Toxicology – Angiotensin-Converting Enzyme (ACE) Inhibitors

Core Concept

ACE inhibitors are widely used cardiovascular and renal medications. Important examples include:

• Captopril
• Enalapril
• Lisinopril
• Ramipril
• Benazepril
• Fosinopril
• Perindopril
• Quinapril
• Trandolapril

They are used for conditions such as:

• Hypertension
• Heart failure with reduced ejection fraction
• Post-myocardial-infarction ventricular dysfunction
• Selected chronic kidney disease, particularly with albuminuria
• Other conditions involving the renin–angiotensin–aldosterone system (RAAS)

Most isolated ACE-inhibitor overdoses produce mild or moderate hypotension, but severe vasodilatory shock can occur after a very large exposure or when important coingestants or comorbidities are present.

There is no routinely required specific antidote.

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Renin–Angiotensin–Aldosterone System

The RAAS helps maintain:

• Blood pressure
• Vascular tone
• Sodium balance
• Extracellular fluid volume
• Renal perfusion

Reduced renal perfusion stimulates renin release.

The sequence is:

Angiotensinogen → angiotensin I → ACE → angiotensin II

Angiotensin II then:

• Produces vasoconstriction
• Stimulates aldosterone secretion
• Promotes sodium retention
• Supports blood pressure and renal hemodynamics

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Mechanism of ACE Inhibition

ACE inhibitors block conversion of:

Angiotensin I → Angiotensin II

Consequences include:

• Reduced vasoconstriction
• Reduced aldosterone secretion
• Reduced sodium retention
• Reduced systemic vascular resistance
• Lower blood pressure

In overdose, excessive loss of angiotensin-II-mediated vascular tone can cause significant hypotension.

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Bradykinin

ACE also normally degrades bradykinin.

ACE inhibition therefore increases bradykinin activity.

This contributes to:

• Vasodilation
• Dry cough
• ACE-inhibitor-associated angioedema

This mechanism is clinically important because ACE-inhibitor angioedema is bradykinin-mediated, rather than a conventional histamine-mediated allergic reaction.

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Toxic Dose

There is no reliable universal toxic dose.

Many isolated ingestions, including some apparently large ones, produce only limited hypotension.

Severity depends on:

• Specific ACE inhibitor
• Amount and formulation
• Baseline blood pressure
• Hydration status
• Renal function
• Age and frailty
• Coingestants
• Concurrent antihypertensive therapy

Clinical findings are more useful than the reported dose alone.

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Principal Acute Toxicity – Hypotension

The major manifestation of acute overdose is:

Reduced angiotensin II → vasodilation → decreased systemic vascular resistance → hypotension

Possible symptoms include:

• Dizziness
• Weakness
• Lightheadedness
• Orthostatic symptoms
• Syncope

Severe poisoning may produce:

• Persistent hypotension
• Altered mental status
• Oliguria
• Acute kidney injury
• Shock

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Heart Rate

Tachycardia may accompany hypotension, but the response is variable.

Bradycardia is not usually the defining feature of isolated ACE-inhibitor overdose.

If substantial bradycardia occurs, consider:

• β-blocker coingestion
• Calcium-channel blocker coingestion
• Digoxin
• Clonidine
• Conduction disease
• Hyperkalemia
• Other causes

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Hyperkalemia

ACE inhibition reduces aldosterone activity.

This decreases renal potassium excretion and can produce:

Hyperkalemia

Risk is greatest with:

• Chronic kidney disease
• Acute kidney injury
• Potassium supplements
• Potassium-sparing diuretics
• Other RAAS inhibitors
• Significant dehydration or renal hypoperfusion

Hyperkalemia may be more clinically important than the initial blood-pressure abnormality in susceptible patients.

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Renal Effects

Angiotensin II normally constricts the efferent arteriole, helping maintain glomerular filtration when renal perfusion falls.

ACE inhibition reduces this compensatory effect.

Consequently, renal function may deteriorate in settings such as:

• Severe volume depletion
• Renal hypoperfusion
• Bilateral renal artery stenosis
• Advanced kidney disease
• Severe hypotension

Monitor creatinine and urine output in clinically significant poisoning.

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ACE-Inhibitor Angioedema

One of the most important adverse effects is bradykinin-mediated angioedema.

It can involve:

• Lips
• Tongue
• Floor of mouth
• Oropharynx
• Larynx

Severe swelling can cause life-threatening airway obstruction.

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Clinical Pattern of Angioedema

Typical findings include:

• Nonpitting swelling
• Lip or tongue enlargement
• Voice change
• Dysphagia
• Drooling
• Throat tightness
• Stridor in advanced disease

Unlike histamine-mediated anaphylaxis, ACE-inhibitor angioedema often occurs without urticaria or generalized pruritus.

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Timing of Angioedema

Angioedema can occur:

• Soon after starting therapy
• Months later
• Even after years of previously tolerated treatment

Therefore, long-term uneventful use does not exclude an ACE inhibitor as the cause.

Angioedema is an adverse drug reaction and does not require an overdose.

