Thursday November 18 2010
The kidneys regulate blood pressure
“Simple surgery offers hope to millions with high blood pressure,” The Independent has reported. The newspaper said that a small trial of a new surgical procedure has shown success in reducing high blood pressure.
Currently, around half of people who are treated for high blood pressure do not respond to their recommended drug treatment. This study explored a new approach to reducing blood pressure through using radio waves to deactivate nerves in the kidneys, which is thought to increase blood flow to the kidney and reduce the activity of the hormone renin, itself linked to raised blood pressure.
In the randomised trial, 52 people received the procedure and 54 people carried on with the normal drug treatment alone. The researchers found that patients who had the surgical treatment showed a much greater reduction in blood pressure over a six-month period than the patients who did not. No serious side effects were detected during the six-month follow-up.
This study has demonstrated a promising new avenue for the treatment of high blood pressure in people who do not respond to conventional treatment. Further trials are needed to monitor the long-term safety and effectiveness in larger numbers of people. Reducing high blood pressure is important in lowering the risk of a number of diseases.
Read our guide to making lifestyle changes that can help you reduce your blood pressure.
Where did the story come from?
The study was carried out at a number of hospitals in Australia, New Zealand and Europe and was funded by Ardian, the US company that manufactures the surgical device used in the study. The study’s researchers are from a number of other medical institutions, including Ardian, the Baker IDI Heart and Diabetes Institute and The Alfred Hospital in Melbourne, Australia. It was published in the peer-reviewed medical journal The Lancet.
In general, the study was accurately reported in a number of newspapers. The description by one paper of the process involving ‘electrical energy’ was slightly inaccurate, as the technique actually uses radio waves, which are part of the electromagnetic spectrum.
What kind of research was this?
This was a small randomised controlled trial looking at an alternative strategy for blood pressure reduction through a process called ‘sympathetic denervation’ of the kidneys. This new technique uses a catheter passed into the blood vessels that supply blood to the kidneys. Bursts of radio-frequency energy are applied to reduce the nerve activity in the walls of the blood vessels. This in turn increases blood flow to the kidney and then a reduction in the activity of the hormone renin, itself linked to raised blood pressure, which is thought to be eventually responsible for the reduced blood pressure.
It is important that new techniques for tackling hypertension are developed, as the condition can have a variety of causes and is associated with an increased risk of a number of serious conditions, including stroke and cardiovascular disease.
Another problem with hypertension is that it can be hard to manage effectively. Although there are a number of drugs that can be used to lower blood pressure, around half of the people treated still have blood pressure higher than the recommended levels. This treatment failure may be due to a number of reasons, including forgetting to take medication regularly, and a biological variation that means some patients do not physically respond to the drugs.
In this research, patients who had agreed to participate were randomly allocated to receive either a surgical procedure and their regular medication or to carry on with their usual drug treatment alone. This is an appropriate way of testing whether or not a new medical procedure works.
Unlike trials of drugs, there was no placebo used here, which means that people knew whether or not they were receiving the treatment being studied. This knowledge can sometimes influence how people respond to a treatment, although the main effect being monitored (blood pressure) is an objective outcome and less likely to be influenced by a person’s knowledge of their allocation to either group.
What did the research involve?
The researchers recruited 106 patients aged between 18 and 85, who had been diagnosed with hypertension (systolic blood pressure of 160mmHg or more, or 150mmHg or more if they had type 2 diabetes). All the participants had been treated with three or more antihypertensive drugs without successful reduction of their blood pressure to target levels. Participants were randomly allocated to receive either the denervation surgery or to carry on with drug treatment. In both groups, patients continued whatever medication they were taking prior to the start of the study.
The patients who received the surgical procedure had a long narrow tube passed up to the kidneys through an artery in their leg. Low-power radio frequency signals were applied to the arteries that supply blood to the kidneys, deactivating the nerves around them.
All the patients had their blood pressure monitored before the study, after six months, and at a number of points in between. As well as repeated measurements undertaken by doctors, the patients were given blood pressure machines to take their own readings at home for two weeks before the study started, as well as three times in the morning and three times in the evening and again at the end of the six months. The researchers used averages of the home measurements at the start of the study and the six month visits for the analysis.
This careful and frequent home measurement helped researchers to check the consistency of their measurements, as sometimes blood pressure can be raised by the experience of having it monitored or treated in a clinical setting.
What were the basic results?
The patients in the surgery group had an average blood pressure reading of 178/96mmHg. Six months after the procedure, their blood pressure had lowered by 32/12mmHg on average. The average reduction in the people who did not have the surgery was 1/0mmHg from a starting average of 178/98mmHg. The difference in blood pressure reduction between the two groups was statistically significant (p<0.0001).
A proportion of people experienced no decrease in systolic blood pressure after six months – 10% of people in the surgery group and 47% of the control group. The target of a systolic blood pressure of less than 140mmHg was achieved by 39% of the people who had the surgery, and 6% of those who did not.
These findings were based on the measurements taken by the researchers when people visited the clinics. Similar effects were seen when they looked at the measurements people had taken themselves at home – 32 patients from the surgery group had an average reduction of 20/12mmHg, while 40 people from the control group showed an average increase of 2/0mmHg.
The researchers did not report any serious side effects of the treatment.
How did the researchers interpret the results?
The researchers say that ‘a significant reduction in blood pressure can be achieved with catheter-based renal denervation’ in patients who had not been successfully treated using conventional drug treatments.
Regarding kidney function, they say ‘there was no evidence of worsening function, suggesting this procedure is safe even in those with mild-to-moderately impaired renal function’.
This study demonstrated that using a simple surgical procedure was effective at lowering the blood pressure of people whose hypertension had not been managed successfully with standard drug treatments. There are, however, some limitations to the study and its findings:
- This study was quite small, and although it detected a significant difference in blood pressure reduction in the treatment and control groups, such a treatment would not become part of routine clinical practice until larger trials are carried out to further assess safety and effectiveness.
- Participants were followed up for six months after the procedure. Future trials following people for longer periods are important to verify that the effects of the treatment are long lasting. They could check, for example, that the nerves do not regrow and to see whether the treatment makes any difference to the risk of other conditions associated with hypertension, such as stroke.
- Although the number of people achieving the target systolic blood pressure of less than 140mmHg was greater in the group who received surgery than in those who did not, 61% of the people who had surgery had still not reached this target six months after the procedure.
- The researchers did not find any significant harmful effects of the procedure, but longer studies may be needed to verify whether any serious adverse events develop over time.
- The people involved in the study knew what treatment they were receiving. Sometimes knowledge of whether or not a treatment has been received can influence participants’ behaviour. For example, the people who knew they were treated might be more motivated to stick to lifestyle choices that reduce blood pressure, or be more accurate when measuring their blood pressure at home. A ‘blinded’ study (such as when the control patients receive ‘sham surgery’) would reduce this problem.
- Those in the surgery group still continued to use their regular medication. Therefore, the study cannot tell us whether or not people receiving the treatment could cease taking their daily medication.
Overall, this research shows a promising new avenue for treatment of hypertension that has not responded well to conventional management with drugs. Larger, long-term trials will help to explore whether the effects of the surgery are long lasting, and whether it helps to reduce the risk of other conditions associated with hypertension.