# Potassium Disorders

## Introduction

Total body potassium stores are approximately 3500 mEq, corresponding to 50 to 55 mEq per kilogram of body weight. The overwhelming majority of potassium, approximately 98 percent, resides within cells, primarily in skeletal muscle, while only 2 percent is found in the extracellular fluid. The normal serum potassium concentration is maintained within a remarkably narrow range of 3.5 to 5.0 mEq/L through the integrated actions of internal balance, governing transcellular potassium shifts, and external balance, governing renal and gastrointestinal potassium excretion. Potassium is the primary determinant of the resting membrane potential of excitable cells, and even modest derangements in serum potassium concentration can produce life-threatening cardiac arrhythmias and profound neuromuscular dysfunction.

## Potassium Physiology

### Internal Potassium Balance (Transcellular Shifts)

The sodium-potassium ATPase, expressed on virtually all cell membranes, is the primary pump responsible for maintaining the steep potassium concentration gradient across the cell membrane, transporting three sodium ions out of the cell in exchange for two potassium ions into the cell. Several factors promote the movement of potassium into cells and lower serum potassium. Insulin stimulates Na-K-ATPase activity independent of its effects on glucose uptake, making it a critical acute regulator of serum potassium. Beta-2 adrenergic agonists such as albuterol and epinephrine activate the Na-K-ATPase through cAMP-mediated phosphorylation. Alkalemia promotes potassium entry into cells as hydrogen ions exit to buffer the extracellular pH, with a rough estimate of a 0.3 to 0.5 mEq/L decrease in serum potassium for each 0.1 unit rise in pH during metabolic alkalosis. Aldosterone has a minor transcellular effect in addition to its more important renal actions.

Conversely, several factors shift potassium out of cells and raise serum potassium. Insulin deficiency or resistance, as seen in diabetic ketoacidosis, allows potassium to leak from cells. Non-selective beta-blockers impair beta-2-mediated potassium uptake. Mineral acidemia causes potassium efflux from cells, though the magnitude varies significantly: mineral acids such as hydrochloric acid cause a more pronounced shift than organic acids, because the organic anions (lactate, beta-hydroxybutyrate) enter cells along with potassium, attenuating the net shift. Hyperosmolality causes solvent drag, with water movement out of cells carrying potassium along, producing an approximately 0.4 to 0.8 mEq/L increase in serum potassium for each 10 mOsm/kg rise in osmolality. Cell lysis from rhabdomyolysis, tumor lysis syndrome, or hemolysis releases massive intracellular potassium stores. Intense exercise can transiently raise serum potassium by 1 to 2 mEq/L through potassium release from contracting skeletal muscle. Succinylcholine and digitalis toxicity also promote transcellular potassium shifts outward.

### External Potassium Balance

Daily potassium intake on a typical Western diet ranges from 40 to 120 mEq per day. The kidneys are responsible for approximately 90 percent of daily potassium elimination, making renal excretion the dominant mechanism for maintaining external potassium balance. The gastrointestinal tract accounts for approximately 10 percent of daily excretion under normal conditions, but this fraction can increase substantially in chronic kidney disease, reaching 30 to 40 percent through upregulation of colonic BK (maxi-K) potassium channels. Skin losses are minimal, typically less than 5 mEq per day.

<image>Diagram showing internal and external potassium balance. On the left, depict a cell with the Na-K-ATPase pump on the membrane, showing 3 Na+ exiting and 2 K+ entering. Include arrows showing factors that drive potassium into cells (insulin, beta-2 agonists, alkalosis) and factors that drive potassium out of cells (acidosis, hyperosmolality, cell lysis, beta-blockers). On the right, show the external balance with daily intake from diet entering the GI tract, with 90% excretion via the kidneys and 10% via the colon. Include the intracellular K+ concentration (140 mEq/L) and extracellular K+ concentration (4.0 mEq/L).</image>

## Renal Potassium Handling

### Proximal Tubule and Loop of Henle

Approximately 65 percent of filtered potassium is reabsorbed in the proximal convoluted tubule, primarily through solvent drag via the paracellular pathway as water is reabsorbed. An additional 25 percent is reabsorbed in the thick ascending limb through paracellular transport driven by the lumen-positive transepithelial voltage generated by NKCC2 activity and ROMK-mediated potassium recycling. By the time tubular fluid reaches the early distal convoluted tubule, only about 10 percent of filtered potassium remains, and the fine-tuning of potassium excretion occurs in the distal nephron.

