4 Veterinary Fluid Therapy: Objectives and Calculations

Fluid Therapy Objectives

• One should have knowledge of water distribution in the body.
• You should calculate fluid deficits.
• Recognize electrolyte and acid-base abnormalities.
• Choose appropriate fluid choices.
• There is no need to memorize fluid #'s but have an idea of their effect.
• Recognize acidifying/alkalinizing fluids.
• Understand which fluids are Na Cl rich versus poor, .9% NaCl is rich, and 0.45% NaCl is poor.

Total Body Water (TBW)

• TBW comprises 60% of body weight (BW).
• Intracellular Fluid (ICF) makes up 40% of BW and 66% of TBW.
• Extracellular Fluid (ECF) is 20% of BW and 33% of TBW.
• Interstitial Volume is 12% of BW and 60% of ECF.
• Blood Volume is 8% of BW and 40% of ECF.

Fluid Deficit Calculation

• Volume needed is calculated by adding % dehydration, maintenance, and ongoing losses.

Dehydration vs Hypovolemia

• Dehydration refers to the loss of total body water.
• Hypovolemia is the decrease in circulatory volume (blood).
• An animal can be dehydrated without being hypovolemic.
• Animals can pull water from the interstitial space to maintain circulatory volume.

Assessment of Hydration

• Mild dehydration is not clinically appreciable if it is less than 5%.
• Clinical findings with dehydration: Tachycardia, reduced jugular fill, cold extremities, tacky mucous membranes and sunken eyes.
• Reduction in body weight is helpful in assessing hydration while being hospitalized.

Laboratory Estimates

• PCV/TP measurements can be affected by splenic contraction or anemia.
• Protein loss via GI, renal or third spacing can affect PCV/TP.
• High Globulin with chronic disease can affect PCV/TP
• Creatinine levels are useful if renal function is normal.
• Urine specific gravity is sensitive for dehydration when renal function is normal.
• Adult horses demonstrate USG >1.030 when dehydrated.
• Young foals demonstrate USG > 1.005 when dehydrated.
• Increased blood lactate (> 2.0) occurs with dehydration.
• Lactate increases with anaerobic metabolism.
• Anemia can cause increased lactate.
• Pneumonia and hypoxemia/hypoxia can cause increased lactate.
• Hypovolemia and dehydration result in poor perfusion, decreased oxygen delivery and increased lactate concentrations.

What Values To Estimate Hydration

• 6% Dehydrated: HR 40-60, CRT 2, PCV/TP 40/7, Creatinine 1.5-2.0
• 8% Dehydrated: HR 60-80, CRT 3, PCV/TP 45/7.5, Creatinine 2.0-3.0
• 10% Dehydrated: HR 80-100, CRT 4, PCV/TP 50/8, Creatinine 3.0-4.0
• 12% Dehydrated: HR >100, CRT >4, PCV/TP >50/8, Creatinine > 4.0
• In reality, estimating percentage dehydrated is a guess, constant reassessment of hydration is necessary.

Maintenance Fluids

• For general maintenance, administer 60 ml/kg/day.
• For a 450 kg horse, this equals 27 L/day, or roughly 25-30 L/day using 5 L bags.
• Alternatively, administer 2.5 ml/kg/hr for general maintenance.
• For a 450 kg horse, this equals 1,125 ml/hr or about 1 L/hr.
• Neonatal foals have a maintenance rate of 80-120 ml/kg/day because of higher total body water.
• Catheters can be positional.

Ongoing Losses

• Nasogastric reflux (post-op colic or Anterior enteritis) should be measured as an ongoing loss, this is easy to measure.
• Diarrhea is an ongoing loss and is harder to measure.
• Third spacing and hemorrhage can be ongoing losses.

Routes of Fluid Administration

• IV administration is necessary for severe dehydration, refluxing horse, or large volumes.
• Oral administration via NG tube is least expensive, and great with large colon impactions (not with refluxing horse).
• SQ is not used.

