Introduction

Angiotensin is substance in the blood that is necessary for the blood vessels to constrict. One way of treating high blood pressure is to block the formation of angiotensin, or block its action on the blood vessels. There are two classes of drugs that make use of this approach. The so-called angiotensin-converting enzyme (ACE) inhibitors block an enzyme in the body that is necessary for the formation of angiotensin, while the angiotensin receptor blockers (ARBs) block the access of angiotensin to its point of action in the blood vessels. It looks like ACE inhibitors and ARBs have the same result - stopping angiotensin contracting the blood vessels - so what, if any, are the differences between them?

Two of the earliest physicians working in the angiotensin field have recently reviewed our knowledge of the ACE inhibitors and ARBs in the medical journal, Lancet. This is a summary of their review, with some additional information.

Effective medical treatment of high blood pressure has been available for the last 50 years, and with every new class of drugs introduced there has been a reduction in the number and severity of the side effects that they cause. This is real progress, as antihypertensive treatment is, usually, a lifelong undertaking; without the 'motivation' provided by obvious disease symptoms, taking a pill a day requires that it doesn't cause side effects.

The ACE inhibitors and the ARBs have side-effect profiles that are almost ideal for effective medications. However, there is still a lot to be learned about the way they work and their full effects on the cardiovascular system.

Advantages of ACE inhibitors and ARBs

In studies where they were compared with dummy tablets (i.e. placebo-controlled studies), ACE inhibitors were shown to reduce the risk of illness and death from heart attack, heart failure, kidney disease, and stroke. The newer ARBs have so far provided similar results for kidney disease and heart failure. In the case of heart failure, giving an ARB (valsartan) to patients already taking an ACE inhibitor improved their outcomes still further.

The so-called LIFE study¹ examined over 9,000 patients with high blood pressure and enlargement of the left heart chamber (ventricular hypertrophy), who were given an ARB (losartan), or another, older class of drug, a beta-blocker (atenolol). After 4 years' treatment there were less illness and deaths due to stroke or heart attack with the ARB, and also a smaller number of these patients developed diabetes for the first time, compared with the beta-blocker. In those LIFE study patients who had diabetes from the beginning of the study, reduction of fatal heart attacks and heart failure were greater with the ARB than with the beta-blocker.

Importantly, measurements of heart enlargement showed that there was a greater reduction with losartan than with atenolol. The finding was not totally surprising, as angiotensin is considered a 'growth factor' for heart muscle, so that blocking it's action would limit heart muscle growth.

Differences between ACE inhibitors and ARBs

The favorable effects of ACE inhibitors and ARBs on blood pressure are virtually the same. They also have similar effects on other angiotensin-related conditions, like heart muscle enlargement and kidney disease. But, how do they differ?

One of the commonest side effects of ACE inhibitors is a dry, persistent cough. It occurs in about 1 in 10 of people taking them. None of the ARB drugs have been found to have this effect, which is gives them a clear advantage over the ACE inhibitors.

Other possible differences between ACE inhibitors and ARBs may be revealed in future studies. In the meantime, we can conclude that the benefits of both are extremely similar, their actions that go beyond merely lowering blood pressure are also similar, but they differ in that the ACE inhibitors have likely to cause an irritating cough in some patients.