VetWrap Spring 2013 - DoveLewis | Emergency Animal Hospital

Fluid Therapy
Hospital Shuttle Service
for Referring Veterinarians
$35 ONE WAY $55 ROUND TRIP
We are happy to offer transport service to and
from our hospital to your practice for your clients’
convenience. Our service includes:
• Delivery of pet to your clinic for discharge
or continued care
• Pick up of stable patients from your clinic for
transport to DoveLewis for overnight care
• Pick up and drop off for all imaging services*
Shuttle Service is available:
7:30AM to 10:00PM, Monday through Friday
For more information, contact:
Brian Stief, Clinic Operations Manager,
971.255.5908, bstief@dovelewis.org.
MeiMei Welker, DVM, Outreach DVM,
971.255.5904, mwelker@dovelewis.org
Please call for hours and availability for shuttle service for imaging services.
A round trip fee is assessed for all imaging services.
Serviceable zip codes:
97006, 97005, 97008, 97034, 97035, 97201, 97202, 97203, 97204,
97205, 97206, 97209, 97210, 97212, 97213, 97214, 97215, 97216,
97217, 97218, 97219, 97220, 97221, 97223, 97225, 97227, 97229,
97230, 97232, 97233, 97236, 97238, 97239, 97266, 97267, 97294.
*Partial service: 97124, 97123 and 97007.
Dove O|N Monitoring—$220
Dove O|N includes exam, ER or ICU monitoring (as
determined by a DoveLewis veterinarian) with fluids,
pain management—antibiotics, or oral medications
as prescribed by the referring veterinarian (if indicated)
and patient status lab work (if necessary).
Packages are intended for stable patients.
Article
Ladan Mohammad-Zadeh, DVM DACVECC
Fluid therapy is a topic that we all learned from day one in
veterinary school. Although it is a fundamental part of many
of our therapies, it is far from a basic concept. This review of
fluid therapy will start with understanding fluid compartments,
osmotic and oncotic pressures, review the fluid types available
and the ‘when’ and ‘where’ of fluid therapy. Lastly, we will
review certain common scenarios in general practice and the
role of fluid therapy in those situations.
Fluid Compartments
Total body water (TBW) is comprised of 60% intracellular
space and 40% extracellular space. The extracellular space is
further divided into intravascular and interstitial (25% and 75%
respectively). There is a minute amount of space accounted for
in synovial, pericardial, pleural and peritoneal fluids. The major
cations in the intracellular space are K+ and Mg++. The major
anions are PO4- and protein. In contrast, the major cations in
the extracellular space are Ca++ and Na+ and the major anions
Cl- and HCO3-. The balance of fluid between the intravascular
space and the interstitium is governed by hydrostatic and
oncotic forces. The balance of fluid between intracellular and
extracellular compartments is governed by the osmotic effect of
Na+. This is true because membranes are virtually impermeable
to ions but allow water to move freely across membranes.
Important Terminology
Osmolality: A count of the total number of osmotically active
particles in a mass of solution (mOsml/kg).
Effective Osmole: Molecule that has the ability to exert an
osmotic force (i.e. can cause water pull). Typically these particles
are not freely moveable across membranes. These include Na+,
K+, and Glucose.
Ineffective Osmole: Molecules that do not exert an osmotic force.
Usually these particles freely move across membranes. They
include urea and HCO3-.
Tonicity = Effective Osmolality: This is the sum of the
concentrations of solutes which have the capacity to exert an
osmotic force across the membrane. Na+ is the main osmole that
affects water balance between the intracellular and extracellular
fluid. In practical terms, tonicity reflects Na+ concentration of a
fluid. The following is a commonly used equation for osmolality.
Eighteen and 2.8 are the molecular weight of the molecule
divided by 10 to get the correct unit conversion. The normal
serum osmolality is 290 – 310 mOsm/L. Cats may run on the
higher end of normal. The following equation is the calculation
for osmolality:
2(Na + K) +
BUN + Glucose
2.8 18
Calculating a patient’s osmolality can help estimate the degree
of intracellular dehydration. The higher the osmolality, the more
water will need to shift out of the intracellular space into the
extracellular space to compensate, resulting in intracellular
dehydration. Severe hyperosmolality can lead to neurologic
signs and weakness.