### Distal Nephron: Site of Regulated K+ Secretion

The principal cells of the connecting tubule and cortical collecting duct are the major site of regulated potassium secretion. These cells express two types of apical potassium channels: ROMK, which mediates constitutive, baseline potassium secretion, and BK (maxi-K) channels, which are flow-dependent and activated by increases in tubular flow rate. The basolateral Na-K-ATPase drives potassium into the cell from the peritubular space, while potassium exits the cell into the lumen through apical ROMK and BK channels. Potassium secretion is driven by three key factors: the lumen-negative transepithelial voltage generated by ENaC-mediated sodium absorption, the intracellular potassium concentration, and the tubular flow rate. In states of potassium depletion, type A intercalated cells reabsorb potassium from the lumen via the apical H-K-ATPase, which simultaneously secretes hydrogen ions.

### Determinants of Distal K+ Secretion

Five principal factors govern the rate of potassium secretion in the distal nephron. First, aldosterone increases ENaC expression and activity, enhances Na-K-ATPase activity, and upregulates ROMK, collectively promoting potassium secretion. Second, increased distal sodium delivery and tubular flow rate, as occurs with diuretic use or volume expansion, activate BK channels and enhance potassium secretion. Third, serum potassium concentration itself directly stimulates aldosterone release from the adrenal zona glomerulosa and enhances apical potassium channel activity. Fourth, acid-base status modulates potassium handling, with alkalosis promoting potassium secretion and acidosis inhibiting ROMK. Fifth, the presence of non-reabsorbable anions such as bicarbonate or beta-hydroxybutyrate in the distal tubular fluid increases lumen negativity and enhances potassium secretion.

### WNK-SPAK/OSR1 Kinase Pathway

The WNK (with-no-lysine) kinase signaling pathway provides an elegant mechanism for coordinating sodium and potassium handling in the distal nephron. WNK1 and WNK4 kinases regulate the activity of NCC, ROMK, and ENaC. During low potassium intake, WNK4 activates NCC in the distal convoluted tubule, increasing sodium chloride reabsorption at that site and reducing sodium delivery to the cortical collecting duct, thereby decreasing potassium secretion and conserving potassium. Conversely, during high potassium intake, WNK4 is inhibited, NCC activity decreases, more sodium is delivered to the collecting duct where it is reabsorbed through ENaC, and potassium secretion is enhanced. Gordon syndrome, also known as pseudohypoaldosteronism type II, results from gain-of-function mutations in WNK kinases that cause constitutive NCC hyperactivity, producing a clinical phenotype of hyperkalemia, hypertension, and metabolic acidosis that responds dramatically to thiazide diuretics.

## Hypokalemia

### Etiologies

Decreased dietary potassium intake rarely causes hypokalemia as the sole cause, because the kidneys possess the remarkable ability to reduce potassium excretion to less than 15 mEq per day. Transcellular shifts are an important cause of acute hypokalemia and include insulin administration, beta-2 agonist therapy, alkalosis, refeeding syndrome, and hypokalemic periodic paralysis (both the genetic channelopathy form and thyrotoxic periodic paralysis).

