Renal Denervation (RDN) and the Treatment of Hypertension – Simplified
Hypertension (high blood pressure) has often been called the ‘silent killer’. This tagline has been well earned because People are often totally unaware of the condition which, if untreated, leads to an increased risk of heart disease and stroke.
Historically, hypertension had been treated using medications and changes to peoples lifestyle and will continue to be treated in that way for the foreseeable future. Unfortunately there is a population of patients (30%) who despite being treated by numerous drugs still continue to have high blood pressure and therefore remain at risk of two of the USA’s biggest killers. If a patient is on 3 or more blood pressure medications and continues to have an average Blood Pressure of 140/90 or above, they are categorised as having ‘resistant hypertension’.
Over the last few years Renal Denervation has gained increasing clinical evidence as a treatment option for these patients or indeed those patients who cannot tolerate the side effects of blood pressure medications.
What is Renal Denervation?
Renal Denervation is a process in which small catheter (tube) is inserted into one of your larger arteries (usually the femoral artery near your groin) and guided under x-ray until it can be steered into your Renal Arteries. The Renal Arteries are the larger blood vessels in your Kidneys. Once in position the catheter is used to ‘burn’ at several points along the Renal Artery Wall creating several areas of ‘Scar Tissue’. This is carried out on any Renal Arteries over 4mm in Diameter on both kidneys.
This procedure is considered minimally invasive because only a small incision above an artery is made and the process is carried out under local anaesthetic.
How does Renal Denervation Work?
Well unfortunately to understand this, I first have to explain a little about the Sympathetic Nervous System. Bodily functions are controlled by two main Nervous Systems, the Sympathetic Nervous System and the Parasympathetic Nervous System.
Though it may not feel like it, as you sit reading this a combination of the two are controlling your breathing rate, heart rate and many other things that you take for granted. In certain situations these nervous systems have the power to initiate much more obvious changes in your bodies behaviour.
An example of this is the FIGHT or FLIGHT mechanism that you are probably familiar with. If not think of a time where you have been in danger or been exposed to something particularly nasty and you will probably recall your heart rate increasing, a feeling of your heart beating extra hard and your breathing rate quickened. This is your Sympathetic Nervous System coming into affect!
So what is happening?
When the Sympathetic Nervous System is ‘Stimulated’ a signal is sent around the network of nerves that causes changes in the bodies behaviour. These changes depend on the organ and the process that the nerve ending innervates (innervate just means to supply with nerves).
If you think of a Fire alarm in a building, when activated this has differing affects on people depending on the personnel that the signal reaches. Most people will leave the building in an orderly function, the Fire Wardens will get their high visibility jacket and start to round people up and the Fire Brigade that are notified will be attracted to the building. This is the same principal – the same signal can have different affects on different organs.
Here is a fairly classic example of the Sympathetic Nervous System increasing Blood Pressure.
The Hypothalamus (Part of the brain) sends an impulse down the Sympathetic Nervous System in response to feedback from your body or from the environment around you…. Like a herd of stampeding elephants for example.
This Signal causes the release of Epinephrine (a form of adrenaline) from the Adrenal Glands into the blood stream via the Renal Arteries. Epinephrine is like a chemical ‘FIRE ALARM’ wherever the signal reaches it invokes a response of some form.
- The Liver – Increase in Glycogen Released into Blood Stream
- The Heart – Increase in Rate and Force of Contraction
- The Lungs – Increase in Breathing Rate and Bronchodilation (Increasing gas exchange)
- Blood Vessels – Vasoconstrict (decrease the diameter) of blood vessels in the skin and digestive system.
If we have a quick review of some of these effects – An increase in heart rate, an increase in force of contraction and vasoconstriction of many blood vessels will all lead to an INCREASE IN BLOOD PRESSURE. This is principally how The Sympathetic Nervous System affects blood pressure.
So how Does Renal Denervation Help?
One of the key mechanisms involves the following…
As I mentioned earlier the Sympathetic Nervous System have many many roles to play. Another one of these roles affects the behaviour/properties of the Renal Arteries. The Renal Arteries are innervated (that word again) by two types of Sympathetic Nerve Fibres – Efferent and Afferent.
When stimulated the efferent nerves increase how much Epinephrine enters the Renal Arteries from the Kidneys.
*To make this simpler I am going to use that fire alarm analogy again!!!!* There is a hall packed full of people, the hall has one useable door and so people leave the hall in dribs and drabs. Then the fire alarm sounds and all five FIRE DOORS burst open. All of a sudden lots of people are able to leave the hall very quickly.
This is the role that the efferent nerves play during Sympathetic Nerve Stimulation – They open the ‘fire doors’ of the Renal Arteries allowing Epinephrine to flood into the blood stream.
IF YOU HAVE HYPERACTIVE RENAL NERVES – Some of these Fire Doors will always be left open – more Epinephrine will enter the blood stream and therefore a persons average blood pressure will be increased!
RENAL DENERVATION BURNS THESE HYPERACTIVE RENAL NERVES and stop this happening!
Results of Renal Denervation
The two largest trials to date carried out by Medtronic the Simplicity HTN-1 and Simplicity HTN-2 have shown average decreases in blood pressure of -33/19 mmHg and -32/12 mmHg respectively.
Companies that Produce Renal Denervation Systems
Medtronic Simplicity was the first RDN device to receive the CE mark of approval in 2010 and now has over 5 years of clinical experience. Other RDN devices are now available but have less clinical data.
- St. Jude Medical’s EnligHTN™ System
- Vessix’s V2™ Renal Denervation System
- Covidien’s OneShot™ System
- Recor’s Paradise™ System
- Terumo’s Iberis™ System.
None of these devices has currently received FDA approval.
I hope this has cleared up a few things and I have noticed the Typo in my Pictures!!! ah well 😉
Time to watch the end of Harry Potter
Thanks for Reading
Image courtesy of dream designs / FreeDigitalPhotos.net