How ARBs Work: The Cardiologist's Guide to Angiotensin Receptor Blockers

Patients ask me about their blood pressure medications all the time, and the ARB class is one where the questions come up most. The names alone are a mouthful — losartan, valsartan, olmesartan, telmisartan, irbesartan, candesartan, azilsartan. Patients want to know what these drugs actually do, why I chose one for them instead of a different class, and whether the differences between them matter. This is an honest explanation, without jargon, of how angiotensin receptor blockers work in the body, why they earned such a central place in modern cardiovascular care, and what I pay attention to when I prescribe them.

The System ARBs Block

To understand ARBs, you have to understand the renin-angiotensin-aldosterone system, or RAAS. This is an ancient hormone circuit that your body evolved to maintain blood pressure and fluid balance. When blood pressure falls or when the kidneys sense low salt, cells in the kidney release an enzyme called renin. Renin cleaves a circulating liver protein into angiotensin I. A second enzyme called angiotensin-converting enzyme, or ACE, then converts angiotensin I into angiotensin II. Angiotensin II is the business end of the system. It constricts blood vessels, drives aldosterone release from the adrenal glands (which causes salt and water retention), stimulates thirst, and promotes cardiac and vascular remodeling over time.

Angiotensin II exerts these effects by binding two receptor subtypes, AT1 and AT2. The AT1 receptor is the one that drives vasoconstriction, sodium retention, cardiac hypertrophy, and fibrosis. The AT2 receptor, when activated, does the opposite — vasodilation, natriuresis, and anti-inflammatory effects. ACE inhibitors reduce the production of angiotensin II upstream. ARBs take a different approach: they block angiotensin II from binding to the AT1 receptor selectively, while leaving the AT2 receptor free to do its beneficial work.

What That Blockade Does Clinically

When you block the AT1 receptor, arterial smooth muscle relaxes and peripheral vascular resistance drops, lowering blood pressure. The adrenal glands make less aldosterone, so the kidneys hold on to less sodium and water. The heart works against lower afterload, which over months and years reverses left ventricular hypertrophy in patients with high blood pressure. In the kidneys, efferent arteriolar dilation lowers glomerular pressure, which protects nephrons from the pressure-driven damage that defines diabetic and hypertensive kidney disease. This is the same set of downstream effects as ACE inhibitors, but produced with a different mechanistic route.

The clinical applications flow from those effects. ARBs are first-line therapy for hypertension. They are pillar therapy for heart failure with reduced ejection fraction, almost always as part of the ARNI combination drug sacubitril-valsartan in modern practice. They are kidney-protective in diabetes with albuminuria, in chronic kidney disease with proteinuria, and after kidney transplant. They reduce risk after heart attack in patients who cannot tolerate ACE inhibitors. They are used for preventing atrial fibrillation recurrence in patients with hypertension and left atrial enlargement. And they are standard therapy for aortopathy, particularly in patients with Marfan syndrome or bicuspid aortic valve with aortic dilation.

ARBs vs ACE Inhibitors

Patients often ask why I am putting them on an ARB instead of an ACE inhibitor, or vice versa. The short answer is that in most conditions the classes are clinically equivalent for hard endpoints. The ONTARGET trial randomized 25,620 high-risk vascular patients to ramipril 10 mg daily, telmisartan 80 mg daily, or the combination, and followed them for a median of 56 months. Event rates for the two single agents were essentially identical — 16.5 percent with ramipril and 16.7 percent with telmisartan, a hazard ratio of 1.01 with a 95 percent confidence interval from 0.93 to 1.10. The combination arm did worse because it increased hypotension, renal dysfunction, and hyperkalemia without improving cardiovascular outcomes. That trial is why I do not combine an ACE inhibitor with an ARB in the same patient, and it is the best evidence that in hard-endpoint terms the two classes are interchangeable in high-risk hypertension.

The practical reason I pick an ARB over an ACE inhibitor comes down to tolerability. ACE inhibitors cause a dry, persistent cough in roughly 5 to 20 percent of patients depending on how strictly you define it — ONTARGET found cough in 4.2 percent of ramipril patients compared with 1.1 percent on telmisartan, and meta-analyses show ACE inhibitors raise cough risk about three-fold compared with ARBs. The mechanism is bradykinin accumulation in the airways, because blocking ACE also blocks bradykinin degradation. Angioedema is rarer but more dangerous — roughly 0.3 percent on ramipril in ONTARGET versus 0.1 percent on telmisartan. ARBs do not interfere with bradykinin, so they cause essentially no cough and substantially less angioedema. For most of my patients I start with whichever class I think they will tolerate best, and I switch to an ARB if an ACE inhibitor produces a cough.

Differences Among the ARBs

Within the class, the drugs are more similar than different, but there are a few distinctions that matter in practice. Losartan is the oldest and cheapest. It has a shorter duration of action than the others. The LIFE trial used losartan 50 to 100 mg once daily and still showed a stroke-reduction benefit over atenolol in hypertensive patients with left ventricular hypertrophy, though in clinic I sometimes split the dose twice daily when blood pressure is tough to control. Valsartan has the largest evidence base in heart failure, both as monotherapy in Val-HeFT and as the ARB component of sacubitril-valsartan in PARADIGM-HF, which made ARNI the standard of care for HFrEF. Telmisartan has the longest half-life of any ARB and unique partial PPAR-gamma agonist activity, which gives it modest insulin-sensitizing effects. Olmesartan and azilsartan are the most potent at lowering blood pressure on a milligram-for-milligram basis. Candesartan has strong evidence for heart failure with preserved ejection fraction from the CHARM program, and irbesartan has strong evidence for diabetic kidney disease from IDNT and IRMA-2.

