Roughly ⅔ of the total body weight is water (men); infants have a little more body water, women have a little less

  ⅔ of water weight is intracellular (mostly muscle)

  ⅓ of water weight is extracellular

•  ⅔ of extracellular water is interstitial

•  ⅓ of extracellular water is in plasma

  Proteins – determine plasma/interstitial compartment osmotic pressures

  Na – determines intracellular/extracellular osmotic pressure

  Volume overload – most common cause is iatrogenic; first sign is weight gain

  Cellular catabolism – can release a significant amount of H₂O

  0.9% normal saline: Na 154 and Cl 154; 3% normal saline: Na 513 and Cl 513

  Lactated Ringer’s (LR; ionic composition of plasma): Na 130, K 4, Ca 2.7, Cl 109, bicarb 28

  Plasma osmolarity: (2 × Na) + (glucose/18) + (BUN/2.8)

•  Normal: 280–295

  Water shifts from areas of low solute concentration (low osmolarity) to areas of high solute concentration (high osmolarity) to achieve osmotic equilibration

ESTIMATES OF VOLUME REPLACEMENT

  4 cc/kg/h for 1st 10 kg

  2 cc/kg/h for 2nd 10 kg

  1 cc/kg/h for each kg after that

  Best indicator of adequate volume replacement is urine output

  During open abdominal operations, fluid loss is 0.5–1.0 L/h unless there are measurable blood losses

  Usually do not have to replace blood lost unless it is > 500 cc

  Insensible fluid losses – 10 cc/kg/day; 75% skin, 25% respiratory, pure water

  Replacement fluids after major adult gastrointestinal surgery

•  During operation and 1st 24 hours, use LR

•  After 24 hours, switch to D5 ½ NS with 20 mEq K⁺

•  5% dextrose will stimulate insulin release, resulting in amino acid uptake and protein synthesis (also prevents protein catabolism)

•  D5 ½ NS @ 125/h provides 150 g glucose per day (525 kcal/day)

GI FLUID SECRETION

  Normal K⁺ requirement: 0.5–1.0 mEq/kg/day

  Normal Na⁺ requirement: 1–2 mEq/kg/day

GI ELECTROLYTE LOSSES

  Sweat – hypotonic (Na concentration 35–65)

  Saliva – K⁺ (highest concentration of K⁺ in body)

  Stomach – H⁺ and Cl⁻

  Pancreas – HCO₃⁻

  Bile – HCO₃⁻

  Small intestine – HCO₃⁻, K⁺

  Large intestine – K⁺

  Gastric losses – replacement is D5 ½ NS with 20 mg K⁺

  Pancreatic/biliary/small intestine losses – replacement is LR with HCO₃⁻

  Large intestine losses (diarrhea) – replacement is LR with K⁺

  GI losses – should generally be replaced cc/cc

  Dehydration (eg marathon runner) – replacement with normal saline

  Urine output – should be kept at least 0.5 cc/kg/h; should not be replaced, usually a sign of normal postoperative diuresis

POTASSIUM (NORMAL 3.5–5.0)

  Hyperkalemia – peaked T waves on EKG; often occurs with renal failure; Tx →

•  Calcium gluconate (membrane stabilizer for heart)

•  Sodium bicarbonate (causes alkalosis, K enters cell in exchange for H)

•  10 U insulin and 1 ampule of 50% dextrose (K driven into cells with glucose)

•  Kayexalate

•  Dialysis if refractory

  Hypokalemia – T waves disappear (usually occurs in setting of overdiuresis)

•  May need to replace Mg⁺ before you can correct K⁺

SODIUM (NORMAL 135–145)

  Hypernatremia – usually from dehydration; restlessness, irritability, seizures

•  Correct with D5 water slowly to avoid brain swelling

  Hyponatremia – usually from fluid overload; headaches, nausea, vomiting, seizures

•  Water restriction is first-line treatment for hyponatremia, then diuresis

•  Correct Na slowly to avoid central pontine myelinosis (no more than 1 mEq/h)

•  Hyperglycemia can cause pseudohyponatremia – for each 100 increment of glucose over normal, add 2 points to the Na value

•  SIADH (syndrome of inappropriate antidiuretic hormone) causes hyponatremia

CALCIUM (NORMAL 8.5–10.0; NORMAL IONIZED CA 4.425.5)

  Hypercalcemia (Ca usually > 13 or ionized > 6–7 for symptoms) – causes lethargic state

•  Breast cancer most common malignant cause

•  Hyperparathyroidism most common benign cause

•  No lactated Ringer’s (contains Ca²⁺)

•  No thiazide diuretics (these retain Ca²⁺)

•  Tx: normal saline at 200–300 cc/h and Lasix

•  For malignant disease → mithramycin, calcitonin, alendronic acid, dialysis

  Hypocalcemia (Ca usually < 8 or ionized Ca < 4 for symptoms) – hyperreflexia, Chvostek’s sign (tapping on face produces twitching), perioral tingling and numbness, Trousseau’s sign (carpopedal spasm), prolonged QT interval; can occur after parathyroidectomy

•  May need to replace Mg⁺ before you can correct Ca

•  Protein adjustment for calcium – for every 1g decrease in protein, add 0.8 to Ca

MAGNESIUM (NORMAL 2.0–2.7)

  Hypermagnesemia – causes lethargic state; usually in renal failure patients taking magnesium containing products

•  Tx: calcium

  Hypomagnesemia – usually occurs with massive diuresis, chronic TPN without mineral replacement or ETOH abuse; signs similar to hypocalcemia

METABOLIC ACIDOSIS

  Anion gap = Na – (HCO₃ + Cl); Normal is < 10–15

  High anion gap acidosis – “MUDPILES” = methanol, uremia, diabetic ketoacidosis, par-aldehydes, isoniazid, lactic acidosis, ethylene glycol, salicylates

  Normal anion gap acidosis – usually loss of Na/HCO₃⁻ (ileostomies, small bowel fistulas)

  Tx: underlying cause; keep pH > 7.20 with bicarbonate; severely ↓ pH can affect myocardial contractility

METABOLIC ALKALOSIS

  Usually a contraction alkalosis

  Nasogastric suction – results in hypochloremic, hypokalemic, metabolic alkalosis, and paradoxical aciduria →

•  Loss of Cl⁻and H ion from stomach secondary to nasogastric tube (hypochloremia and alkalosis)

•  Loss of water causes kidney to reabsorb Na in exchange for K⁺ (Na/K ATPase), thus losing K⁺ (hypokalemia)

•  Na⁺/H⁻ exchanger activated in an effort to reabsorb water along with K⁺/H⁻ exchanger in an effort to reabsorb K⁺ → results in paradoxical aciduria

•  Tx: normal saline (need to correct the Cl- deficit)

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