Fluids
http://www.pearsonhighered.com/samplechapter/0131186116.pdf
Total Body Water
- 50-70% of total body weight
- i.e. 40L in 70 kg male
- More in thin people [less fat]
- More in younger people
- 2/3 Intracellular (~60%)
- Most in skeletal muscle (M>F)
- 1/3 Extracellular (~40%)
- ¾ Extravascular (~30%)
- ¼ Intravascular/plasma (~10%)
- Most fluid exchange occurs here
Plasma Osmolality = 2 [Na] + Glucose + BUN
- 2 salts and a sugar BUN
- Osmolality active particles determines amount of water in a compartment - osmotic effects
Types of fluid
- Non-isotonic fluid – Difference in osmolality – there is an osmotic gradient
-- Based on Starling forces – the balance between plasma hydrostatic P + oncotic P
- Isotonic fluids – Equal gains and losses within ECF (no osmolar changes)
Colloid vs. Crystalloid - Volume expansion (intravascular)
- Important for management of hypovolemia and dehydration
- Maintain BP
- Crystalloid – High saline, Ringer's, D5W
- Goal is to expand intravascular space
- Iso-osmotic - only 2/3 stay intravascular
- 2L or 20 mg/kg bolus (2 large bore IVs)
- Colloid – Albumin, blood products (Stimulate liver to release albumin)
- Stay intravascular if capillary intact
- Only give if:
- Hypovolemic after 2L Crystalloid
- Excess Na/water but hypovolemic (i.e. CHF, ascites)
- Cannot make albumin (i.e. CLD, transplant pt., malnutrition, resection)
- Hemorrhagic or coagulopathy
Complications:
- More PE or Respiratory failure; more expensive; transfusion reations
Dehydration - common
- GI loss (V/D, suction, fistula)
- Third spacing w/ injury
Intra/retro abdominal infection, peritonitis, obstruction
- Burns
- Fever
- Osmotic diuresis (urea, mannitol, glucose)
- Post-op/low input during op
- Renal function important to monitor:
- Even w/o intake - excrete 800 ml/day of urine waste
- Fever and hyperventilation increase amount
- Distal tubules work w/ Na-K/H
- Aldosterone and ACTH
Fluid Loss
Free water deficit
- N Body Water (0.6 x kg) - current body water
- CBW = NBW x (N serum Na/measured serum Na)
- Fever (2-2.5 ml/kg/day per degree Celsius)
- Loss of Fluid (Vomit, NG, fistula)
- Third-spacing (trauma, inflamed, op)
- Burns
- Osmotic diuresis (urea, mannitol, glucose)
Management:
- Replace fluids in 24 hrs
- Maintenance fluids for next 24 hours
- Rehydration important to balance ECF and ICF
Rehydration Method
- Must maintain N body intake + provide fluid to make up for the fluid loss
- Weight (in kg) dependent
Estimated maintenance fluid
4-2-1 Rule
- 4 ml/kg/hr - 10 kg
- 2 ml/kg/hr - 11-20 kg - (+40 ml/hr from above)
- 1 ml/kg/hr - >20 kg (+60 ml/hr from above)
Replacement:
- 1st hour – Volume expander (Crystalloid – 2L or 20 ml/kg NS or Ringer's)
Maintenance:
- Next 8 hours – Total fluid given = Expected maintenance fluid + 1/2 calculated loss
- Next 16 hours – Total fluid given = Expected maintenance fluid + Other 1/2 calculated loss
- Longer time-frame means slower rate of infusion
- Calculated loss is the amount of fluid lost
Complications:
- Large V fluid => Can lead to edema
- CHF, Ascites, Pulmonary edema
- Large amount dextrose => Can lead to hyperglycemia
- Large amounts of NS => Can lead to hyperchloremic metabolic acidosis => affects O2 transportation
- Non-anion gap metabolic acidosis
- Ringer's lactate give when hypovolemic + metabolic alkalosis (i.e. NG tube, vomiting)
- May worsen when lactate is metabolized
Excess fluid
Types:
- Isotonic – Increased ECF w/ balance w/ ICF
- Iatrogenic (TPN)
- 2o to renal insufficiency, cirrhosis or CHF
- Hypotonic – Insufficient little salt in replacement for GI losses
- Third spacing
- Increased ADH w/ surgical stress or SIADH
- Hypertonic – Excess Na w/o adequate water (water out of cells)
- Rapid infusion of non-electrolyte osmotically-active solutes
- Glucose and mannitol.
Management:
- Restrict Na+ and fluids
- If hypertonic hypervolemia – Replace free water
- Correct hypertonicity => then remove excess fluid
- NS or ½ NS (not too fast) w/ Lasix
- If hypotonic hypervolemia – Replace saline
- Correct hypotonicity => self-corrects hypervolemia
- NS or Ringers; hypertonic saline if severely symptomatic
- Or Lasix 10-50 mg + Replace K+ - do not overdo
- Maintains kidney/brain perfusion
- On PRN basis: Cardiotonic drugs, O2 + ventilation
Hydration status
Hypovolemia:
- Tachycardia, low pulse pressure, Postural hypotension - BP remains low if 20-30% of total blood lost
- CVS signs first => CNS signs => Tissue signs after 24 hrs
On exam:
- Assess neck veins, tissue turgor, cool extremities, dry mucus membranes
Hypervolemia:
On exam:
- JVP elevated, Rales, S3 HS, Edema
- Assess input/output:
- Normal output: 0.5 cc/kg/hr for adult, 1 cc/kg/hr for kids
- Low: Hypovolemia, renal failure, low flow states
- High: Hypervolemia, diabetes insipidus, osmotic diuresis, post-obstructive diuresis
Investigation:
- Check LBC daily
- BUN/Cr <15 is adequate hydration
- BUN/Cr >20 + FENa (Fractional excretion of Na) <1% point to hypovolemia
