Fluid and Electrolyte Balance

Due to the neonates unique fluid spaces, maintenance of proper fluid balance and frequent monitoring are essential. The 4 decisions that have to be made in neonatal fluid therapy are: the type of fluids, the rate of administration, the goals of the therapy and the limits of specific fluid therapies.
Editor's Comment - For an emergency fluid resuscitation protocol go directly to the end of the chapter.

I. Guidelines for Fluid Therapy:

Editor's Comment - Remember this take home point: Results of studies suggest that administration of an isotonic electrolyte fluid containing a physiologic concentration of sodium may not be appropriate for use in neonatal foals that require maintenance fluid therapy [1,2].

• Neonatal Characteristics

• Increased risk of infection requires absolute adherence to strict asepsis protocols for intravenous catheterization and fluids.
• Fluid and energy intake are directly linked on the normal all milk diet.
• Any medical or physical disorder which impairs nursing or milk availability will have immediate and profound effects on energy and fluid balance.
• In normal foals urine output is very high and fecal output is low.
• Body fluid compartments differ. Relative to body weight total body water, plasma volume and extracellular fluid volume (ECF) are much larger in foals. Compared to adults, water content of the body is 10% higher (75-80% vs. 65-70%). The extracellular and intracellular ratio is 50%-50% in neonates compared to 40%-60% in adults. 75% of the extracellular fluids are within the interstitium. The colloid oncotic pressure of the plasma is lower than in adults (20 mmHg vs. 25 mmHg), therefore increased hydrostatic pressure can lead to interstitial edema more rapidly than in adults (overhydration) [3,4].
• Premature animals may have even larger body fluid compartments relative to body weight.
• Foals have a higher surface area to body weight ratio and a higher resting respiratory rate. These factors contribute to larger insensible fluid losses and a greater tendency to lose body heat in cool or drafty environments.

• Fluid Intake

• Fluid intake as milk in the newborn nursing foal approaches 80 to 100 ml/kg/day (4 to 5 L/day in a 50 kg-foal) taken as small frequent feedings which may range from 100 to 300 ml every hour or two. Milk intake may approach 150-250 ml/kg/day in slightly older foals!! This large fluid intake is necessary to provide adequate energy from the all milk diet and to maintain appropriate digestive processes. Normal urine output is high, (5-10 ml/kg/h), and specific gravity is low (<1010 g/l).

• Fluid Therapy

• The first goal is adequate transfer and absorption of colostral immunoglobulins (See Colostrum - Assessment of and Sources for Foals).
• Route of fluids: Several categories with regard to route of administration for fluid therapy requirements.

• Orphan foal, bright, alert, standing, good suck reflexes, no serious medical problems.

• Bottle or bucket feeding 
• Nurse mare

• Weak or ill foal, suck reflex present but weak, able to maintain sternal position for part of the time, no obvious gastrointestinal abnormality, gestational age >320 days.

• Feeding via naso-gastric tube.

• Weak or ill foals with weak to no suck reflex. Dehydrated, hypoglycemic, recumbent foals with obvious gastrointestinal disease, or major fluid deficits due to diarrhea, evaporative losses.

• Intravenous fluids.
• Parenteral feeding.

• Guidelines for Initial Fluid
  It is absolutely essential that the initial body weight be measured and subsequently recorded at regular time intervals.

• Types of Fluids

• Crystalloids

• Isotonic: Lactated Ringer's, Normosol R, Plasmalyte 148, 0.9% sodium chloride, isotonic bicarbonate)
  (Editor's Comment - Use this for replacement of deficits and NOT maintenance.
• Hypertonic: (5% bicarbonate, 7.2% saline, 0.9 % sodium chloride with 5% dextrose)
• Hypotonic: (Plasmalyte 56, Normosol M, 0.45% Saline, 2.5% Dextrose)
  Editor's Comment - Use these for maintenance fluids.

• Colloids:

• Protein based: plasma, whole blood, hemoglobin products, albumin
• Polysaccharide based: hetastarch, pentastarch, dextran.

• Distribution of Fluids
  Isotonic fluids distribute within the EC according to volume (75% interstitium, 25% plasma).
  Hypertonic fluids remain in the EC space and draw fluid from IC space. Hypotonic fluids distribute in the EC and IC evenly until osmotic forces equalize.
  Colloids distribute within the EC plasma space only (unless capillary leak is present).
• Maintenance - Intravenous Fluids 
  Editor's Comment - Studies have shown that giving normal neonatal foals isotonic fluids produces Na retention and other adverse problems. Use hypotonic fluids for maintenance and use isotonic for initial replacement.
  These recommendations provide the maintenance requirements of the weak or recumbent foal that requires intravenous therapy but is not severely dehydrated, hypotensive or in shock.
  Editor's Comment - In foals with birth hypoxia do not overhydrate.

