Fluid and Electrolyte Disorders

Stephen Freedman

Introduction: Fluid Compartments

Total body water (TBW) = 67% intracellular fluid (ICF) + 33% extracellular fluid (ECF)
ECF = 25% intravascular + 75% interstitial + transcellular
As a percentage of body weight, TBW varies inversely with advancing age

Maintenance Fluid and Electrolyte Requirements

Insensible losses = 400-600 mL/BSA (m²)/day plus urine and stool output
Since weight is easily available, used as an adequate alternative to surface area

Electrolyte Requirements

▪	Sodium:	1-3 mmol/kg/day
▪	Potassium:	1-3 mmol/kg/day

Table 24.1 Fluid Requirements

WEIGHT (KG)	MAINTENANCE FLUID REQUIREMENT/24 HOURS	MAINTENANCE FLUID REQUIREMENT/HOUR
< 10 kg	100 mL/kg	4 mL/kg
1-20 kg	1,000 mL + 50 mL for each kg > 10 kg	40 mL + 2 mL for each kg > 10 kg
> 20 kg	1,500 mL + 20 mL for each kg > 20 kg	60 mL + 1 mL for each kg > 20 kg

Disorders of Sodium Homeostasis

Hypernatremia (serum sodium > 145 mmol/L)

Etiology

Increased total body Na from excess intake or hyperaldosteronism
Pure water loss with normal body Na
- Insensible losses, renal (diabetes insipidus), inadequate access to water
Decreased total body water from diarrhea, vomiting, or renal cause
Normal total body Na and water with abnormal central regulation
- Hypothalamic abnormality (essential hypernatremia, hypodipsia)

Clinical Manifestations

Depend on volume status, degree of hypernatremia, and rate of rise
Dry mucous membranes, irritability, weakness, lethargy, coma, seizures
Increased extracellular osmolarity protects perfusion and results in doughy skin

Investigations

Approach to Diagnosis

Treatment

If circulatory compromise, bolus 20 mL/kg normal saline
Calculate free water deficit:
- Free water deficit = 0.6 × weight × [(plasma Na/145) − 1]
- Simple calculation = 4 mL × weight (kg) × desired Δ Na
- 4 mL/kg water lowers serum Na by 1 mEq/L
Goal is to reduce serum Na by < 10-12 mmol/L/day to prevent cerebral edema
Fluid requirement = free water deficit plus ongoing losses plus maintenance
Give only 75% maintenance fluids due to increased ADH
Usually reasonable to start with D5W + 0.45 NS solution with frequent monitoring of Na
Add K after urine output established and if normal renal function
Monitor Na q 1-4 h initially with glucose, calcium, CNS status
Admit if symptomatic or Na > 160 mmol/L without an obvious cause
If severe hypernatremia Na > 170 mmol/L, do not correct serum Na < 150 in first 48-72 hours; consult nephrology and ICU

Hyponatremia (serum sodium < 135 mmol/L)

Etiology

Pseudohyponatremia (normal total body water and Na) due to hyperosmolar states (hyperglycemia), extreme hyperlipidemia, hyperproteinemia
Edema and hyponatremia: CHF, hypoalbuminemia, cirrhosis, acute renal failure
Dehydration and hyponatremia
- Vomiting, diarrhea, tube drainage, renal losses, third space losses
Increased total body water with normal total body Na
- SIADH, primary polydipsia, hypotonic feeds, hypothyroidism
  
  Note: hospitalized patients receiving hypotonic maintenance fluids may have high levels of circulating ADH and are at risk of developing severe hyponatremia