Resistant hypertension: a complete guide to high blood pressure that does not respond

Resistant hypertension is the reason a meaningful proportion of patients are referred to my clinic. The label is often applied prematurely, and a substantial number of patients labelled resistant turn out to have controllable blood pressure once the right things are investigated. This article walks through how I approach the assessment.

Resistant hypertension is the term used when blood pressure remains above target despite the use of three or more antihypertensive medications at adequate doses, including a diuretic. It affects around 10 per cent of patients with hypertension and is one of the main reasons people are referred to specialist hypertension clinics. The condition matters because uncontrolled hypertension carries substantial cardiovascular risk, and because the great majority of resistant cases can be brought under control with the right approach.

This guide walks through what resistant hypertension is, what to rule out before concluding that it is genuine, what underlying causes to consider, and what specialist treatment can offer.

The definition of resistant hypertension has been refined over the years. The current consensus is that the term applies when blood pressure is above target (typically 140/90 in clinic, 135/85 on home or ambulatory monitoring) despite at least three antihypertensive medications, one of which is a diuretic, all at adequate doses. The term controlled resistant hypertension is sometimes used for patients whose blood pressure is at target on four or more medications, on the basis that they have demonstrated resistance even though current control is good.

Before concluding that hypertension is genuinely resistant, several things need to be ruled out. The first is white coat effect, in which blood pressure is high in clinic but normal at home. This is a common explanation for apparently resistant hypertension and is excluded with home or ambulatory monitoring. A 24-hour ambulatory blood pressure monitor, in particular, gives a definitive picture of true blood pressure throughout daily activities and sleep. Many patients labelled as resistant turn out to have well-controlled blood pressure outside the clinic.

The second is non-adherence to medication. This is the single most common reason for apparently resistant hypertension, and it is more common than most patients (or doctors) appreciate. Studies that have measured drug levels in blood or urine in patients with resistant hypertension consistently find that around 30 to 50 per cent are not taking all the medications prescribed. Reasons include side effects, cost, complex regimens, lack of perceived benefit, and simple forgetfulness. Non-adherence is not a moral failing. It is a common human response to long-term medication for an asymptomatic condition. Identifying it requires honest conversation and sometimes objective testing.

The third is suboptimal medication choice. The right combination matters. The classical first-line combination is an ACE inhibitor or angiotensin receptor blocker plus a calcium channel blocker plus a thiazide diuretic, all at appropriate doses. Patients on three medications that do not represent this combination, or on inadequate doses, may have apparently resistant hypertension that responds promptly to optimisation. Patients on a thiazide diuretic at low dose, in particular, often improve with dose escalation or a switch to a longer-acting agent such as indapamide or a thiazide-like diuretic such as chlorthalidone.

The fourth is lifestyle factors that work against the medications. Excessive salt intake reduces the effectiveness of all antihypertensive medications. The threshold for hypertension benefit is around 5 to 6 grams of salt per day, and patients eating substantially more (which most people on a typical Irish diet do) often see meaningful improvement when this is addressed. Excessive alcohol intake raises blood pressure and can produce apparent resistance. Weight gain raises blood pressure. Lack of physical activity contributes. Use of NSAIDs (such as ibuprofen and diclofenac) raises blood pressure and can interfere with the effects of antihypertensive medications, particularly ACE inhibitors and diuretics. Oral contraceptives, steroids, decongestants, and certain herbal preparations can all contribute.

Once these factors are addressed, true resistant hypertension prompts a search for an underlying cause. This is where secondary hypertension becomes important.

Primary aldosteronism is the most common identifiable cause of resistant hypertension. It is caused by overproduction of aldosterone from the adrenal glands, either bilaterally (from hyperplasia of both glands) or from a single adenoma. The traditional teaching that it was rare was wrong. Modern screening shows that primary aldosteronism accounts for 10 to 20 per cent of cases of resistant hypertension and around 5 per cent of all hypertension. It is dramatically underdiagnosed in routine practice. Screening is with the aldosterone-to-renin ratio in the blood. Confirmation requires further hormonal testing, often including a saline suppression test, and imaging to distinguish a single adenoma from bilateral disease. Treatment is with adrenalectomy for a single adenoma or with mineralocorticoid receptor antagonists (spironolactone or eplerenone) for bilateral disease. Identifying primary aldosteronism can transform blood pressure control and reduces cardiovascular risk independently of the blood pressure effect.

Renal artery stenosis is another important cause. Narrowing of one or both arteries supplying the kidneys triggers excessive renin release and hypertension. The two main types are atherosclerotic disease (typical in older patients with other vascular risk factors) and fibromuscular dysplasia (typically affecting young to middle-aged women). Diagnosis is by ultrasound Doppler, CT angiography, or MR angiography. Treatment depends on the type. Fibromuscular dysplasia often responds well to angioplasty. Atherosclerotic disease is generally managed medically, with stenting reserved for selected cases where medical therapy is inadequate.

Obstructive sleep apnoea is increasingly recognised as a cause of resistant hypertension. Repeated apnoeic episodes during sleep produce surges in sympathetic activity that raise blood pressure. The diagnosis is by sleep study. Treatment with continuous positive airway pressure (CPAP) can improve blood pressure significantly in patients with significant apnoea. Asking about snoring, daytime sleepiness, and witnessed apnoeas is part of the assessment of any patient with resistant hypertension.

Chronic kidney disease both causes and is caused by hypertension. Patients with significant CKD almost always have hypertension that requires intensive management. The relationship is bidirectional and managing both together is the core of nephrology care for these patients.

Other secondary causes to consider include pheochromocytoma (a rare adrenal tumour, screened with urinary or plasma metanephrines), Cushing’s syndrome, coarctation of the aorta (typically diagnosed in childhood but occasionally first identified in adults), and thyroid disease.

Specialist treatment of resistant hypertension begins with optimising the existing regimen and adding the right fourth-line agent. The PATHWAY-2 trial established spironolactone as the most effective fourth-line agent in most cases. It substantially outperformed alternatives in patients with resistant hypertension on optimal triple therapy. Spironolactone is given at 25 to 50 mg daily, with potassium monitoring. Eplerenone is an alternative for patients who cannot tolerate spironolactone due to its side effects (particularly breast tenderness or enlargement in men).

For patients in whom spironolactone is not appropriate or not effective, alternatives include beta-blockers, alpha-blockers (such as doxazosin), centrally acting agents (such as moxonidine), and in selected cases hydralazine or minoxidil. Renal denervation, a procedure that disrupts sympathetic nerve activity around the kidney arteries, is available in specialist centres for selected patients and has growing evidence of benefit.

Lifestyle measures continue to matter throughout. Sustained weight loss, salt restriction, regular exercise, moderation of alcohol, treatment of any sleep apnoea, and avoidance of medications that raise blood pressure all add to the effect of pharmacological therapy. The combination of optimised medication, attention to lifestyle, and treatment of any secondary cause achieves good blood pressure control in the great majority of patients with resistant hypertension.

The practical messages for patients are clear. If your blood pressure is persistently high despite three or more medications, it is worth pursuing specialist assessment. The diagnostic yield is meaningful, particularly for primary aldosteronism, which remains substantially underdiagnosed. The treatment options are wider than commonly appreciated. Good blood pressure control is achievable in most cases, and the long-term cardiovascular benefits are substantial.

I see private patients at Blackrock Clinic, The Beacon Hospital, Bon Secours Dublin, the Hermitage Medical Centre, and St Vincent’s Private Hospital. If you would like a consultation about your kidney health, you or your GP can contact my secretary through drrorymcquillan.ie. Most patients are seen within two to three weeks of referral.