Chapter Fourteen, part 1. Hypovolemic States

Outline

Chapter 14

- Hypovolemic States

- Etiology

- True volume depletion occurs when fluid is lost from from the extracellular fluid at a rate exceeding intake

- Can come the GI tract

- Lungs

- Urine

- Sequestration in the body in a “third space” that is not in equilibrium with the extracellular fluid.

- When losses occur two responses ameliorate them

- Our intake of Na and fluid is way above basal needs

- This is not the case with anorexia or vomiting

- The kidney responds by minimizing further urinary losses

- This adaptive response is why diuretics do not cause progressive volume depletion

- Initial volume loss stimulates RAAS, and possibly other compensatory mechanisms, resulting increased proximal and collecting tubule Na reabsorption.

- This balances the diuretic effect resulting in a new steady state in 1-2weeks

- New steady state means Na in = Na out

- GI Losses

- Stomach, pancreas, GB, and intestines secretes 3-6 liters a day.

- Almost all is reabsorbed with only loss of 100-200 ml in stool a day

- Volume depletion can result from surgical drainage or failure of reabsorption

- Acid base disturbances with GI losses

- Stomach losses cause metabolic alkalosis

- Intestinal, pancreatic and biliary secretions are alkalotic so losing them causes metabolic acidosis

- Fistulas, laxative abuse, diarrhea, ostomies, tube drainage

- High content of potassium so associated with hypokalemia

- [This is a mistake for stomach losses]

- Bleeding from the GI tract can also cause volume depletion

- No electrolyte disorders from this unless lactic acidosis

- Renal losses

- 130-180 liters filtered every day

- 98-99% reabsorbed

- Urine output of 1-2 liters

- A small 1-2% decrease in reabsorption can lead to 2-4 liter increase in Na and Water excretion

- 4 liters of urine output is the goal of therapeutic diuresis which means a reduction of fluid reabsorption of only 2%

- Diuretics

- Osmotic diuretics

- Severe hyperglycemia can contribute to a fluid deficit of 8-10 Iiters

- CKD with GFR < 25 are poor Na conservers

- Obligate sodium losses of 10 to 40 mEq/day

- Normal people can reduce obligate Na losses down to 5 mEq/day

- Usually not a probl