Renal potassium losses, identified by a urine potassium excretion exceeding 20 mEq/L or a transtubular potassium gradient greater than 3, can be subdivided based on the presence or absence of hypertension. Hypokalemia with hypertension suggests mineralocorticoid excess states including primary aldosteronism, Cushing syndrome, renovascular disease, Liddle syndrome, apparent mineralocorticoid excess from 11-beta-hydroxysteroid dehydrogenase type 2 deficiency or licorice ingestion, and congenital adrenal hyperplasia. Hypokalemia without hypertension is most commonly caused by diuretics (both loop and thiazide types), but also includes Bartter syndrome, Gitelman syndrome, renal tubular acidosis types I and II, hypomagnesemia, and vomiting or nasogastric suction, which causes renal potassium loss through bicarbonaturia and secondary hyperaldosteronism rather than direct gastrointestinal potassium loss.

Gastrointestinal potassium losses, identified by a urine potassium concentration less than 20 mEq/L reflecting appropriate renal conservation, include diarrhea, laxative abuse, villous adenoma, and intestinal fistulae.

### Clinical Manifestations

The cardiac manifestations of hypokalemia progress from U waves, T-wave flattening, and ST-segment depression through QT prolongation and increased susceptibility to digitalis toxicity to atrial and ventricular arrhythmias including torsades de pointes. Neuromuscular manifestations include weakness, cramps, and ascending paralysis that may progress to rhabdomyolysis in severe cases. Renal consequences of chronic hypokalemia include nephrogenic diabetes insipidus from AQP2 downregulation, metabolic alkalosis from increased hydrogen ion secretion and enhanced ammoniagenesis, chronic tubulointerstitial nephritis (hypokalemic nephropathy), and renal cyst formation. Gastrointestinal manifestations include ileus and constipation.

### Management

Oral potassium chloride is preferred for non-urgent potassium replacement, typically given in doses of 40 to 100 mEq per day in divided doses. Intravenous potassium chloride is indicated for severe hypokalemia below 2.5 mEq/L or when oral administration is not feasible, at a rate of 10 to 20 mEq per hour via a peripheral line or up to 40 mEq per hour via a central line in the ICU with continuous cardiac monitoring. As a rough approximation, each 10 mEq of potassium replacement raises serum potassium by approximately 0.1 mEq/L, though this relationship is highly variable. Correction of concurrent hypomagnesemia is essential, as magnesium depletion causes renal potassium wasting through activation of ROMK channels and impairment of Na-K-ATPase function, rendering potassium supplementation ineffective until magnesium is repleted. Potassium-sparing diuretics including amiloride, spironolactone, and eplerenone are useful adjunctive therapies.

<image>ECG rhythm strip comparison showing the progressive electrocardiographic changes of hypokalemia: normal tracing at K+ 4.0 mEq/L, then mild hypokalemia (3.0 mEq/L) showing T-wave flattening and U-wave appearance, moderate hypokalemia (2.5 mEq/L) showing ST depression and prominent U waves, and severe hypokalemia (2.0 mEq/L) showing T-U wave fusion, prolonged QT/QU interval, and risk of torsades de pointes. Label each key waveform change clearly.</image>

## Hyperkalemia

### Etiologies

Before investigating true hyperkalemia, pseudohyperkalemia must be excluded. This artifact results from in vitro potassium release during or after phlebotomy and can be caused by hemolysis during blood draw, extreme leukocytosis exceeding 100,000 per microliter, thrombocytosis exceeding 500,000 per microliter, prolonged tourniquet application, or repeated fist clenching. When pseudohyperkalemia is suspected, a whole blood sample processed rapidly by the laboratory or an arterial blood gas potassium measurement can confirm the true value.

Transcellular shifts causing hyperkalemia include mineral acidosis, insulin deficiency, beta-blocker use, digitalis toxicity, succinylcholine administration, hyperkalemic periodic paralysis, arginine hydrochloride infusion, tumor lysis syndrome, and rhabdomyolysis.

