Fluid and Sodium Disorders Study Notes

Fluid Physiology & Body Water

• Total Body Water (TBW) proportion by population group:

  • Infants: $0.7\,\text{L/kg}$

  • Children & Adult Males: $0.6\,\text{L/kg}$

  • Adult Females: $0.5\,\text{L/kg}$

  • Elderly ($>70\,\text{years}$): $0.45\,\text{L/kg}$

• Daily fluid requirement: $25\text{--}30\,\text{mL/kg/day}$ (approximately $2500\,\text{mL/day}$).

• Plasma osmolality range: $280\text{--}310\,\text{mOsm/L}$.

• Primary fluid regulation hormones: Arginine vasopressin (AVP / ADH), aldosterone, angiotensin II, brain/atrial natriuretic peptides (BNP/ANP), and cortisol.

Hypervolemia Management

• Clinical manifestations: Shortness of breath, elevated blood pressure, jugular venous distension (JVD), pulmonary crackles, and peripheral/sacral pitting edema.

• Common etiologies: Heart failure, cirrhosis, kidney disease, critical illness, and drugs (corticosteroids, NSAIDs, thiazolidinediones, excessive IV fluids).

• Non-pharmacologic management: Dietary sodium restriction ($1\text{--}2\,\text{g/day}$) and fluid restriction ($1\text{--}2\,\text{L/day}$).

• Pharmacologic therapy: Intermittent loop diuretics (furosemide, torsemide, bumetanide, ethacrynic acid).

• Loop Diuretic Conversions:

  • Oral equivalent doses: Oral furosemide $40\,\text{mg}$ = torsemide $20\,\text{mg}$ = bumetanide $1\,\text{mg}$ = ethacrynic acid $50\,\text{mg}$.

  • IV to PO dosing ratios: Furosemide 1:2 (IV $20\,\text{mg}$ = PO $40\,\text{mg}$); Torsemide 1:1; Bumetanide 1:1; Ethacrynic acid 1:1.

  • Ethacrynic acid is non-sulfonamide based and reserved for true sulfa allergies.

• Adverse effects: Volume depletion, hypotension, hypokalemia, hypomagnesemia, hyperglycemia, hyperuricemia, hypochloremic metabolic alkalosis, and ototoxicity.

Hypovolemia & IV Fluid Therapy

• Clinical manifestations: Thirst, dizziness, muscle cramps, hypotension, tachycardia, poor skin turgor, dry mucous membranes, reduced JVP, and elevated BUN/SCr ratio ($>20:1$).

• Oral Rehydration Therapy (ORT): Preferred for mild-to-moderate hypovolemia in alert patients. Requires glucose-sodium co-transport solutions (Pedialyte with $45\,\text{mEq/L}$\,$\text{Na}^+$, DripDrop ORS with $\sim 60\,\text{mEq/L}$\,$\text{Na}^+$). Plain water and commercial sports drinks are inappropriate for ORT.

• 5 Rs of IV Fluid Therapy: Resuscitation, Routine maintenance, Replacement, Redistribution, and Reassessment.

• Fluid Resuscitation Protocol:

  • Indicated for shock signs: Systolic BP $<100\,\text{mmHg}$, HR $>90\,\text{bpm}$, or RR $>20\,\text{breaths/min}$.

  • Bolus: $500\text{--}1000\,\text{mL}$ IV crystalloid over $15\text{--}30\,\text{minutes}$; repeat as needed to a maximum of $2000\,\text{mL}$ total ($\sim 30\,\text{mL/kg}$).

• Routine Maintenance IV Fluids: $25\text{--}35\,\text{mL/kg/day}$. Typical fluid: D5W with $0.45\%$\,NaCl plus $20\text{--}40\,\text{mEq/L}$\,KCl.

• Fluid Properties:

  • $0.9\%$\,NaCl (Normal Saline): Isotonic ($308\,\text{mOsm/L}$). High volumes cause hyperchloremic metabolic acidosis.

  • Balanced Crystalloids (Lactated Ringer's, Plasma-Lyte A): Preferred in sepsis, trauma, DKA, and surgery. Avoid LR in traumatic brain injury or severe liver disease.

  • $5\%$\,Dextrose (D5W): Isotonic in container, functions as hypotonic free water in vivo after dextrose metabolism.

  • Colloids (Albumin $5\%$, $25\%$): Provides intravascular volume expansion; indicated for third-spacing in cirrhosis or nephrotic syndrome.

Hyponatremia (Serum Sodium < 136 mEq/L)

• Diagnostic Subtypes:

  • Hypovolemic: $\text{TBW}\downarrow$, $\text{Na}^+\downarrow\downarrow$. Urine $\text{Na}^+ < 20\,\text{mEq/L}$ (extrarenal loss) vs $> 20\,\text{mEq/L}$ (renal loss). Treatment: $0.9\%$\,NaCl.

  • Euvolemic: $\text{TBW}\uparrow$, total $\text{Na}^+$ normal. Causes include SIAD, primary polydipsia, and adrenal insufficiency. SIAD features Urine $\text{Na}^+ > 30\,\text{mEq/L}$ and concentrated urine. Treatment: Fluid restriction ($<1\,\text{L/day}$), urea, oral NaCl tabs with loop diuretics, or Vaptans.

