Definition and causes.

Isotonic contraction is defined as volume contraction in which sodium and water are lost in isotonic proportions. Hence, although there is a decrease in the total volume of extracellular fluid, there is no change in osmolality. Causes of isotonic contraction include vomiting, diarrhea, kidney disease, and misuse of diuretics. Isotonic contraction is characteristic of cholera, an infection that produces vomiting and severe diarrhea.

Treatment.

Lost volume should be replaced with fluids that are isotonic to plasma. This can be accomplished by infusing isotonic (0.9%) sodium chloride in sterile water, a solution in which both sodium and chloride are present at a concentration of 145 mEq/L. Volume should be replenished slowly to avoid pulmonary edema.

Definition and causes.

Hypertonic contraction is defined as volume contraction in which loss of water exceeds loss of sodium. Hence, there is a reduction in extracellular fluid volume coupled with an increase in osmolality. Because of extracellular hypertonicity, water is drawn out of cells, thereby producing intracellular dehydration and partial compensation for lost extracellular volume.

Causes of hypertonic contraction include excessive sweating, osmotic diuresis, and feeding excessively concentrated foods to infants. Hypertonic contraction may also develop secondary to extensive burns or disorders of the central nervous system (CNS) that render the patient unable to experience or report thirst.

Treatment.

Volume replacement in hypertonic contraction should be accomplished with hypotonic fluids (eg, 0.11% sodium chloride) or with fluids that contain no solutes at all. Initial therapy may consist simply of drinking water. Alternatively, 5% dextrose can be infused intravenously. (Since dextrose is rapidly metabolized to carbon dioxide and water, dextrose solutions can be viewed as the osmotic equivalent of water alone.) Volume replenishment should be done in stages. About 50% of the estimated loss should be replaced during the first few hours of treatment. The remainder should be replenished over 1 to 2 days.

Definition and causes.

Hypotonic contraction is defined as volume contraction in which loss of sodium exceeds loss of water. Hence, both the volume and osmolality of extracellular fluid are reduced. Because intracellular osmolality now exceeds extracellular osmolality, extracellular volume becomes diminished further by movement of water into cells.

The principal cause of hypotonic contraction is excessive loss of sodium through the kidneys. This may occur because of diuretic therapy, chronic renal insufficiency, or lack of aldosterone (the adrenocortical hormone that promotes renal retention of sodium).

Treatment.

If hyponatremia is mild, and if renal function is adequate, hypotonic contraction can be corrected by infusing isotonic sodium chloride solution for injection. When this is done, plasma tonicity will be adjusted by the kidneys. However, if the sodium loss is severe, a hypertonic (eg, 3%) solution of sodium chloride should be infused. Administration should continue until plasma sodium concentration has been raised to about 130 mEq/L. Patients should be monitored for signs of fluid overload (distention of neck veins, peripheral or pulmonary edema). When hypotonic contraction is due to aldosterone insufficiency, patients should receive hormone replacement therapy along with intravenous infusion of isotonic sodium chloride.

Causes.

Respiratory alkalosis is produced by hyperventilation. Deep and rapid breathing increases CO₂ loss, which in turn lowers the pCO₂* of blood, and thereby increases pH. Mild hyperventilation may result from a number of causes, including hypoxia, pulmonary disease, and drugs (especially aspirin and other salicylates). Severe hyperventilation can be caused by CNS injury and hysteria.