It is easy to confuse the terms osmolality and tonicity. It is helpful
to think in terms of tonicity representing the sodium content
and osmolality representing all of the effective osmoles in a fluid.
For example, 0.9% NaCl is both isotonic (154 mEq/L NaCl) and
iso-osmolar (308 mOsm/L). Five percent dextrose in water, D5W,
is hypotonic (0 mEq NaCl) but iso-osmolar (252 mOsm/L) because
of the dextrose concentration. For IV solutions that are meant to be
used over many hours to days, solutions must be iso-osmolar in order
to decrease the risk of phlebitis. For short term infusions however,
it is acceptable to infuse hyperosmolar solutions such as 7.5% NaCl
which is both hypertonic (1232 mEq Na/L) and hyperosmolar (2464
mOsm/L). Mannitol is unique because technically it is hypotonic, but
is hyperosmolar (1098 mOsm/L) and does cause fluid shifts out of the
intracellular and interstitial spaces. This is why it is primarily used to
treat cerebral edema.
Oncotic Pressure
Colloid osmotic pressure is the pressure exerted against the flow
of water out of the intravascular space. Hydrostatic force can be
seen as its opposite, that is, the pressure of water flow out of the
intravascular space into the interstitium. Colloid osmotic pressure
(COP) is synonymous with oncotic pressure. Albumin imparts
80% of the COP in the intravascular space. Thus, it is easy to
understand how hypoalbuminemia and the associated drop in COP
results in fluid shifts out of the intravascular space. Starling’s forces
describe the effect of hydrostatic pressure and oncotic pressure on
both the interstitial side and capillary side. It dictates the direction
of water in or out of the plasma space. In arterial capillaries, the
hydrostatic pressure is slightly greater than oncotic pressure,
favoring filtration, or movement of the water into the interstitium.
On the venous side, oncotic pressure is slightly higher favoring
fluid movement into the plasma space.
Fluid Therapy Plan
When you are deciding on a fluid therapy plan, it might be useful to
pose the following questions: Is fluid therapy indicated? What type
of fluid should be given? By what route should the fluid be given?
How rapidly should fluid be given? How much fluid should be given?
Assessing hydration is something we all do as part of our complete
physical exam. Determining the level of dehydration in our patients
is a combination of physical exam parameters (usually mucous
membrane and skin tent), history of losses or lack of water intake,
and lab parameters such as PCV/TS and urine specific gravity. Skin
tent is not a reliable indicator of interstitial hydration in pediatric
patients and geriatric patients. Levels of dehydration include: 5-6%
mild, 7-10% moderate, and 10-12% severe. A patient is clinically
moribund at a level of 15% dehydration.
To answer the question of what type of fluid should be given,
there are only a few major categories of fluid types to choose
from. Crystalloids are solutions containing electrolyte and nonelectrolyte
solutes capable of entering all body fluid compartments.
Within one hour of administration, two thirds redistributes into the
interstitium and one third stays in the intravascular space. Isotonic
fluids are meant for interstitial rehydration or maintenance. Hypotonic
fluids target intracellular rehydration. Hypertonic fluids are
meant for intravascular volume expansion or to create interstitial
volume dehydration. Most crystalloids, other than 0.9% NaCl and
0.45% NaCl/2.5% dextrose, are considered balanced solutions.
A balanced solution is buffered, isotonic and contains electrolytes.
Saline is an acidic fluid and when given in large volumes may
exacerbate underlying acidosis or delay normalization of pH.
Balanced crystalloids are buffered, making them more alkaline.
Other terms that are used to describe crystalloids are replacement
and maintenance. Replacement fluids contain an electrolyte profile
similar to extracellular space (higher Na+ and less K+), while
maintenance fluids have an electrolyte profile similar to intracellular
space (lower Na+, higher K+). The advantages of a maintenance
fluid type is that it decreases the Na+ load long term and
6 VetWrap Volume 7 Issue 2 Spring 2013 Spring 2013 Volume 7 Issue 2 VetWrap 7