  • Hypervolemic: $\text{TBW}\uparrow\uparrow$, $\text{Na}^+\uparrow$. Caused by heart failure, cirrhosis, nephrotic syndrome, or acute renal failure. Treatment: Fluid and sodium restriction, loop diuretics, Vaptans.

• Safety Limits for Sodium Correction:

  • Rapid correction of chronic hyponatremia ($>48\,\text{hours}$) causes Osmotic Demyelination Syndrome (ODS).

  • Goal correction rate: $4\text{--}8\,\text{mEq/L}$ increase in the first $24\,\text{hours}$ (maximum $10\,\text{mEq/L}$ per $24\,\text{hours}$; limit to $4\text{--}6\,\text{mEq/L}$ if high ODS risk).

• Emergency Management for Severe Symptoms (Seizures, Coma):

  • Bolus $3\%$\,NaCl $100\,\text{mL}$ IV over $10\,\text{minutes}$ (may repeat up to 2 times until symptoms improve or serum sodium rises $4\text{--}6\,\text{mEq/L}$).

• Pharmacologic Therapies:

  • Tolvaptan (Samsca): Oral V2 receptor antagonist ($7.5\text{--}60\,\text{mg}$ daily). Maximum duration $30\,\text{days}$. Do not combine with fluid restriction.

  • Conivaptan (Vaprisol): Continuous IV V1a/V2 antagonist used for up to $4\,\text{days}$.

  • Urea (Ure-Na): Medical food generating osmotic diuresis ($30\text{--}60\,\text{g/day}$).

Hypernatremia (Serum Sodium > 145 mEq/L)

• Subtype & Etiology Overview:

  • Hypovolemic: Water loss exceeds sodium loss. Initial treatment is volume expansion with isotonic crystalloids ($0.9\%$\,NaCl or LR) until stable, followed by hypotonic fluid replacement.

  • Euvolemic: Pure water loss from Diabetes Insipidus (AVP disorders).

  • Central DI: Deficient AVP secretion. Treatment: Desmopressin (DDAVP) ($10\,\text{mcg}$ intranasally or PO $0.05\,\text{mg}$ BID).

  • Nephrogenic DI: Renal resistance to AVP. Treatment: HCTZ, amiloride, sodium restriction, NSAIDs.

  • Hypervolemic: Sodium overload. Treatment: Discontinue sodium sources, IV D5W plus loop diuretics.

• Correction Rates:

  • Acute ($<48\,\text{hours}$): Reduce serum sodium by $1\text{--}2\,\text{mmol/L/hr}$ to target $145\,\text{mmol/L}$ within $24\,\text{hours}$.

  • Chronic ($>48\,\text{hours}$): Reduce serum sodium by $< 0.5\,\text{mmol/L/hr}$ ($6\text{--}11\,\text{mmol/L/day}$) to prevent cerebral edema.

• Free Water Deficit Formula:

  • $\text{Free Water Deficit (L)} = \text{TBW\%} \times \text{Weight (kg)} \times \left(\frac{\text{Serum Na} - 140}{140}\right)$

Questions & Clinical Scenarios

• Question: What is the estimated total body water in a healthy $70\,\text{kg}$ male?

  • Answer: $42\,\text{L}$ ($70\,\text{kg} \times 0.6\,\text{L/kg}$).

• Question: A patient receives torsemide $10\,\text{mg}$ IV BID. What oral bumetanide regimen is equivalent?

  • Answer: $0.5\,\text{mg}$ PO BID. (Torsemide $10\,\text{mg}$ IV BID = $20\,\text{mg}$ IV daily = $20\,\text{mg}$ PO daily = $1\,\text{mg}$ PO bumetanide daily, divided as $0.5\,\text{mg}$ PO BID).

• Question: An 18-year-old female presents with 3 days of emesis, weakness, dizziness, and orthostatic hypotension (supine BP $110/70\,\text{mmHg}$, standing BP $85/65\,\text{mmHg}$). What is the optimal initial therapy?

  • Answer: $0.9\%$\,NaCl $1\,\text{L}$ IV over $30\,\text{minutes}$ (fluid resuscitation for acute hypovolemia).

• Question: A 67-year-old male with chronic kidney disease presents with 2+ leg edema, elevated JVD, and serum sodium $132\,\text{mEq/L}$. What disorder is present and how should it be managed?

  • Answer: Hypervolemic hyponatremia. Manage with dietary sodium restriction, fluid restriction, and furosemide.

• Question: A 30-year-old runner finishes a marathon experiencing headache, confusion, orthostatic hypotension, and serum sodium $124\,\text{mEq/L}$. What is the recommended treatment?

  • Answer: $3\%$\,NaCl $100\,\text{mL}$ IV bolus over $10\,\text{minutes}$ (indicated for symptomatic severe hyponatremia).

• Question: A 19-year-old female presents with polyuria, nocturia, polydipsia, serum sodium $148\,\text{mEq/L}$, and urine osmolality $180\,\text{mOsm/L}$. What is the diagnosis and appropriate treatment?

  • Answer: Euvolemic hypernatremia secondary to Central Diabetes Insipidus. Treat with desmopressin $10\,\text{mcg}$ intranasally at bedtime.