Chapter 56 - Disorders of Sodium and Water Balance

Abstract

Disorders of sodium (Na+) and water balance commonly are encountered in critically ill patients. Critical illness, multiple-organ dysfunction, fluid therapy, and the numerous additional interventions applied in the routine care of patients admitted to the intensive care unit can interfere with the complex mechanisms that maintain total body sodium and water homeostasis. Sodium is the principal osmolyte in the extracellular fluid and hence determines the size of the extracellular volume. Loss of sodium and water can cause life-threatening hypovolemia, and resuscitation with sodium-containing fluids is essential. In contrast, gain of sodium and water expands the extracellular volume and can cause significant morbidity because of respiratory, circulatory, and renal failure. Besides the effect on extracellular volume, sodium and water balance determines extracellular tonicity. Cells are bathed in salt water, and normonatremia—normal tonicity—is pivotal for optimal cell size and function. Disorders of sodium and water balance disturb the cellular environment by causing hyponatremia or hypernatremia. Hyponatremia and hypernatremia are frequent in the critically ill patient and associated with significant morbidity and mortality. A rapid change in serum sodium concentration can result in brain damage and death because of brain edema or osmotic demyelination. Addressing disorders of sodium and water balance, including the safe treatment of patients admitted to the intensive care unit and the prevention of fluid and electrolyte imbalances, is essential in critical care therapy. To achieve this, clinicians must have a solid understanding of what determines serum sodium concentration in the individual patient.