Acute Kidney Injury & Acid-Base Disorders

In Prerenal AKI (true hypovolemia, hemorrhage, or decreased effective arterial blood volume in CHF/cirrhosis), intact renal tubular epithelial cells respond to angiotensin II, aldosterone, and antidiuretic hormone (ADH) by avidly reabsorbing sodium, water, and urea. Consequently, urine sodium is low (U_Na <20 mEq/L), fractional excretion of sodium is <1% (FE_Na <1%), urine osmolality is concentrated (>500 mOsm/kg), and passive proximal tubular urea reabsorption elevates the serum BUN/Creatinine ratio to >20:1 (NOTE: if the patient is taking loop or thiazide diuretics, natriuresis falsely elevates U_Na and FE_Na, so Fractional Excretion of Urea [FE_Urea ≤35%] must be used instead!). In contrast, once prolonged ischemia or nephrotoxins cause sloughing of tubular brush-border cells—Intrinsic Acute Tubular Necrosis (ATN)—the injured tubules cannot reabsorb sodium or concentrate urine, yielding U_Na >40 mEq/L, FE_Na >2%, isosthenuric urine osmolality (<350 mOsm/kg), a normal BUN/Cr ratio (10–15:1), and pathognomonic muddy brown granular casts.

Ischemic ATN preferentially damages the metabolically demanding straight segment (S3) of the proximal tubule and the thick ascending limb of the loop of Henle in the outer medulla, whereas nephrotoxic ATN affects the proximal convoluted tubule—triggered by aminoglycosides, iodinated contrast, amphotericin B, cisplatin, myoglobinuria (rhabdomyolysis: blood positive on dipstick with zero RBCs on microscopy), or hemoglobinuria. Distinguish ATN from Acute Interstitial Nephritis (AIN)—a Type IV hypersensitivity reaction occurring 5 to 14 days after starting beta-lactams (penicillins/cephalosporins), sulfonamides, NSAIDs, rifampin, or proton pump inhibitors—which presents with fever, maculopapular rash, peripheral eosinophilia, sterile pyuria, white blood cell casts, and eosinophiluria; AIN is treated by stopping the offending drug (plus corticosteroids if renal recovery stalls). Regardless of AKI etiology, emergent hemodialysis is indicated for the refractory AEIOU criteria: Acidosis (pH <7.1), Electrolytes (refractory hyperkalemia >6.5 mEq/L or ECG changes), Ingestions (lithium, methanol, ethylene glycol, salicylates), Overload (diuretic-refractory pulmonary edema), and Uremia (uremic pericarditis, encephalopathy, or platelet bleeding).

When evaluating a patient with a severe High Anion Gap Metabolic Acidosis (HAGMA) and altered mental status, calculate the Serum Osmolal Gap = Measured Serum Osmolality minus Calculated Osmolality, where Calculated Osmolality = 2 × [Na⁺] + (Glucose / 18) + (BUN / 2.8) + (Ethanol / 4.6). An Osmolal Gap >10 mOsm/kg accompanied by HAGMA indicates acute ingestion of a toxic alcohol metabolized by alcohol dehydrogenase and aldehyde dehydrogenase into organic acids: Methanol (windshield washer fluid/moonshine) is metabolized to formic acid, causing optic disc hyperemia, visual 'snowstorm,' and blindness plus bilateral putaminal necrosis on CT; Ethylene glycol (antifreeze) is metabolized to glycolic and oxalic acid, causing flank pain, hypocalcemia, cranial neuropathies, acute renal failure, and envelope-shaped calcium oxalate monohydrate crystals in the urine (which fluoresce under Wood lamp if fluorescein was added to antifreeze). Both are treated immediately with Fomepizole (competitive alcohol dehydrogenase inhibitor) and hemodialysis. In contrast, Isopropyl alcohol (rubbing alcohol) is metabolized to acetone (a ketone body, not an organic acid), producing an elevated Osmolal Gap, positive serum/urine ketones, and CNS depression WITHOUT metabolic acidosis (normal pH and normal anion gap).

