Besides helping in the diagnosis of hyperkalemia, ECG also helps in determining its severity.
High potassium levels produce certain changes, which manifest on the graph or strip of the 12 lead electrogram or ECG.
How hyperkalemia affects the heart and shows ECG changes?
Hyperkalemia (too much potassium in the blood) is caused by a shift of potassium from the intracellular compartment to the extracellular compartment (which is the bloodstream).
98 percent of the body potassium is stored in the various body cells such as the cells of the muscles, bone and the body organs. Only 2 percent circulates in the blood and the extracellular fluid.
Too much potassium in the circulating blood changes the voltage or the electrical stability of the cardiac cell membrane.
It reduces myocardial sensitivity and that of SA node, the AV node and conducting tissues.
The cardiac muscle cells then become more excitable, which causes irregular beating of the heart (arrhythmias) and corresponding ECG changes.
Hyperkalemia can be mild, moderate or severe. All these types can produce changes in the electrocardiogram.
As the hyperkalemia progresses, it leads to the depression of the SA node leading to reduced impulse generation. It also leads to reduced impulse conduction by the AV node, the bundle of His, bundle branches, and Purkinje fibers.
All these are parts of the heart’s electric impulse conduction system called His-Purkinje system.
This results in bradycardia, conduction blocks and finally cardiac arrest.
Progressive hyperkalemia and corresponding ECG changes
Normal potassium levels in the blood are between 3.6 to 5.2 millimoles per liter (mmol/L) or 3.5 and 5.0 milliEquivalents per liter (mEq/L).
Hyperkalemia is classified as
- mild (serum K 5.5–6.0 mEq/L),
- moderate (serum K 6.1–6.9 mEq/L ) or
- severe (serum K >7.0 mEq/L).
As potassium levels rise, certain manifestations on the ECG graph begin to show.
Potassium blood level greater than 7 mEq/L can lead to significant complications in the hemodynamic and neurologic functions.
Levels exceeding 8.5 mEq/L can lead to respiratory paralysis or cardiac arrest and can immediately be fatal.
ECG changes in hyperkalemia have an orderly progression, which roughly corresponds with the serum potassium level.
Serum potassium > 5.5 mEq/L
Early ECG changes in hyperkalemia with serum potassium more than 5.5 mEq/L is associated with repolarization abnormalities. They include
- Peaked T waves with a narrow base more evident in the precordial leads. This is usually the earliest sign of hyperkalemia.
- Shorter QT interval
- ST-segment depression
Serum potassium > 6.5 mEq/L
With serum potassium > 6.5 mEq/L, there is progressive paralysis of the atria
Changes seen will be
- Wide and flat or disappearing P wave
- Longer PR segment
- Absent P waves eventually
Serum potassium > 7.0 mEq/L
With serum potassium > 7.0 mEq/L conduction abnormalities and associated bradycardia present
- Prolonged QRS interval with an extraordinary QRS complex
- Severe AV block with slow junctional and irregular ventricular rhythms
- Sinus bradycardia or slow atrial fibrillation
- Development of a sine wave appearance (a pre-terminal rhythm)
S. potassium > 8.0 mEq/L
At a serum potassium level higher than 8.0 mEq/L, the ECG shows the following changes:
- Absent P wave
- Progressive widening of the QRS complex
- Conduction block (Intraventricular, bundle branch, fascicular blocks)
Serum potassium > 9.0 mEq/L
At this level, hyperkalemia causes cardiac arrest. On ECG you will see
- Asystole (heart stops beating)
- Ventricular fibrillation
- Pulseless electrical activity (PEA) with peculiar, wide complex rhythm
A word of extreme caution
Though ECG findings generally compare with the potassium level, the above facts do not always hold true. In some patients, the serum potassium level may not correlate with the ECG changes as explained above.
It has been seen that patients with relatively normal ECGs may develop sudden hyperkalaemic cardiac arrest and life-threatening cardiac arrhythmias can occur without a warning at almost any potassium level in hyperkalemia.