Hypokalemia

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.

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).

Key Distinctions

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.

Replacement & Nursing

Potassium replacement depends on severity. Mild hypokalemia (3.0–3.4 mEq/L) in a stable patient is corrected with oral supplements — potassium chloride (KCl) 20–40 mEq PO given with a full glass of water and food to prevent GI irritation. Never crush extended-release tablets. For moderate to severe hypokalemia (< 3.0 mEq/L) or when the patient cannot take PO, IV replacement is used. The critical safety parameters for IV potassium: never exceed 10 mEq/hour via peripheral line (up to 20 mEq/hour via central line with continuous cardiac monitoring), never give IV push or bolus, and the maximum concentration for peripheral infusion is 40 mEq/L. Higher concentrations require central access. Always use an infusion pump — gravity drip is never acceptable. During IV infusion, place the patient on continuous cardiac monitoring and assess the IV site frequently because KCl is a vesicant that causes phlebitis and tissue necrosis if it infiltrates. Before administering potassium, verify adequate urine output (≥ 0.5 mL/kg/hr) — impaired renal excretion increases hyperkalemia risk. Recheck the potassium level after replacement. Concurrently assess magnesium — hypomagnesemia makes hypokalemia refractory to correction, so magnesium must be repleted first or simultaneously.

Key Distinctions

Don't confuse the 10 mEq/hour peripheral IV limit with the 20 mEq/hour central line limit — NCLEX tests both. Students often forget that IV potassium is never given by push under any circumstance; this distinguishes it from most other IV electrolyte replacements. Oral KCl requires food and fluids to protect the GI mucosa — giving it on an empty stomach causes ulceration, not just nausea.

Clinical Pearl

IV potassium: never push, always pump, always monitor. If the heart rate changes during infusion, stop and reassess — the potassium is talking to the myocardium.