For most patients with hypertension alone, any ARB will work. I reach for losartan in cost-sensitive patients, valsartan when I anticipate transitioning to sacubitril-valsartan, telmisartan for patients with metabolic syndrome, and candesartan or irbesartan when I have a specific heart failure or kidney indication where the trial data is strongest.

What I Monitor

Two laboratory values matter when I start or titrate an ARB: serum creatinine and potassium. Because ARBs dilate the efferent glomerular arteriole, glomerular filtration pressure drops, and a small initial rise in creatinine of up to 30 percent is expected and acceptable. A bump beyond that, or a rise that continues over weeks, suggests renovascular disease, volume depletion, or concomitant nephrotoxic exposure and warrants investigation. Potassium also rises modestly because aldosterone suppression reduces renal potassium excretion. In patients with advanced kidney disease, type 4 renal tubular acidosis, or combined use with spironolactone or potassium supplements, this can become clinically dangerous. I check labs within two weeks of starting or titrating and periodically thereafter.

Blood pressure response is what I titrate to. In uncomplicated hypertension the goal is below 130/80 mmHg in most guidelines, with lower targets in patients with known cardiovascular disease, diabetes, or kidney disease. When monotherapy does not achieve that, I add a calcium channel blocker or a thiazide diuretic before considering beta-blockers or additional agents, and that sequence is supported by ACCOMPLISH and the broader hypertension literature.

Who Should Not Take an ARB

ARBs are contraindicated in pregnancy because they cause fetal renal agenesis and oligohydramnios, and women of reproductive age on an ARB need reliable contraception or should be switched if pregnancy is planned. They should not be used in patients with bilateral renal artery stenosis because they can precipitate acute kidney injury by removing the angiotensin-II-dependent efferent arteriolar tone that maintains glomerular filtration in those patients. History of angioedema with an ACE inhibitor is a relative contraindication — the cross-reactivity rate is low, approximately 3.5 percent in published series with wide confidence intervals, but the consequences of airway angioedema are severe enough that I usually avoid ARBs in that setting. Severe hyperkalemia is another reason to hold or discontinue.

ARBs and Modern Cardiovascular Therapy

The role of ARBs has evolved. Twenty years ago the debate was whether they were equivalent to ACE inhibitors. That question is now settled, and in most hard-endpoint contexts they are — current heart-failure guidelines use "ACE inhibitor or ARB" language rather than positioning one as superior, and both classes reduce all-cause mortality by roughly 20 to 30 percent compared with placebo in HFrEF. The more interesting developments are combination therapies where the ARB is a backbone. Sacubitril-valsartan brings in the neprilysin inhibitor sacubitril to preserve natriuretic peptides and produced an unambiguous mortality benefit over enalapril in PARADIGM-HF. The addition of SGLT2 inhibitors to background ARB therapy has produced further kidney and heart failure benefit in DAPA-HF, EMPEROR, DAPA-CKD, and EMPA-KIDNEY. The MRA class, particularly finerenone in the FIDELIO-DKD and FIGARO-DKD trials, layers on top of ARB therapy in diabetic kidney disease for still more benefit.

The takeaway from that literature is that ARBs are rarely doing all the work. They are a foundation on which other RAAS-modulating and natriuresis-modulating drugs build. When a patient tells me they are on an ARB plus an SGLT2 inhibitor plus a GLP-1 receptor agonist plus a statin, what I am seeing is good care — a stack of interventions each targeting a different pathway, each proven to reduce events, working together.

What This Means for You

If your cardiologist has prescribed an ARB, you are on one of the most studied and safest classes of cardiovascular drugs in existence. Take it at the same time every day. Call if you get a bad cough, unusual swelling around the face or tongue, lightheadedness, or muscle weakness that might suggest high potassium. Do not stop it abruptly without talking to your clinician, particularly if you have heart failure or kidney disease where abrupt withdrawal can cause rebound. If you are pregnant or trying to become pregnant, tell your doctor immediately so we can switch you to a pregnancy-safe alternative.

And if you are considering whether to start one, the evidence is that patients who take their ARBs consistently have fewer heart attacks, fewer strokes, less kidney disease progression, less heart failure, and fewer deaths. That is a medication worth taking.

References

1. ONTARGET Investigators. Telmisartan, ramipril, or both in patients at high risk for vascular events. N Engl J Med. 2008;358:1547-1559.

2. McMurray JJ, Packer M, Desai AS, et al. Angiotensin-neprilysin inhibition versus enalapril in heart failure. N Engl J Med. 2014;371:993-1004.

3. Dahlöf B, Devereux RB, Kjeldsen SE, et al. Cardiovascular morbidity and mortality in the Losartan Intervention For Endpoint reduction in hypertension study (LIFE). Lancet. 2002;359:995-1003.

4. Lewis EJ, Hunsicker LG, Clarke WR, et al. Renoprotective effect of the angiotensin-receptor antagonist irbesartan in patients with nephropathy due to type 2 diabetes. N Engl J Med. 2001;345:851-860.

5. Pfeffer MA, Swedberg K, Granger CB, et al. Effects of candesartan on mortality and morbidity in patients with chronic heart failure: the CHARM-Overall programme. Lancet. 2003;362:759-766.

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8. Whelton PK, Carey RM, Aronow WS, et al. 2017 Guideline for the prevention, detection, evaluation, and management of high blood pressure in adults. Hypertension. 2018;71:e13-e115.