• Fluids:

• Plasmalyte 56, Normosol M: These fluids are hypotonic (110 mEq/l osmalarity). They contain less sodium (40 mEq/l), more potassium (13 mEq/l). They also contain magnesium instead of calcium. Acetate (16 mEq/l) is the only alkalinizing part in these fluids. Chloride to sodium ration is 1:1, which is a disadvantage for patients with hyperchloremic acidosis.
• Sodium chloride 2.5% and dextrose 2.5%. The solution lacks potassium or other electrolytes as well as alkalinizing agents. Ideal for patients with hyperkalemia.
• Dextrose in water (5%). Isotonic as a solution, but as glucose is metabolized it is then a source of free water. It should not be utilized in replacing fluid deficits or hypovolemia. It provides 170 kcal/l energy and lacks electrolytes.

• Volume

• 80 to 120 ml/kg/day (4-6 L in a 50-kg foal)
• Palmer's estimate is 100 ml/kg for the first 10 kg bodyweight, 50 ml/kg for the second 10 kg bodyweight and 25 ml/kg for the remainder of the body weight [5].
• Warm fluids to body temperature and best if administered as continuous IV infusion (flow rate 3-5 ml/kg/hr).
• As foals gain strength oral administration of liquid nutrients (milk or milk replacer) should supplement and eventually supplant IV therapy.

• Monitoring maintenance fluid therapy. The most critical component of the plan for fluid therapy is the clinical and laboratory assessment of response to therapy and resulting adjustments in the volume, rate or composition of administered fluids.

• Glucose

• Q4H or more often.
• Adjust flow to maintain glucose between 80-160 mg/dl (4.5-9 mmol/l).

• PCV and TPP - SID-BID. Normal range for foals is broad. Increasing PCV and TPP or developing signs of hypovolemia are indications for more sodium containing fluid.
• Plasma electrolytes, bicarbonate - SID. Monitor more frequently if status changes.
• Body weight. Record body weight at least BID. Foals on maintenance fluids should have small daily weight gain. Significant weight loss indicates inadequate, inappropriate therapy, or excessive fluid losses and the cause should be determined. Large, rapid and continued weight gain suggests fluid accumulation and the cause should be determined.
• Urine output. Urine output should be high6 ml/kg/h. Oliguria or anuria in the face of fluid therapy indicates a serious problem and the cause must be promptly and fully determined. Editor's Comment - if the foal's urine is concentrated and output is low and the foal is gaining too much weight from fluid retention-may have inappropriate ADH syndrome and have to lower rate and amounts of fluid.

II. Estimation of Fluid Deficits

• Clinical markers

• Clinical markers in hypovolemia

• Mentation
• Heart rate
• Pulse quality
• Extremity temperature
• CRT
• Mucous membrane color
• Jugular refill
• Urine production

• Clinical markers of dehydration

• Tacky mucous membrane
• Dry cornea
• Reduced skin turgor
• Sunken eyes

• Classification of dehydration

• Mild: 5% of body weight
• Moderate: 10% of body weight
• Severe: 15% of body weight

