Order: 375 mg PO. Available: 125 mg tablets. How many tablets will the nurse administer?

Choice A rationale

Hyponatremia occurs in SIADH due to the excessive secretion of antidiuretic hormone, which causes the kidneys to reabsorb an inappropriate amount of free water. This water retention dilutes the concentration of sodium in the extracellular fluid, leading to serum sodium levels falling below the normal range of 135 to 145 mEq/L. The resulting dilutional hyponatremia can lead to cellular edema and significant neurological complications if the water intake is not strictly restricted to manage the imbalance.

Choice B rationale

Hypernatremia is not associated with SIADH because the primary pathophysiology involves water retention rather than water loss or sodium excess. Hypernatremia, defined as a serum sodium level greater than 145 mEq/L, is typically seen in conditions like diabetes insipidus where there is a deficiency of antidiuretic hormone or a lack of renal response to it. In SIADH, the body retains too much water, which consistently lowers the sodium concentration through dilution rather than raising it through dehydration.

Choice C rationale

Concentrated urine is a hallmark of SIADH because the high levels of circulating antidiuretic hormone act on the renal collecting ducts to increase water permeability. This results in maximum water reabsorption back into the systemic circulation, leaving very little water to be excreted. Consequently, the urine produced is highly concentrated with a high specific gravity, typically exceeding 1.030, and a high urine osmolality, reflecting the body's inability to excrete excess water despite low serum osmolality levels.

Choice D rationale

Polyuria is the excretion of large volumes of dilute urine, which is the opposite of what occurs in SIADH. Patients with SIADH actually experience oliguria because the kidneys are reabsorbing almost all filtered water under the influence of excessive antidiuretic hormone. Polyuria is a clinical manifestation of diabetes insipidus or osmotic diuresis, where the kidneys fail to concentrate urine. In SIADH, the volume of urine output is significantly decreased while the concentration of the urine remains abnormally high.

Choice E rationale

Low serum osmolality is a direct result of the excessive water retention seen in SIADH. As the body reabsorbs free water in the distal tubules and collecting ducts, the blood becomes diluted, lowering the concentration of particles. Normal serum osmolality ranges from 275 to 295 mOsm/kg. In SIADH, this value drops below 275 mOsm/kg. This hypoosmolar state occurs simultaneously with the production of concentrated urine, which is a diagnostic indicator that the ADH secretion is inappropriate.

Choice A rationale

Water remains evenly distributed between compartments only when the intravenous fluid administered is isotonic, such as 0.9 percent normal saline. Isotonic solutions have an osmolality similar to that of intracellular and extracellular fluids, resulting in no net movement of water across the cell membrane. Hypertonic solutions, by definition, have a higher solute concentration than the cytoplasm of the cells, which creates an osmotic gradient that necessitates the movement of water.

Choice B rationale

Water moves into cells, causing them to swell, when a hypotonic solution is administered. Hypotonic fluids, such as 0.45 percent normal saline, have a lower osmolality than the fluid inside the cells. This causes water to shift from the intravascular space into the intracellular space to equalize concentrations. In the case of hypertonic saline, the concentration of solutes in the blood is higher than in the cells, which prevents water from entering.

Choice C rationale

Sodium does not passively diffuse into cells in significant quantities to equalize osmotic pressure because the cell membrane is selectively permeable and utilizes the sodium-potassium pump to maintain gradients. While some movement occurs, the primary mechanism for balancing the osmotic pressure difference created by hypertonic saline is the movement of water. Osmosis dictates that the solvent moves toward the higher solute concentration, rather than the solute moving to fill the cells.

Choice D rationale

Hypertonic saline has a higher osmolality than the intracellular fluid. When this solution is introduced into the extracellular space, it creates an osmotic pull that draws water out of the cells and into the blood vessels. This process causes the cells to shrink, a process known as crenation. This shift helps expand the intravascular volume in cases of severe dehydration but must be monitored closely to prevent cellular damage and fluid overload.