American Heart Association meeting in Chicago this year to treat severe, drug–refractory hypertension. Made by CVRx, this device functions by stimulating the carotid bulb at the bifurcation (branch point) of the internal and external carotid arteries to activate the baroreceptor reflex.

The baroreceptor reflex is a normal reflex that the body uses to quickly provide more or less blood pressure to the brain with changes in posture. Elevation in blood pressures causes mechanical stretch of receptors (“baroreceptors”) in the carotid sinus. When stretched, these cells fire electrical impulses faster to stimulate centers in the brain that are responsible for deactivating sympathetic stimuli while activating the parasympathetic (or heart rate slowing and vasodilating portion of the body’s blood-pressure-regulating system to lower blood pressure. By stimulating these baroreceptors, researchers hope to use the body’s own baroreceptor reflex to drive blood pressure lower. The report of piloted patients appeared promising enough for the Food and Drug Administration to grant an Investigational Device Exemption (IDE) to expand the evaluation of the device to a larger cohort of patients.

Figure from the CVRx website

Early European results were reported for 12 patients at the European Society of Hypertension meeting in June 2006. In this study, after three months of active therapy, systolic blood pressure was reduced by an average of 24 mmHg (189 mmHg vs. 165 mmHg). In the preliminary US trial presented at the AHA meeting, three months of active therapy reduced systolic blood pressure by an average of 22 mmHg (180 mmHg vs. 158 mmHg) and diastolic blood pressure by an average of 18 mmHg (105 mmHg vs. 87 mmHg), using office cuff measurements. While data are preliminary at present, this device might become promising.

But there are some significant hurdles for the company to clear before the device can become reality.

First, because hypertension is painless and causes no symptoms until end-organ damage occurs, treating patients with an implantable device will be a hard sell for physicians. Furthermore, many patients with hypertension are simply left untreated by their physicians, or receive inadequate medical therapy. Confounding social and psychological problems, like alcoholism and medication non-compliance, further complicate therapy of hypertension. To think that such cases of hypertension warrant therapy with an implantable device is misguided.

Vanity, as well, will be difficult to overcome. From my experience with pacemakers, patients do not like their scar and associated discomfort occasionally experienced from pacemaker implantation beneath the collar bone (clavicle). Fortunately a shirt can usually cover the scar. But will patients want scars on both side of their neck with this device?

Safety issues also need to be addressed more completely. If the device is too effective in an individual patient and drop their blood pressure precipitously, how will it be inhibited? No sensor exists in the device to sense its effects on a persons’ blood pressure – certainly this is not a minor issue. Further, the baroreflex can cause profound slowing of the heart rate, or even asystole (no heart rate). No mechanism for pacing the heart (or a means of sensing the patient’s heart rate) exists in the device’s current design should a slow heart rhythm occur.

Finally, I hope that patients with renovascular hypertension are sufficiently screened before enrolling in a study using this device. Blocked arteries leading to the kidneys can result in profound medically-refractory blood pressure elevations and should be excluded before implanting this device.

In summary, the concept is interesting, but significant refinements are required before this device reaches the broader public. For now, treating hypertension early and often with conventional medical therapy and careful counselling is much more likely to be cost-effective at stemming hypertension’s end-organ complications.



Anonymous said...

Dr. Wes, I'm not sure I understand all of the "med speak," but am I right in assuming that this device would only lower the systolic pressure by an average of 22 points, and the diastolic by 18?

If that's so, wouldn't most HTN patients still need medications? Or is this intended to only treat mild HTN?

I think it may be "a hard sell," as you put it, if patients have an implant (and the necessary surgery) ... including scars, and still need their hypertension meds.

.... said...

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