The principle goals of fluid and electrolyte therapy are the restoration and maintenance of homeostasis by restoring cellular perfusion, normal cellular functions, fluid volume, electrolyte and acid-base balance.
In order to understand the need of an animal to fluid therapy, three basic body changes must be kept in mind, these changes are; hydration status of the body, electrolytes changes, and changes of the acid-base balance.
I-HYDRATION STATE OF THE ANIMAL
Water constitutes more than 70% of the animal’s body; 50% intra-cellular fluid and 20% extra-cellular fluid. The 20 % extra-cellular fluid is classified into 15% (in the interstitial space and in the body cavities like GIT and peritoneal cavity etc…) and the other 5% is intra-vascular.
Body of the animal
The animal suffers from dehydration when the animal losses large amount of water outside the body as during severe diarrhea, severe vomiting, hot weather with water withheld, etc… all the previously mentioned condition cause direct loss of body water and the final result is called dehydration. Dehydration doesn’t only mean loss of water outside the body, but the animal may also suffer from dehydration without loss of water outside the body. This occurs when the ratio between the mentioned body fluids disturbed, and an example of this is the sequestration of the body fluid in the rumen or the intestine as a result of obstruction of the bowel.
Severity of dehydration
Body weight deficit %
> 10 %
Dehydration up to 5% is clinically inapparent, but over 5% is clear and the other signs of dehydration are clear, and over 12-15% dehydration, it is said that the animal suffers from shock. Detection of the dehydration can be done either clinically by using skin fold test or laboratory by detection of the PCV in the light of total plasma proteins (TPP), hemoglobin concentration, and kidney function tests. The skin fold test is performed over sides of the neck by picking up and pinching of a skin fold. It is used as a rough guide for degree of dehydration.
In good hydration state the skin fold should flatten within 1-2 sec and percentage of dehydration measured with the consumed seconds for flattening of the skin fold. On reaching 7 seconds (7% dehydration) the animal will be so dehydrated that the eyes seem sunken. The major problem with dehydration is the adverse effect on the cardiovascular system with consequent reduction of tissue perfusion.
This finally will lead to, oliguria, low blood pressure, absence of hemorrhage at surgery site, tachycardia, cellular ischemia and acidosis as a result of glycolysis.
It is a group of extra and intra cellular cations (+) and anions (-). Water in the body never moves freely, but it usually moves with electrolytes. The loss or sequestration of water in the body is usually associated with disturbances in electrolytes, and if these disturbances are not corrected, the animal will die.
Na+ 131-147 mEq/L
HCo- 3 20
Organic acids 10
It is the major extra-cellular cation, responsible for half the plasma osmolarity, plays an important role in maintaining normal distribution of water, and controls osmotic pressure of extra-cellular fluid. The plasma sodium level in normal horses is 131-147 mEq /L.
It is the major intra-cellular cation and the extra-cellular level is very low. Red blood cells and muscles contain high amount of potassium, so hemolysis and muscular activity elevate the serum level of potassium. The plasma potassium level in normal horses is 1.7-4.93 mEq/L.
It is the major extra-cellular anion. The plasma chloride level in normal horses is 94-113 mEq/L.
Acid-base balance is carefully regulated with normal blood pH ranging from 7.36-7.44. Blood pH is determined by the concentration of free hydrogen ions and the animal suffers from acidosis when pH lowered than 7.35 and pH values less than 7.2 are life threatening, but significant alkalosis is uncommon in surgical patients. The state of acid-base balance or imbalance of the extra-cellular fluid depends upon the relative quantities of carbonic acid and base bicarbonate present in the extra-cellular fluid. Normally they are present in the ratio of 1part carbonic acid: 20 parts bicarbonate. When the pH increases over 7.8 or decreases than 6.8, the animal dies. Alteration in the acid-base balance is either respiratory or metabolic in origin.
H2O + CO2 H2CO3 H+ + HCO3-
It is a condition where the CO2 is retained as a result of hypoventilation leading to respiratory distress, depression of the CNS, weakness, cyanosis, and finally coma. The condition is characterized by retention of CO2 and H2CO3 and excess H+. The compensatory mechanism of the kidney is the retention of HCO3- and secretion of H+. Laboratory analysis of the blood and urine will reveal dropping in the pH. Treatment of this affection is directed toward proper ventilation and using of alkalizing solution like sodium bicarbonate, lactate or acetate.
