Water and Electrolyte Regulation
Fluid and electrolyte disturbances are common in critically ill neurosurgical patients, as central nervous system (CNS) damage may affect the brain’s ability to keep regulating their homeostasis. The potential consequences of fluid and electrolyte disturb
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9.3 Water and Electrolyte Regulation Ludwig Schürer, Stefan Wolf, and Christianto B. Lumenta
9.3.1 Introduction Fluid and electrolyte disturbances are common in critically ill neurosurgical patients, as central nervous system (CNS) damage may affect the brain’s ability to keep regulating their homeostasis. The potential consequences of fluid and electrolyte disturbances on the predamaged brain are profound and influence the development of cytotoxic or vasogenic brain edema, loss of cerebral autoregulation, increased intracranial pressure or decreased perfusion pressure. Early recognition and appropriate management of fluid and electrolyte disorders are important to minimize secondary brain damage.
9.3.2 Basics 9.3.2.1
9.3.2.3
Osmolarity, Osmolality and Tonicity
To better understand the differences between osmolarity, osmolality and tonicity, these terms are defined in Table 9.3.1.
Body Water
Total body water (TBW) varies considerably (45–80%, mean approx. 55%) and depends on age, sex and leanness. TBW is distributed in two compartments. Three fifths represent the intracellular fluid compartment and two fifths are found extracellularly. The extracellular fluid compartment can be divided into the intravascular (25%) and the interstitial space (75%). 9.3.2.2
Since the distribution of sodium essentially corresponds to the extracellular volume and water is freely permeable within the fluid spaces, two rules of thumb can be formulated: 1. Primary disturbances of the sodium concentration result in variations of the extracellular space, given that the system for osmotic regulation is intact. 2. Disturbances of the water content are followed by alterations of the intra- and extracellular volume. If the ratio of water content to normally dissolved solutes is changing, an alteration of osmolarity is the consequence.
Electrolytes
In the extracellular compartment, sodium and chloride are predominant, whereas in the intracellular space, potassium and phosphate are prevailing. Extracellular sodium normally ranges from 135–145 mEq/L. Only 2% of the sodium is located intracellularly. The opposite holds for potassium. Nearly all potassium is found intracellularly; only 2% is found extracellularly. The normal extracellular potassium level is 3.5–5.0 mEq/L. The high intracellular to extracellular gradients of sodium and potassium are maintained by the sodium/potassium ATPase pump and are required to maintain a normal membrane potential.
9.3.3 Regulation of Body Water and Osmolarity Figure 9.3.1 summarizes volume and osmoregulation. In healthy individuals, plasma sodium and osmolarity are maintained within a markedly narrow range. This stability is achieved primarily by adjusting total body water to keep it in balance with sodium. Baroreceptors in the aorta and the atrium of the heart register blood volume and -pressure and modulate fluid intake (thirst) and fluid excretion (kidney). Antidiuretic Hormone. The most important control mechanism is the release of antidiuretic hormone (ADH). ADH is secreted in parallel with risi
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