Hyperkalemia vs Hypokalemia: The One Cue That Gives It Away
Potassium questions feel like a coin flip because both directions cause weakness, both cause dysrhythmias, and both show up in "which finding does the nurse expect" stems. Here's the thing your lecture buried on slide 40: one cue separates them every time. It's the T wave. Peaked T wave means the potassium is high. Flat T wave with a U wave means it's low. Get that cold and the rest of the question falls into place.
The range, and why the window is so small
Normal serum potassium is 3.5–5.0 mEq/L. Below 3.5 is hypokalemia, above 5.0 is hyperkalemia. That's a window of 1.5 — compare it to sodium, which gets a 10-point window (135–145) and still causes trouble at the edges. Potassium is that tightly controlled because it sets the resting electrical charge of every muscle cell, and the heart is a muscle that can't afford surprises. Almost all of the body's potassium lives inside cells; the serum number is the sliver outside — which is why shifts across the cell membrane, not just gains and losses, can swing the lab.
The one cue: read the T wave
When the exam gives you an ECG description, that description is the answer key. Memorize both directions as a story, not a list.
- Hyperkalemia — the T wave goes tall, narrow, peaked ("tented"). As potassium keeps climbing: PR interval lengthens, P wave flattens and disappears, QRS widens, and the tracing degrades toward a sine wave, ventricular fibrillation, or asystole. The heart slows down and blocks before it stops.
- Hypokalemia — the T wave goes flat or inverted, a U wave appears after it (a little hump the T wave normally hides), and the ST segment depresses. Ectopy shows up: premature ventricular contractions, then ventricular dysrhythmias. The heart gets irritable before it gets dangerous.
Everything else, side by side
Once you've placed the T wave, the rest of the findings sort themselves. The pattern to hold onto: hyperkalemia makes smooth muscle and nerves hyperactive while it makes the heart slow. Hypokalemia makes everything slow and low, but the heart irritable.
| System | Hyperkalemia (K+ > 5.0) | Hypokalemia (K+ < 3.5) |
|---|---|---|
| ECG | Peaked T waves → prolonged PR, flat P, wide QRS → sine wave, VF, asystole | Flattened or inverted T, U wave, ST depression → PVCs, ventricular dysrhythmias |
| Heart rate / pulse | Bradycardia, heart block, irregular rhythm | Weak, thready, irregular pulse; palpitations |
| Skeletal muscle | Early twitching and cramping, then weakness ascending from the legs to flaccid paralysis; paresthesias (numbness, tingling) | Generalized weakness, leg cramps, decreased deep tendon reflexes, flaccid paralysis if severe |
| GI | Hyperactive bowel sounds, diarrhea, abdominal cramping, nausea | Hypoactive or absent bowel sounds, constipation, abdominal distension, paralytic ileus |
| Respiratory | Shallow breathing late, from respiratory muscle weakness | Shallow, ineffective respirations from weak respiratory muscles — check this early |
| Neuro / other | Restlessness, irritability, oliguria if kidneys are the cause | Lethargy, confusion, polyuria, and the acid-base twin: metabolic alkalosis |
Causes, written the way the stem writes them
NCLEX almost never says "the client has hyperkalemia." It describes a client whose kidneys stopped working, or whose cells got crushed, or whose blood went acidic, and expects you to hear the potassium in the story. Learn the cues, not the list.
The potassium is high because...
- The kidneys can't dump it. Acute kidney injury, chronic kidney disease, oliguria, rising creatinine and BUN. Cue words: "urine output 15 mL/hr," "missed dialysis," "creatinine 4.2." This is the number-one cause on the exam and in real life.
- Cells broke open and spilled it. Burns, crush injuries, rhabdomyolysis, tumor lysis after chemotherapy, hemolysis, massive transfusion of older blood. Cue words: "pinned under a vehicle," "48 hours after a 40% burn," "dark tea-colored urine."
- Acidosis pushed it out of the cell. Hydrogen ions move into cells, potassium moves out to balance the charge. DKA is the classic — total-body potassium is actually low, but the serum reads high until the acidosis corrects. Cue words: "pH 7.24," "Kussmaul respirations," "glucose 480."
- No aldosterone to trade it away. Adrenal insufficiency (Addison's). Cue words: "bronze skin," "hypotension, hyponatremia, hyperkalemia together."
- Incidental cues. "On a potassium-sparing diuretic" or "uses a salt substitute" is the stem tipping its hand. Note it; don't build the answer on it.
The potassium is low because...
- It left through the GI tract. Vomiting, NG tube to suction, diarrhea, laxative overuse. Cue words: "NG to low intermittent suction for three days," "nausea and vomiting for 48 hours." Gastric losses also cause metabolic alkalosis, which drives more potassium into cells — a double hit.
- It left through the kidneys. Diuresis of any kind. Cue words: "polyuria," "takes a loop diuretic each morning" (incidental), "high aldosterone" (Cushing's, hyperaldosteronism), high-dose steroid states.
- Alkalosis or insulin pushed it into the cell. The mirror image of the acidosis shift. Watch the DKA client whose treatment is working: as the acidosis resolves and insulin runs, serum potassium drops fast, which is exactly why those clients are on a monitor.
- It never came in. NPO for days, malnutrition, alcohol use disorder, anorexia. Cue words: "NPO since Tuesday, receiving plain dextrose fluids."
The priority assessment
For either direction, the first assessment is the same: the heart. Apical pulse for rate and rhythm, then a continuous cardiac monitor. The findings that kill are electrical, and you can't see electricity by asking about leg cramps. After the heart, in order:
- Respiratory effort and depth. Both imbalances weaken the diaphragm. A client breathing shallowly at 26 with a K+ of 2.4 is closer to a ventilator than the stem admits.
- Muscle strength and reflexes. Hand grips, leg lifts, deep tendon reflexes. Ascending weakness is a trend, and trends are what Next Gen questions are built on.
- Bowel sounds and urine output. They tell you cause and direction in one exam.
- The IV bag. Glance at what's hanging. Potassium is never given IV push, never undiluted, and never faster than policy allows on a pump.
Fully worked question
- Notify the provider of the laboratory results.
- Place the client on a continuous cardiac monitor.
- Remove the potassium-rich foods from the breakfast tray and request a renal diet.
- Assess lower-extremity strength and deep tendon reflexes.
Answer: 2. Every cue in the stem points the same direction — a kidney that can't excrete, oliguria, a potassium above 6, and symptoms (ascending weakness, hyperactive GI). The threat to this client's life is a dysrhythmia in the next few minutes, and you can't see a dysrhythmia without a monitor. It takes thirty seconds, it changes what you say on the phone, and it's the assessment that directly addresses the lethal problem. Assess before you act, and assess the thing that kills first.
Why the others are wrong: Option 1 is necessary and it's your second move, but "first" is the word being tested — the provider will ask what the rhythm shows, and you'll have nothing to say. Option 3 is a correct intervention on the wrong timescale; the orange juice is not what stops this heart in the next ten minutes. Option 4 is an assessment, which is why it's tempting, but muscle strength tells you what you already know; it doesn't tell you whether the QRS has started to widen.