Toxicology – Hyperkalemia

Definition

Hyperkalemia is an abnormally elevated serum potassium concentration, generally >5.0–5.5 mmol/L, depending on the laboratory reference range.

The major danger is disruption of cardiac conduction and neuromuscular function, potentially causing fatal dysrhythmias.

Pathophysiology

Potassium is predominantly intracellular. An increase in extracellular potassium alters the resting membrane potential of excitable cells.

Progressive hyperkalemia causes:

Membrane depolarization → sodium-channel inactivation → impaired conduction → muscle weakness and cardiac conduction abnormalities

Severe hyperkalemia can progress to:

• Bradycardia
• AV block
• Wide-complex rhythms
• Ventricular dysrhythmias
• Cardiac arrest

Major Mechanisms

Hyperkalemia generally results from one or more of four mechanisms:

1. Pseudohyperkalemia

• Hemolysis during blood collection
• Prolonged tourniquet use or fist clenching
• Marked thrombocytosis
• Marked leukocytosis
• Specimen handling problems

2. Reduced renal potassium excretion

• Acute kidney injury
• Chronic kidney disease
• Hypoaldosteronism
• Adrenal insufficiency

3. Potassium shift from cells into plasma

• Acidemia
• Insulin deficiency
• Hyperosmolar states
• Digoxin toxicity
• Extensive tissue injury

4. Increased potassium load

• Potassium supplements
• Excessive IV potassium
• Massive cellular destruction
• Tumor lysis syndrome

Significant potassium intake alone rarely produces severe hyperkalemia when renal function is normal.

Drug-Related Causes

Important medications associated with hyperkalemia include:

• ACE inhibitors
• Angiotensin-receptor blockers
• Potassium-sparing diuretics
• NSAIDs
• Trimethoprim
• Heparin
• Beta-blockers
• Calcineurin inhibitors
• Potassium supplements

Digoxin toxicity is particularly important in toxicology.

Tissue Destruction

Potassium is released from damaged cells.

Causes include:

• Rhabdomyolysis
• Crush injury
• Severe seizures
• Hyperthermia
• Tumor lysis
• Hemolysis

Severe sympathomimetic poisoning can indirectly cause hyperkalemia through hyperthermia, seizures, and rhabdomyolysis.

Digoxin Toxicity

Acute digoxin poisoning can produce significant hyperkalemia because inhibition of the Na⁺/K⁺-ATPase prevents normal cellular potassium uptake.

Associated findings may include:

• Nausea and vomiting
• Bradycardia
• AV block
• Ventricular dysrhythmias
• Visual abnormalities
• Hyperkalemia

In acute digoxin poisoning, hyperkalemia can be an important marker of severe toxicity.

Clinical Features

Mild hyperkalemia may be completely asymptomatic.

Neuromuscular manifestations can include:

• Paresthesias
• Muscle weakness
• Reduced reflexes
• Flaccid paralysis in severe cases

Cardiovascular manifestations include:

• Bradycardia
• Hypotension
• Conduction abnormalities
• Ventricular dysrhythmias
• Cardiac arrest

ECG Changes

Classically described progression includes:

Peaked T waves → PR prolongation → P-wave flattening/loss → QRS widening → sine-wave pattern → ventricular fibrillation/asystole

However, this progression is not reliably sequential.

A patient with severe hyperkalemia may have relatively modest ECG abnormalities, while dangerous dysrhythmias can occasionally develop without the complete classic sequence.

Therefore, neither the ECG nor potassium concentration alone perfectly predicts cardiac risk.

Pseudohyperkalemia

Always consider whether an unexpectedly elevated potassium result is genuine.

Common clues include:

• Laboratory report of hemolysis
• Difficult venipuncture
• Prolonged tourniquet application
• Marked leukocytosis
• Marked thrombocytosis
• No clinical explanation for the result

When the patient is stable and the result is unexpected, potassium can be repeated using a properly collected sample.

Do not delay emergency treatment to repeat the sample when severe hyperkalemia is clinically credible or ECG toxicity is present.

Evaluation

Important initial investigations include:

• Repeat potassium when pseudohyperkalemia is suspected
• Renal function
• Glucose
• Bicarbonate
• Calcium and magnesium
• ECG
• Continuous cardiac monitoring in significant cases

Additional testing depends on the suspected cause:

• Creatine kinase for rhabdomyolysis
• Digoxin concentration
• Blood gas for significant acid-base disturbance
• CBC when marked leukocytosis or thrombocytosis is possible
• Evaluation for adrenal insufficiency when clinically appropriate

Management Principles

Treatment has three major objectives:

1. Stabilize the myocardium

2. Shift potassium into cells

3. Remove potassium from the body

The urgency depends on the overall clinical picture, particularly:

• ECG abnormalities
• Degree and rate of potassium elevation
• Symptoms
• Renal function
• Ongoing potassium release
• Underlying cause

Severe hyperkalemia with cardiac toxicity is a medical emergency.

