ECG not only helps in the diagnosis of hyperkalemia but indicates the severity of the condition as well. This is a fatal condition and a real medical emergency which requires quick diagnosis. This is when ECG often comes handy.
Definition: Serum potassium (K+) > 5 mEq/l
Electrophysiologic basis of ECG changes:
a. Mild hyperkalemia: causes elevation of the Maximum Diastolic Potential (MDP) in the SA node. It can sometimes have positive chronotropic effects.
b. Severe hyperkalemia: the more positive MDP leads to the inactivation of Na+ channels and to a reduction in the slope and amplitude of APs in the AV node (negative dromotropic effect). Moreover, the K+ conductance (gK) rises, and the Prepotential (slow depolarization) slope becomes flatter due to a negative chronotropic effect. Faster myocardial repolarization decreases the cytosolic Ca2+ conc.
c. Extreme cases: the pacemaker is also brought to a standstill (cardiac paralysis).
- At K = 5.0 to 6.0 mEq/L, rapid repolarization causes peaked T waves (best seen in leads V2 to V4).
- At K = 6.0 to 6.5 mEq/L, decrease in conduction causes prolonged PR and QT intervals.
- At K = 6.5 to 7.0 mEq/L, P waves are diminished and ST segment may be depressed.
- At K = 7.0 to 8.0 mEq/L, P waves disappear, QRS widens, and irregular idioventricular rhythm appears.
- At K = 8.0 to 10.0 mEq/L, QRS merges with T wave to produce classic sine wave (QRS-T fusion – a sinusoidal waveform).
- At K = 10.0 to 12.0 mEq/L, ventricular fibrillation and diastolic arrest occur.
Note: Hyperkalemia causes tall, peaked (tented) T waves. ECG signs may be absent if the onset of hyperkalemia is slow – as seen in chronic renal failure – even though the serum potassium is in the range of 7 – 7.5meq/L.