9.2 Potassium, Calcium, Magnesium & Phosphate Disorders

Severe Hyperkalemia: Pathophysiology and Electrocardiographic Cascade

Potassium is the predominant intracellular cation ($140-150\text{ mEq/L}$ intracellularly vs. $3.5-5.0\text{ mEq/L}$ extracellularly). The resting membrane potential ($V_m$) of cardiac myocytes is determined by the ratio of intracellular to extracellular potassium concentration, as defined by the Nernst equation:

$V_m \approx -61.5 \times \log_{10}\left(\frac{[\text{K}^+]_{\text{in}}}{[\text{K}^+]_{\text{out}}}\right)$

As extracellular potassium rises, the resting membrane potential becomes less negative (partially depolarized, shifting from $-90\text{ mV}$ toward $-70\text{ mV}$). While closer to threshold potential, this persistent baseline depolarization paradoxically inactivates fast voltage-gated sodium channels, depressing Phase 0 cardiac depolarization velocity, slowing intraventricular conduction, and disrupting normal repolarization.

The Electrocardiographic Cascade

The ECG manifestations of hyperkalemia follow a classically progressive, sequential morphology:

1. Peaked T Waves ($K \approx 5.5-6.5\text{ mEq/L}$): Narrow-based, tall, symmetric, "tented" T waves, most prominent in precordial leads (V2-V4), reflecting accelerated Phase 3 repolarization via an enhanced rapid delayed rectifier potassium current ($I_{\text{Kr}}$), whose conductance rises with extracellular potassium.
2. Conduction Delays ($K \approx 6.5-7.5\text{ mEq/L}$): Prolongation of the PR interval, flattening and eventual disappearance of P waves (sinoatrial arrest with junctional escape rhythms), and ST-segment depression.
3. QRS Complex Widening ($K \approx 7.0-8.0\text{ mEq/L}$): Diffuse, non-specific intraventricular conduction delay; QRS widens symmetrically and merges with the T wave.
4. Sine-Wave Pattern ($K > 8.0-9.0\text{ mEq/L}$): Complete fusion of the widened QRS complex and the T wave into a smooth, sinusoidal undulating waveform. This is a pre-terminal rhythm indicating imminent degenerating ventricular fibrillation, pulseless electrical activity (PEA), or asystole.

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Three-Phase Emergency Management of Hyperkalemia

Emergency treatment of severe hyperkalemia follows a structured three-phase hierarchy:

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Comprehensive Hyperkalemia Pharmacotherapy Protocol

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Hypokalemia and Refractory Potassium Homeostasis

Hypokalemia ($[\text{K}^+] < 3.5\text{ mEq/L}$, severe $< 2.5\text{ mEq/L}$) hyperpolarizes myocardial cell membranes, prolongs ventricular repolarization, and promotes automaticity and re-entrant tachyarrhythmias (ventricular tachycardia, ventricular fibrillation, Torsades de Pointes).

Electrocardiographic Findings

• Progressive ST-segment depression;
• T wave flattening or inversion;
• Prominent U waves (positive deflections immediately following the T wave, best seen in leads V2-V4);
• Apparent QT interval prolongation (actually representing QU interval prolongation);
• Atrial and ventricular premature contractions, supraventricular tachycardias, and lethal ventricular arrhythmias.

Repletion Protocols and Infusion Safety Limits

• Every $1.0\text{ mEq/L}$ drop in serum potassium below $4.0\text{ mEq/L}$ corresponds to a total body potassium deficit of approximately $200\text{ to }400\text{ mEq}$.
• Peripheral IV Infusion Limit: Maximum rate is $10\text{ mEq/h}$ (concentration $\le 40\text{ mEq/L}$) to prevent severe peripheral venous burning, chemical phlebitis, and sclerosis.
• Central Venous Infusion Limit: In urgent ICU settings with continuous cardiac rhythm monitoring, infusions of $20\text{ mEq/h}$ (concentrations up to $20-40\text{ mEq}/100\text{ mL}$) may be delivered via a dedicated central venous line.

The Critical Role of Magnesium in Refractory Hypokalemia

A frequent clinical failure in the ICU is refractory hypokalemia that fails to correct despite aggressive intravenous potassium repletion. The underlying cause is unrecognized concurrent hypomagnesemia.

