ACE Inhibitors and ARBs

• The RAAS Cascade and Angiotensin II Generation: The renin-angiotensin-aldosterone system is activated by decreased renal perfusion pressure, sympathetic nervous system stimulation, or hypokalemia. Renin, released from juxtaglomerular cells in the kidney, cleaves angiotensinogen (hepatic origin) to form angiotensin I, a biologically inactive decapeptide. Angiotensin-converting enzyme (ACE), a zinc-dependent dipeptidyl carboxypeptidase found predominantly on pulmonary capillary endothelium but also expressed in vascular endothelium, kidneys, and heart, catalyzes the conversion of angiotensin I to angiotensin II, an octapeptide that serves as the primary bioactive effector molecule. This conversion represents the rate-limiting step in the RAAS and is the target of ACE inhibitors. Notably, angiotensin II can also be generated through alternative pathways involving chymase and other serine proteases, which explains why ARBs are sometimes preferred in certain clinical contexts—they block angiotensin II action regardless of its synthetic route.

• Angiotensin II Receptor Signaling and Vascular Effects: Angiotensin II exerts its systemic effects through two main receptor subtypes: AT1 receptors (predominantly responsible for pathological effects including vasoconstriction, sympathetic activation, and inflammation) and AT2 receptors (generally counterregulatory, promoting vasodilation and anti-inflammatory effects). ACE inhibitors reduce angiotensin II production, thereby decreasing stimulation of both AT1 and AT2 receptors; ARBs selectively block AT1 receptors while potentially allowing compensatory AT2 receptor stimulation (a theoretical advantage, though clinical significance remains debated). AT1 receptor activation triggers multiple intracellular signaling cascades: (1) phospholipase C activation, increasing inositol trisphosphate and diacylglycerol, leading to intracellular calcium mobilization and vascular smooth muscle contraction; (2) protein kinase C activation, promoting MAPK/ERK1/2 phosphorylation and cell proliferation; (3) NADPH oxidase activation, generating reactive oxygen species (ROS) that promote oxidative stress, inflammation, and endothelial dysfunction; (4) JAK/STAT pathway activation, driving pro-inflammatory gene transcription. By blocking these pathways, ACE inhibitors and ARBs reduce immediate vasoconstriction and chronic vascular remodeling. Additionally, angiotensin II stimulates the sympathetic nervous system via pre-ganglionic and ganglionic mechanisms, increasing norepinephrine release; RAAS blockade reduces this sympathetic overdrive, contributing to blood pressure reduction and improved autonomic balance.

• Aldosterone Suppression and Sodium-Volume Homeostasis: Angiotensin II is the primary physiologic stimulus for aldosterone synthesis and release from adrenal zona glomerulosa cells. Aldosterone binds to mineralocorticoid receptors on principal cells of the collecting duct, promoting epithelial sodium channel (ENaC) expression and activity, leading to enhanced sodium reabsorption and water retention by osmotic coupling. It also activates Na-K-ATPase and increases ROMK (renal outer medullary potassium) channel reabsorption, resulting in net hypokalemia as potassium is secreted into the tubular lumen. By reducing aldosterone synthesis, ACE inhibitors and ARBs decrease sodium reabsorption, promoting natriuresis and reducing extracellular fluid volume—a crucial mechanism in hypertension and heart failure. Notably, aldosterone suppression leads to hyperkalemia as a consequence of reduced urinary potassium excretion, a critical adverse effect requiring monitoring, especially in patients with renal impairment or those receiving concurrent potassium supplementation or other potassium-sparing agents.