Decreased renal potassium excretion is the most clinically significant category and can result from reduced GFR (in AKI or advanced CKD, though potassium typically remains normal until GFR falls below 15 to 20 mL/min due to colonic compensation), hypoaldosteronism (from adrenal insufficiency, hyporeninemic hypoaldosteronism in type IV RTA and diabetic nephropathy, or drugs including ACE inhibitors, ARBs, spironolactone, eplerenone, heparin, ketoconazole, trimethoprim, and calcineurin inhibitors), aldosterone resistance (pseudohypoaldosteronism type I, Gordon syndrome, calcineurin inhibitors), or reduced distal sodium delivery (severe heart failure, severe volume depletion).

### Clinical Manifestations

The cardiac toxicity of hyperkalemia follows a characteristic progression: peaked T waves appear first, followed by P-wave flattening, PR prolongation, QRS widening, the emergence of a sine wave pattern, and ultimately ventricular fibrillation or asystole. Neuromuscular manifestations include paresthesias, weakness, and ascending paralysis that can mimic Guillain-Barre syndrome. Metabolic consequences include non-anion gap metabolic acidosis due to impaired ammoniagenesis and hydrogen ion secretion in the collecting duct.

### Acute Management (K+ >6.0 mEq/L or ECG changes)

The acute management of hyperkalemia follows a three-pronged approach. Cardiac membrane stabilization is the first priority, achieved with calcium gluconate 10 percent solution, 10 to 20 mL administered intravenously over 2 to 5 minutes. Calcium gluconate acts within 1 to 3 minutes and lasts 30 to 60 minutes; it does not lower serum potassium but stabilizes the cardiac membrane against arrhythmias. Calcium chloride, which contains three times the elemental calcium, is reserved for cardiac arrest situations and must be given via central line due to its tissue toxicity.

Transcellular shifting agents represent the second step. Regular insulin 10 units intravenously combined with 25 grams of dextrose (D50W) lowers potassium by 0.5 to 1.2 mEq/L with onset in 15 to 30 minutes and duration of 4 to 6 hours; glucose must be monitored every hour for at least 5 hours because hypoglycemia occurs in up to 75 percent of patients who do not receive adequate dextrose. Nebulized albuterol at 10 to 20 mg lowers potassium by 0.5 to 1.0 mEq/L but should be used cautiously in patients with cardiac disease. Sodium bicarbonate 150 mEq intravenously has minimal acute efficacy as monotherapy but may be more effective in patients with concurrent metabolic acidosis.

| Intervention | Mechanism | Dose | Onset | Duration | K⁺ Reduction | Key Consideration |
|-------------|-----------|------|-------|----------|--------------|-------------------|
| Calcium gluconate 10% | Membrane stabilization (does NOT lower K⁺) | 10–20 mL IV over 2–5 min | 1–3 min | 30–60 min | None | First priority with ECG changes |
| Insulin + dextrose | Transcellular shift (Na-K-ATPase) | 10 units regular insulin + 25 g D50W | 15–30 min | 4–6 hrs | 0.5–1.2 mEq/L | Monitor glucose q1h × 5 hrs; hypoglycemia in ~75% without adequate dextrose |
| Nebulized albuterol | Transcellular shift (β2-mediated) | 10–20 mg nebulized | 15–30 min | 2–4 hrs | 0.5–1.0 mEq/L | Caution in cardiac disease; additive with insulin |
| Sodium bicarbonate | Transcellular shift | 150 mEq IV | 30–60 min | 2–4 hrs | Minimal alone | Most effective with concurrent metabolic acidosis |
| Furosemide | Renal K⁺ excretion | 40–80 mg IV | 30–60 min | 6 hrs | Variable | Requires adequate renal function |
| Sodium zirconium cyclosilicate (Lokelma) | GI K⁺ binding | 10 g TID × 48 hrs, then 5–10 g daily | 1–2 hrs | Ongoing | 0.7–1.0 mEq/L | Faster onset than patiromer; preferred for acute settings |
| Patiromer (Veltassa) | GI K⁺ binding | 8.4 g daily | 4–7 hrs | Ongoing | 0.7 mEq/L | Too slow for acute management; enables continued RAAS blockade |
| Hemodialysis | Direct K⁺ removal | — | Immediate | Session-dependent | 1–2 mEq/L per hour | Definitive for severe/refractory hyperkalemia |

