10.9 Electrolyte Imbalances and Dysrhythmia Risk

Why Electrolytes Are a Rhythm Problem

Test-plan item II.E.3 is examined almost entirely through the monitor. Every cardiac myocyte's action potential depends on transmembrane ion gradients, so an electrolyte shift is a change in the resting membrane potential, the rate of phase 0 depolarization, or the duration of repolarization — which is to say, a change in the ECG and in arrhythmia risk. On a cardiac unit the derangements are largely iatrogenic: loop diuretics waste potassium and magnesium, aggressive decongestion produces contraction alkalosis, RAAS inhibitors and mineralocorticoid receptor antagonists retain potassium, and the antiarrhythmics we use prolong the QT. The nurse who checks the magnesium before escalating potassium replacement, or who recognizes a QTc creeping past 500 ms on a sotalol load, prevents the arrest.

The Reference Table

Potassium

Hyperkalemia

Causes on a cardiac unit: acute kidney injury and CKD, ACE inhibitors, ARBs, sacubitril-valsartan, spironolactone and eplerenone, potassium-sparing diuretics, trimethoprim-sulfamethoxazole and heparin (both suppress aldosterone effect), beta blockers, tissue breakdown from rhabdomyolysis or hemolysis on mechanical circulatory support, massive transfusion, and metabolic acidosis. Always ask whether the value is pseudohyperkalemia from a hemolyzed sample, a tight tourniquet with fist-clenching, or marked leukocytosis or thrombocytosis — but never delay treating a patient with ECG changes while you wait for a redraw.

ECG progression — memorize the order:

1. Peaked, narrow-based, symmetric T waves (roughly 5.5–6.5 mEq/L), tallest in the precordial leads.
2. PR prolongation and P-wave flattening, then loss of the P wave (6.5–7.5).
3. QRS widening into a bizarre, non-specific intraventricular conduction delay (7.0–8.0).
4. Sine wave — the widened QRS merges with the T wave (above 8.0).
5. Ventricular fibrillation or asystole.

The emergency sequence:

Hypokalemia

Causes: loop and thiazide diuretics (the dominant cause in cardiology), GI losses, insulin and beta-2 agonists driving potassium intracellularly, alkalosis, hypomagnesemia, hyperaldosteronism, and refeeding.

ECG: ST-segment depression, T-wave flattening then inversion, prominent U waves, and apparent QT prolongation that is really a fused QU interval. Consequences: increased automaticity, PVCs, atrial and ventricular tachycardia, torsades, and marked potentiation of digoxin toxicity.

Replacement rules:

• Every 10 mEq of potassium raises the serum level about 0.1 mEq/L — total body deficit is far larger than the serum number suggests (a serum potassium of 3.0 mEq/L reflects a deficit of roughly 200–400 mEq).
• Peripheral infusion: no more than 10 mEq/h at a concentration no greater than 10 mEq per 100 mL. Faster or more concentrated infusion causes phlebitis and severe pain.
• Central infusion: up to 20 mEq/h with continuous ECG monitoring; rates up to 40 mEq/h are reserved for life-threatening hypokalemia with arrhythmia under direct monitoring.
• Correct magnesium first. Hypomagnesemia removes the inhibition on renal outer medullary potassium channels, so the kidney dumps every milliequivalent you give. Hypokalemia that will not correct is hypomagnesemia until proven otherwise — this is the single most tested electrolyte relationship on the CMC.
• Target 4.0–5.0 mEq/L in acute myocardial infarction, heart failure and any patient with ventricular ectopy; recheck 1–2 hours after an IV dose.
• Watch for rebound hyperkalemia in patients on RAAS inhibitors or with AKI.

Magnesium, Calcium, Sodium and Phosphate

Magnesium

Hypomagnesemia is nearly universal in diuretic-treated heart failure, alcohol use disorder, diarrhea, proton pump inhibitor use, amphotericin, cisplatin, post-cardiopulmonary bypass states and refeeding. It produces torsades de pointes, atrial fibrillation (magnesium repletion reduces post-cardiac-surgery AF), refractory hypokalemia and refractory hypocalcemia, plus tremor, tetany and seizures. Serum magnesium is a poor reflection of total body stores because most magnesium is intracellular — a "normal" level does not exclude deficiency.

Torsades treatment: magnesium sulfate 1–2 g IV, given over 5–20 minutes (or as a push over about 2 minutes if the patient is pulseless), repeated once if needed, followed by an infusion of 0.5–1 g/h — and it is given regardless of the serum magnesium level. Magnesium suppresses the early afterdepolarizations that trigger torsades without shortening the QT. Adjuncts: correct potassium to 4.5–5.0 mEq/L, stop every QT-prolonging drug, and overdrive pace at 90–110/min (or use isoproterenol) for pause-dependent recurrent torsades, since increasing the rate shortens the QT. Defibrillate if it degenerates to VF; unsynchronized shock is used for polymorphic VT because the monitor cannot reliably synchronize.

Hypermagnesemia is essentially a disease of renal failure plus magnesium-containing antacids, laxatives or repletion, and of obstetric magnesium infusions. Clinical sequence: loss of deep tendon reflexes at roughly 4–6 mEq/L (the earliest reliable sign, and a nursing assessment), then hypotension and bradycardia, then respiratory depression at 8–12, then complete heart block and cardiac arrest above 12–15. Treatment is IV calcium gluconate 1–2 g as the physiologic antagonist, stopping the source, loop diuretics with saline if renal function permits, and dialysis in renal failure.

