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Electrolyte disturbances and ECG changes

Created: 18/8/2006
Updated: 10/8/2006
The normal state of cardiac cell membrane polarisation is dependent upon the maintenance of a normal ionic balance across the membranes, with K+ being the most important. Because changes in intracellular K+ concentration are proportionately much smaller than changes in extracellular K+ concentration, it follows that the absolute level of extracellular K+ concentration is the single most important factor affecting the cell membranes.

Hyperkalaemia: ECG changes

All of the ECG changes that occur with a raised K+ concentration are non-specific and may affect any part of the ECG.

The typical progressive changes of hyperkalaemia are as follows:

  1. Appearance of tall, pointed, narrow T waves. 
  2. Decreased P wave amplitude, decreased  R wave height, widening of QRS complexes, ST segment changes (elevation/depression), hemiblock (esp. left anterior) and 1st degree heart block. 
  3. Advanced intraventricular block (very wide QRS with RBBB, LBBB, bi- or tri-fascicular blocks) and ventricular ectopics.
  4. Absent P waves, very broad, bizarre QRS complexes, AV block, VT, VF or ventricular asystole.

Click here for a larger image

Advanced hyperkalaemia

Marked widenening of the QRS duration combined with tall, peaked T waves are suggestive of advanced hyperkalaemia. Note the absence of P waves, suggesting a junctional rhythm, but in hyperkalaemia the atrial muscle may be paralysed while still in sinus rhythm. The sinus impulse conducts to the AV node through internodal tracts, without activating the atrial muscle.


ECG changes in decreasing order of frequency are: 

  • ST segment depression, decreased T wave amplitude, increased U wave height 
  • Cardiac arrhythmias 
  • Prolongation of the QRS duration, increased P wave amplitude and duration

Various types of arrhythmias may occur in hypokalaemia. These may include atrial and ventricular ectopics, atrial tachycardia, heart blocks, VT and VF.


The main change is reduction in the Q-T interval on the ECG. The T wave duration is unaffected but the ST segment duration is shortened. Patients with hypercalcaemia have an increased sensitivity to digitalis and may present with a variety of arrhythmias.


The main ECG change is prolongation of the Q-T interval. There is no increase in T wave duration but the ST segment is prolonged.


It is useful to remember the following associations:

In hypomagnesaemia, there is flattening of the T waves, ST segment depression, prominent U waves and, occasionally, a prolonged P-R interval occurs.

In hypermagnesaemia, there may be a prolonged P-R interval and widened QRS complexes.

ECG changes of hypomagnesaemia resemble that of hypokalaemia

ECG changes of hypermagnesaemia resemble that of hyperkalaemia

Hypokalaemia, hypomagnesaemia and hypercalcaemia aggravate digitalis toxicity


No significant changes on the ECG occur with hypo-/hypernatraemia.

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