Fluids and Electrolytes Cristina E. Pinal, DO PGY4 Nephrology Fellow

Why is this important? Because you and your patients are mostly water…and electrolytes!

Key concepts to understand Total Body Water is approximately 60% of body weight Percentage highest in babies and adult males Percentage lowest in adult females and obese people How is this distributed? Intracellular fluid (ICF) = 2/3 TBW Extracellular fluid (ECF) = 1/3 TBW

TBW ICF ECF Major cations are k+ and MG2+ Major anions are protein and organic phosphates (adp, atp, amp) ECF Interstitial fluid and plasma Major cation is NA+ Major anions are cl- and HCO3- Plasma is ¼ of ECF and 1/12 of of TBW (1/4 x 1/3 = 1/12) Plasma proteins – albumin and globulins Interstitial fluid is ¾ of ECF and ¼ of TBW (3/4 x 1/3 = 3/12 = ¼) Composition is similar to plasma but has little protein

Shifts of water between compartments

When you give isotonic NaCL ECF volume increased Plasma proteins and hematocrit may decrease because of dilution Arterial blood pressure increases (yay!) because ECF is increasing Isosmotic expansion Uses: For fluid losses, volume contraction (dehydration, hypovolemia), etc

diarrhea ECF decreases; osmolarity is unchaged Plasma protein/hematocrit increase (concentration of protein and RBCs) Arterial blood pressure decreases as ECF volume decreases Isosmotic volume contraction fluid is secreted from the GI tract with same osmolarity as that of the ECF; thus, no change in the ECF osmolarity. So no shift in body fluid between the ICF and the ECF. So, only total volume of the ECF contracts due to its loss within in the diarrheal fluid. Give isotonic fluid back

Alteration of fluids

Fluid states in disease

Types of fluids 1) Crystalloid – contains chemicals of simple ionic structure that can pass semi permeable membrane (water follows crystalline component by osmosis) 2) Colloid – contain large molecules that cannot pass semi permeable membrane (increase oncotic pressure)

Types of fluids

Fluid distribution

Sodium homeostasis -sodium is major cation that determines serum osmolality This regulates water flow Total amount of sodium in the body is a component of water balance Hyponatremia, hypernatremia Hyponatremia- for assessment you will need: Urine sodium, urine osmolality, serum osmolality, urine potassium and volume status TSH Uric acid

Potassium homeostasis Most abundant intracellular cation Hypokalemia (GI losses, diuretics, metabolic alkalosis, hypomagnesemia) Hyperkalemia (renal failure, metabolic acidosis, rhabdomyolysis, digoxin)

Potassium stores in body

Magnesium homeostasis Intracellular cation Associated with potassium and calcium homeostasis A decrease in intracellular magnesium, (via deficiency), releases the magnesium- mediated inhibition of ROMK channels and increases potassium secretion.  (hypomagnesemia associated with loop and thiazide diuretics, GI losses, trauma, burns, sepsis) Hypermagnesemia (renal failure or iatrogenic)

Magnesium absorption and elimination

Hyperkalemia