Fluid and Electrolyte Therapy :

 

Purposes of fluid administration during the peri-anesthetic period:

• Replace insensible fluid losses (evaporation, diffusion) during the anesthetic period 

• Replace sensible fluid losses (blood loss, sweating) during the anesthetic period 

• Maintain an adequate and effective blood volume 

• Maintain cardiac output and tissue perfusion 

• Maintain patency of an intravenous route of drug administration 

 

Review normal body water distribution:

• 1 gm = 1 mL; 1 kg = 1 liter; 1 kg = 2.2 lbs 

• Total body water: 60% of body weight 

• Intracellular water: 40% of body weight 

• Extracellular water (plasma water + interstitial water): 20% of body weight 

• Interstitial water: 20 % of body weight 

• Plasma water: 5 % of body weight 

• Blood volume: 9 % of body weight (blood volume = plasma water + red blood cell volume) 

• Intercompartmental distribution of water maintained by hydrostatic, oncotic, and osmotic forces 

• Daily water requirement = 1-3 mL/kg/hr (24-72 mL/kg/day) 

  • 50 ml x body weight (kg) provides rough estimate for daily requirement

  • Requirements vary with age, environment, disease, etc… 

 

Fluid movement across capillary membranes:

• Filtration is governed by Starling’s equation

• Net driving pressure into the capillary = [(Pc – Pi) –  (πp – πi)]

• Pc = capillary hydrostatic pressure (varies from artery to vein)

• Pi = interstitial hydrostatic pressure (0)

• πp = plasma oncotic pressure (28 mmHg)

• πi = interstitial oncotic pressure (3 mmHg)

• If COP in the capillaries decreases lower than the COP in the interstitium, fluid will move out of the vessels and edema will develop.

• Plasma colloid osmotic pressure

• Plasma proteins are primary determinant for the plasma colloid osmotic (oncotic) pressure

• One gram of albumin exerts twice the colloid osmotic pressure of a gram of globulin

• Because there is about twice as much albumin as globulin in the plasma, about 70 % of the total colloid osmotic pressure results from albumin.

 

Diagram to show the Starling forces in capillaries 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

• What happens if an animal loses 500 mL of blood?

  • The capillary hydrostatic pressure (Pc) drops especially at the venous end.

  • The net pressure into capillary increases and the balance is no longer maintained so fluid is retrieved into the circulation from the interstitium until Pc is restored.

 

Sodium:

• The major component of ECF

• Osmotic concentration is regulated by maintaining sodium balance and provides osmotic forces to maintain water balance in interstitial fluid compartment 

• Generally, water and sodium disturbances occur simultaneously 

• Sodium levels indicate overall fluid balance 

• Sodium levels are regulated by the kidney, through aldosterone & other related factors 

 

Potassium:

• The major component of ICF

• 98% of total body potassium is located intracellularly 

• Provides osmotic forces to maintain water balance in intracellular fluid compartment 

• Plasma potassium levels may not reflect total body potassium levels! Because it is indirect measure of intracellular K+

• Potassium imbalances result in altered function of excitable membranes (eg heart, CNS) 

• Normal renal function is required to prevent hyperkalemia 

• Hypokalemia should be treated slowly

  • Do not exceed 0.5- 1 mEq K+/kg/hr, also maximum concentration 40 mEq/L.

 

Calcium:

• Vital ion in normal neuromuscular activity, cardiac rhythm and contractility, cell membrane function, and coagulation 

• Highly protein bound; total plasma calcium levels vary with plasma albumin levels, however ionized calcium levels may remain constant 

 

Chloride:

• The major component of ECF

• Renal regulation of electroneutrality usually results in an inverse relationship between Cl- and HCO3- 

• Tends to follow Na+, so chloride deranges, in general, do not need to be directly corrected

 

Bicarbonate:

• Part of the major buffer system in the body 

 

Other Anions:

• Plasma proteins, organic acids, sulphates 

• Not routinely measured 

• Constitute the "anion gap" 

 

Changes in fluid and electrolyte balance in response to disease processes:

• May vary widely 

• Hypovolemia is common! 

• Electrolyte changes are variable 

• Goal is to correct fluid and electrolyte imbalances before anesthesia, if possible 

 

Changes in fluid and electrolyte balance in response to anesthesia: 

• Many anesthetic agents produce vasodilation and hypotension relative hypovolemia! 

