All kidney patients, whether they are on haemodialysis, CAPD, or have received a kidney transplant, discover early in their treatment that the nursing and medical staff helping with their care spend a lot of effort checking their blood pressure. In due course, most patients do this for themselves and keep careful and accurate records. Although all kidney patients and many members of the general public now know the importance of keeping high blood pressure under control, perhaps fewer people understand exactly why this matters so much.
For patients with kidney disease a sensible concern with blood pressure is particularly important. Why should this be?
The links between kidney disease and high blood pressure go back a long way. It was Richard Bright, a physician at Guys Hospital in London, who first described the relationship in 1836. He noted that patients with scarred kidneys also had signs of damage which were in fact due to severe hypertension (high blood pressure). At this time Bright could only speculate, as accurate measurement of blood pressure was not yet possible. We now know that most patients with kidney failure tend to have high blood pressure. This can be due to an accumulation of salt and water in the body or sometimes as a result of the damaged kidneys releasing too much renin, an enzyme which is very important in the hormone systems which keep blood pressure up. To complicate the situation, some people develop kidney failure because they have high blood pressure, although this is rare. Kidney function remains normal in most people who have “essential hypertension” (about one in ten of the population) and high blood pressure causes only about one in twenty cases of serious kidney failure.
It has become clear in the last decade that once kidneys are damaged – whatever the cause – the high blood pressure which may be provoked by the damage will itself worsen the kidney scarring, making a vicious circle. For this reason much effort is made, in caring for patients with impaired kidney function who are not yet dependant on dialysis, to keep blood pressure as normal as possible. The hope is that the need for dialysis treatment may be delayed for some time.
Why does blood pressure remain important once patients begin routine treatment with haemodialysis, CAPD, or receive a kidney transplant?
First, the level of blood pressure gives a useful guide to “volume status”. If insufficient body fluid has been removed by dialysis therapy the body fluid volumes automatically increase and blood pressure rises; put simply, the blood vessels become over distended by the excessive fluid. Similarly, if too much fluid is removed by dialysis then blood pressure falls, especially when standing up (“postural hypertension”), which can cause feelings of extreme tiredness and dizziness. Hence blood pressure, together with body weight, is a yardstick for judging whether too little or too much fluid has been removed during dialysis. Although transplant patients do not have a problem excreting fluid from the body, they do tend to develop high blood pressure and this happens for a number of reasons, including renin secretion from the patients own scarred kidneys and side effects of prednisone and cyclosporin.
Apart from the potential harmful effects of high blood pressure on the kidneys, several large studies involving thousands of participants with high blood pressure have shown that it is a very important risk factor which increases the chances of suffering a stroke or heart attack later in life. They have also shown that lowering blood pressure to normal with treatment reduces this risk. These conclusions are very important for kidney patients because it is common for them to be exposed to high blood pressure and it appears likely that if hypertension is not adequately treated in dialysis and transplant patients they may be at a particularly high risk of suffering a heart attack or stroke.
For these reasons, checking blood pressure carefully is a central feature both of visits to outpatients clinics and in kidney patients own self care. There is increasing use too of “ambulatory” blood pressure measurement, when a blood pressure cuff is worn for 24 hours and inflated automatically every thirty or sixty minutes. The blood pressure measurements are stored for later analysis by a processor. This gives a very accurate picture of blood pressure level, avoiding the problem of the “alarm reaction” which some patients experience when they visit clinics.
Control of high blood pressure in kidney patients follows two simple principles. First, fluid volume overload is corrected by altering the haemodialysis or CAPD regime appropriately, or by use of diuretics such as frusemide in patients who are not dependant on dialysis. Then, specific antihypertensive drugs are prescribed. These may include drugs which relax the blood vessels, such as the calcium antagonists nifedipine or verapamil, or the vasodilators hydralazine or minoxidil. There are also drugs which reduce the force of the heart beat (betablockers) such as atenolol or metoprolol, and converting enzyme inhibitors such as captopril or enalapril which reduce the effects of the renin hormonal system. Several other new forms of blood pressure treatments are now also available and they are all safe and effective.