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
| 1–2 L/day |
| 500–1,000 mL/day |
| 500–1,000 mL/day |
| 500–1,000 mL/day |
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)
![](https://i0.wp.com/basicmedicalkey.com/wp-content/uploads/2017/06/image00503.gif?w=960)
![](https://freepngimg.com/download/social_media/63059-media-icons-telegram-twitter-blog-computer-social.png)
Stay updated, free articles. Join our Telegram channel
![](https://clinicalpub.com/wp-content/uploads/2023/09/256.png)
Full access? Get Clinical Tree
![](https://videdental.com/wp-content/uploads/2023/09/appstore.png)
![](https://videdental.com/wp-content/uploads/2023/09/google-play.png)