The kidneys play a pivotal role in maintaining blood pressure, and blood pressure, in turn, affects the health of the kidneys.
THE leading cause of death in Malaysia is coronary heart disease. According to the Statistics Department, it constituted 12.9% of medically certified deaths in 2008, with pneumonia second at 7.0%, and strokes (cerebrovascular disease) third at 6.6%.
Data from the Health Ministry reveal that heart diseases and diseases of the pulmonary circulation were the most common causes of death in its hospitals at 16.09% in 2009, with septicaemia second at 13.82%, and cerebrovascular disease fifth at 8.43%.
High blood pressure (hypertension) is a common medical condition in Malaysia. According to the National Health and Morbidity Survey (NHMS) 2006, the prevalence was 32.2% of the population aged 18 years and above.
The disturbing finding was that only 35.8% of those with hypertension were aware of their condition, and of these, 87.7% were then currently on treatment. Of those who were aware of their hypertension and were receiving treatment, only 26.3% were found to have their hypertension controlled. The overall control of hypertension was a shocking 8.2%.
Hypertension is the most important modifiable risk factor for coronary heart disease, cerebrovascular disease, kidney failure (end-stage renal disease) and peripheral vascular disease. As such, the identification and treatment of patients with hypertension, together with the promotion of a healthy lifestyle, will assist in decreasing the prevalence of the condition.
Hypertension is either primary (essential) or secondary. The former comprises 90% or more of all cases whilst the latter comprises the rest. The majority of the secondary causes of hypertension are due to conditions in the kidneys (renal).
The kidneys play a pivotal role in maintaining blood pressure within a healthy range, and blood pressure, in turn, also affects the state of health of the kidneys.
Regulation of blood pressure
The regulation of blood pressure is complex. Although blood pressure is a product of heart (cardiac) output and peripheral vascular resistance, both are affected by multiple factors. The factors that affect cardiac output include sodium intake, renal function, and hormones.
The factors that affect peripheral vascular resistance include the sympathetic nervous system, humoral factors, and local autoregulation. The humoral factors are mediated by compounds that constrict or dilate the blood vessels, e.g. angiotensin, prostaglandins, etc.
Autoregulation is mediated by contraction and expansion of the volume in the blood vessels, which is regulated by the kidneys, and transfer of fluids across the blood capillaries.
The interactions between cardiac output and peripheral vascular resistance maintain a set blood pressure for a person. Changes in the various factors that affect cardiac output and peripheral vascular resistance would lead to the development of hypertension. Changes in blood viscosity, blood velocity, vascular wall thickness and circulating blood volume can all lead to the development of hypertension.
When blood pressure remains high over a period of time, it is called hypertension. Many people consider that a blood pressure below 140/90 mm Hg is normal. However, experts recommend that normal blood pressure with respect to cardiovascular risk be set at 120/80 mm Hg or less. Blood pressure of 120-139/80-99 mm Hg is considered as pre-hypertension, a category of individuals who are at risk of progressing to hypertension and in whom lifestyle modifications are essential preventive measures.
Hypertension and kidney disease
Hypertension results in the heart having to work hard, and also causes damage to blood vessels all over the body. Should the blood vessels in the kidney be damaged, its function of removal of waste materials and excess fluid from the body will be affected.
The extra fluid in the blood vessels would then increase the blood pressure further, with the vicious cycle continuing if untreated or inadequately treated.
Although hypertension is treated when it is diagnosed, the incidence of end-stage renal disease has not decreased. There are several reasons for this and they include the nature of hypertensive disease, failure to reduce the blood pressure to a level that provides protection, poor compliance with treatment prescribed, and the concomitant presence of diabetes.
Pressure in the glomeruli in the kidneys in hypertension leads to the development of sclerosis and impairment of renal function.
The likelihood of developing end-stage renal disease is increased in patients with diabetic nephropathy and hypertension. The increased glomerular pressure results in the presence of albumin in the urine in microscopic amounts (microalbuminuria).