Metabolic alkalosis (pH >7.45, elevated HCO₃^-, with compensatory hypoventilation increasing PaCO₂ by 0.7 mmHg per 1 mEq/L rise in HCO₃^-) is stratified diagnostically using a spot Urine Chloride (U_Cl) concentration: (1) Saline-Responsive Metabolic Alkalosis (U_Cl <20 mEq/L) results from extracellular chloride and volume depletion—classically recurrent vomiting or nasogastric tube suction (loss of gastric HCl plus secondary hyperaldosteronism from hypovolemia) or prior/remote loop or thiazide diuretic use; volume repletion with intravenous 0.9% Normal Saline restores distal chloride delivery and allows the cortical collecting duct pendrin exchanger to excrete bicarbonate. (2) Saline-Resistant Metabolic Alkalosis (U_Cl >20 mEq/L) reflects autonomous mineralocorticoid excess or ongoing tubular transport defects: if the patient is hypertensive, evaluate for Primary Hyperaldosteronism (Conn syndrome), Cushing syndrome, or Liddle syndrome; if the patient is normotensive, evaluate for Bartter syndrome (thick ascending limb NKCC2 defect mimicking loop diuretics, with hypercalciuria), Gitelman syndrome (DCT NCC defect mimicking thiazides, with hypocalciuria and severe hypomagnesemia), or active surreptitious diuretic abuse.

In prerenal azotemia (hypovolemia, hemorrhage, congestive heart failure, cirrhosis/hepatorenal syndrome), decreased renal perfusion activates the RAAS and sympathetic nervous system. Angiotensin II constricts the efferent arteriole, which preserves GFR while reducing downstream peritubular capillary hydrostatic pressure and increasing Filtration Fraction (FF = GFR / RPF). Because more protein-free fluid is filtered at the glomerulus, the blood entering the peritubular capillaries has a markedly elevated oncotic pressure (π_c) and low hydrostatic pressure (P_c). These Starling forces drive avid proximal tubular reabsorption of sodium and water, which passively pulls urea back into the blood. Simultaneously, Antidiuretic Hormone (ADH / vasopressin) inserts aquaporin-2 channels and urea transporters in the medullary collecting duct to further reabsorb water and urea. Because creatinine is freely filtered and actively secreted without ever being reabsorbed, serum BUN rises disproportionately to creatinine, yielding a BUN : Creatinine ratio > 20:1, Urine Osmolality > 500 mOsm/kg, and Urine Na⁺ < 20 mEq/L.

The Fractional Excretion of Sodium (FeNa) measures the percentage of filtered sodium excreted in the urine: FeNa (%) = [(U_Na × P_Cr) / (P_Na × U_Cr)] × 100. In prerenal AKI with intact tubular epithelium, aldosterone and Angiotensin II reclaim > 99% of filtered sodium (FeNa < 1%). In intrinsic Acute Tubular Necrosis (ATN), sloughed necrotic tubular cells cannot reabsorb sodium, leaking sodium into the urine (FeNa > 2%). However, if a prerenal patient has received a loop or thiazide diuretic (or has chronic kidney disease or aldosterone deficiency), urinary sodium wasting falsely elevates FeNa > 1%. Because urea reabsorption in the proximal tubule is unaffected by loop/thiazide diuretics, clinicians calculate the Fractional Excretion of Urea (FeUrea): FeUrea < 35% confirms Prerenal AKI, whereas FeUrea > 50% indicates ATN. Also remember that an upper GI bleed elevates BUN/Cr > 30:1 due to intestinal digestion and hepatic urea conversion of blood hemoglobin proteins.

Postrenal AKI accounts for 5–10% of cases and is most common in older men with benign prostatic hyperplasia (BPH) or prostate cancer, women with pelvic/cervical malignancy encasing both ureters, patients with retroperitoneal fibrosis (methysergide, IgG4 disease, lymphoma), or neurogenic bladder (anticholinergics, spinal cord injury, multiple sclerosis). Because a single normal kidney can maintain a normal serum creatinine, ureteral stones cause AKI only if there is bilateral ureteral obstruction or obstruction of a solitary functioning kidney (e.g., renal transplant). Initial evaluation starts with Foley urethral catheterization (immediately diagnostic and therapeutic for bladder outlet obstruction, yielding high post-void residual > 200–500 mL) and Renal Ultrasound to detect hydronephrosis. After decompressing severe bilateral obstruction, watch closely for post-obstructive diuresis (polyuria > 200 mL/hr caused by urea osmotic diuresis and tubular ADH resistance), replacing 50% of urine output with 0.45% NaCl to prevent hypovolemic shock and hypokalemia.

On board examinations, ischemic ATN selectively damages two specific nephron segments located in the outer medulla: (1) the straight segment (S3 / pars recta) of the proximal convoluted tubule, and (2) the medullary Thick Ascending Limb (mTAL) of the loop of Henle. This exquisite sensitivity results from a physiological mismatch between oxygen supply and demand: both segments perform massive active ATP-dependent sodium transport via basolateral Na⁺/K⁺-ATPase pumps, yet the renal medulla receives < 10% of total renal blood flow and operates at a borderline hypoxic baseline (PO₂ ~10–20 mmHg) because oxygen diffuses directly from descending to ascending vasa recta capillaries via countercurrent exchange.