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Airway Management

Airway assessment is the highest priority in significant ACE-inhibitor angioedema.

Concerning findings include:

• Progressive tongue swelling
• Floor-of-mouth swelling
• Voice change
• Drooling
• Dysphagia
• Stridor
• Respiratory distress

If airway compromise is developing:

Secure the airway before swelling progresses to the point that intubation becomes extremely difficult.

Severe cases may require advanced airway expertise and preparation for a surgical airway.

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Epinephrine, Antihistamines, and Steroids

A critical distinction is that ACE-inhibitor angioedema is primarily bradykinin-mediated.

Therefore:

• Antihistamines
• Corticosteroids
• Epinephrine

do not directly reverse the underlying bradykinin mechanism and may have limited benefit in isolated ACE-inhibitor angioedema.

However, if the diagnosis is uncertain and anaphylaxis is possible, epinephrine should not be withheld from a patient with a compatible life-threatening allergic reaction.

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Bradykinin-Targeted Therapies

Several therapies have been studied or used for severe ACE-inhibitor angioedema, including agents that target bradykinin pathways and, in some settings, plasma-derived products.

Evidence has been mixed, and no pharmacologic treatment should delay definitive airway management.

The key principle is:

Airway protection takes precedence over medication.

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ACE-Inhibitor Cough

A persistent dry cough can occur during therapeutic treatment.

This is associated with increased bradykinin and related mediators.

The cough is:

• Usually nonproductive
• Not evidence of pulmonary edema by itself
• Reversible after discontinuation, although resolution may take longer than only a few days in some patients

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Other Chronic Adverse Effects

Less common adverse effects include:

• Dysgeusia, particularly with captopril
• Rash
• Renal dysfunction
• Rare hepatic injury
• Rare hematologic abnormalities

These are not the defining findings of acute overdose.

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Diagnosis

ACE-inhibitor poisoning is usually a clinical diagnosis based on:

• Medication history
• Amount and timing
• Vital signs
• Renal function
• Potassium
• Coingestants

Serum ACE-inhibitor concentrations are not useful for routine emergency management.

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Laboratory Evaluation

A small, asymptomatic isolated exposure may require little testing.

For symptomatic or substantial poisoning, useful tests include:

• Electrolytes
• Potassium
• Bicarbonate
• Creatinine
• Glucose

Additional testing depends on clinical severity and suspected coingestants.

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ECG

An ECG is appropriate when there is:

• Significant hypotension
• Hyperkalemia
• Syncope
• Intentional overdose
• Suspected cardiovascular coingestion

Marked conduction abnormalities are not typical of isolated ACE-inhibitor poisoning and should prompt investigation for:

• Hyperkalemia
• β-blockers
• Calcium-channel blockers
• Sodium-channel blockers
• Other coingestants

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Differential Diagnosis of Hypotension

Toxicologic possibilities include:

• β-blockers
• Calcium-channel blockers
• α₁ antagonists
• Clonidine and other imidazolines
• Nitrates
• Tricyclic antidepressants
• Other vasodilators

Nontoxicologic possibilities include:

• Dehydration
• Sepsis
• Hemorrhage
• Cardiogenic shock
• Adrenal crisis
• Anaphylaxis
• Autonomic dysfunction

Severe or unusual findings should not automatically be attributed to the ACE inhibitor.

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Initial Management

Management centers on:

Airway assessment → circulation/perfusion → renal and potassium evaluation → supportive care

For uncomplicated isolated overdose, supportive treatment is usually sufficient.

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IV Fluids

Hypotension may respond to appropriate isotonic crystalloid, especially when volume depletion is contributing.

However, fluid administration should be individualized in patients with:

• Heart failure
• Renal impairment
• Pulmonary edema

The goal is restoration of adequate perfusion rather than administration of a fixed volume.

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Trendelenburg – Modern Correction

Routine Trendelenburg positioning is obsolete for treatment of hypotension.

It provides no reliable sustained improvement in organ perfusion and may worsen respiratory mechanics.

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Vasopressors

Persistent shock despite appropriate initial fluid resuscitation may require vasopressor therapy.

Norepinephrine is generally a reasonable first-line vasopressor for persistent vasodilatory shock.

The historical dopamine-first approach is no longer routinely preferred.

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Refractory ACE-Inhibitor Shock

Rare severe overdose can produce hypotension that is unusually resistant to conventional vasopressors because the RAAS pathway itself is blocked.

Angiotensin II has been used in selected cases of severe refractory ACE-inhibitor-associated vasodilatory shock.

This is a specialist/critical-care intervention rather than routine therapy for ordinary ACE-inhibitor ingestion.

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Atropine

Atropine is useful only when clinically significant bradycardia contributes to poor perfusion.

It is not an antidote to ACE inhibition and should not be routinely administered simply because the patient is hypotensive.

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Hyperkalemia Management

Clinically important hyperkalemia should be managed according to severity.

