Potassium Imbalances: ECG Recognition and Clinical Significance

1. Flattened T waves: Because the repolarization process is altered, the T wave loses its normal amplitude and appears shallow or flat.
2. U waves: This is a hallmark sign. A U wave is a small deflection that follows the T wave. In hypokalemia, this wave becomes prominent and easily distinguishable from the T wave.
3. ST Segment Depression: In more severe cases, the ST segment may dip below the isoelectric line.

Exam Tip: If a question asks you to identify a rhythm characterized by a "prominent wave following the T wave," think Hypokalemia and look for the U wave.

Hyperkalemia: High Potassium Levels

Hyperkalemia occurs when serum potassium levels exceed the normal range (typically $> 5.0$ mEq/L). This is arguably more dangerous in the acute setting because high extracellular potassium can prevent cells from repolarizing properly, potentially leading to cardiac arrest.

ECG Manifestations of Hyperkalemia

Hyperkalemia is often described as a progressive series of ECG changes that worsen as potassium levels rise. The most recognizable early sign is:

1. Peaked T waves: Unlike the flat T waves seen in hypokalemia, hyperkalemia produces T waves that are tall, narrow, and "tented" or "peaked".
2. Widened QRS Complex: As the potassium levels continue to climb, the conduction through the ventricles slows down, causing the QRS complex to widen.
3. Loss of P waves: In severe cases, the atrial activity (P wave) may disappear entirely as the heart enters a state of electrical instability.

Summary Comparison Table

To help differentiate these two critical states, use the following comparison of their primary ECG findings:

Clinical Application: A Worked Example

Scenario: You are reviewing an ECG for a patient who has been taking potent loop diuretics (which can cause potassium loss). You notice the T waves are very shallow and there is a distinct, small wave appearing just after each T wave.

Step 1: Identify the T wave morphology. The T waves are flat/shallow. This suggests an abnormality in repolarization.

Step 2: Identify the secondary wave. The presence of a wave following the T wave is a U wave.

Step 3: Correlate with the clinical history. The patient is on diuretics, which are known to cause potassium depletion.

Conclusion: The ECG findings of flat T waves and U waves, combined with diuretic use, are highly indicative of Hypokalemia.

By mastering these visual cues, you can move from simply "looking" at a monitor to actively "interpreting" the life-threatening electrolyte shifts occurring within your patient.