Anesthesia Providers and Fluid-Electrolyte Physiology

Understanding fluid and electrolyte physiology is crucial for anesthesia providers, especially when dealing with patients who undergo surgery and experience perioperative fluid and electrolyte imbalances. Total body water (TBW) consists of extracellular fluid (ECF) and intracellular fluid (ICF). ECF includes intravascular fluid (plasma) and interstitial fluid, while ICF is mainly composed of proteins and other dissolved molecules inside cells. Key electrolytes such as sodium, potassium, calcium, magnesium, and phosphate play critical roles in maintaining osmotic pressures within these compartments.

Fluid compartments are dynamic and influenced by several factors, including the patient's age, body weight, stress response, and disease state. A healthy 70-kg adult male has approximately 42 L of total body water, with 14 L in the intravascular space and 27 L in the interstitial space. Intracellular fluid accounts for about two-thirds of the total body water, with a significant portion being proteins and other dissolved molecules.

Anesthesia can affect fluid and electrolyte physiology in various ways. General anesthesia may cause vasodilation, leading to hypovolemia and decreased organ perfusion. Additionally, surgery itself can produce fluid shifts and imbalances, necessitating careful monitoring and management of both fluid and electrolyte levels.

Hypernatremia occurs when serum sodium levels exceed 145 mmol/L, while hyponatremia refers to sodium levels below 135 mmol/L. Hyponatremia is common in the perioperative setting and may result from a variety of factors, including fluid overload and loss of sodium in urine due to diuretics or renal dysfunction. Symptoms of severe hyponatremia can range from nausea, confusion, seizures, to coma and potentially death. Treatment typically involves repletion of sodium and careful monitoring of the patient's progress.

Hypokalemia (serum potassium <3.5 mmol/L) and hyperkalemia (>5 mmol/L) can also arise during anesthesia and surgery. Hypokalemia may lead to arrhythmias, particularly in patients who receive beta-blockers or digoxin. On the other hand, hyperkalemia may cause cardiac instability if uncontrolled. Most cases of hyperkalemia are managed by removing potassium from the IV fluid, but serious cases may warrant direct administration of insulin or glucose to facilitate potassium uptake by cells.

The management of fluid and electrolyte imbalances in the perioperative setting involves appropriate choice of intravenous fluids and close monitoring of the patient's electrolyte levels. Orally administered solutions containing water and electrolytes (e.g., oral rehydration therapy, ORT) can also be useful for preoperative fluid and electrolyte management. However, caution must be taken to avoid aspiration, particularly during induction of anesthesia.