Principles include:

• ECG assessment
• Cardiac membrane stabilization when indicated
• Intracellular potassium shifting
• Correction of contributing abnormalities
• Potassium removal when required
• Dialysis in selected severe cases, particularly with renal failure

Treatment is based on the potassium level, ECG, clinical condition, renal function, and trajectory.

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GI Decontamination

Do not induce vomiting.

Routine gastric lavage is obsolete.

Activated charcoal may occasionally be considered after a substantial recent ingestion when:

• Presentation is sufficiently early
• The airway is safe
• Expected benefit outweighs aspiration risk

Most uncomplicated ACE-inhibitor exposures do not require aggressive GI decontamination.

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No Routine Specific Antidote

There is no standard antidote required for ordinary ACE-inhibitor poisoning.

Management consists primarily of:

• Hemodynamic support
• Renal monitoring
• Hyperkalemia treatment when necessary
• Airway management for angioedema
• Treatment of coingestants

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Observation

The older universal 4–6-hour observation rule should not be applied mechanically.

Observation depends on:

• Specific agent
• Amount
• Formulation
• Symptoms
• Blood-pressure trajectory
• Renal function
• Potassium
• Coingestants

Clinically important hypotension usually develops relatively early after immediate-release ingestion, but persistent toxicity can occur after substantial exposures or in patients with impaired physiology.

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Admission

Hospital management is appropriate for:

• Persistent hypotension
• Shock
• Significant hyperkalemia
• Acute kidney injury
• Altered mental status
• Significant coingestion
• Angioedema involving the tongue, floor of mouth, pharynx, or larynx
• Any threatened airway

Severe shock or airway compromise warrants intensive care.

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Pregnancy – Major Modern Correction

The historical FDA pregnancy letter categories are obsolete.

ACE inhibitors are generally avoided during pregnancy, particularly because fetal RAAS blockade later in pregnancy can cause serious fetal toxicity.

Potential complications include:

• Fetal renal dysfunction
• Oligohydramnios
• Impaired skull ossification
• Pulmonary developmental complications secondary to oligohydramnios
• Neonatal renal failure
• Fetal or neonatal death in severe exposure

Pregnancy exposure should prompt obstetric and medication review rather than reliance on the old “Category D” designation.

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Pregnancy Testing

The older statement that every woman of childbearing age must undergo pregnancy testing before ACE-inhibitor therapy is overly rigid as a universal toxicology rule.

Medication counseling and pregnancy assessment should be individualized according to clinical circumstances and current prescribing guidance.

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Safeguarding

Rigid historical age cutoffs for neglect, abuse, or intentional poisoning are inappropriate.

Pediatric exposure should instead be evaluated according to:

• Developmental ability
• Medication accessibility
• Circumstances
• Consistency of the history
• Recurrent unexplained poisoning
• Other safeguarding concerns

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Prognosis

Most isolated ACE-inhibitor overdoses have a favorable outcome with supportive care.

Poor outcomes are more likely with:

• Massive exposure
• Severe persistent hypotension
• Significant hyperkalemia
• Acute kidney injury
• Serious cardiovascular coingestants
• Prolonged hypoperfusion
• Airway compromise from angioedema

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Important Modernization of the Older Source

• ACE inhibitors reduce angiotensin II and aldosterone while increasing bradykinin activity.
• Acute overdose primarily causes vasodilatory hypotension.
• Hyperkalemia and renal dysfunction are especially important in susceptible patients.
• ACE-inhibitor angioedema is predominantly bradykinin-mediated, not classic histamine-mediated allergy.
• Angioedema may develop even after years of therapy.
• Airway management is the priority in progressive tongue/pharyngeal/laryngeal swelling.
• Antihistamines and corticosteroids do not directly reverse bradykinin-mediated angioedema.
• If true anaphylaxis remains possible, epinephrine should still be used appropriately.
• Routine serum ACE-inhibitor concentrations have no clinical role.
• Trendelenburg positioning is obsolete.
• Norepinephrine is generally favored over dopamine for persistent vasodilatory shock.
• Angiotensin II has a specialized role in selected refractory shock.
• Atropine is useful only when clinically important bradycardia is actually present.
• Ipecac and routine gastric lavage are obsolete.
• Activated charcoal has only a selective early role.
• Fixed observation periods should be replaced by symptom-, renal-, potassium-, and exposure-based assessment.
• Historical FDA pregnancy letter categories are obsolete.

Key Points

• ACE inhibition → ↓ angiotensin II → vasodilation and hypotension.
• ↓ aldosterone → risk of hyperkalemia.
• Reduced efferent arteriolar constriction can worsen renal function in susceptible patients.
• ↑ bradykinin → cough and angioedema.
• Most isolated overdoses are relatively mild, but severe vasodilatory shock can occur.
• Monitor blood pressure, potassium, creatinine, and ECG when clinically indicated.
• Treat hypotension supportively; persistent vasodilatory shock may require a vasopressor.
• Progressive ACE-inhibitor angioedema is primarily an airway emergency.
• There is no routinely required specific antidote for uncomplicated ACE-inhibitor overdose.