"Blood pressure lowered by nerve surgery," reports The Daily Telegraph based on the results of a study in rats. Although similar surgery has been performed to treat shortness of breath, we will have to wait for the results of clinical trials before we know whether the operation lowers blood pressure in people.
Researchers investigated how interrupting nerve signals to the carotid body, a small nodule on the side of each carotid artery in the neck, could lower blood pressure.
The carotid body detects the amount of oxygen and carbon dioxide in the blood, and causes breathing and blood pressure to change until blood oxygen levels are back to normal. Oversensitivity of the carotid body is one cause of blood pressure problems.
The current research repeated earlier work that showed that interrupting nerve signals to the carotid body lowers blood pressure in rats with high blood pressure. Researchers also looked at how interrupting signals to the carotid body reduces blood pressure.
For people whose hypertension does not respond well to treatment, doctors may not be able to tell what is causing their condition. For this reason, it is difficult to say what proportion of people with hypertension could be helped by this surgery. A small trial of the technique on people is already underway.
Where did the story come from?
The study was carried out by researchers from the University of Bristol, the University of São Paulo, and Coridea NC1, an idea generator, technology incubator and consulting firm.
It was funded by the British Heart Foundation and Cibiem NC1. Cibiem is a medical device company which focuses on carotid body modulation for the treatment of diseases such as hypertension, heart failure, diabetes and renal failure.
The study was published in the peer-reviewed journal Nature Communications.
The conclusions of the research were well reported by the media, although the number of people the surgery could potentially help may have been somewhat overestimated.
The Daily Mail says that the treatment could help 2.5 million people with "uncontrolled" hypertension, but the surgery may not help – or be appropriate for – all of these people.
What kind of research was this?
This was an animal study looking into how interrupting nerve signals to the carotid body, a small nodule on the side of each carotid artery, could lower blood pressure. The carotid body, or "carotid sinus", detects the amount of oxygen and carbon dioxide in the blood, and is responsive to changes in pressure in the arteries. Oversensitivity of the carotid sinus – for example, to pressure or massage – can be a cause of blood pressure and heart rate problems in some people.
A previous study by the current researchers found that interrupting nerve signals to the carotid body lowers blood pressure in a rat model of hypertension (high blood pressure).
These findings have led to a clinical trial in a specific group of people with hypertension caused by problems with their sympathetic nervous system, and who have not responded to other treatment. The sympathetic nervous system is involved in the "fight or flight" response and causes increases in heart rate and blood pressure.
What did the research involve?
Put simply, the researchers performed a number of experiments on rats with hypertension ("spontaneously hypertensive" rats) and normal rats to investigate how surgically interrupting nerve signals to the carotid body lowers blood pressure.
What were the basic results?
The researchers found that giving spontaneously hypertensive rats pure oxygen deactivates the carotid body and causes blood pressure to fall. It also causes a decrease in sympathetic nerve activity in the kidneys (the kidneys are involved in the regulation of blood pressure).
They found that interrupting nerve signals to the carotid body had a similar effect. Interrupting nerve signals to the carotid body caused:
- the feedback loop that maintains blood pressure (the "baroreflex") to improve
- a reduction in the presence of immune cells in heart tissue, which is associated with many forms of hypertension
- improved kidney function
There has also been interest in interrupting kidney nerve signals for the treatment of hypertension. The researchers found that interrupting kidney nerve signals and signals to the carotid body had an additional effect in rats.
How did the researchers interpret the results?
The researchers conclude that interrupting nerve signals to the carotid body reduces blood pressure. They propose that this could translate into a beneficial treatment for patients with hypertension caused by problems with sympathetic nervous system signalling.
Previous research already found that interrupting nerve signals to the carotid body lowers blood pressure in rats with high blood pressure. These findings have led to a clinical trial in a specific group of people who have hypertension caused by signalling problems within the sympathetic nervous system, and who have not responded to other treatment.
The results of this clinical trial, which is already underway, will tell us if this approach is effective in people whose hypertension is caused by nerve signalling problems.
In many people with hypertension that does not respond well to treatment, the biological mechanisms that underlie their condition are unknown. It is therefore difficult to say what proportion of people with hypertension this type of intervention could help, if it is proven to be safe and effective by further trials.
It is also worth bearing in mind that all surgery has risks, and this technique may cause unforeseen side effects. While the results showed a statistically significant reduction in a blood pressure measure in rats, it remains to be seen whether this will have any impact on the health (or reduced mortality) of people with high blood pressure.