A. Normal Urine Output

1. Normal is >30mL/hr, 1200-1400 mL/day (highly dependent on fluid intake)
2. Solutes (waste products) - urea (uremic)
3. Renal perfusion is best indicator of cardiac output in most patients
4. Urine production is best indicator of renal perfusion / function

B. Insensible Losses

1. 400-600 mL/day
2. Respiratory
3. Sweating
4. Stool (100-200cc/day)

C. Water Metabolism (Gain) ~100mL/day created
D. Total Maintenance Fluids

1. Adults
   1. About 1.5-2 liters per day
   2. This is ~80mL/hr for 70kg person
   3. Consider using 1.1-1.3mL/hr/kg for maintenance
2. Children:
   1. The "4:2:1" rule is often used to calculate the rate (mL/hr) based on weight (wt)
   2. Rate = 4mL/kg/hr (up to 10kg) + 2mL/kg/hr (10-20kg) + 1mL/kg/hr (>20kg)
3. In general, D5-1/2NS+20mEq/L KCl or Lactated Ringer's should be used for maintenance

E. Electrolytes

1. Mainly Na+ at K+
2. Potassium
   1. Regulated primarily by aldosterone (stress) leading to K+ secretion
   2. Maintenance: need ~40 mEq/day for adults (2 liters at 20mEq/L)
3. Sodium
   1. Aldosterone regulation retains sodium
   2. Normal daily intake is 6-10 gm of NaCl (min ~ 2g)
   3. Avoid hyponatremia
   4. Most patients alternate D5/0.25NS and D5/0.5NS all with 20mEq/L K+
4. Magnesium - follow levels in serum (poor marker for intracellular magnesium levels)
5. Calcium - follow serum levels corrected for albuminl; maintain in renal failure

F. Electrolyte Concentrations in Various Body Fluids

1. Urine Output Down
2. Orthostatic Changes: HR increases >10bpm; BP decresaes >15mm when supine to standing
3. Tachycardia - increased heart rate (HR; early change)
4. BUN Increase (Increased resorption)
5. Capillary Refill Down
6. Skin Turgor Down
7. Blood Pressure Drops (Late change)

H. Dehydration in Children [3]

1. Prediction of 5% in children relies on signs:
2. Abnormal capillary refill time: 4.1X risk
3. Abnormal skin turgor: 2.5X risk
4. Abnormal respiratory pattern: 2.0X risk
5. Combinations of these signs much more accurate than any one sign
6. Very difficult to estimate level of dehydration in children

I. Monitoring Fluid Deficits

1. Maintenance fluids - urine and insensible losses
2. Abnormal losses - drains, fistulas, burns
3. Initial fluid deficit
   1. Operations - replacement usually by anesthesiologist
   2. Replacement for "Third Spacing" ~10-15 mL/kg body weight
   3. Replacement of crystalloid (normal saline) for blood (colloid) ~3mL:1mL blood lost
4. An abnormal plasma Na+ concentration is indicative of a disorder of volume homeostasis

J. Fluid Replacement Solutions [1]

1. Ringer's Lactate generally preferred over D5-NS due to buffering activity
2. Albumin 4% did not improve outcomes over NS for fluid resuscitation in intensive care [2]
3. In post-hoc study of critically ill patients with traumatic brain injury, albumin was associated with higher morality than saline for resuscitation [4]

References

1. Adrogue HJ and Madias NE. 2000. NEJM. 342(20):1493
2. SAFE Study Investigators. 2004. NEJM. 350(22):2247
3. Steiner MJ, DeWalt DA, Byerley JS. 2004. JAMA. 291(22):2746
4. SAFE Study Investigators. 2007. NEJM. 357(9):874