Chp. 4 Fluid Therapy Flashcards
When is fluid therapy indicated?
-Multitude of circumstances in which animal is unable to compensate for changes in fluid, EL balance
-Nature of therapy specific to each circumstance, based on individual patient’s needs
What is the preferred ROA for fluids? Why?
IV -> more precise, controlled delivery to the patient and closer to the heart/circulation
What are common sites for IV catheterization in dogs and cats?
-cephalic
-medial (cat), lateral (dog) saphenous
-jugular veins
Are there other routes of administration that can be used for fluid administration?
-IO, IP, PO, subcutaneous
-IO = more invasive, helpful with small patients when IV catheterization is challenging, fluids can be delivered at similar rate
-Other routes have limitations: slower/unreliable absorption, potential to cause peritonitis, infection
What terminology used to classify types of fluids?
-crystalloids
-colloids
Definition of crystalloids
Those containing ions/solutes that redistribute to all fluid compartments within the body
Colloid solutions
Contain large molecular weight substances that remain predominantly within the vascular compartment
What defines a balanced or unbalanced crystalloid solution?
-Balanced if components are similar to those in ECF
-Unbalanced if not the case
Crystalloids classified based on tonicity. What is the basis of this classification?
-Hypertonic, hypotonic, isotonic based on osmolarity relative to that of blood and ECF
Some crystalloid solutions are considered acidifying. What is an example of such a solution and why is that term used?
-NaCl = acidifying crystalloid solution -> unbalanced solution that contains no bicarbonate precursor
-When large volumes administered rapidly, may result in dilution acidemia (by virtue of diluting plasma bicarbonate)
Despite the acidifying effect of NaCl, there are circumstances in which normal saline is the replacement fluid of choice. What are some examples?
-Metabolic alkalosis (pyloric outflow tract obstruction)
-Patient that is hyperkalemic as a result of its primary disease (Addison’s disease, ruptured bladder)
In hyperkalemic patients, although normal saline is useful bc it does not contain K, an acidifying solution may be undesirable. Is there a way to modify saline to decrease this acidifying effect?
-Can add Nabicarb to solution
-Since plasma normally contains 18-24mEq/L of bicarbonate, amount can be added to 1L saline to minimize acidifying effect of saline alone
What does replacement therapy mean?
-At presentation, hydration status will determine initial replacement fluid therapy
-Addresses deficit fluid volume
-Ex: if 10kg patient is 10% dehydrated, then 1L balanced electrolyte solution will constitute the replacement component of fluid to be administered
What does maintenance therapy mean?
-Directed at maintaining normal sensible losses (eg urinary production) and insensible losses (respiratory, cutaneous)
-Sometimes termed normal ongoing losses
-Based on patient’s body weight, maintenance energy requirements
What does the term ongoing losses refer to?
Vomiting, diuresis
-Losses can be quantified -> weighing animal as needed during the day, and then replaced by a volume per weight basis
What are maintenance energy requirements in a cat?
50-60 kcal/kg/day
What are maintenance energy requirements in a dog?
70-80 kcal/kg/day
What is normal urine output in small animal patients?
1-2mL/kg/hr
What is the commonly used daily fluid requirement for small animal patients?
25-50mL/kg/day
Ideally, what types of fluids should be used to replace losses?
Isotonic solutions containing composition similar to that of ECF generally used for replacement of losses
–maintenance fluids contain lower sodium and higher potassium levels that better match composition of sensible and insensible losses
–Solutions to replace abnormal ongoing losses should be selected based on presumed or measured composition of the loss
What are some examples of commercially available solutions for replacement therapy?
LRS, Normosol-R, Plasma-Lyte 148/Plasma-lyte R, normal saline (0.9%)
What are some examples of commercially available solutions available for maintenance therapy?
Normosol-M, Plasma-Lyte 56/Plasma-Lyte M, half strength saline (0.45%)
What are the main components in replacement solutions?
-Isotonic, contain electrolytes in a similar composition to that of plasma
-Na 130-154mEq/L
-Cl 80-154 mEq/L
-K 4-5mEq/L
Some solutions contain other ions including Ca, Mg, and alkalinizing substance or bicarbonate precursor in the form of acetate, lactate, or gluconate
What are main components in maintenance solutions?
-Generally hypotonic
-Na 40-55
-K 13-20
-Cl 40-55
May also contain bicarbonate precursors, other ions, dextrose
What are the main components in ongoing losses solutions?
No commercially available solutions specifically meet this requirement
Often replacement solution supplemented with K is used to meet this need
What is the recommended maximal rate of IV administration of potassium?
-0.5mEq/kg/hr
What are signs that potassium administration might be too rapid?
-tall-tented T waves
-widened QRS complexes
-prolonged P-R interval
-Absence of P waves
-Bradycardia
Cardiac arrest will result if hyperkalemia is not treated
Although D5W is commercially available and could be considered an isotonic solution (osmolality 250mOsm/L), rapid administration/large doses may result in patient compromise. Explain?
While isotonic in the fluid preparation, when given to the patient, the dextrose is utilized by cells, leaving behind only free water that is hypotonic -> may lead to cellular edema, lysis
What is the recommended max rate for IV administration of potassium?
0.5mEq/kg/hr
How would you administer dextrose to a patient that needed it?