• Bradykinin Accumulation and Cough/Angioedema: ACE is identical to kininase II, the primary enzyme responsible for degradation of bradykinin, an inflammatory mediator derived from high-molecular-weight kininogen through the contact activation (intrinsic coagulation) pathway. Bradykinin promotes vasodilation through endothelial B2 receptor stimulation (increasing nitric oxide and prostacyclin production), increases vascular permeability, and stimulates sensory nerve endings in airways and throat, triggering cough and potentially angioedema. ACE inhibitors, but not ARBs, accumulate bradykinin by inhibiting its degradation; this bradykinin accumulation accounts for the characteristic dry, persistent cough affecting 5-10% of ACE inhibitor users and the rare but serious complication of angioedema (incidence 0.1-0.5%, higher in African American and Asian populations, potentially related to genetic polymorphisms in ACE and bradykinin receptor genes). ARBs lack this effect because they do not inhibit kininase II, making them a suitable alternative in bradykinin-mediated adverse effects. Angioedema is typically characterized by subcutaneous or mucosal swelling affecting lips, tongue, throat, and sometimes gastrointestinal tract, occurring within hours to weeks of drug initiation, and represents an absolute contraindication to further ACE inhibitor use due to risk of airway compromise.

• Hemodynamic Effects and Renal Protection: RAAS blockade produces a characteristic pattern of hemodynamic changes: systemic arterial vasodilation (reducing peripheral vascular resistance and mean arterial pressure), preferential efferent arteriolar dilation in the kidney (greater dilation than afferent arteriolar dilation, reducing glomerular capillary hydrostatic pressure), and mild reduction in cardiac preload (due to natriuresis and venous dilation). In the kidney, angiotensin II preferentially constricts efferent arterioles more than afferent arterioles; ACE inhibitors and ARBs preferentially dilate efferent arterioles, reducing intraglomerular pressure and proteinuria—a mechanism distinct from the blood pressure-lowering effect and particularly valuable in diabetic nephropathy where glomerular hyperfiltration drives progressive kidney disease. Over time, reduced intraglomerular pressure, decreased angiotensin II-mediated fibroblast activation and TGF-β production, and reduced oxidative stress and inflammation combine to slow the progression of glomerulosclerosis and tubulointerstitial fibrosis. In heart failure, chronic ACE inhibition reduces cardiac afterload and preload, decreasing myocardial wall stress and oxygen demand; importantly, it also prevents/reverses cardiac remodeling by blocking angiotensin II-mediated cardiomyocyte hypertrophy, apoptosis, and fibroblast proliferation, improving ejection fraction and reducing sudden cardiac death risk.

• Nitric Oxide Enhancement and Endothelial Function: Beyond RAAS blockade, ACE inhibitors may enhance endothelial function by increasing bradykinin (which stimulates endothelial B2 receptors to produce nitric oxide and prostacyclin), reducing angiotensin II-mediated oxidative stress (ROS inactivates nitric oxide), and improving insulin sensitivity (reducing insulin resistance-related endothelial dysfunction). These pleiotropic effects contribute to improved vascular compliance, reduced atherosclerotic progression, and cardioprotection beyond simple blood pressure reduction—a concept supporting the use of ACE inhibitors/ARBs even in hypertensive patients with relatively normal blood pressures but high cardiovascular risk or organ damage.

• Essential Hypertension: Essential (primary) hypertension, accounting for 90-95% of hypertension cases in developed nations, involves complex interactions between genetic predisposition (heritability ~50%), environmental factors (sodium intake, obesity, stress, alcohol, physical inactivity), and neurohumoral dysregulation including RAAS activation. Overactivity of the RAAS contributes to hypertension in approximately 10-20% of hypertensive patients, particularly in those with low-renin hypertension (suggesting volume-dependent mechanisms) or high-renin hypertension (suggesting RAAS-dependent mechanisms); however, even in apparently "low-renin" hypertension, ACE inhibitors/ARBs provide benefit through local tissue RAAS blockade and non-blood pressure mechanisms. Risk factors for RAAS-dependent hypertension include renal artery stenosis, chronic kidney disease, aldosteronism, and genetic polymorphisms in ACE, angiotensinogen, and AT1 receptor genes.

