Chapter 15: Fluid and Electrolytes

INTRODUCTION

PRINCIPLES OF FLUIDS AND ELECTROLYTES

Physiologic mechanisms regulate volume, electrolytes, osmolality, and acid–base within a narrow homeostatic range. Alterations in homeostasis such as from renal dysfunction and other disease states, deficiency or excess of fluids, and/or electrolytes may lead to perturbations that need correction.

Therapy requires determination and correction of the underlying cause and likely an intervention to correct the resultant fluid and electrolyte disturbances, some of which may be life-threatening and need prompt attention. Therapy needs to be given at the appropriate rate and for the appropriate duration. The assessment and management of various electrolyte disorders are discussed in this chapter.

GOALS OF IV FLUID THERAPY

• Provide maintenance fluids, electrolytes, and nutrition

• Resuscitation in cases of shock

• Correct volume depletion

• Correct electrolyte and/or acid–base disorders

• Correct ongoing loss of fluid and electrolytes

Steps in ordering IVFs

1. Patient assessment: Determine volume status and presence of electrolyte or acid–base disturbances. Assess for underlying medical conditions such as congestive heart failure (CHF), chronic kidney disease, diabetes mellitus (DM), liver disease, etc., and for ongoing losses of fluid and electrolytes such as diarrhea, NG tube drainage, acute pancreatitis, etc.

2. Selection of appropriate IVFs, including additives such as potassium or magnesium.

3. Selection of the rate of administration.

4. Ongoing monitoring and reassessment and adjustments as indicated.

FLUID COMPARTMENTS

Total Body Water (TBW)

TBW varies due to age, gender, and body composition. TBW is approximately 80% in a full-term infant, 60% in a lean male adult, and 40–50% in obese patients and the elderly.

Below are calculations for a 70-kg male with normal body habitus (1 kg = 1 L)

TBW = 0.6 × Body wt (kg) = 42 L

• 66.6% × TBW = intracellular fluid (ICF) = 28 L

• 33.3% × TBW = extracellular fluid (ECF) = 14 L

  • 25% × ECF = Intravascular fluid (plasma) = 3.5 L

  • 75% × ECF = Interstitial fluid = 10.5 L

Total Blood Volume

Total blood volume = 5600 mL (8% of body water in a 70-kg man)

Red Blood Cell Mass

• Man, 20–36 mL/kg (1.15–1.21 L/m²)

• Woman, 19–31 mL/kg (0.95–1.0 L/m²)

The distribution of TBW is important in understanding how ...