Prerenal FeNa
FeNa < 1% and BUN/Cr > 20:1 (kidneys retain sodium avidly)
ATN Hallmark
Muddy brown granular casts; FeNa > 2%; urine osm < 350
Winter's Formula
Expected PaCO2 = 1.5 × [HCO3-] + 8 ± 2 (respiratory compensation)
Anion Gap Equation
AG = Na+ - (Cl- + HCO3-) (normal = 8–12 mEq/L)
Prerenal vs. Intrinsic vs. Postrenal AKI Matrix
Differentiating the etiology of an abrupt rise in serum creatinine:
| Diagnostic Parameter | Prerenal Azotemia | Intrinsic (Acute Tubular Necrosis) | Postrenal (Obstructive) |
|---|---|---|---|
| Pathophysiology | Renal hypoperfusion (dehydration, HF, sepsis) | Tubular epithelial necrosis (ischemic or nephrotoxic) | Bilateral outflow tract obstruction (BPH, stones, tumor) |
| BUN / Creatinine Ratio | > 20:1 (proximal tubule reabsorbs urea) | < 15:1 (damaged tubule cannot reabsorb) | Variable (initially > 20:1, then < 15:1) |
| Fractional Excretion of Sodium (FeNa) | < 1% | > 2% (impaired tubular sodium reabsorption) | > 2% (in chronic obstruction) |
| Urine Osmolality | > 500 mOsm/kg (concentrated) | < 350 mOsm/kg (isosthenuric) | < 350 mOsm/kg |
| Urine Sodium | < 20 mEq/L | > 40 mEq/L | > 40 mEq/L |
| Urine Sediment | Hyaline casts or normal | Muddy brown granular casts, renal tubular cells | Crystals, hematuria, or bland |
Systematic 4-Step Acid-Base Interpretation
Follow this exact algorithmic sequence on all ABG board vignettes:
Step 1: Determine Primary Process
Check pH: < 7.35 = Acidemia; > 7.45 = Alkalemia. Match with PaCO2 (respiratory) or HCO3- (metabolic).
Step 2: Check Anion Gap (If Metabolic Acidosis)
AG = Na - (Cl + HCO3). If AG > 12: High Anion Gap Metabolic Acidosis (HAGMA). Causes: MUDPILES (Methanol, Uremia, DKA, Propylene glycol, Iron/Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates).
Step 3: Calculate Winter's Formula for Compensation
Expected PaCO2 = 1.5 × [HCO3-] + 8 ± 2. If measured PaCO2 > expected = concomitant respiratory acidosis (hypoventilation). If measured PaCO2 < expected = concomitant respiratory alkalosis.
Step 4: Check Delta-Delta (In HAGMA)
ΔΔ = (ΔAG) / (ΔHCO3) = (AG - 12) / (24 - HCO3). If ratio < 0.8: concomitant Non-Anion Gap Acidosis (NAGMA). If ratio > 2.0: concomitant Metabolic Alkalosis.
COMLEX / OMM Integration
NBOME High-Yield Correlate
Renal Autonomics & Chapman Reflexes
- Autonomic Innervation: Sympathetic preganglionic fibers for the kidneys and upper ureters originate from
- Parasympathetics:
- Chapman Reflex:
T10–T11 and synapse in the aorticorenal and superior mesenteric ganglia.- Parasympathetics:
Vagus nerve (CN X) innervates the kidneys and upper ureters!- Chapman Reflex:
1 inch superior and 1 inch lateral to the umbilicus anteriorly.
Board Traps & Common Distractors
- Trap: Using FeNa in a patient on loop diuretics. Diuretics force natriuresis, causing FeNa to be falsely elevated > 1% in prerenal states. Use Fractional Excretion of Urea (FeUrea < 35% indicates prerenal) instead.
- Emergent Indications for Dialysis (AEIOU): Acidosis (refractory pH < 7.1), Electrolytes (refractory K+ > 6.5 with ECG changes), Ingestions (toxic alcohols, lithium, salicylates), Overload (refractory pulmonary edema), Uremia (pericarditis, encephalopathy, bleeding).