Electrolyte disorders are extremely common in critically ill patients.
Hyponatremia is the most common electrolyte disorder; rapid correction with hypertonic saline is acceptable only when hyponatremia is known to be acute; otherwise correction should not exceed 8 mmol/L in 24 hours.
Severe hyperkalemia (≥6.5 mmol/L) is a medical emergency that requires immediate intervention.
While symptomatic hypocalcemia requires calcium supplementation, it is very uncommon; in contrast, asymptomatic hypocalcemia is very common in ICU patients and does not require treatment.
Hypomagnesemia is common and most often related to diuretic use and alcoholism.
Severe hypophosphatemia is most commonly seen with refeeding syndrome.
Total body water is distributed across three major compartments: the intracellular fluid (ICF), the interstitium, and the vascular space (the latter two combined constitute the extracellular fluid [ECF]). Regulation of the intracellular volume is achieved partly by regulation of the plasma osmolality through changes in water balance. Since sodium (Na+) concentration is the main determinant of plasma osmolality, disorders of water balance are manifested by changes in the plasma Na+ concentration. Hence, increased plasma Na+ or hypernatremia reflects an increase in plasma osmolality due to deficit of total body water, whereas decreased plasma Na+ or hyponatremia reflects a decrease in plasma osmolality due to excess total body water. An important concept is that disorders associated with an abnormal plasma Na+ concentration are not disorders of a primary change in total body Na+.
The relationship between plasma Na+ concentration to plasma osmolality is further discussed below.1 The effective plasma osmolality or tonicity is determined by those osmoles that act to hold water in the ECF and the main determinant is the plasma Na+ (urea is an ineffective osmole, and glucose at normal concentrations accounts for only 5 mOsm/kg). A simplified relationship between osmolality and plasma Na+ can be presented as follows.
Since body fluids are in osmotic equilibrium,
Further, since exchangeable Na+ salts are the primary effective extracellular solutes and exchangeable potassium (K+) salts are the primary effective intracellular solutes, the following relationship is obtained:
So, the above relationship can be simplified by combining [103-1a] and [103-1b]:2