• Results in alterations in sympathetic nervous system activity and the endocrine system 

• Redistribution of blood flow with changes in vascular resistance 

• Reduction in urinary flow rate, renal blood flow, and glomerular filtration rate seen with withholding water (fasting), anesthetic drug effects, and increased ADH levels 

• These effects can be eliminated or reduced by "filling the tank" with crystalloids 

 

Maintenance vs. replacement therapy:

• Maintenance fluid therapy (plasmalyte 56, 0.45 NaCL with dextrose etc) is designed to meet the patient's ongoing sensible and insensible fluid losses with normal fluid volume over 1 – 2 days; in the normal animal this is primarily water loss, with a lesser degree of electrolyte loss. 

• Replacement fluid therapy (LRS, Plasmalyte A, Normosol, 0.9 NaCL, etc)  is designed to replace existing fluid deficits; this usually requires replacement of both water and electrolytes 

• The optimal fluid type for each of the above settings depend upon serum electrolytes, acid-base status, and concurrent administration of drugs and blood products. 

 

Precautions for rapid fluid administration:

• In dogs, maximum fluid administration rate is 90 ml/kg/hr (1 blood volume/hour) 

• In cats, maximum fluid administration rate is 60 ml/kg/hr (1 blood volume/hour) 

• Monitor cardiopulmonary status carefully. 

• Monitor packed cell volume and total plasma protein levels: maintain PCV > 20% and TPP > 4 g/dl 

 

Normal fluid administration rates during anesthesia:

• Dogs, cats: 10 - 20 ml/kg/hr 

• Horses, cattle: 5 - 10 ml/kg/hr 

 

Physiologic parameters useful for fluid therapy planning:

 

Types of fluids available and general indications for their use:

• Crystalloid = a solution of crystalline solid dissolved in water
• Colloids = a suspension of particles in a liquid ie, does not cross a semipermeable membrane, so exerts a colloid oncotic pressure

 

Crystalloids: Replacement fluids 

• Generally are polyionic isotonic fluids 

• Ringer's, Lactated Ringer's (LRS), PlasmaLyte 148, PlasmaLyte A are all polyionic isotonic crystalloid fluids that closely mimic plasma electrolyte concentrations (with or without bicarbonate precursors) 

• 0.9% NaCl (normal saline) is an isotonic solution of Na, Cl, and water 

• 5% dextrose is an isotonic solution of dextrose in water; the dextrose is rapidly metabolized, thus this essentially results in the administration of free water 

• Commonly administered during general anesthesia to diminish the cardiovascular effects of anesthetic drugs and replace ongoing fluid losses 

• Usually administered at 10-20 ml/kg/hr in small animal 

• Usually administered at 5-10 ml/kg/hr in large animals

• May need to infuse 40 – 90 ml/kg/hr during shock using multiple catheters or fluid pumps 

• Replace acute blood loss by administering 3 volumes of crystalloid solution for each 1 volume of blood lost 

 

Crystalloids: Maintenance fluids 

• Are hypotonic crystalloids that are low in sodium, chloride, and osmolality, but high in potassium compared to normal plasma compositions. 

  • Eg, 0.45 % sodium chloride, 2.5 % dextrose with 0.45 % saline, 2.5 % dextrose with half strength LRS, Normosol M, Normosol M in 5 % dextrose, PlasmaLyte 56 in 5% dextrose, and Plasmalyte 56.

• Generally polyionic isotonic or hypotonic fluids 

• Used for long term fluid therapy, such as the ICU setting; not generally used during anesthesia 

• Generally are low in Na and Cl, and high in K 

• May or may not contain dextrose 

 

Crystalloids: Hypertonic fluids 

• Hypertonic saline (7.5% NaCl) has been indicated in some shock states to maintain cardiovascular function; pulls fluid into intravascular space by osmosis by creating transient hypernatremia. Dose is 4 ml/kg.  Must follow with isotonic, polyionic fluids

• Generally used to treat particular deficits (eg 10% dextrose given to a hypoglycemic neonatal foal) or to treat edema (eg mannitol) 

• Usually must be given cautiously 

 

• ions are presented as mEq/l

• M = Maintenance; R = Replacement 

 

Colloids:

• Synthetic colloids are polydisperse (various molecular weight) and do not readily cross semipermeable membrane.