Acute Tubular Necrosis progresses through three distinct clinical stages: (1) Initiation Phase (hours to days): the acute ischemic or toxic insult reduces GFR as tubular epithelial cells lose brush borders and polarity; (2) Maintenance (Oliguric) Phase (typically 1 to 3 weeks): GFR stabilizes at its nadir (5–10 mL/min) due to intratubular cast obstruction and afferent arteriolar vasoconstriction triggered by tubuloglomerular feedback (high NaCl delivery to the macula densa constricts the afferent arteriole via adenosine). During this oliguric phase (< 400 mL urine/day), patients are at peak risk for life-threatening volume overload, hyperkalemia, metabolic acidosis, and uremia; and (3) Recovery (Polyuric / Diuretic) Phase: tubular epithelial cells regenerate along the basement membrane, clearing luminal casts and restoring glomerular filtration before the immature regenerating tubular cells regain their capacity to reabsorb sodium, potassium, and water. This produces massive polyuria (3–5+ L/day) with life-threatening hypovolemia, hypokalemia, and hypomagnesemia.

A classic board distractor asks you to differentiate Contrast-Induced AKI from Renal Atheroembolic Disease (Cholesterol Crystal Embolization) in an elderly vasculopath who develops renal failure after cardiac catheterization, coronary angiography, or aortic surgery. Contrast nephropathy presents within 24–48 hours of dye exposure with an isolated creatinine bump that resolves within 7 days and lacks systemic signs. Conversely, Cholesterol Atheroembolism occurs when mechanical catheter manipulation scrapes ulcerative aortic atherosclerotic plaques, showering cholesterol crystals into renal arcuate/interlobular arteries and lower-extremity arterioles. It presents days to weeks later with progressive, often irreversible renal failure accompanied by livedo reticularis, 'purple/blue toes' with intact pedal pulses, Hollenhorst retinal plaques, peripheral blood eosinophilia, eosinophiluria, and hypocomplementemia (low C3). Renal biopsy reveals pathognomonic biconvex, needle-shaped clefts within occluded arterioles where cholesterol crystals dissolved during tissue fixation.

While the classic triad of Acute Interstitial Nephritis (AIN)—fever, maculopapular rash, and peripheral eosinophilia—is memorable, all three features appear simultaneously in only 10–15% of patients. On board vignettes, always suspect AIN when an inpatient or outpatient develops a rising serum creatinine 1 to 2 weeks after starting a beta-lactam antibiotic (nafcillin, piperacillin, cephalexin), a sulfonamide (TMP-SMX, furosemide, thiazide), an NSAID, or a Proton Pump Inhibitor (omeprazole, pantoprazole). Note that PPI-induced AIN often presents insidiously weeks to months after initiation without fever or rash! Urinalysis is the diagnostic cornerstone, showing sterile pyuria (WBCs with negative bacterial culture), White Blood Cell (WBC) casts (which localize inflammation strictly to the kidney, seen only in AIN and acute pyelonephritis), mild sub-nephrotic proteinuria, hematuria, and eosinophiluria on Hansel or Wright stain.

The renal medulla and papillae exist in a hypertonic, hypoxic microenvironment perfused exclusively by the fragile vasa recta. In analgesic nephropathy (chronic ingestion of NSAIDs combined with acetaminophen or aspirin), NSAIDs block vasodilatory prostaglandins while acetaminophen metabolites deplete glutathione, causing oxidative papillary necrosis and chronic tubulointerstitial nephritis. Similarly, in Sickle Cell Trait (HbAS) as well as Sickle Cell Disease (HbSS), the extreme hypertonicity, hypoxia, and acidosis of the renal medulla induce sickling of erythrocytes within the vasa recta, causing microinfarctions of the renal papillae. Consequently, a young Black patient presenting with painless gross hematuria or flank pain with sloughed tissue and isosthenuria (inability to concentrate urine) has renal papillary necrosis from sickle cell trait until proven otherwise.

When a patient with underlying chronic kidney disease (eGFR < 45–60 mL/min/1.73m²), diabetes mellitus, or heart failure requires an urgent contrast-enhanced study (such as coronary angiography for STEMI or CT angiography for aortic dissection), life-saving imaging should never be withheld. However, three evidence-based preventive measures must be executed: (1) Administer IV Isotonic Crystalloid (0.9% Normal Saline at 1 mL/kg/hr for 6–12 hours pre- and post-procedure) to expand intravascular volume, dilute tubular contrast concentration, and suppress renin/vasopressin; (2) Use the lowest possible volume of iso-osmolar or low-osmolar non-ionic contrast media; and (3) Temporarily hold nephrotoxic medications (NSAIDs, diuretics) and hold Metformin for 48 hours (to prevent metformin-associated Type B lactic acidosis if contrast AKI occurs). Based on the PRESERVE trial, routine oral N-acetylcysteine (NAC) and IV sodium bicarbonate offer no additional benefit over isotonic normal saline.