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• Mechanism: The Renal Outer Medullary Potassium (ROMK) channel in the apical membrane of the cortical collecting duct secretes potassium into the tubular lumen. Intracellular magnesium ions ($Mg^{2+}$) physically bind and occlude the pore of the ROMK channel from the cytoplasmic side, acting as a molecular brake on potassium secretion (inward rectification). In hypomagnesemia, this intracellular blockade is lost, resulting in uninhibited, continuous potassium leakage into the tubular lumen.
• Clinical Action: Whenever managing hypokalemia in the ICU, always measure and aggressively replete magnesium. Intravenous Magnesium Sulfate $2\text{ g}$ IV over 30 to 60 minutes must be co-administered to restore ROMK channel inhibition and allow successful potassium retention.

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Hypercalcemia Crisis and Acute Hypocalcemia

Calcium Physiology and Protein Binding

Only the free, ionized calcium fraction ($1.15\text{ to }1.33\text{ mmol/L}$) is biologically active. Approximately 40-45% of total serum calcium is bound to albumin. In critically ill patients with severe hypoalbuminemia, total serum calcium is falsely depressed, necessitating calculation of corrected calcium or direct ionized calcium measurement:

$\text{Corrected Total Calcium (mg/dL)} = \text{Measured Calcium (mg/dL)} + 0.8 \times (4.0 - \text{Albumin [g/dL]})$

Hypercalcemic Crisis (Total $\text{Ca} > 14\text{ mg/dL}$, i.e., $> 3.5\text{ mmol/L}$)

Hypercalcemia in the ICU is driven primarily by Hypercalcemia of Malignancy (80% mediated by Parathyroid Hormone-Related Protein [PTHrP] in solid tumors; 20% by osteolytic bone metastases or $1,25(\text{OH})_2\text{D}$ production in lymphoma) or Primary Hyperparathyroidism.

• Clinical Manifestations: Mental status depression, stupor, coma, muscle weakness, polyuria (via nephrogenic DI), nephrocalcinosis, acute kidney injury, pancreatitis, and shortened QT interval predisposing to ventricular arrhythmias.
• Treatment Protocol:
  1. Volume Resuscitation (Hour 0-24): Infuse $0.9\%$ Normal Saline at $200\text{ to }300\text{ mL/h}$ (titrated to restore intravascular volume and maintain urine output at $100-150\text{ mL/h}$). Saline expands intravascular volume, enhances glomerular filtration of calcium, and increases proximal tubular sodium delivery, which inhibits passive paracellular calcium reabsorption.

     Tip

     Loop diuretics (furosemide) should never be administered routinely in hypercalcemia until the patient is completely volume replete. Premature loop diuretic administration worsens volume depletion, reduces GFR, and paradoxically elevates serum calcium.

  2. Calcitonin (Rapid Onset Bridge): Administer Salmon Calcitonin $4\text{ Units/kg}$ SQ or IM every 12 hours. Calcitonin inhibits osteoclast bone resorption and promotes renal calcium clearance within 2 to 4 hours, lowering serum calcium by $1\text{ to }2\text{ mg/dL}$. However, its clinical utility is limited by tachyphylaxis (downregulation of calcitonin receptors) within 24 to 48 hours.
  3. Bisphosphonates (Definitive Long-Acting Therapy): Administer Zoledronic Acid $4\text{ mg}$ IV over 15 minutes (or pamidronate $60-90\text{ mg}$ IV over 2-4 hours). Bisphosphonates induce osteoclast apoptosis. Onset is delayed ($48\text{ to }72\text{ hours}$), with peak nadir at 4 to 7 days. Caution: Zoledronic acid carries nephrotoxic potential and requires dose adjustment or caution in severe renal impairment (eGFR $< 30\text{ mL/min}$).
  4. Denosumab (RANKL Monoclonal Antibody): Administer Denosumab $120\text{ mg}$ SQ (with additional doses on days 8 and 15 in refractory cases). Denosumab blocks the receptor activator of nuclear factor-kappa B ligand (RANKL), halting osteoclast maturation and survival. It is the treatment of choice for severe hypercalcemia of malignancy in patients with severe renal failure where bisphosphonates are contraindicated.
  5. Glucocorticoids: Hydrocortisone $100\text{ mg}$ IV every 8 hours (or prednisone $60\text{ mg}$ PO) is indicated specifically for hypercalcemia mediated by elevated calcitriol ($1,25(\text{OH})_2\text{D}$), seen in granulomatous diseases (sarcoidosis, berylliosis, tuberculosis) and certain lymphomas.
  6. Hemodialysis: Emergent hemodialysis against a low-calcium or zero-calcium dialysate is reserved for patients with severe hypercalcemic crisis ($> 18\text{ mg/dL}$), refractory neurological collapse, or concurrent anuric renal failure or heart failure that precludes saline hydration.

