1 Body Water Distribution & Dehydration Assessment

Distribution of Water in the Body

• Total body water (TBW) is 60% of body weight
• Intracellular fluid (ICF) accounts for 40% of body weight and 66% of TBW
• Extracellular fluid (ECF) accounts for 20% of body weight and 33% of TBW
• Interstitial volume is 12% of body weight and 60% of ECF
• Blood volume is 8% of body weight and 40% of ECF

Volume of Fluid Needed

• Usually calculated for a 12 to 24 hour period
• Total volume fluid needed = % dehydration + maintenance + ongoing losses

Assessment of Dehydration

• Mild dehydration is not clinically appreciable (<5%)
• Clinical signs of dehydration include tachycardia, reduced jugular fill, cold extremities, tacky mucous membranes, and sunken eyes
• A reduction of body weight can be an indicator of fluid therapy success
• PCV increases due to splenic contraction in dehydration (normal range 32-45%)
• Plasma protein increases with dehydration (normal range 6.5-7.5 g/dl); disease processes can affect TP
• Creatinine concentration increases with dehydration if renal function is normal
• Urine specific gravity > 1.030 suggests dehydration in animals with normal renal function; young foals have lower USG (1.001-1.005)
• Blood lactate concentrations > 2.0 mmol/L may indicate tissue hypoxia due to dehydration

Dehydration level estimates

• 6% dehydration: HR 40-60, CRT 2 sec, PCV/TP 40/7, Creatinine 1.5-2.0 mg/dl
• 8% dehydration: HR 60-80, CRT 3 sec, PCV/TP 45/7.5, Creatinine 2.0-3.0 mg/dl
• 10% dehydration: HR 80-100, CRT 4 sec, PCV/TP 50/8, Creatinine 3.0-4.0 mg/dl
• 12% dehydration: HR >100, CRT >4 sec, PCV/TP >50/8, Creatinine >4.0 mg/dl

Maintenance Fluids

• Maintenance fluid requirement varies with age and environment
• Adult horse: 60 ml/kg/day or 30 ml/lb/day; a 500 kg horse requires 30 L a day (2.5 ml/kg/hr)
• Neonatal foal requires a higher rate: 80-120 ml/kg/day
• Foals with over-hydration concerns may receive less fluid based on a formula
  • 1st 10 kg of body weight: 100 ml/kg/day
  • 2nd 10 kg of body weight: 50 ml/kg/day
  • Additional kg of body weight: 25 ml/kg/day

Ongoing Losses

• Reasons for losses include colitis, nasogastric reflux, renal failure, sweat loss, third spacing of fluid, and hemorrhage
• Assessing clinical signs and lab data is important to determine if fluid therapy goals are being met

Route of Fluid Administration

• Influenced by volume needed, patient condition, type of fluid, cost, and availability

Oral Fluid Administration

• Easiest and least costly route
• Indications include mild dehydration and impactions
• Given as a bolus (4 L q 30-60 minutes) or even as a CRI
• Patient should not be refluxing

Intravenous Fluid Administration

• Jugular vein is most commonly used; cephalic, lateral thoracic, and saphenous veins are alternatives
• Indicated for critically ill patients requiring high rates and volumes of fluids
• Can be expensive, complications can include thrombophlebitis

Subcutaneous Fluid Administration

• Not commonly used due to limited subcutaneous space and volume capacity

Rate of Fluid Administration

• Animals in shock and significant dehydration require faster fluid administration
• Determine % dehydrated and calculate the total fluid deficit
• Most clinicians administer ¼ of the calculated fluid deficit initially, then reassess
• Maximum rate for shock is 60-80 ml/kg/hr
  • For a 500 kg horse, this is 30-40 L in 1 hour or 15-20 L in 30 minutes
• Rapid administration can be achieved with a fluid pump and large bore catheters
• Overhydration is concerning in foals and miniature horses
• Continuous rate infusion (CRI) is used for replacement of maintenance and ongoing losses
• Adult horses: use drop/second to administer CRI, estimating fluid remaining in the bag
• Miniature horses/Foals: use a fluid pump to administer CRI
• Bolus fluid administration corrects dehydration and can be used in ambulatory foals with mild-moderate diarrhea

Types of Fluids to Give

• Crystalloids are most commonly used
  • Contain electrolytes and non-electrolyte substances and distribute to the ECF within minutes
  • Isotonic polyionic fluids (LRS, Plasmalyte A) are common in equine medicine

Crystalloid Fluid Properties

• 0.9% NaCl: mOsm/L 308, Na+ 154 mEq/L, Cl- 154 mEq/L, pH 5
• Lactated Ringers: mOsm/L 273, Na+ 130 mEq/L, Cl- 109 mEq/L, K+ 4 mEq/L, Ca++ 3 mEq/L, pH 6.5
• Plasmalyte A: mOsm/L 298, Na+ 140 mEq/L, Cl- 98 mEq/L, K+ 5 mEq/L, Mg++ 3 mEq/L, pH 7.4
• 5% Dextrose: mOsm/L 252, pH 4
• 5% NaHCO3: mOsm/L 1190, Na+ 595 mEq/L, pH 8

Acidifying and alkalinizing fluids

• Fluids can be divided into acidifying and alkalinizing fluids based on the buffer
• Buffers include lactate (metabolized to HCO3 in the liver), acetate, and gluconate

Lactic Acid

• Increased lactic acid can be caused by either increased production or reduced clearance
• Increased production is most commonly due to tissue hypoxia
• Clinical trials have shown that lactate containing solutions do not exacerbate lactic acidosis due to hypoperfusion
• LRS should be used in caution in an animal with lactic acidosis and liver failure however

