Fluid Compartments, Types, and Routes & Dehydration/Maintenance

Total body water

60% of body weight

Total body water pediatric patients

70% of body weight

Total body water pediatric patients

Use the lean body mass (70% of body weight) then 60% of that

Blood volume dogs

8-9% BW

Blood volume cats

5-6% BW

Forces between ICF and ECF

-permeable to H2O based on concentration

-impermeable to electrolytes and proteins

-OSMOTIC FORCES

Forces within ECF and between INT/IV

-freely permeable to electrolytes based on concentration and H2O follows

-impermeable to proteins and large molecules

-STARLINGS FORCES

Osmolality is proportion to

The number of non-dissociable ions

What determines movement of fluid between ECF and ICF?

Osmolality

What is the main component of osmolality?

Na

Cat and dogs normal osmolality

300mOsm/L

Osmolality equation & distribution of ions

Effective osmoles

-generate osmotic pressure that results in movement of water

-determines tonicity of compartment

-Na, K

Ineffective osmoles

-have osmotic potential but can diffuse!

-no osmotic pressure, no movement of water

-contributes to osmolality but not tonicity

-urea, glucose, D5W acts like H2O

Starling forces equation and diagram

Hydrostatic pressure

-Pc-Pi

-fluid leakage OUT of vessul

-capillary dependent on venous pressure

-venous stasis = increase pressure

Oncotic presure

-fluid to go IN to vessels

-depends on proteins, primarily albumin (# = concentration/molecular wt)

Dog and cat capillary oncotic pressure normals

Dogs = 21-25mmHg

Cats = 23-28mmHg

Filtration coefficient (Kfc)

-will increase as capillary SA increases

-depends on leakiness of vessels

-liver/kidney = leaky

-brain/mm = tight

Reflection coefficient

Permeability of the capillary bed to the protein. Liver low, kidney high

Lungs

Low Kfc & high reflection coefficient = NOT very leaky

Endothelial glycocalyx

Mesh like gel covering the endothelial cells that acts as a barrier to protect the endothelium from things inside of the vessel

Hypotonic osmolality

0.45% NaCl = 154mOsm/L

Isotonic osmolality

0.9% NaCl = 310mOsm/L

Hypertonic osmolality

7.5% NaCl = 1300mOsm/L

Fluid electrolyte divisions

NaCl (LRS) or electrolytes (plasmalyte)

Fluid acid base divisions

-none = acidifying

-lactate (LRS) or gluconate/acetate = alkalinizing

Hypotonic fluid behavior

-distributed in ICF and ECF

-depends on Na in solution

-isotonic portion goes to 100% ECF

-water portion = 67% ICF, 33% ECF

Hypotonic fluids indications

-maintenance

-free water deficit (hypernatremia)

-maintenance when Na restriction is needed (cardiac/renal dz)

Hypotonic fluids contraindications

-shock

-hyponatremia

Hypotonic fluids side effects

-hyponatremia

-rapid admin = acute changes in blood osmolarity/fluid shufts

-neuro sequels and RBC damage

Isotonic fluids behavior

Stays in the ECF (3/4 interstitium, 1/4 intravascular), takes 20-30 min to get to the interstitium

Isotonic fluids indications

-shock

-rehydration

-replacement of loss

Isotonic fluids contraindications

-low oncotic pressure

-severe cardiac/renal dz

-severe, acute bleeding (no large volumes)

-free H2O loss/gain (Na disturbances)

Isotonic fluids side effects

-tissue edema with large volume

-worsening/creation of acid-base disorders

-pro-inflammatory effects

Hypertonic fluids behavior

-fluid from ICF —> IV

-rapidly redistributes across all compartments

-improved cardiac contractility

Hypertonic fluids indications

-shock

-TBI

-hyponatremia

Hypertonic fluids contraindications

-chronic hyponatremia

-severe dehydration

Hypertonic fluids side effects

-cannot be redosed

-transient hypernatremia

-if you give it too fast, can cause vasodilation/bradycardia

Colloids definition

Contains larger molecules that do not readily cross capillary membranes

Colloids behavior

-stays in vascular space

-contributes to oncotic pressure

Synthetic colloids

HES that has prolonged vascular expansion than crystalloids. Used in shock cases and for oncotic support for hypoproteinemia

Side effects of synthetic colloids

-dose dependent coagulopathy >20ml/kg/day! But is less concerning with vetstarch

-renal injury (unclear mechanism, evidence in humans but is poor in animals)

Natural colloids — plasma

-usually use FFP as CRI because canine serum albumin is challenging to buy

-use in acute blood loss, coagulopathy, hypoalbuminemia

Natural colloids — HSA

-stays in the vasculature and pulls fluid from interstitium and ICF

-side effect = type I/III hypersensitivity

-ONE DOSE ONLY!!!

-use in severe sepsis and shock

Disadvantages of peripheral IV

Only for relative isotonic samples and usually cannot be used for sampling

Central line

-safer admin of hypo and hypertonic fluids

-serial blood sampling

-more expensive

-do not give for TBI, active bleeding, or high risk of bleeding

Dog/cat IO placement

Proximal humerus, femur, tibia

Bird IO placement

Distal ulnar, proximal tibia

SQ fluids

-isotonic cause hypo/hyper can cause tissue injury

-NOT for severe dehydration or shock

Chart for route

What is dehydration?

Isotonic fluid (salt and water) loss

% dehydration =

% body wt (kg) loss

Dehydration chart

Mucus membrane moistness is influenced by

-evaporation (panting)

-oral ulceration, uremia, etc

-tear production influences (KCS, fan in front of dog, etc)

Skin elasticity influenced by

-youth more elastic than old

-body fat

-breed differences

Eye position

Late finding and is influenced by ocular disease and BCS/nutritional dz

Changes in body weight

Corrected 5% dehydration SHOULD increase patient body weight by 5%

Thirst mechanism

-baroreceptors sense decrease in volume and signals ADH and RAAS

-increase in osmolality of EC space is sensed by osmosreceptors and makes animal seek out water

Hypovolemia

ONLY seen with 10% plus dehydration

Fluid type for dehydration

Isotonic crystalloid

Usual time frame for dehydration

8-12 hours

What impacts how fast?

-severe dehydration = faster

-pt tolerating fluid deficit w/o changes = can be slower

-heart/kidney/lung dz = slower

-cats are less tolerant of larger volumes

More aggressive therapy means

More aggressive monitoring

Maintenance for fluid type

Usually isotonic with K+ or hypotonic is true maintenance and can be good for the renal and heart patients