Hypokalemia — Recognition
The patient's ECG shows a new U wave and flattened T waves — but the cardiac monitor won't alarm for these subtle changes. Knowing what hypokalemia looks like before it becomes lethal is your job.
Core Concept
Hypokalemia is a serum potassium below 3.5 mEq/L. Potassium is the primary intracellular cation and drives resting membrane potential in excitable tissues — skeletal muscle, smooth muscle, and especially cardiac muscle. When extracellular K+ drops, the resting membrane becomes hyperpolarized, meaning cells need a stronger stimulus to fire. This explains the clinical picture: skeletal muscle weakness (often legs first, ascending), diminished or absent deep tendon reflexes, decreased bowel motility progressing to paralytic ileus, and fatigue. Cardiac effects are the real danger. ECG changes follow a predictable sequence: flattened T waves → ST depression → prominent U waves (a small wave after the T wave) → dysrhythmias including PVCs, ventricular tachycardia, and cardiac arrest. Common causes include loop and thiazide diuretics, vomiting, NG suction, diarrhea, and alkalosis — which shifts K+ into cells and drops the serum level even further. Hypokalemia also potentiates digoxin toxicity, so always check K+ in patients on digoxin. Assessment priorities: check the ECG before acting on the number, auscultate bowel sounds (hypoactive suggests worsening), and evaluate respiratory muscle strength in severe cases (below 2.5 mEq/L).
Watch Out For
Don't confuse hypokalemia ECG changes (flattened T waves, U waves) with hyperkalemia ECG changes (tall peaked T waves, widened QRS). Students mix up U waves — they appear in hypokalemia, not hyperkalemia. Alkalosis causes hypokalemia (K+ shifts into cells); acidosis causes hyperkalemia (K+ shifts out) — remember the inverse relationship between pH and potassium.
Clinical Pearl
Think of hypokalemia as everything going flat and slow: flat T waves, flat reflexes, flat (hypoactive) bowels, flat energy. The U wave is the unique red flag.
Test Your Knowledge
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