1. Cardiac Membrane Stabilization

Intravenous calcium is used when significant hyperkalemia is producing concerning ECG abnormalities.

Calcium:

• Stabilizes the cardiac membrane
• Acts rapidly
• Does not lower serum potassium

Its effect is temporary, so potassium-shifting and elimination therapies must also be initiated.

Calcium gluconate and calcium chloride are both used clinically, with calcium chloride delivering more elemental calcium but carrying greater risk of tissue injury if extravasated.

Calcium and Digoxin Toxicity

Older teaching warned that IV calcium in digoxin poisoning could precipitate catastrophic dysrhythmias—the historical “stone heart” concern.

Modern evidence does not support treating calcium as absolutely contraindicated when a patient with suspected digoxin toxicity has life-threatening hyperkalemia.

However, the definitive treatment for severe digoxin poisoning is digoxin immune Fab, and toxicology consultation is appropriate.

2. Shift Potassium Intracellularly

Insulin with glucose

Insulin activates cellular potassium uptake and can rapidly lower extracellular potassium.

The major complication is hypoglycemia, so glucose must be monitored closely for several hours.

Beta₂-adrenergic agonists

Nebulized beta₂ agonists such as albuterol can promote intracellular potassium uptake.

They are generally used as an adjunct rather than the sole treatment for severe hyperkalemia.

Sodium bicarbonate

Bicarbonate is most useful when hyperkalemia occurs with significant metabolic acidosis.

Its potassium-lowering effect is inconsistent in patients without substantial acidemia, so it should not routinely replace insulin or other established therapies.

3. Remove Potassium

Intracellular shifting is temporary. When total-body potassium is excessive, potassium must ultimately be eliminated.

Methods include:

• Renal excretion when kidney function permits
• Potassium-binding agents in appropriate nonemergent settings
• Hemodialysis

Hemodialysis

Hemodialysis provides rapid and reliable potassium removal.

It is particularly important when there is:

• Severe hyperkalemia with kidney failure
• Refractory hyperkalemia
• Recurrent hyperkalemia despite temporizing treatment
• Severe ongoing potassium release
• Inadequate renal elimination

Emergency membrane stabilization and intracellular shifting should not be delayed while dialysis is being arranged.

Potassium-Binding Resins

The source emphasizes sodium polystyrene sulfonate (SPS/Kayexalate), but its role has changed substantially.

SPS:

• Has a delayed and somewhat unpredictable effect
• Is not appropriate as the sole emergency treatment for life-threatening hyperkalemia
• Can cause significant gastrointestinal adverse effects

Use with sorbitol is particularly problematic because of an association with serious intestinal injury.

Newer potassium binders have roles in selected patients, especially chronic hyperkalemia, but they do not replace immediate cardiac stabilization and rapid potassium-shifting therapy in an unstable patient.

Digoxin-Associated Hyperkalemia

When severe acute digoxin poisoning is responsible:

Digoxin immune Fab is the definitive antidotal treatment.

Fab binds circulating digoxin and reverses inhibition of Na⁺/K⁺-ATPase.

As toxicity reverses, extracellular potassium can move back into cells, so potassium may fall rapidly.

Serial potassium monitoring is therefore essential.

Monitoring

Significant hyperkalemia requires:

• Continuous cardiac monitoring
• Serial ECGs when appropriate
• Serial potassium measurements
• Glucose monitoring after insulin therapy
• Renal-function monitoring

Also identify whether potassium is continuing to enter the circulation from:

• Rhabdomyolysis
• Tumor lysis
• Tissue ischemia
• Ongoing potassium administration

Key Points

• Hyperkalemia can cause rapidly fatal cardiac conduction abnormalities.
• Major mechanisms are reduced renal excretion, intracellular-to-extracellular shifting, increased potassium load, and pseudohyperkalemia.
• Always consider pseudohyperkalemia, particularly with a hemolyzed specimen or unexpected result.
• Do not delay treatment to repeat the potassium when severe hyperkalemia and cardiac toxicity are clinically credible.
• ECG changes classically progress from peaked T waves to QRS widening and a sine-wave pattern, but actual progression is unpredictable.
• Emergency management follows three principles: stabilize the heart → shift K⁺ into cells → remove K⁺ from the body.
• IV calcium stabilizes the myocardium but does not lower potassium.
• Insulin promotes intracellular potassium uptake; monitor carefully for hypoglycemia.
• Bicarbonate is most useful when significant metabolic acidosis accompanies hyperkalemia.
• Hemodialysis is highly effective for severe or refractory hyperkalemia, particularly with renal failure.
• Sodium polystyrene sulfonate is not an adequate emergency treatment, and routine administration with sorbitol is no longer favored.
• Acute digoxin poisoning can cause hyperkalemia through Na⁺/K⁺-ATPase inhibition; severe toxicity is treated with digoxin immune Fab.