Extracellular potassium concentration directly governs the resting membrane potential of cardiac myocytes via the Nernst equation. As serum K⁺ rises above 5.5 mEq/L, the resting membrane potential becomes less negative (partially depolarized), which initially accelerates phase 3 repolarization (via I_Kr channel activation) before inactivating fast voltage-gated Na⁺ channels and slowing phase 0 conduction velocity. This produces a strict chronological ECG progression in Hyperkalemia: (1) Tall, narrow-based, symmetric 'peaked' (tented) T waves (K⁺ 5.5–6.5), (2) Prolonged PR interval and flattening/loss of the P wave (K⁺ 6.5–7.5), (3) Widening of the QRS complex (K⁺ 7.0–8.0), and (4) Fusion of the wide QRS and T wave into a sinusoidal 'Sine Wave' pattern (> 8.0) degenerating into ventricular fibrillation or asystole. Conversely, Hypokalemia (K⁺ < 3.5 mEq/L) causes T-wave flattening/inversion, ST depression, prominent U waves (best seen in leads V₂–V₃), and pseudo-QT prolongation predisposing to Torsades de Pointes (always replete Magnesium first in refractory hypokalemia!).

When conservative medical therapy fails in AKI, emergent hemodialysis is initiated for any of the AEIOU criteria: (1) A — Acidosis: severe metabolic acidosis (pH < 7.10) refractory to IV sodium bicarbonate or unable to tolerate sodium/volume load; (2) E — Electrolytes: severe hyperkalemia (K⁺ > 6.5 mEq/L) refractory to medical shifting/excretion or accompanied by persistent ECG changes; (3) I — Ingestions (Dialyzable Toxins): water-soluble, low-volume-of-distribution, non-protein-bound toxins—memorize Methanol, Ethylene Glycol, Lithium, Salicylates (Aspirin), Theophylline, and Valproate (note: tricyclic antidepressants, digoxin, beta-blockers, and benzodiazepines are NOT dialyzable); (4) O — Overload: diuretic-refractory cardiogenic/pulmonary edema causing hypoxemia; and (5) U — Uremia: symptomatic uremic complications, specifically uremic pericarditis (friction rub, pleuritic pain—heparin-free dialysis is mandatory to prevent hemorrhagic tamponade!), uremic encephalopathy (asterixis, confusion, seizures), or refractory uremic bleeding. Uremic bleeding is caused by guanidinosuccinic acid impairing platelet GP IIb/IIIa aggregation and adhesion (prolonged bleeding time with normal platelet count, normal PT, and normal aPTT); treat acutely with DDAVP (Desmopressin), which releases von Willebrand factor and Factor VIII from endothelial Weibel-Palade bodies.

When evaluating a patient with Normal Anion Gap (Hyperchloremic) Metabolic Acidosis (NAGMA, Anion Gap 8–12 mEq/L)—remembered by HARDASS (Hyperalimentation/0.9% NS, Acetazolamide/Addison disease, Renal Tubular Acidosis, Diarrhea, Uteroenterostomy, Spironolactone, Saline)—the next diagnostic step is calculating the Urine Anion Gap (UAG) = (U_Na + U_K) − U_Cl to estimate urinary ammonium (NH₄⁺) excretion. In Lower GI Diarrhea (bicarbonate lost in stool), healthy kidneys compensate by excreting abundant NH₄Cl, raising urinary chloride above Na⁺+K⁺ and yielding a NEGATIVE Urine Anion Gap ('NeGUTive'). Conversely, in Renal Tubular Acidosis (RTA), impaired renal acid excretion prevents NH₄Cl synthesis, yielding a POSITIVE Urine Anion Gap: Type 1 (Distal) RTA (alpha-intercalated H⁺-ATPase failure: urine pH > 5.5, hypokalemia, calcium phosphate kidney stones, Sjögren/amphotericin), Type 2 (Proximal) RTA (PCT HCO₃^- reabsorption defect: urine pH < 5.5 once serum HCO₃^- drops, hypokalemia, Fanconi syndrome/multiple myeloma/acetazolamide), and Type 4 RTA (hyporeninemic hypoaldosteronism in diabetes/ACEi/spironolactone/TMP-SMX: urine pH < 5.5 with HYPERKALEMIA).