• Clinicopathologic and biochemical markers

• Serial PCV and TP: Clinical evidence of hypovolemia or marked increase in PCV and TP are grounds for continued administration of sodium-containing replacement fluids.
• Lactate: End product of anaerobic metabolism; will increase in shock due to inadequate oxygen delivery to the tissues. Obtaining a normal lactate is the end-point of fluid resuscitation. Monitor lactate during fluid replacement
  Causes for increased lactate are: tissue hypoxia (hypovolemia, seizures, exercise) enhanced glycolysis (SIRS, cathecholamines, thiamine deficiency, alkalosis, salycilates, B2-agonists, theophylline) and reduced clearance due to liver or renal dysfunction [6].
• Serum creatinine: might be increased after parturition due to placental compromise, and decreased oxygen delivery. Elevated levels should normalize within 2448 hours after birth with a gradual decrease.
• Hyponatremia (i.e., Na <130 mEq/L) represents a "relative water excess". It may indicate excessive loss of sodium and potassium as with diarrhea, adrenal insufficiency, excessive administration and/or retention of free water which also results in increased body weight, ruptured bladder with third space fluid accumulation in the abdomen, false hyponatremia due to hyperlipemia, hyperglycemia, and/or hyperproteinemia.
• Hypernatremia (i.e., Na >146 mEq/L) represents relative water deficit.
• Hyperkalemia (i.e., K >5.5 mEq/L) is commonly associated with volume depletion, metabolic acidosis, renal compromise, and ruptured bladder. Correction of hyperkalemia depends on restoration of effective circulating blood volume to assure adequate renal function. Administration of glucose at rates sufficient to produce hyperglycemia will tend to drive potassium into the cells. Avoid potassium-containing fluids.
• Hypokalemia (i.e., K <3 mEq/L) may develop following correction of a metabolic acidosis or as the result of potassium depletion if potassium is not provided. If renal function is normal, hypokalemic patients can tolerate 10 to 40 mEq/L of potassium in IV fluids given at a moderate to slow rate (max rate 0.5 mEq/kg/h).
• Hypoglycemia and hyperglycemia is a common and continuing problem for the neonatal foal. Maintenance of energy reserves may require parenteral nutrition as detailed in Parenteral Nutrition.
• Metabolic acidosis if sufficiently severe to require bicarbonate administration necessitates close monitoring of acid-base balance (initially every 30 to 60 minutes). Arterial samples are absolutely critical in foals with compromised respiratory function especially if assisted ventilation is required. Venous blood gases are adequate to assess metabolic acid-base abnormalities but should be relied upon only when respiratory function is normal. After the initial administration the objective should be the gradual and progressive correction of metabolic acidosis over a 12-to 24-hour period. Excessive, rapid bicarbonate administration may cause respiratory depression and CO2 retention or hypokalemia. Mixed acid-base abnormalities occur frequently in foals.

• Other monitoring tools (See Daily Plan and Monitoring of the Critical Care Patient)

• Mean arterial blood pressure (MAP)
• Central venous pressure (CVP)
• Urine output and characteristics

III. Resuscitation and Replacement

Resuscitation procedures will vary with the primary cause. The following approach will apply to the resuscitation of the premature foal, the dehydrated, volume depleted foal with diarrhea, intestinal obstruction, sepsis or shock. Careful monitoring of response to therapy should be performed at short time intervals with therapy adjusted, according to the monitored response and determination of the primary causal factors.

1. Principal Problems.

1. Plasma volume depletion - sodium-containing replacement fluid (Ringer, Lactated Ringer's Normosol R, Plasmalyte 148, 0.9% saline).
2. Hyperglycemia.
3. Metabolic acidosis: restoration of blood volume may ameliorate an acidosis and prompt fluid replacement may be all that is required. Sodium bicarbonate should only be given cautiously, if necessary (i.e. severe metabolic acidosis base deficit >8-10 mEq/L) preferably as an isotonic fluid. Blood gases must be closely monitored (i.e., every 60 minutes at first) particularly in patients requiring oxygen or assisted ventilation. Persistence of a base deficit greater than 6mEq/L in the face of volume replacement provides grounds for bicarbonate administration.
   Calculation of bicarbonate required:
   HCO3 (mEq) = Body weight (Kg) X 0.4 (L/Kg) X base deficit (mEq/L). The rate of bicarbonate administration will vary with degree of acidosis and primary disease process.
   With a severe metabolic acidosis up to one half of the calculated bicarbonate requirement can be administered in the first hour. The remaining bicarbonate is given over the subsequent hours as indicated by monitoring of acid-base balance. Rapid complete or excessive correction of acid-base abnormalities can have severe adverse CNS repercussions. Isotonic bicarbonate is better than hypertonic (5%, 8.4%). To make isotonic bicarbonate add 150 mEq (mmol) Sodium-bicarbonate (NaHCO3) to 1 L sterile water.

2. Resuscitation - Intravenous fluids [7].

1. Crystalloids

1. Lactated Ringers: first line fluid in resuscitation. It may be disadvantageous in patients with hyperchloremic metabolic acidosis and with hyperkalemia.
2. Normosol R, Plasmalyte 148: these are similar to LRS with few notable differences. They contain magnesium instead of calcium; therefore can be administered with plasma or whole blood. They contain acetate and gluconate instead of lactate. This can be an advantage in patients with liver failure. They contain less chloride and more sodium, therefore the strong ion difference is larger (SID=47 mEq/l)
3. 0.9% sodium chloride: can only be used in special situations due to the 0 potassium content (uroperitoneum, acute renal failure, HYPP crisis). Due to high chloride content, saline is not recommended in metabolic acidosis.
4. Isotonic Bicarbonate: best indications are inorganic metabolic acidosis, uroperitoneum. One has to monitor these patients for hypokalemia, metabolic acidosis, hypocalcemia. Bicarbonate administration should be performed cautiously in foals with hypoventilation.