Acute Hypocalcemia (Ionized $\text{Ca} < 0.9\text{ mmol/L}$)

• Etiologies: Massive blood transfusion (citrate anticoagulation binds ionized calcium), post-parathyroidectomy ("hungry bone syndrome"), severe acute pancreatitis (saponification of mesenteric fat), severe sepsis, severe hypomagnesemia.
• Clinical Manifestations: Neuromuscular irritability, perioral and fingertip paresthesias, painful carpopedal spasms, Chvostek's sign (facial twitching upon tapping the facial nerve anterior to the ear), Trousseau's sign (carpal spasm induced by inflating a sphygmomanometer cuff above systolic blood pressure for 3 minutes), and QT interval prolongation (due to lengthening of Phase 2 ST segment), predisposing to Torsades de Pointes.
• Treatment: In acute symptomatic hypocalcemia, administer $1\text{ to }2\text{ g}$ of Calcium Gluconate IV in $50\text{ to }100\text{ mL}$ D5W over $10\text{ to }20\text{ minutes}$, followed by a continuous infusion of elemental calcium at $1\text{ to }2\text{ mg/kg/h}$. Concurrently check and correct serum magnesium.

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Severe Hypophosphatemia and Refeeding Syndrome

Phosphate is essential for cellular bioenergetics (ATP generation), structural cell membrane integrity (phospholipids), and tissue oxygen unloading (2,3-diphosphoglycerate [2,3-DPG]). Severe hypophosphatemia is defined as serum phosphate $< 1.0\text{ mg/dL}$ ($< 0.32\text{ mmol/L}$).

Clinical Manifestations of ATP and 2,3-DPG Depletion

• Pulmonary: Diaphragmatic contractile failure, respiratory muscle fatigue, and inability to liberate (wean) from mechanical ventilation;
• Cardiovascular: Impaired myocardial contractility, acute heart failure, decreased stroke volume, and ventricular dysrhythmias;
• Neurological: Metabolic encephalopathy, delirium, seizures, paresthesias, ascending motor paralysis mimicking Guillain-Barré syndrome;
• Hematologic & Muscular: Acute rhabdomyolysis, impaired erythrocyte membrane deformability leading to hemolytic anemia, and leukocyte dysfunction.

Refeeding Syndrome

Occurs when malnourished, starved, or chronically alcoholic patients receive aggressive enteral or parenteral nutrition:

1. Chronic starvation depletes whole-body phosphate, potassium, magnesium, and thiamine, although serum levels may appear falsely normal due to hemoconcentration and intracellular shifts.
2. Carbohydrate administration stimulates a massive surge in endogenous insulin secretion.
3. Insulin triggers rapid intracellular uptake of glucose, phosphate, potassium, and magnesium, while simultaneously activating cellular glycolysis and glycogen synthesis.
4. Acute, severe depletion of extracellular phosphate, potassium, and magnesium follows within 24 to 72 hours, precipitating fatal cardiac arrhythmias, acute cardiogenic pulmonary edema, and sudden arrest.
5. Prevention & Protocol: Before initiating nutrition in high-risk patients, measure baseline electrolytes; administer prophylactic thiamine ($100-200\text{ mg}$ IV); start caloric intake at a conservative $10-15\text{ kcal/kg/day}$; and proactively replete phosphate ($> 2.5\text{ mg/dL}$), potassium ($> 4.0\text{ mEq/L}$), and magnesium ($> 2.0\text{ mg/dL}$).

