Hyperkalaemia

Normal level of Potassium is vital for regulating the normal electrical activity of the heart.
Increased extracellular potassium level reduces myocardial excitability, with depression of both pacemaking and conducting tissues.

Progressively worsening hyperkalaemia leads to suppression of impulse generation by the SA node and reduced conduction by the AV node and His-Purkinje system, resulting in bradycardia and conduction blocks and ultimately cardiac arrest.

Definitions

Hyperkalaemia is defined as a potassium level > 5.5 mEq/L

Moderate hyperkalaemia is a serum potassium > 6.0 mEq/L

Severe hyperkalaemia is a serum potassium > 7.0 mE/L

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Effects of hyperkalaemia on the ECG

Serum potassium > 5.5 mEq/L is associated with repolarization abnormalities:

Peaked T waves (usually the earliest sign of hyperkalaemia)

Serum potassium > 6.5 mEq/L is ASSOCIATED

with progressive paralysis of the atria:

P wave widens and flattens

PR segment lengthens

P waves eventually disappear

Serum potassium > 7.0 mEq/L is ASSOCIATED

with conduction abnormalities and bradycardia:

Prolonged QRS interval with bizarre QRS morphology

High-grade AV block with slow junctional and ventricular escape rhythms

Any kind of conduction block (bundle branch blocks, fascicular blocks)

Sinus bradycardia or slow AF

Development of a sine wave appearance (a pre-terminal rhythm)

Serum potassium level of > 9.0 mEq/L causes cardiac arrest due to: