Assessment of volume status

Volume Status - The Body's Balance

• Hypovolemia (Dehydration):

  • Vitals: Tachycardia, hypotension, orthostasis.
  • Signs: ↓ skin turgor, dry membranes, flat neck veins.
  • Labs: BUN/Cr > 20:1, ↑ hematocrit, Urine Na+ < 20 mEq/L.

• Hypervolemia (Overload):

  • Vitals: Hypertension, bounding pulses.
  • Signs: Jugular venous distention (JVD), pulmonary crackles, pitting edema.
  • Labs: ↓ BUN/Cr, ↓ hematocrit (dilutional), Urine Na+ > 20 mEq/L.

⭐ In hypovolemia, a BUN/Cr ratio > 20:1 is a classic indicator of pre-renal azotemia, as the kidneys avidly reabsorb urea with sodium and water.

Physical Exam - Hands-On Assessment

• Hypovolemia (Volume Depletion)

  • Vitals: ↑ Heart Rate, ↓ Blood Pressure, Orthostatic hypotension.
  • Neck Veins: ↓ Jugular Venous Pressure (JVP < 3 cmH₂O).
  • Skin & Mucosa: ↓ Skin turgor (tenting), dry mucous membranes, delayed capillary refill (>2 sec).
  • Urine: ↓ Urine output (<0.5 mL/kg/hr).

• Hypervolemia (Volume Overload)

  • Vitals: ↑ Blood Pressure.
  • Neck Veins: ↑ JVP (>8 cmH₂O), positive hepatojugular reflux.
  • Cardiopulmonary: S3 gallop, pulmonary edema (crackles/rales).
  • Extremities: Pitting peripheral edema.
  • Abdomen: Ascites.

⭐ Orthostatic Hypotension: A sensitive early indicator of hypovolemia. Defined as a drop in systolic BP by ≥20 mmHg or diastolic BP by ≥10 mmHg within 3 minutes of standing.

Lab & Imaging - The Inside Story

• Hypovolemia Markers (Dehydration):

  • Labs:
    • BUN/Cr Ratio: >20:1 (prerenal azotemia)
    • Hematocrit/Albumin: ↑ (hemoconcentration)
    • Urine Sodium: <20 mEq/L (kidneys conserve Na)
    • Fractional Excretion of Na (FENa): <1%
    • Urine Osmolality: >450 mOsm/kg (concentrated)
  • Formula: FENa = $100 \times \frac{(Urine_{Na} \times Plasma_{Cr})}{(Plasma_{Na} \times Urine_{Cr})}$

• Hypervolemia Markers (Fluid Overload):

  • Labs: ↓ Hct/Albumin (hemodilution); BNP >100 pg/mL
  • Imaging:
    • CXR: Pulmonary edema, Kerley B lines, pleural effusion.
    • IVC Ultrasound: Distended (>2.1 cm) with <50% respiratory collapse.

⭐ In oliguria, a FENa <1% suggests a prerenal (volume-responsive) cause, while FENa >2% points towards Acute Tubular Necrosis (ATN).

Invasive Monitoring - The Deep Dive

• Arterial Line: Provides continuous, real-time blood pressure and mean arterial pressure (MAP). Essential for titrating vasoactive infusions and managing hemodynamically unstable patients. Allows for frequent arterial blood gas sampling.
• Central Venous Pressure (CVP): Measures right atrial pressure to estimate right ventricular preload. Normal: 2-8 mmHg. Note: CVP is a static measure; trends over time provide more reliable information for fluid management.
• PA Catheter:

  ⭐ Measures PCWP, a surrogate for left atrial pressure. PCWP > 18 mmHg suggests cardiogenic pulmonary edema.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypovolemia presents with tachycardia, hypotension, poor skin turgor, and orthostatic changes.
• Key lab findings suggesting dehydration include a BUN/Cr ratio > 20:1, low urine sodium (< 20 mEq/L), and FeNa < 1%.
• Hypervolemia is characterized by jugular venous distention (JVD), pulmonary crackles, and pitting peripheral edema.
• An S3 gallop on cardiac auscultation is a specific sign of significant volume overload.
• Invasive monitoring shows low CVP and PCWP in hypovolemia, and high CVP/PCWP in hypervolemia.