Intravenous Phosphate Repletion Protocol

• Severe hypophosphatemia ($< 1.0\text{ mg/dL}$): Administer $0.24\text{ to }0.48\text{ mmol/kg}$ of IV Sodium Phosphate or Potassium Phosphate infused over $4\text{ to }6\text{ hours}$ (maximum infusion rate $7\text{ mmol/h}$).
• Select potassium phosphate if concurrent hypokalemia exists; select sodium phosphate if serum potassium is normal or elevated.

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Tumor Lysis Syndrome (TLS): Cairo-Bishop Criteria and ICU Protocol

Tumor Lysis Syndrome represents the massive, acute release of intracellular contents following the destruction of rapidly proliferating, chemosensitive malignant cells (typically high-grade hematologic malignancies: Burkitt lymphoma, acute lymphoblastic leukemia [ALL], acute myeloid leukemia [AML], or bulky diffuse large B-cell lymphoma).

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Cairo-Bishop Diagnostic Definitions

1. Laboratory TLS (LTLS)

Established when two or more of the following metabolic disturbances occur simultaneously within 3 days before or 7 days after the initiation of cytotoxic chemotherapy:

• Hyperuricemia: Serum uric acid $\ge 8.0\text{ mg/dL}$ (or $25\%$ increase from baseline);
• Hyperkalemia: Serum potassium $\ge 6.0\text{ mEq/L}$ (or $25\%$ increase from baseline);
• Hyperphosphatemia: Serum phosphorus $\ge 4.5\text{ mg/dL}$ (or $25\%$ increase from baseline);
• Hypocalcemia: Serum total calcium $\le 7.0\text{ mg/dL}$ or ionized calcium $< 1.12\text{ mmol/L}$ (or $25\%$ decrease from baseline).

2. Clinical TLS (CTLS)

Established when Laboratory TLS is present accompanied by at least one of the following clinical complications:

• Acute kidney injury (serum creatinine $\ge 1.5\times$ upper limit of normal or baseline);
• Cardiac arrhythmia or sudden cardiac death;
• Generalized seizures, tetany, or neuromuscular irritability.

Comprehensive Management Protocol in the ICU

1. Aggressive Isotonic Crystalloid Hydration:
   • Administer balanced crystalloids or $0.9\%$ saline at $2\text{ to }3\text{ L/m}^2/\text{day}$ (or $150\text{ to }200\text{ mL/h}$) to maintain high tubular flow and dilute intraluminal solute concentration.
   • Resuscitation target: Urine output $> 100\text{ mL/m}^2/\text{h}$ (approximately $> 2\text{ mL/kg/h}$). Add loop diuretics (furosemide) only if urine output fails to match high fluid intake after adequate volume repletion.
   • Note on Urinary Alkalinization: Historically, sodium bicarbonate infusions were used to alkalinize urine (target urine $\text{pH} > 7.0$) to increase uric acid solubility. Urinary alkalinization is no longer recommended: while it solubilizes uric acid, alkaline urine markedly decreases calcium-phosphate solubility, promoting fatal metastatic calcium phosphate precipitation in renal tubules and myocardium.
2. Uric Acid Lowering Therapy:
   • Rasburicase (Recombinant Urate Oxidase): Directly converts insoluble uric acid into allantoin, an inert, highly water-soluble metabolite readily excreted by the kidneys (solubility is $10\times$ higher than uric acid). Dose: $0.15\text{ to }0.2\text{ mg/kg}$ IV over 30 minutes (or a single fixed dose of $3\text{ to }6\text{ mg}$ IV in non-extreme cases). It drops serum uric acid levels within 4 hours.

     Warning

     Rasburicase Contraindication: Strictly contraindicated in patients with Glucose-6-Phosphate Dehydrogenase (G6PD) deficiency. Urate oxidase generates hydrogen peroxide ($H_2O_2$) during the enzymatic breakdown of uric acid. In G6PD-deficient erythrocytes that lack glutathione antioxidant capacity, $H_2O_2$ induces severe methemoglobinemia and acute intravascular hemolytic anemia.