• Diabetic Nephropathy and Chronic Kidney Disease: Diabetes mellitus, the leading cause of end-stage renal disease worldwide, promotes glomerulosclerosis and tubulointerstitial fibrosis through hyperglycemia-induced oxidative stress, advanced glycation end-product (AGE) formation, and RAAS activation. In diabetic kidneys, intrarenal angiotensin II is markedly elevated, promoting mesangial cell proliferation, podocyte injury, glomerular basement membrane thickening, and proteinuria. ACE inhibitors and ARBs are particularly indicated in diabetic patients because they reduce glomerular capillary pressure and protein filtration independent of systemic blood pressure lowering, slowing progression from microalbuminuria to proteinuria to ESRD—a mechanism demonstrated across multiple landmark trials (Collaborative Study Group Trial of Captopril in Insulin-Dependent Diabetes Mellitus). Risk factors for rapid diabetic nephropathy progression include poor glycemic control, hypertension, elevated baseline proteinuria, and genetic predisposition (familial clustering of ESRD in diabetes families).

• Heart Failure (Reduced Ejection Fraction): Heart failure with reduced ejection fraction (HFrEF, EF ≤40%) involves multiple compensatory mechanisms including RAAS activation, which initially maintains cardiac output through increased contractility and systemic vasoconstriction but ultimately promotes maladaptive remodeling. The transition from acute decompensation to chronic progression is driven by sustained angiotensin II-mediated cardiomyocyte loss (apoptosis and necrosis), interstitial fibrosis, and chamber dilation. Landmark trials (CONSENSUS, SOLVD, AIRE) demonstrated that ACE inhibitors reduce mortality by 15-27% in HFrEF, making them foundational therapy; ARBs provide similar benefits in ACE inhibitor-intolerant patients. Risk factors for HFrEF progression include ischemic etiology (prior MI), hypertension, diabetes, and genetic predisposition (familial cardiomyopathy mutations).

• Acute Coronary Syndrome and Post-MI Cardioprotection: Following acute myocardial infarction, acute RAAS activation serves as a compensatory mechanism to maintain perfusion but drives adverse ventricular remodeling (dilation, wall thinning) in the infarct zone and remote myocardium, promoting progressive dysfunction and sudden cardiac death. ACE inhibitors initiated within hours to days post-MI reduce 30-day mortality by ~7% and long-term mortality by ~20%, with benefit stratified by risk (greatest in anterior MI, Killip class II-III, EF <40%), as demonstrated in multiple trials (ISIS-4, GISSI-3, SAVE). ARBs provide similar benefit in ACE inhibitor-intolerant patients or as additive therapy with ACE inhibitors in high-risk subgroups.

• Hypertensive Emergency and Acute Decompensated Heart Failure: While not typically first-line acute agents, ACE inhibitors (particularly enalapril IV) and ARBs (losartan) can be used in hypertensive emergencies with left ventricular dysfunction or acute pulmonary edema due to their afterload-reducing properties; however, short-acting agents like sodium nitroprusside, nitroglycerin, or nicardipine are generally preferred due to more rapid and titratable hemodynamic effects.

• Proteinuric Kidney Disease (Non-Diabetic): Beyond diabetic nephropathy, ACE inhibitors and ARBs reduce proteinuria and slow progression in diverse proteinuric glomerulonephritides, including IgA nephropathy, membranoproliferative glomerulonephritis, and focal segmental glomerulosclerosis, through mechanisms of glomerular capillary pressure reduction and anti-inflammatory/anti-fibrotic effects independent of blood pressure lowering.

• Left Ventricular Hypertrophy: Chronic hypertension drives left ventricular hypertrophy (LVH) through sustained pressure overload and angiotensin II-mediated cardiomyocyte growth. ACE inhibitors and ARBs regress LVH more effectively than other antihypertensive classes (approximately 13% reduction in left ventricular mass index vs. 5% for beta-blockers), potentially through direct anti-growth signaling independent of blood pressure reduction.