Calcium

Use ionized calcium in critical illness — total calcium is bound to albumin and misleads. If only total calcium is available, corrected calcium = measured calcium + 0.8 x (4.0 − serum albumin). Alkalosis increases binding and lowers ionized calcium without changing total calcium, so hyperventilation produces symptomatic hypocalcemia at a "normal" total level.

Citrate is a nursing-critical concept. Banked blood and CRRT citrate anticoagulation both chelate ionized calcium. In citrate accumulation the total calcium rises while the ionized calcium falls, and a total-to-ionized calcium ratio above 2.5 is the diagnostic clue; it presents as hypotension, prolonged QT and a widening anion gap in a patient who looks like they should be improving.

Calcium chloride versus gluconate: 10 mL of 10% calcium chloride contains about 272 mg (13.6 mEq) of elemental calcium; 10 mL of 10% calcium gluconate contains about 93 mg (4.65 mEq) — chloride delivers roughly three times as much calcium per volume and works faster, but it is a vesicant that requires central access and causes severe tissue necrosis on extravasation. Gluconate is the peripheral agent. Never run calcium in the same line as sodium bicarbonate (precipitation) or with ceftriaxone.

Sodium

Hyponatremia in heart failure is dilutional, driven by non-osmotic vasopressin release in response to a low effective arterial blood volume, and it is one of the strongest independent predictors of mortality in decompensated heart failure — a sodium under 130–135 mEq/L marks a poor-prognosis patient, and the treatment is decongestion and improved perfusion, not sodium administration. Other causes: thiazides (far more than loops), SIADH, adrenal insufficiency, and excessive free water.

Correction limits are the safety rule: no more than 8 mEq/L in 24 hours (many centers use 6–8, with an absolute ceiling of 10–12 in the most chronic patients). Over-rapid correction of chronic hyponatremia causes osmotic demyelination syndrome — a delayed, often irreversible quadriparesis, dysarthria, dysphagia and locked-in syndrome appearing 2–6 days later. Risk is highest with sodium under 120 mEq/L, alcohol use disorder, malnutrition, hypokalemia and liver disease. Watch for autocorrection: once the stimulus is removed (volume repletion, stopping a thiazide), a brisk water diuresis can overshoot the limit without any further intervention.

Severe symptomatic hyponatremia (seizure, coma, obtundation) is treated with 3% hypertonic saline 100–150 mL IV over 10–20 minutes, repeated up to three times, targeting a rise of 4–6 mEq/L to stop the symptoms — that increment relieves cerebral edema and is well within the 24-hour cap. Tolvaptan, a vasopressin V2 antagonist, is a hospital-initiated aquaretic (15 mg daily, titrated to 30–60 mg, limited to 30 days) used for hypervolemic and euvolemic hyponatremia; do not fluid-restrict during the first 24 hours because that compounds the correction rate, and it is contraindicated in liver disease.

Phosphate

Hypophosphatemia is under-appreciated and highly relevant to the cardiac ICU. Phosphate is the substrate for ATP and 2,3-diphosphoglycerate, so depletion causes reduced myocardial contractility, an acute reversible cardiomyopathy, arrhythmias, diaphragmatic weakness and failure to wean from mechanical ventilation, plus a leftward oxyhemoglobin shift that impairs tissue oxygen delivery. Levels under 1.0–1.5 mg/dL are symptomatic.

Refeeding syndrome is the classic trigger: a malnourished patient — alcohol use disorder, cardiac cachexia, prolonged NPO status, post-arrest — receives carbohydrate, insulin surges, and phosphate, potassium and magnesium all move intracellularly within 24–72 hours. Prevention is the nursing intervention: start feeding at 25–50% of goal calories, give thiamine before or with the first carbohydrate load, monitor phosphate, potassium and magnesium at least daily for 3–5 days, and advance slowly. Replace with potassium phosphate or sodium phosphate roughly 15–45 mmol IV over 4–6 hours, choosing the sodium salt when potassium is already high and watching for hypocalcemia and, in renal impairment, hyperphosphatemia.

QT Prolongation in Cardiac Care

Drugs used every day on a cardiac unit prolong repolarization: amiodarone (long QT but paradoxically low torsades risk), sotalol, dofetilide, ibutilide, procainamide, quinidine, disopyramide, plus haloperidol and droperidol, ondansetron, methadone, azithromycin and fluoroquinolones, fluconazole, citalopram and escitalopram, hydroxyzine, and several antiemetics. Risk multiplies with hypokalemia, hypomagnesemia, hypocalcemia, bradycardia, female sex, structural heart disease, hepatic or renal impairment, and drug combinations.

Action thresholds: a QTc above 500 ms, or an increase of more than 60 ms from baseline, requires notification, review and usually discontinuation of the offending agent plus aggressive potassium and magnesium repletion. During a sotalol or dofetilide load the QTc is measured before each dose and 2–3 hours after it, and the drug is held or stopped per protocol. Use the same lead for serial QT measurement, measure in the lead with the longest clearly defined T wave, exclude the U wave, and remember that Bazett's correction over-corrects at rapid rates — in AF with RVR, calculate over several beats or use an alternative correction. Continuous QT monitoring should be enabled on the bedside monitor for any patient loading a class III antiarrhythmic.