It is now abundantly clear that hypertension is a major risk factor in the development of diabetic complications, both macrovascular and microvascular. Therefore its detection and treatment are of vital importance in overall diabetic management. Hypertension is common in diabetes: it affects 10-30% of white patients with type 1 diabetes and 30-50% of those with type 2 diabetes. It is a major risk factor for stroke and coronary artery disease, but also aggravates nephropathy and retinopathy. Blood pressure starts to rise when microalbuminuria develops, and the close link between hypertension and nephropathy may be explained by genetic factors leading to an increased susceptibility to develop both associated with increased sodium-lithium counter-transport activity in red blood cells. It is now postulated that the common link between obesity, diabetes, hyperlipidemia and hypertension in type 2 diabetes is insulin resistance and associated hyperinsulinemia, either inherited or perhaps acquired through malnutrition in early life. Occasionally hypertension is associated with renal artery stenosis, and this should be investigated, and ACE inhibitors and angiotensin II receptor antagonists avoided, when this clinical suspicion arises.
Targets for blood pressure lowering should be lower than for the non-diabetic population because of the major adverse effect of hypertension in this group: a blood pressure of 130/80 should be aimed for and perhaps even lower in high-risk patients, e.g. those with nephropathy. The Blood Pressure Control Study incorporated into the UKPDS demonstrated that a tight blood pressure control policy achieving a mean blood pressure of 144/82 gave a reduced risk for any diabetes-related end point, diabetes-related deaths, stroke, microvascular disease, heart failure and progression of retinopathy. It also demonstrated that in many patients combination therapy was necessary to achieve this level of blood pressure control. However, effective blood pressure reduction is likely to be more achievable than effective blood glucose control.
ACE inhibitors are the first choice to treat diabetic hypertension, as not only are they effective but also they delay the progression of diabetic retinopathy and diabetic nephropathy (perhaps in the latter by reducing intraglomerular pressure). Such agents include captopril, enalapril, lisinopril, fosinopril and ramipril. The effect of ramipril in reducing the rates of death, myocardial infarction and stroke in a broad range of high-risk patients, about 40% of whom had diabetes, was demonstrated in the Heart Outcomes Prevention Evaluation Study (HOPE study) and is likely to extend to other drugs in this group.
When ACE inhibitors cannot be tolerated, e.g. owing to dry cough, then an angiotensin II receptor antagonist such as losartan, valsartan or irbesartan should be substituted. Beta-blockers are also helpful antihypertensive agents, especially post-myocardial infarct or when there is co-existing angina. They should be avoided in bronchial asthma or when there is severe peripheral vascular disease. Bendrofluazide at low dose (2.5mg/day) effectively lowers blood pressure, but should be avoided in type 2 diabetes for fear of aggravating hyperglycemia. Indapamide may be a useful substitute. Calcium-channel blockers and a-receptor blockers, such as doxazosin are useful addon drugs to achieve satisfactory blood pressure control.
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