Therapeutic Effects (Intended Outcomes)

• Blood Pressure Reduction: ACE inhibitors and ARBs reduce systolic and diastolic blood pressure by 8-15 mmHg and 5-10 mmHg, respectively, with dose-dependent effects and individual variability based on baseline renin activity, salt sensitivity, renal function, and genetic factors. The blood pressure-lowering effect develops gradually over 2-4 weeks, with maximal effect at 4-8 weeks, reflecting the time required for RAAS downregulation and vascular remodeling. Patients may initially experience dizziness, lightheadedness, or syncope due to orthostatic hypotension, particularly in volume-depleted states or in conjunction with diuretics; this "first-dose effect" occurs in <5% of patients but warrants caution in elderly or renally compromised individuals.

• Reduction in Proteinuria and Slowing of Renal Decline: In patients with chronic kidney disease and proteinuria (>0.5 g/day), ACE inhibitors and ARBs typically reduce proteinuria by 30-50% within 4-12 weeks, independent of blood pressure reduction. This antiproteinuric effect is mediated by glomerular capillary pressure reduction and is a strong predictor of renoprotection—studies demonstrate that patients achieving >30% proteinuria reduction experience slower decline in glomerular filtration rate (GFR). Baseline serum creatinine may transiently increase by 10-30% within weeks of initiation; this reflects the reduction in intraglomerular pressure and is generally reversible and does not indicate progressive kidney disease (distinct from acute kidney injury, which occurs with severe renal artery stenosis or profound volume depletion).

• Improvement in Heart Failure Symptoms: In patients with HFrEF, ACE inhibitors and ARBs improve exercise tolerance, reduce dyspnea, decrease orthopnea and paroxysmal nocturnal dyspnea, and improve overall functional class (New York Heart Association classification) within 4-12 weeks. These symptomatic improvements reflect reduced cardiac afterload and preload (due to systemic and pulmonary vasodilation and natriuresis) and improved coronary perfusion pressure.

• Prevention of Ventricular Remodeling Post-MI: Following acute MI, ACE inhibitor therapy prevents left ventricular dilation and dysfunction in the weeks to months post-event, reducing the incidence of subsequent heart failure development from approximately 20% to 10-12% in high-risk populations.

Adverse Effects (Unintended Consequences)

• Persistent Dry Cough: A dry, irritating cough develops in 5-10% of ACE inhibitor recipients, typically within weeks to months of initiation, and

Class effects (both ACE inhibitors and ARBs)

• Hyperkalemia: aldosterone suppression reduces principal-cell ENaC/ROMK-driven potassium secretion, producing hyperkalemia, and in susceptible patients (CKD, diabetes) a hyperchloremic non-anion-gap acidosis resembling type 4 RTA. Risk multiplies with CKD, diabetes, potassium supplements, MRAs, trimethoprim-sulfamethoxazole, and NSAIDs.
• Functional acute kidney injury: efferent arteriolar dilation lowers glomerular capillary pressure, so GFR falls when perfusion is already marginal — bilateral renal artery stenosis (or stenosis in a solitary kidney), volume depletion, or concurrent NSAID plus diuretic (the triple whammy). A creatinine rise up to roughly 30% that plateaus is hemodynamic and expected; a larger or progressive rise warrants holding the drug and evaluating for stenosis or hypovolemia.
• Symptomatic hypotension: most pronounced with the first dose in high-renin, diuretic-treated, or hyponatremic heart failure patients.
• Fetal toxicity: FDA boxed warning. Second- and third-trimester exposure causes fetal renal hypoperfusion and anuria with oligohydramnios, limb contractures, pulmonary hypoplasia, and calvarial hypoplasia (Potter sequence phenotype). ACOG advises discontinuing RAAS blockers when pregnancy is recognized; all ACE inhibitors, including captopril, are contraindicated — captopril's short half-life makes it useful for rapid titration, not for pregnancy.

ACE-inhibitor-specific (bradykinin-mediated)

• Dry cough and angioedema: kininase II inhibition raises bradykinin and substance P. Angioedema is non-pruritic, non-urticarial, may be delayed months to years, and is disproportionately common in Black patients.
• Captopril's sulfhydryl group: dysgeusia, rash, and rare neutropenia/agranulocytosis.