The renin angiotensin system in the kidneys is involved in the development of hypertension. The consumption of angiotensin converting enzyme (ACE) inhibitors, which reduces glomerular pressure, has been shown to slow down the progression to end-stage renal disease in diabetic nephropathy, even if there is no hypertension. However, this beneficial impact of the ACE inhibitors is less clear in patients who are not diabetic.
The NHMS 2006 reported that the overall control of hypertension was a shocking 8.2%. The survey also reported that overall prevalence of diabetes mellitus (known and newly diagnosed) was 11.6%, with the national prevalence of known and newly diagnosed diabetes among adults above 30 years having risen from 8.3% in NHMS in 1996 to 14.9% in NHMS 2006.
The 16th report of the Malaysian Dialysis and Transplant Registry 2008 reported that a total of 18,856 patients were reported to the registry as being on renal dialysis at the end of 2008, with a prevalence rate of 626 per million per year. These facts do not augur well for the nation’s health.
Hypertension is common in those with renal disease, with the majority of patients with end-stage renal disease having hypertension. This has been mainly attributed to volume expansion and the activation of the rennin angiotensin system in the kidneys.
Damage to the kidneys may be without symptoms and is often detected through medical examinations and laboratory investigations, which would provide information about renal function. An elevation of the serum creatinine would indicate that the main function of the kidneys, i.e. glomerular filtration, is impaired.
The presence of protein in the urine (proteinuria) is another indicator of impaired renal function. However, proteinuria is also present in other conditions, e.g. heart disease, damaged blood vessels.
Microalbuminuria is an early indicator of diabetic nephropathy and those with increased risk of cardiovascular morbidity and mortality.
It is currently recommended that diabetics be screened for microalbuminuria. Such screening in hypertensives who do not have diabetes has not been established.
There are occasions when hypertension is due to renal artery stenosis. If the clinical features suggest this condition and if a corrective procedure is considered, imaging studies like computed tomography (CT) angiography and magnetic resonance angiography (MRA) would be performed. Both imaging studies are not without risk.
The risk of CT angiography is that of dye nephropathy, especially in diabetics with chronic renal disease. The risk of MRA is that of nephrogenic systemic fibrosis due to the gadolinium used in the MRA.
Additional tests may be done depending on the clinical situation.
Medicines would be required in most patients to control the hypertension. There are two groups of medicines that lower blood pressure and have a protective effect on the kidneys in diabetics, i.e. ACE inhibitors and angiotensin receptor blockers (ARBs). Both ACE inhibitors and the ARBs also reduce proteinuria and reduce the progression to end-stage renal disease in non-diabetics.
Controlling blood pressure
The recommendation of the National Heart, Lung, and Blood Institute (NHLBI) in the United States is that people with kidney disease should use whatever treatment necessary, including lifestyle changes and medicines, to keep their blood pressure below 130/80.
It has been calculated that a blood pressure reduction of 2mm Hg reduces the risk of stroke by 15% and the risk of coronary artery disease by 6% in a given population.
The NHLBI recommends lifestyle changes to control the blood pressure:
> Maintenance of the body weight at a level close to normal. Choose fruits, vegetables, grains, and low-fat dairy foods.
> Limitation of the daily salt (sodium) intake to 2,000mg or lower if there is high blood pressure. Read nutrition labels on packaged foods to inform oneself of how much sodium there is in one serving. Keep a sodium diary.
> Getting plenty of exercise, i.e. at least 30 minutes of moderate activity on most days of the week.
> Avoidance of excessive alcohol consumption. The limit for men is two drinks (two 360ml servings of beer, two 150ml servings of wine or two 45ml servings of “hard” liquor) a day. The limit for women is no more than a single serving a day because metabolic differences make women more susceptible to the effects of alcohol.
> Limitation of caffeine intake.
> Cessation or reduction of smoking and avoidance of illicit drugs are also essential preventive measures.
Source: Dr Milton Lum