Replacement Fluids

• Should have electrolyte concentrations similar to that in the ECF (LRS)
• Exception is hypertonic saline (7% NaCl) which will expand the intravascular volume quickly; follow with isotonic fluids

Maintenance Fluids

• Should preferably have less sodium, and more calcium, magnesium and potassium (15-30 mEq/L)
• There are no commercial maintenance fluids for horses
• Will often add calcium gluconate (25-50 ml to a 5 L bag of LRS) and potassium chloride (20 mEq/L to LRS)

Electrolytes

• Electrolyte, acid-base abnormalities and hypoalbuminemia determine the best type of fluids chosen
• Sodium is a major component of extracellular fluid
• Hyponatremia (<120 mEq/L) can cause cerebral edema: correct slowly; choices include LRS and 0.9% NaCl
• Hypernatremia (>160 mEq/L) can cause cerebral dehydration, blindness, depression, seizures: correct slowly; choices include 5% dextrose, 2.5% dextrose in 0.45% NaCl
• Potassium is a major component of intracellular fluid; acid-base changes affect potassium concentrations
  • Acidosis promotes potassium leaving intracellular space
  • Alkalosis promotes potassium entering intracellular space
• Hyperkalemia (> 5mEq/L): causes cardiac arrhythmia
  • Choices include dextrose and calcium gluconate and NaHCO3
• Hypokalemia (< 3.0 mEq/L): leads to neuromuscular, GI and cardiac conduction abnormalities
  • Supplement/treat when potassium < 3.0 mEq/L
  • Supplement/treat when anorexia for several days
  • Supplement/treat if potassium low normal and there is an ongoing acidosis
  • Maximal rate to supplement is 0.5 mEq/kg/hr
• K+ Deficit Calculation: (4.0 – [K+]) x Vd(0.4) x BW (kg)
• Chloride is a major extracellular anion
  • Can be sequestered or lost with proximal GI disease
  • Hypochloremia: can occur with proximal enteritis, loss of salvia, colitis and renal failure; fluid choices: 0.9% NaCl; LRS
  • Hyperchloremia: less common; renal tubular acidosis, severe colitis; correct with 5% dextrose
• Calcium: total calcium = protein bound(albumin) + ionized
  • Hypocalcemia occurs with blister beetle and GI disturbances; correct with 23% Cagluconate
  • Hypercalcemia is rare, reported in chronic renal failure horses or neoplasia
• Dextrose: treats hypoglycemia; treat hyperkalemia or hypersosmolar states

Acid-Base Status

• H+ + HCO3- ↔ H2CO3 ↔ H2O + CO2
• Normal pH range: 7.35-7.45
• Normal HCO3 range: 22-26 mEq/L
• Normal PaCO2 range: 35-45 mmHg
• Assess pH, HCO3, and PaCO2 to determine acid-base status:
  • Acidosis: pH < 7.4
  • Alkalosis: pH > 7.4
  • Metabolic acidosis: HCO3 < 24
  • Metabolic alkalosis: HCO3 > 24
  • Respiratory alkalosis: PaCO2 < 35 mmHg
  • Respiratory acidosis: PaCO2 > 45-50 mmHg

Common Acid-Base Abnormalities with GI Disease

• Metabolic acidosis
  • Lactic acidosis (due to tissue hypoxia/poor perfusion)
  • Loss of HCO3 (loss in feces with diarrhea or urine)
• Metabolic alkalosis: Hypochloremic metabolic alkalosis due to proximal enteritis or esophageal obstruction

Treatment of Metabolic Acidosis

• Correct dehydration and provide volume expansion
  • Dehydrated horses often have high lactate
  • Clinical evidence of dehydration indicates use of LRS or Plasmalyte
• Reassess blood gas
• Bicarbonate is indicated when HCO3 < 10-15 mEq/L despite fluid therapy, use this formula to work out the deficit:
  • HCO3 deficit = (24 – HCO3) x Vd X Body weight
  • Vd = 0.3 to 0.6 (Use 0.4 to 0.5 in most cases, high end for neonates)
  • Replace half the calculated deficit in the first 1-2 hours, and the rest over 12-24 hours
• 5% NaHCO3 is 595 mEq/L of HCO3, but is hypertonic; ideally should dilute
• 1.3% NaHCO3 is 156 mEq/L and is isotonic, but must be made (not commercially available)
• Oral NaHCO3 can be given: 1 gm NaHCO3 = 12 mEq of HCO3

Anion Gap

• Anion Gap (AG) = Differences between measured cations and measured anions; categorize acid-base abnormalities
• AG = (Na+ + K+) – (Cl- + HCO3); reference range is 10-17
• Provides a measure of unmeasured anions (lactate, ketones...)
• High Anion Gap: Lactate (metabolic acidosis with an increase in anion gap)
• Normal Anion Gap: Metabolic acidosis with renal tubular acidosis and diarrhea

Colloids

• Colloids: more of the administered solution remains in the plasma compartment
• Use of colloids is indicated in patients with hypoalbuminemia (albumin < 1.5 gm/dl - the edema threshold)
• Choices: Plasma, Hetastarch and whole blood
  • Hetastarch will increase oncotic pressure, but have no effect on albumin concentrations
  • Whole blood is usually given when blood transfusion is necessary to increase oxygen carrying capacity

Summary of Fluid Therapy

• Fluid therapy is common in veterinary medicine
• Fluid therapy plans are continually changed based on clinical and laboratory reassessment of the patient