Contraindications and monitoring

• Absolute contraindications: pregnancy, prior ACE-inhibitor angioedema or hereditary angioedema, bilateral renal artery stenosis. Do not combine ACE inhibitor plus ARB plus/minus aliskiren — ACC/AHA hypertension and AHA/ACC/HFSA heart failure guidance cite excess hyperkalemia, hypotension, and AKI without benefit; aliskiren is specifically contraindicated with RAAS blockers in diabetes.
• Monitoring: recheck serum creatinine and potassium after initiation or dose escalation — KDIGO specifies within 2–4 weeks; many US practices check at 1–2 weeks — then periodically thereafter. The ADA Standards of Care likewise call for periodic creatinine and potassium monitoring. Counsel on sick-day withholding during vomiting, diarrhea, or hypovolemia.

Reversal

• No specific antidote: stop the drug and give volume for hypotension.
• Angioedema: airway assessment comes first, and early intubation (with surgical airway readiness for tongue/laryngeal involvement) should not be delayed while awaiting a drug response, because bradykinin-mediated swelling responds poorly to pharmacotherapy. IM epinephrine 0.3–0.5 mg of 1:1000 into the anterolateral thigh is given if airway compromise is suspected, though the mechanism is bradykinin rather than histamine, so antihistamines and corticosteroids are typically ineffective. Icatibant and C1-esterase inhibitor have been used off-label with mixed trial results.
• Hyperkalemia: IV calcium gluconate for ECG changes, insulin with dextrose to shift, and potassium binders (patiromer, sodium zirconium cyclosilicate) to permit continued RAAS blockade.

• Cough is bradykinin, not histamine: a persistent nocturnal dry cough weeks after starting lisinopril is the classic stem. Single best next step: switch to an ARB (losartan, valsartan), which does not inhibit kininase II. Distractor to avoid — adding an antitussive or blaming reflux/asthma before stopping the drug.
• Angioedema is a hard stop: never rechallenge with any ACE inhibitor. Onset can be years after initiation, so recent drug initiation is not required. An ARB may be considered cautiously later, but the exam answer for acute swelling is airway first, then discontinue.
• Bilateral renal artery stenosis: the buzzword stem is an older smoker with an abdominal bruit whose creatinine jumps sharply days after starting an ACE inhibitor. Mechanism: loss of angiotensin II–mediated efferent tone in a kidney dependent on it. A modest (~≤30%) creatinine rise that stabilizes is expected and is not a reason to stop.
• Pregnancy: RAAS blockers are contraindicated in all trimesters; the fetal picture is oligohydramnios, renal failure, and calvarial hypoplasia. ACOG-preferred agents in pregnancy are labetalol, nifedipine, or methyldopa.
• The one association examiners love: scleroderma renal crisis — abrupt malignant hypertension with microangiopathic hemolysis and rising creatinine in diffuse systemic sclerosis. Treatment is a short-acting ACE inhibitor (captopril), started even when creatinine is already elevated. ARBs are not equivalent here.
• Heart failure framing: per the 2022 AHA/ACC/HFSA guideline, guideline-directed therapy for HFrEF is four classes — ARNI (preferred over ACE inhibitor/ARB), beta blocker, MRA, and SGLT2 inhibitor. Switching from an ACE inhibitor to sacubitril/valsartan requires a 36-hour washout to avoid additive bradykinin-mediated angioedema.
• Albuminuria drives the choice: ADA and KDIGO recommend an ACE inhibitor or ARB (not both) for hypertensive patients with diabetes and albuminuria, titrated to the maximum tolerated dose — the renoprotection is from lowered intraglomerular pressure, partly independent of systemic BP.
• Common distractor: ARBs still cause hyperkalemia, fetal toxicity, and hemodynamic AKI. Only the cough and angioedema risk fall meaningfully when switching classes.