Acute kidney injury and electrolyte emergencies share a common thread: the pathophysiology is traceable from mechanism to lab finding to clinical consequence to intervention. The clinician who understands the chain does not need to memorize the management — they can derive it. The clinician who memorizes without mechanism will misapply it under the pressure of an unfamiliar presentation.

Acute kidney injury: classification by mechanism.

The KDIGO (Kidney Disease: Improving Global Outcomes) criteria define AKI as any of the following: serum creatinine increase of ≥0.3 mg/dL within 48 hours; serum creatinine increase to ≥1.5 times baseline within 7 days; or urine output <0.5 mL/kg/hour for 6 or more hours. These thresholds reflect the kidneys’ functional reserve: creatinine does not rise until approximately 50% of functional nephrons are impaired, meaning early AKI can be missed if only creatinine is tracked.

AKI is classified by the location of the problem relative to the nephron.

Prerenal AKI is the most common type and the most reversible. Reduced renal perfusion pressure — from hypovolemia, hypotension, heart failure, or vasoconstriction — leads to reduced GFR without intrinsic tubular damage. The kidney attempts to compensate by maximally reabsorbing sodium and water: urine sodium is low (<20 mEq/L), urine is concentrated (osmolality >500 mOsm/kg), and the BUN:creatinine ratio is elevated (>20:1). Prerenal AKI responds to volume restoration and perfusion pressure support. Left untreated, sustained hypoperfusion progresses to intrinsic injury (acute tubular necrosis).

Intrinsic AKI involves damage to the kidney itself. Acute tubular necrosis (ATN) — from ischemia (prolonged prerenal insult) or nephrotoxins (aminoglycosides, contrast, NSAIDs, myoglobin) — is the most common intrinsic pattern. The damaged tubules can no longer concentrate urine or conserve sodium: urine sodium is high (>40 mEq/L), urine is isoosmotic, and the BUN:creatinine ratio normalizes (<10–15:1). Muddy brown granular casts in the urine are the microscopic hallmark of ATN. Recovery follows tubular regeneration over 1–4 weeks if the insult is removed.

Postrenal AKI results from obstruction to urinary flow. Bilateral obstruction (or unilateral in a solitary kidney) is required to cause AKI. Bladder outlet obstruction from prostatic hypertrophy, malignancy, or neurogenic bladder is the most common cause. Foley catheter placement resolves outlet obstruction immediately; upper tract obstruction requires urology or interventional radiology. Post-obstructive diuresis — massive urine output after relief of chronic obstruction — is a real complication requiring careful fluid management.

Life-threatening electrolyte emergencies.

Several electrolyte abnormalities are immediately life-threatening and require recognition on the monitor or physical exam before the lab result is back.

Hyperkalemia is the electrolyte emergency with the most immediately lethal cardiac complication. As potassium rises, the cardiac conduction system depolarizes progressively: peaked T waves → PR prolongation → QRS widening → sine wave pattern → ventricular fibrillation. Management is sequential and mechanism-targeted:

  • Calcium gluconate or chloride (1–2 g IV over 10 minutes): membrane stabilization. Does not lower potassium. Acts within minutes. Indicated for any hyperkalemia with ECG changes.
  • Insulin (10 units regular) + dextrose (25 g D50): shifts potassium into cells. Onset 15–30 minutes, lasts 4–6 hours. Glucose is given simultaneously to prevent hypoglycemia.
  • Sodium bicarbonate (in metabolic acidosis): shifts potassium intracellularly by raising pH. Less reliable than insulin as a monotherapy.
  • Kayexalate or patiromer: removes potassium from the body via the gut. Onset is slow (hours); not for acute management.
  • Dialysis: definitive removal in severe or refractory hyperkalemia.

Symptomatic hyponatremia (sodium <120–125 mEq/L with seizures, altered consciousness, or severe symptoms) is treated with 3% hypertonic saline. The target correction rate is 1–2 mEq/L per hour until symptoms resolve, then no more than 8–10 mEq/L per 24 hours total. Overcorrection causes osmotic demyelination syndrome (central pontine myelinolysis) — an irreversible neurological injury that develops over 24–48 hours after too-rapid correction. Chronic hyponatremia (present for >48 hours) carries the highest risk for osmotic demyelination because the brain has adapted by extruding osmolytes. Acute hyponatremia (onset <48 hours) can be corrected more rapidly without the same risk. The clinical challenge: the onset timing is often unknown.

Critical hypokalemia (potassium <2.5 mEq/L with dysrhythmia or respiratory muscle weakness) requires aggressive parenteral replacement. Oral replacement is preferred when the gut works and the situation is not immediately life-threatening. IV potassium should not exceed 10–20 mEq/hour through peripheral access (pain, phlebitis risk) or 20–40 mEq/hour through central access with continuous cardiac monitoring. Co-existing hypomagnesemia prevents potassium repletion — magnesium must be repleted in parallel.

Hypercalcemia in the high-acuity setting most commonly reflects malignancy (PTH-related peptide, osteolytic metastases) or primary hyperparathyroidism. Symptomatic hypercalcemia (>12–14 mg/dL with symptoms, or any level with severe symptoms) is treated: isotonic saline hydration first (to increase urinary calcium excretion), followed by calcitonin for rapid early effect (works within hours, tachyphylaxis develops in 24–48 hours), followed by bisphosphonates (onset 48–72 hours, more durable effect). Steroids are effective in specific hypercalcemia mechanisms (vitamin D-mediated, hematologic malignancy).

The exam relevance.

AKI and electrolyte questions on CCRN and CEN test pattern recognition from a minimal clinical dataset. A patient with BUN:creatinine ratio of 28:1, low urine sodium, and concentrated urine is prerenal until volume is restored. A patient with peaked T waves and potassium of 7.2 needs calcium before anything else — not kayexalate, not bicarbonate first. A patient with sodium of 115 who had it for three weeks should not have their sodium corrected by more than 8–10 mEq in the first 24 hours regardless of how symptomatic they are. The mechanism is the answer. The lab is the prompt.