Fluid Management
Posted by Anil

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