Potassium elimination is the third and definitive approach. Furosemide 40 to 80 mg intravenously promotes kaliuresis if renal function is adequate. Sodium zirconium cyclosilicate (Lokelma) at 10 grams three times daily for 48 hours followed by 5 to 10 grams daily binds potassium in the gastrointestinal tract with onset in 1 to 2 hours. Patiromer (Veltassa) at 8.4 grams daily has a slower onset of 4 to 7 hours and is not suitable for acute management. Sodium polystyrene sulfonate (Kayexalate) at 15 to 30 grams orally or rectally has a slow and questionable onset and carries a risk of intestinal necrosis, particularly in the sorbitol-containing preparation, and has been largely supplanted by newer agents. Hemodialysis is the definitive treatment for severe or refractory hyperkalemia.

### Chronic Management

Long-term management of hyperkalemia involves dietary potassium restriction to less than 2 to 3 grams per day, optimization of diuretic therapy, discontinuation or dose reduction of offending medications when possible, and use of patiromer or sodium zirconium cyclosilicate to enable continued RAAS inhibitor therapy. Fludrocortisone at 0.1 mg daily is an option for patients with documented hypoaldosteronism but must be used cautiously given the risks of hypertension and edema.

## Transtubular Potassium Gradient (TTKG)

The transtubular potassium gradient estimates the potassium concentration in the cortical collecting duct lumen relative to the peritubular capillary and is calculated as the ratio of urine potassium to plasma potassium divided by the ratio of urine osmolality to plasma osmolality. A TTKG greater than 7 in the setting of hyperkalemia suggests an appropriate renal response and points to an extrarenal cause. A TTKG less than 3 in hyperkalemia indicates impaired renal potassium excretion and suggests hypoaldosteronism or aldosterone resistance. Conversely, a TTKG less than 3 in the setting of hypokalemia suggests appropriate renal potassium conservation and points toward extrarenal loss. The TTKG has limitations: it assumes the presence of ADH activity and adequate cortical collecting duct sodium reabsorption, and it is less reliable at extremes of urine osmolality or sodium concentration.

<image>Clinical algorithm for the diagnostic approach to hyperkalemia. Start with confirming true hyperkalemia by ruling out pseudohyperkalemia (check for hemolysis, leukocytosis, thrombocytosis). Then branch into transcellular shift (assess for acidosis, insulin deficiency, beta-blockers, cell lysis) versus decreased renal excretion. For decreased renal excretion, branch into reduced GFR (AKI, CKD) versus normal GFR. For normal GFR, check aldosterone and renin levels: low renin and low aldosterone suggests hyporeninemic hypoaldosteronism (type IV RTA); high renin and low aldosterone suggests adrenal insufficiency; high renin and high aldosterone suggests aldosterone resistance (pseudohypoaldosteronism, calcineurin inhibitors). Include TTKG values at each decision node.</image>

## Key Clinical Pearls

- Always check and replace magnesium in refractory hypokalemia; hypomagnesemia causes ROMK-mediated renal potassium wasting that is resistant to potassium supplementation alone
- In DKA, serum K+ may be normal or elevated on presentation despite profound total body K+ depletion; insulin therapy will rapidly lower serum K+; replace K+ before insulin if K+ <3.5 mEq/L
- Calcium gluconate does NOT lower serum potassium; it stabilizes the cardiac membrane and should be given first in hyperkalemia with ECG changes
- Sodium zirconium cyclosilicate (Lokelma) has a faster onset than patiromer (Veltassa) and is preferred for more acute settings; both enable continued RAAS blockade in CKD patients
- The AMBER trial demonstrated that patiromer enabled continued spironolactone therapy in resistant hypertension patients with CKD and hyperkalemia

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