General Pathology High Yield #7: Dehydration and its treatment

information from lecture notes, especially Renal pp. 16-21.

Remember that the body is approximately 60% water by weight. It is distributed into 3 compartments:

• intracellular (2/3 of body water)
• transcellular (cerebrospinal, intraocular, pleural, peritoneal, synovial fluids)
• extracellular (~1/3 of body water) - divided into 3/4 interstitial, ¼ intravascular

Clinical signs of extracellular volume depletion:

• orthostatic hypotension occurs when >10% of intravascular volume is lost. Shock occurs at >20% volume loss.
• interstitial volume depletion is manifest as decreased skin turgor (less apparent in elderly due to reduced skin elasticity)
• transcellular volume reduction: dry eyes and mouth

Plasma osmolality (plasma sodium concentration) may reflect alterations in water balance and distribution. In dehydration, plasma Na may be high, low, or normal. ▫Increased plasma protein concentration (and thus plasma oncotic pressure) occurs in dehydration due to water deprivation, prolonged sweating, vomiting, and diarrhea. ▫Plasma protein concentration is decreased in nephrosis (characterized by proteinuria), malnutrition or severe liver disease (decreased albumin production), and capillary injury due to burns or skin inflammation (plasma protein leaks into the interstitial space). In these cases, water may be redistributed to the interstitial space (edema) leading to dehydration.

▫Plasma protein concentration is normal in hemorrhage; volume is depleted.

Notes: Sweat is hypotonic - so excess sweating may result in hypernatremia.

GI secretions are isotonic with plasma, with elevated potassium concentrations -

so diarrhea may result in hypokalemia and metabolic acidosis, while vomiting may result in metabolic alkalosis from loss of gastric H+ ions.

Causes of Edema: ↑hydrostatic pressure (congestive heart failure), ↓ plasma proteins, ↑vascular permeability, or lymphatic obstruction - affects effective circulating volume.

Volume depletion occurs when sodium losses are greater than sodium gains (negative Na balance). Normally the kidneys can efficiently conserve sodium even if dietary intake is restricted.

Causes of Sodium Depletion:

Extra-renal:

GI: vomiting or diarrhea

Skin: burns, skin inflammatory diseases, sweating

Third Space Sequestration: ascites, bowel obstruction, peritonitis, internal hemorrhage

Renal:

Normal kidney function: diuretics, hypoaldosteronism

Renal damage: chronic renal failure, ‘salt-wasting’ interstitial nephritis, tubular necrosis

Treatment of dehydration: Isotonic saline infusion or packed rbcs if hemorrhage. Treat the cause!

Oral rehydration: World Health Organization (WHO) formula - developed for cholera.

Contains isotonic salt and sugar to take advantage of the Na-glucose cotransporter on the small bowel epithelium.