Situation 
Shock
 Decreased blood pressure
 Increased lactic acid
 Decreased urine output

Hypovolemia +/ electrolyte abnormalities
 "Preshock" (downtrends in blood pressure or urine output, increasing tachycardia)
 Diabetic ketoacidosis
 Rhabdomyolysis
 Postobstructive diuresis
 Hypovolemic acute prerenal failure
 Pancreatitis

Electrolyte abnormalities +/ hypovolemia
 Hypovolemic hyponatremia
 Hypernatremia
 Hypokalemia
 Hyperchloremic acidosis

NPO 
Choice of fluids 
 NS
 LR
 Albumin
 Hespan (but numerous contraindications)

Typically composition is directed by published guidelines (as in DKA) or fluids can be formulated by the considerations for maintenance fluids given in the column furthest to the right. Remember that 40 mEq of potassium in a peripheral line has a maximal infusion rate of 125 mL/hr 
Hypovolemic hyponatremia
 0.9% saline ("NS", 154 mEq Na/L)
 3% saline (513 mEq Na/L)
Hypernatremia
 0.45% saline ("1/2 NS", 77 mEq Na/L)
 D5 (0 mEq Na/L)
Hypokalemia
 Given normal renal function, assume 20 mEq of potassium increases serum potassium 0.25 mEq/L
Hyperchloremic acidosis
 Assuming mild hyponatremia or normal sodium consider 150 mEq NaHCO3 in 1000 mL of D5 or free water (1)

Based on electrolytes, specifically: sodium, potassium, chloride, bicarbonate, and glucose (and occasionally phosphate) as well as blood pressure. If they are hypertensive consider hypotonic (1/2 NS) rather than isotonic (NS, LR) solutions even if they are mildly hyponatremic.
 If Na is within normal limits < 140 use NS, > 140 use 1/2 NS
 If K is within normal limits < 4 add 20 mEq KCl
 If hypoglycemic or marginally hyperglycemic use the D5 variant of the 1/2 NS or NS chosen above

Rate 
20 mL/kg of crystalloid (NS or LR) over 1520 minutes (thus a pressure bag is needed) 
1001000 mL/hr titrated to whatever volume deficit you are correcting 
Hypovolemic hyponatremia
 The initial rate to correct by 10 mEq/L in 24 hours with NS is by rough calculation 1.35 x wgt [kg] for the rate in mL/hr. If using 3% saline multiply by 0.3 (2)
 Titrate the rate to serial BMP, if life threatening hyponatremia (e.g. seizures) correction may be more rapid for the first few hours but still < 1012 mEq/L over 24 hours
Hypernatremia
 The initial rate to correct by 10 mEq/L in 24 hours with D5 is by rough calculation 1.35 x wgt [kg] for the rate in mL/hr. If using 0.45% saline multiply by 2.
 Titrate the rate to serial BMP, such that the rate of correction < 1012 mEq/L over 24 hours
Hypokalemia
 Maximal correction via peripheral line is 40 mEq/L in 500 mL NS over four hours or 125 mL/hr
Hyperchloremic acidosis
 As per "rehydration" rates

402010 "rule" (for patients with normal electrolyte hemostatic mechanisms)
 40 mL/kg/hr for the first 10 kg
 20 mL/kg/hr for the second 10 kg
 10 mL/kg/hr for each additional 10 kg

How do you know it's working 
 Blood pressure increases
 Lactic acid decreases
 Urine output increases

 Blood pressure, heart rate, and urine output improve ("preshock")
 Anion gap decreases (diabetic ketoacidosis)
 CK and renal function improve (rhabdomyolysis)
 Replace 50% of urine output per hour (postobstructive diuresis)
 Urine output increases and renal function improves (hypovolemic acute prerenal failure)
 Amylase and lipase improve (pancreatitis)

Hypovolemic hyponatremia
 Sodium corrects by < 2 mEq/L every 4 hours, thus check BMP or ISTAT every 24 hours and titrate rate and composition of fluids appropriately
Hypernatremia
 Sodium corrects by < 2 mEq/L every 4 hours, thus check BMP or ISTAT every 24 hours and titrate rate and composition of fluids appropriately
Hypokalemia
 Electrolyte correction monitoring at most Q8H
Hyperchloremic acidosis
 Electrolyte correction monitoring at most Q8H

The patient remains hemodynamically stable without electrolyte abnormalities or worsening renal function 