2. Colloids

1. Biologic colloid solutions:

1. Plasma is an excellent colloid for neonates. The administration of large doses of plasma (20-40 ml/kg) is not cost prohibitive. The COP of plasma is approximately 20-25 mmHg. In addition to oncotic effects, it also serves as a carrier for hormones, drugs and is a good source of clotting factors. Foals should be monitored for adverse reactions.
2. Whole blood may be indicated as a colloid in cases of hemorrhagic shock. Complications associated with transfusion are due to incompatible red blood cells and transferred leukocytes. Citrate toxicity and hypocalcemia are potential side effects too.

2. Synthetic colloids:

1. Hydroxyethyl starch (hetastarch) is the most commonly used colloid in equine medicine in the USA. The COP is approx. 30 mmHg, so it is a more cost-effective colloid than plasma [7,9]. The primary side effects are coagulopathies through reductions in clotting factor VIII and vWF. Platelet count may be altered too.
   Recommended dose in adult horses is 8-10 ml/kg.
2. Pentastarch: medium molecular weight colloid, the COP is 40 mmHg. Other advantage is the larger volume expansion compared to other synthetic colloids.
3. Dextrans and Polygelatins are also used for the treatment of hypoproteinemia and decreased colloid oncotic pressure, but allergic reactions and coagulation disturbances appear to be more common in horses.
4. Synthetic oxygen carrying substances like Oxyglobin® (Biopure, Cambridge, MA) contain polymerized bovine hemoglobin. Hemoglobin also provides oncotic effect.

3. Volume

1. 10-20 ml/kg for crytalloids initially
2. 2-10 ml/kg for colloids
3. Reassess for the additional needs (pulse quality, CRT, lactate, arterial blood pressure, CVP, monitor for signs of pulmonary edema, SQ edema)
4. Initial crystalloid doses may have to be repeated 1-4 times.
5. It is nearly impossible to accurately predict the fluid requirements of these patients. Initial estimates can be made and then adjustments made based on response to therapy. As a general rule: the more critical the patient, the shorter interval for assessment and reevaluation of therapy.

IV. Case Based Approach

Obtunded, recumbent, 12 hour old, 45 kg thoroughbred foal. T=95F, R=24, P=84, MM: pale, CRT:4.5 sec, cold extremities, ileus. Marked hypovolemia, lactate = 7.8 mEq/l, glucose = 54 mg/dl (3 mmol/l), IgG<200 mg/dl, MAP = 44 mmHg
Initial management:

Intranasal oxygen therapy
Sternal recumbency
Broad spectrum antimicrobials
Slow warming (1-2°F/hour)

Fluid therapy:

1. 20 ml/kg (1L) replacement crystalloid (LRS, Normosol-R, or Plasmalyte-148) with 20 cc 50% Dextrose added - give over 30 minutes
   Reassessed clinical parameters:
   Obtunded, R = 28, P = 80, T = 94.5°F, lactate = 6.9 mEq/l, glucose = 92 mg/dl, MAP=56 mmHg
2. Give another 20 ml/kg (1L) replacement crystalloid (LRS, Normosol, or Plasmalyte) with 20cc 50% Dextrose over 30 minutes
3. Give 20 ml/kg (1L) Hi-gamma Plasma over 1 hour
   Reassessed clinical parameters:
   Holding the head, moderate suckle reflex, R=36, P=92, T=97.5°F, lactate=3.8 mEq/l, glucose=135 mg/dl (7.5 mmol/l), MAP=74 mmHg
   Now switch to maintenance fluids: Continue fluid therapy at lower rate of 4-5 ml/kg/h (1L fluid q4-6 hours).
   Continue monitoring (Physical exam, MAP, Lactate, Electrolytes, Blood gas, etc.)
4. If still no marked improvement with fluid resuscitation and the foal is rehydrated, then proceed to inotropic therapy (dobutamine, See Resuscitation Part 2 - Evaluation of Cardiovascular Status & Shock (SIRS)). If inotropic therapy fails add vasopressor as well (norepinephrine, vasopressin).