   • Allopurinol: A competitive xanthine oxidase inhibitor ($300\text{ mg/day}$ PO or $200-400\text{ mg/m}^2/\text{day}$ IV). Allopurinol blocks the formation of new uric acid from hypoxanthine and xanthine, but does not degrade pre-existing uric acid crystals. It is used for low/intermediate-risk prophylaxis, not for acute established severe hyperuricemia.
3. Strict Calcium Management Rule in TLS:
   • In Tumor Lysis Syndrome, the calcium-phosphate product often exceeds $60\text{ to }70\text{ mg}^2/\text{dL}^2$.
   • Do NOT administer intravenous calcium to correct asymptomatic hypocalcemia in TLS! Exogenous calcium combines with excess circulating phosphate, driving precipitous systemic calcium phosphate crystal precipitation in the renal tubulointerstitium, cardiac conduction tissue, and pulmonary capillaries, rapidly exacerbating acute tubular necrosis and inducing fatal arrhythmias.
   • Intravenous calcium gluconate is indicated only if the patient develops life-threatening manifestations of hypocalcemia: tetany, generalized seizures, or documented ECG evidence of severe conduction block or refractory QTc prolongation.

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Potassium Pitfalls: Pseudohyperkalemia, Drug Causes and Hypokalemia Workup

Pseudohyperkalemia

A high potassium result with a normal ECG should prompt a search for pseudohyperkalemia, where potassium leaves cells in the tube rather than in the patient:

• Hemolysis from a difficult draw, small needles, or vigorous shaking.
• Repeated fist clenching and prolonged tourniquet time.
• Marked thrombocytosis (often above about $500{,}000/\mu\text{L}$) or leukocytosis (for example, chronic lymphocytic leukemia with very high counts), because cells release potassium as the sample clots.

Confirm with a heparinized plasma sample or a whole-blood potassium on the blood gas analyzer, run promptly without pneumatic-tube transport. Do not give potassium-lowering therapy for a result that the clinical picture does not support.

Drug-Induced and Endocrine Hyperkalemia

Hypokalemia: Where Is the Potassium Going?

• Transcellular shift: insulin, beta-2 agonists, alkalemia, refeeding, hypothermia, thyrotoxic or familial periodic paralysis, and barium or chloroquine toxicity. Total body potassium is normal, so replacement can cause rebound hyperkalemia.
• Vomiting or nasogastric suction: gastric fluid holds little potassium. The loss is mainly renal: metabolic alkalosis delivers bicarbonate to the collecting duct, and volume depletion activates aldosterone. The urine chloride is low (below about $20\text{ mEq/L}$) even when urine sodium is not.
• Diarrhea: direct gastrointestinal loss with a normal anion gap metabolic acidosis.
• Renal potassium wasting: a spot urine potassium-to-creatinine ratio above about $13\text{ mEq/g}$ (about $1.5\text{ mEq/mmol}$) points to renal loss: diuretics, mineralocorticoid excess, magnesium depletion, amphotericin B, renal tubular acidosis, or osmotic diuresis. A lower ratio suggests extrarenal loss or a shift.

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Magnesium Disorders

Hypomagnesemia

• Causes: chronic proton-pump inhibitor use, loop and thiazide diuretics, alcohol use disorder, diarrhea and malabsorption, refeeding, and nephrotoxic drugs (aminoglycosides, amphotericin B, cisplatin, foscarnet, pentamidine, calcineurin inhibitors, EGFR antibodies such as cetuximab).
• Consequences: refractory hypokalemia (loss of magnesium's block of ROMK channels), hypocalcemia (impaired PTH secretion and PTH resistance), tremor, tetany, seizures, QT prolongation, torsades de pointes, and atrial fibrillation.
• Treatment: give $1\text{--}2\text{ g}$ IV magnesium sulfate for symptomatic deficiency, over about 5 to 15 minutes in torsades with a pulse (faster only in pulseless arrest), then a slower infusion because much of a rapid bolus is excreted. Reduce doses in kidney failure.

Hypermagnesemia

Hypermagnesemia is almost always iatrogenic with reduced kidney function: magnesium sulfate infusions for preeclampsia, magnesium-containing laxatives, enemas, or antacids.

Treatment: stop all magnesium; give IV calcium (calcium gluconate $1\text{--}2\text{ g}$) to antagonize the membrane effects; give isotonic fluid with a loop diuretic if kidney function allows; and use hemodialysis for severe toxicity with kidney failure. Support ventilation if reflexes and respiratory effort are lost.