-If fluids not administered too rapidly, dextrose may be added to commercial replacement solution to achieve desired dextrose concentration
-Maintenance solutions containing dextrose also available for patients in whom slower administration is appropriate
What is the recommended ‘maintenance’ fluid administration rate for anesthetized patient?
5-10mL/kg/hr
Why does maintenance rate under GA exceed rate suggested previously as adequate to replace normal sensible and insensible losses?
-Anesthetic maintenance differs from traditional maintenance -> rate higher under GA
-Compensation for normal sensible, insensible losses
-Also have to maintain normal vascular volume to ensure adequate tissue perfusion, esp important considering normal homeostatic mechanisms may be altered in these patients
-Fluids may be administered to counteract hemodynamic changes that occur with anesthetic drugs (eg VD) +/- to compensate for volume losses (ie hemorrhage) related to primary disease or procedure
Are there patients in home a lower anesthetic maintenance administration rate should be considered?
-CHF
-Regurgitant murmurs
-Pulmonary edema
-Patients with clinically significant reductions in HCT or TP values
What is the recommended fluid administration rate to replace blood loss in a patient with normal PCV and TP? What is the basis for this guideline?
-3x vol lost if crystalloid solutions used
-Approx 2/3 of crystalloid solution administered will redistribute to the interstitial and intracellular fluid spaces within 20-30min
-Only about 1/3 amt of crystalloid given will remain in the vascular space
What is the role of anions such as lactate, acetate, gluconate in the crystalloid solutions?
-Anions, when metabolism zed (largely by oxidation) utilize a hydrogen ion and generate a bicarbonate equivalent to that contained in the ECF compartment
-Help maintain normal blood pH, may be referred to as alkalizing solutions
Are there circumstances in which a specific anion may be preferred?
-Lactate, acetate predominate as anions present in commercially available fluids - both result in an equivalent alkalinizing effect, circumstances in which one may be preferred
What are the indications for colloid use?
-May be useful in patients that require rapid IV expansion (eg hypovolemia, hemorrhage) or patient needs oncotic support (eg hypoproteinemia, hypoalbuminemia)
How do colloids work?
-Colloidal solutions contain large molecular weight particles that remain within the vascular space, help maintain oncotic pressure by holding or withdrawing water into the vascular space
-help maintain vascular volume for hours to days
What determines the duration of effect of different colloid solutions?
-Molecular weight of the compound: larger the particles, longer the compound will remain within the vascular space to generate its effect
-Number of particles also significant –> will determine osmotic capacity of the fluid
Are these values for molecular weight and number readily available?
-Average molecular weight = M subscript w
-Number molecular weight (total weight of all molecules, divided by number of molecules) = M subscript n
What is the disadvantage of acetate-containing solutions?
Rapid administration may result in significant hypotension mediated via vasodilation
In what patients has there been historical concern for lactate administration and why?
-Severe hepatic dz -> enzyme systems responsible for clearance of lactate may become saturated
-Cancer -> lactate metabolism may be impaired or be associated with a high-energy expenditure
-Acetate, which is metabolized at sites throughout the body may be preferred in these patients
What are some of the commercially available colloid solutions?
-Dextran 70
-Dextran 40
-Hetastarch
-Pentastarch
-Gelatin
How and at what rate should colloids be administered? Is there a maximal limit?
-Given IV
-Hetastarch, Dextran 70 can be administered at rates of 1-2mL/kg/hr if used for ongoing oncotic support
-Alternatively, rapid administration up to 20mL/kg used in a patient in acute hypotensive, hypovolemic crisis
Are there other solutions that may be considered colloids?
Blood, its components
What are some concerns when administering colloids?
-Primary concern = influence of colloidal solutions on coagulation
-Both direct effects (interference with platelet function), indirect effects (by virtue of dilution of coagulation factors, etc)
-Hemodilution, reduced HCT
What are some concerns when administering colloids?
-Primary concern = influence of colloidal solutions on coagulation
-Both direct effects (interference with platelet function), indirect effects (by virtue of dilution of coagulation factors, etc)
-Hemodilution, reduced HCT may be clinically significant with sustained or large volume colloid use
-Recent literature –> AKI
-Anaphylactic reactions reported in humans, not documented in dogs and cats
What are the guidelines for rate of administration of replacement solutions?
- Can be rapidly administered as determined by needs of patient
- Frequently half calculated deficit admin in 10-20min –> P’s hydration status and demeanor reassessed and rate adjusted
What are the guidelines for rate of administration of maintenance solutions?
- Admin more slowly in keeping with normal sensible and insensible losses
- Ongoing losses generally replaced over a few hours
- Rate of administration limited by maximal rate of administration of K and type of fluid needed
What is the treatment for hyperkalemia?
- IV admin of 10mg/kg CaCl
- 0.5-1.0mEq/kg NaBicarb
- Admin of K-free fluids
- K-wasting diuretics
- Insulin and glucose/dextrose
- GI exchange resins
What are some fluid incompatibilities to be aware of?
- Fluids containing calcium should be mixed with bicarbonate-containing solutions or citrated blood products –> may precipitate
- Inotropes (dobutamine) should not be administered with NaBicarb-containing solutions –> NiBarb may reduce efficacy of dobutamine