Bolus - Rate of Fluids

• A 10% dehydrated horse has a 50 L deficit.
• Administer 10-20 ml/kg at presentation.
• Administer 5-10 L in a 500 kg horse.
• The maximum shock dose (not really used) is 60-80 ml/kg/hr = 24-32 L in 1 hr.
• Reassess patient and add rest of fluids over 24 hours.

Types of Fluids to Give

• Crystalloids (several) are available.
• Choice depends on replacement of dehydration, maintenance or electrolyte & acid-base.
• LRS is an alkalinizing fluid with mOsm/L: 273.
• LRS includes Na: 130 mEq/L, Cl: 109 mEq/L, K 4 mEq/L, and Ca: 3 mEq/L.
• Lactate anion is metabolized to HCO3 in the liver.
• 0.9% NaCl is an acidifying fluid with mOsm/L of 308.
• 0.9% NaCl includes Na and Cl both at 154 mEq/L.
• Plasmalyte A is an alkalinizing fluid with mOsm/L 298.
• Plasmalyte A contains Na 140 mEq/L, Cl 98 mEq/L, K 5 mEq/L Mg 3 mEq/L.
• Acetate is metabolized to HCO3 in plasma.
• 0.45% NaCl is an acidifying fluid, but HYPOTONIC.
• 2.5% dextrose can be added to make 0.45% NaCl isotonic.
• 0.45% NaCl is used in some patients that develop hypernatremia on IV fluids (foals).
• 5% dextrose is isotonic, but only dextrose.
• 5% or 8.4% NaHCO3 is commercially available but very hypertonic.
• It is diluted to 1.3% to make it isotonic.
• Hypertonic saline is used in select situations.

Lactic Acidosis

• Lactic acidosis is common in dehydrated animals/poor perfusion.
• LRS still used in these patients provided no liver failure.
• If you rehydrate and reassess and the patient is no longer dehydrated and has a metabolic acidosis then administer NaHCO3.
• To treat metabolic acidosis, correct dehydration and then reassess.
• Do not give HCO3- before volume expansion.
• Volume expansion is key with lactic acidosis.
• Saline is mildly acidifying but not the best choice.
• LRS and Plasmalyte are both alkalinizing.
• Increased production of lactate can be caused by tissue hypoxia (dehydration).
• The liver metabolizes lactate and HCO3.
• Be careful using LRS if there is liver failure.

Supplement HCO3

• HCO3 deficit = (24-Patients HCO3) x VD x BW kg.
• The Volume of Distribution (VD) is from 0.3 to 0.6.
• A higher number is needed for neonates.
• Replace ½ deficit and rest over 12-24 hours.
• Do not use with Ca++ fluids.
• 5% NaHCO3 = 595 mEq/L HCO3, is hypertonic at 1190 mOsm/L.
• 1.3% NaHCO3 = 156 mEq/L HCO3, and is Isotonic.
• You have to dilute hypertonic to isotonic.

Other Bicarbonate Options

• Oral administration is easy.
• 1 gm NaHCO3 = 12 mEq HCO3.
• Divide into doses for 24 hrs.

Hope – 3 day filly diarrhea

• PCV 50% TP 8.5 gm/dl (all high).
• Creatinine 3.9 mg/dl (high).
• Hyponatremia = 121 mmol/L (135-144).
• Hypochloremia = 89 mmol/L (95-105).
• pH = 7.25 (rr 7.4).
• HCO3 = 12 mEq/L (rr 24).
• PCO2 = 39 mm Hg (rr 35-45).
• Lactate = 5 mmol/l (rr < 1).

Fluid Therapy- Hope

• Extremely dehydrated, dealing with metabolic acidosis due to lactic acidosis.
• The best fluid type for Hope differs from livestock.
• After 48 hrs, pH = 7.2, HCO3 = 14 mEq/L, Lactate = 0.8 mmol/L, dealing with metabolic acidosis but is not dehydrated.