• Hypertonicity pulls fluids into the vascular space and increase blood volume which effect is longer lasting compared to crystalloid therapy.

• Solutions of starch or dextrans (of various molecular weights) 

• Smaller volumes of colloids are as effective as larger volumes of crystalloids in maintaining intravascular fluid volume 

• Historically have had a number of problems associated with their use, including allergic reactions, impaired coagulation, and renal damage; solutions available now have less problems associated with their use 

• Expensive compared to crystalloids 

 

Composition of Several Colloidal Fluids:

 

Fluid type and volume ratio for plasma volume restoration:

 

Whole blood:

• Contains it all: colloids (plasma proteins), clotting factors including platelets, red blood cells for oxygen carrying capacity 

• Relatively easy to collect and store 

• Indications: acute blood loss, concurrent anemia and hypoproteinemia, clotting defects 

• Stored blood is not quite as useful as fresh blood: reduced oxygen carrying capacity (review 2,3-DPG), platelets are inactive, clotting factors may be degraded 

• A blood filter must be always used to sieve microthrombi from the blood product.

• 5 – 15 ml/kg/hr rate is used to treat acute hypovolemia, and 40-60 ml/kg/hr can be used in life-threatening emergency.

• In massive transfusion, defined as blood volume replacement greater than 1.5 times the recipient volume, abnormal bleeding may occur.

• This homeostatic defects is characterized by oozing from the operative wound, mucous membranes, and intravenous puncture sites. 

• Blood Types and crossmatching 

  • Crossmatching between donor and recipient will minimize a fatal outcome.

  • There are about 12 types in dogs but DEA 1.1, 1.2, 1.7 are most antigenic.

  • Cats have AB blood group system; the most common being type A.

  • Always administer slowly in the beginning so as to allowing adequate time to detect any adverse reactions, such as rashes, edema, vomiting, fever, DIC, dyspnea, hypotension, unconsciousness and tachycardia

 

Packed red blood cells:

• Red cell fraction of separating plasma from whole blood 

• Usually has a PCV of 70% 

• Useful in treating anemia 

• Reduces risk of fluid overload 

• Reconstitute with equal volumes of 0.9% saline 

 

Plasma:

• Two types: fresh or frozen 

• Fresh plasma contains colloids, active platelets, and clotting factors 

  • useful in treating coagulation defects 

• Frozen plasma can be stored for periods up to a year; serve as a source of colloids (plasma proteins); often collected from stored whole blood when the red cell fraction is no longer viable 

  • useful in treating hypoproteinemia and maintaining normal colloidal osmotic pressure 

 

Principles of blood and plasma transfusions: 

• Consider transfusion if PCV < 20% and/or TPP < 4 gm/dl 

• Transfuse appropriate blood components 

• Administration rate: < 10 ml/kg/hr (unless in crisis) 

 

Complications of blood and plasma transfusions: 

• Immune response to red cell antigens 

• Immune response to white cell antigens 

• In vitro (storage) changes 

• Coagulation defects 

• Citrate intoxication 

• Hyperkalemia 

• Hypothermia 

• Sepsis 

 

New product - Oxyglobin®

• Purified bovine hemoglobin in lactated ringer's soln 

• Doesn't contain red blood cells - instead contains crosslinked hemoglobin molecules 

• Plasma half life is 30-40 hours 

• Approved for use in dogs, but may also be used in other species 

• Provides oxygen carrying capacity and oncotic pressure (consists of large protein molecules) - improves oxygenation & provides volume expansion 

• Hemoglobin molecules disperse throughout the plasma 

• Expensive, although has several advantages over whole blood or packed red cells 

  • Don't need to maintain donors 

  • Long shelf life - 2 years 

  • Doesn't require refrigeration 

  • Doesn't require blood typing or cross matching 

  • No risk of bacterial or viral contamination 

  • Few reported adverse effects 

  • Immediate availability a major advantage over blood products in crisis situations 

• However, doesn't provide other components of whole blood that may be desired in some conditions - eg clotting factors 

• Analagous product, Hemopure®, is undergoing FDA trials for use in human medicine