It is a condition of loss of high amount of H2CO3 and in turn H+, with retention of HCO3- as a result of hyperventilation. Clinical signs include hyperpnea, hyperactive tendon reflexes, and CNS stimulation or excitation with convulsion. The pathogenesis is excessive release of CO2 and the compensatory mechanism of the kidney is to excrete HCO3- with retention of H+. Laboratory analyses of the urine and blood will reveal rising of the urine pH. Treatment of such affection includes administration of acidifying solutions like NaCl.
It is a state of retention of H+ with loss of HCO3-, it occurs as a result of excess acid production or excess loss of base. Clinical signs are characterized by depression of the CNS with hyperpnea. The pathogenesis is that ketones and excess Cl- replace HCO3-, thus lowering pH while the compensatory mechanism includes hyperactive respiration to remove H2CO3 and the kidneys save HCO3- and excrete H+. Laboratory analysis will reveal low urine pH, blood pH, serum HCO3-, and rising of the serum K+. Treatment of such condition includes administration of alkalizing agents like sodium bicarbonate, lactate or acetate.
It is a condition of excessive retention of HCO3- with reduction of the H+. Clinical signs include CNS excitation like tetany and convulsion and muscular hyper-tonicity with dyspnea. The pathogenesis is the depletion of Cl- with subsequent rising of the HCO3- and blood pH, while the compensatory mechanism includes rising of the ability of the kidney to excrete HCO3- with retention of H+. Laboratory analysis reveals elevation of the HCO3- and lowered Cl- and K+. Treatment is directed toward administration of acidifying solution like NaCl.
Acid-base imbalance directly affects the electrolytes, when acidosis ensues, the high level of extra-cellular H+ shifts intra-cellular and exchanges with intra-cellular K+. On the other hand, alkalosis causes the extra-cellular K+ to shift intra-cellular and exchanges with intra-cellular H+ so in cases of acidosis, the high extra-cellular K+ is false and the animal may suffer from reduction of the total amount of K+.
How to use fluid therapy?
On using fluid therapy, three basic questions must be kept in mind; what is the type of fluid therapy to be used? what is the amount of fluid therapy to be used?, and what is the rate of injection?
The fluid plan can be made without knowledge of laboratory result by using the anticipated alterations of the disease and isotonic polyionic fluid as lactated Ringer’s is suitable for this stage, and later on the plan can be remodeled on knowing the laboratory results. The fluid should be used at the same body temperature.
Most of animals admitted to your clinic affected with acidosis. The state of acidosis ensues as a result of anaerobic glycolysis, and these animals suffer from hyperglycemia. Accordingly, incorporation of glucose in the fluid therapy is contraindicated unless the laboratory results revealed the need of the animal to glucose. Improper use of glucose may aggravate the condition of dehydration by stimulating renal excretion.
The amount of fluid therapy can be classified into replacement dose and maintenance dose. Priority must be given to the volume of replacement (degree of dehydration X animal weight) injected in a rate of 10-20ml/kg/hour, while the maintenance requirements are 50ml/kg/day and injected in a rate of 2-4ml/kg/hour.
Ringer’s solutions, containing lactate or acetate as bicarbonate precursors, closely resemble plasma composition. Lactate, acetate, and gluconate are used commonly as base sources because bicarbonate solutions are unstable and calcium carbonate precipitates in calcium-containing solutions. Lactate and gluconate induce alkalinization only after hepatic metabolism whereas acetate metabolism occurs in tissues and muscles.
Fluid therapy should be used cautiously in patients with CNS edema, congestive heart failure, or severe respiratory diseases as intensive fluid therapy can exacerbate the problem of fluid accumulation, and administration of sodium rich solutions should be avoided in patients with cardiac diseases to avoid volume overload.
The rapid rate of intravenous fluid administration is not hazardous in adult horses, however, the rapid expansion of circulating volume may cause diuresis before diffusion into ECF compartment, but in foals, this rapid injection rate can result in pulmonary and or cerebral edema. Generally, 10-20 ml/kg/hour for rapid replacement and 2-4-ml/kg/hour maintenance injection rate can be tolerated by animals.
A horse of 400 kg admitted to your clinic and suffers from strangulating obstruction of the large colon and 9% dehydration. Mention the type, amount, and rate of injection of fluid therapy.
Type: Ringer’s lactate
Rate of injection
= 36 L
(4-8 L/ hr)
50 X 400
= 20 L
Replacement dose should be injected to the animal at the 1st day then the maintenance dose is injected, however, the next day the animal is injected with maintenance dose only and this is repeated until the animal become able of taking reasonable amount of food and fluids orally, at this point the animal should be assessed to determine if fluid therapy should be continued or not.