Fluid Therapy Basics

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Fluid Therapy BasicsModule 1 / Dr. Elisa Mazzaferro

Welcome to the rst module of AbbottAnimal

HealthCE.coms uid therapy series, Fluid Therapy

Basics. My name is Dr. Elisa Mazzaferro, with Wheat

Ridge Animal Hospital in Wheat Ridge, Colorado.

School may seem like it was so long ago, even if

you graduated and became a veterinarian or technician

last year! Fluid therapy is not meant to be confusingor time consuming. AbbottAnimalHealthCE.coms uid

therapy series of 10 modules is meant to challenge and

augment your uid therapy knowledge.This introduc-

tory module covers the basics and will set the stage for

the future uid therapy modules, in which youll learn

practical information you can immediately use in your

practice, how to choose and administer the right type

of uid, place and care for catheters, use crystalloids

and colloids, administer uids during emergencies,

anesthesia and surgery, and monitor your patients.

Slide 1

In addition to setting the foundation for these future

uid therapy modules, this module has several objectives.

First, we will dene and then develop a full understanding

of perfusion and homeostasis. Next we will learn about

body water composition and how uid moves between

the different cell compartments. Then well review the

maintenance uid requirements for animals.Well then discuss the importance of determining both

a patients sensible and insensible uid losses. Lastly, well

wrap up by learning the importance of properly diagnosing

a patient with dehydration versus hypovolemic shock.

But before we start, youll take a short pretest to

stimulate your brain cells and help you recall some basic

concepts of uid therapy. Then, well dive in to solidify the

concepts, so that uid therapy is simple, easy, and fun!

Slide 2

Question 1: An animal presents to you in hypo-

volemic shock. All of the following are examples of

parameters to assess perfusion except for which one

listed here?

Is your answer skin tenting? Then you are abso-

lutely right! Hypovolemic shock involves decreased

uid volume within the intravascular space. As such,

decreased intravascular volume results in decreased

tissue perfusion. Perfusion parameters that can be

assessed include capillary rell time, blood pressure,

heart rate, and temperature of extremities. Conversely,

dehydration refers to decreased uid in the interstitial

and intracellular spaces. Increased skin tenting is

associated with dehydration.

Slide 3

Question 2: An intravenous catheter was placed in the

lateral saphenous vein 48 hours ago. This patient requires

aggressive uid diuresis for acute renal failure and is do-

ing well at this time. Another veterinarian in your practice

suggests that the catheter should be removed and replaced

because it has been in for too long. How should you reply?

What did you say? If you thought B and C provided

the correct reply to your colleague, youre right. Several

recent studies have documented that the length of time that

an intravenous catheter is left in place is not signicantly

associated with catheter-related problems. The catheter can

remain in place as long as it is not contributing to a fever,

and as long as it is owing without signs of pain, swelling, or

discharge at the catheter site. Large bore, shorter catheters,

such as 18 gauge, one and one-quarter inch, are the best

for administration of intravenous uid boluses. However,

large volumes of uids can also be administered through a

longer, central venous catheters, when necessary.

Slide 4

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Question 3: A two-year-old intact female West High-

land white terrier presents to you with a four-day history of

progressive vomiting and diarrhea. Her mucous membranes

are dry, and skin tenting is markedly increased. While you

are waiting for the results of a CBC, serum chemistry, and

urinalysis, you elect to administer intravenous uids. What

uid would you choose?

The correct answer is any of the above except B. The

patient is showing signs of dehydration, with increased skin

tenting, a history of both vomiting and diarrhea, and dry mu-

cous membranes. Many intravenous crystalloid uids can be

used to replenish interstitial, intracellular, and intravascular

uid decits.

However, a 0.45% sodium chloride uid with supplemen-

tal dextrose is not a balanced electrolyte solution, and is not

appropriate for rehydration. A 0.9% sodium chloride uid is

an isotonic crystalloid that can be used, although is less bal-

anced than lactated Ringers or Normosol-R.

Slide 5

Question 4: What is an example of an acidifying iso-

tonic crystalloid solution?

The correct answer is 0.9% sodium chloride (NaCl).

This acidifying isotonic crystalloid solution contains no

buffers and can contribute to a strong ion difference when

administered. Lactated Ringers and Normosol-R both

contain buffers, and as such, are not acidifying. 5% dex-

trose in water and 0.45% sodium chloride are hypotonic

crystalloids that do not contain buffers.

Slide 6

Question 5: As you are walking out the door for lunch, your as-

sociate calls for assistance in the surgery room. He has just dropped

an ovarian pedicle and cant nd the bleeder. The abdomen is lling

with blood rapidly. As you glove up, what should you ask yourself?

You should be asking, What is the animals blood pressure? Inmany instances, the loss of blood volume can be managed simply by

relling the intravascular space with a crystalloid, unless hemorrhage

is severe. The body will regenerate red blood cells as needed.

In an emergency situation, blood pressure is the rst parameter

that you can measure to estimate organ perfusion. Titrating intra-

venous crystalloid or colloid uids to maintain normotension is the

goal to maintain vital organ perfusion in cases of hemorrhage. You

may need to change the anesthetic depth as well as add a positive

inotropic drugs, such as dopamine, or pressors, in addition to crystal-

loid and colloid uids or blood products, when necessary. In this case,

nding the ovarian pedicle and ligating the bleeder usually is a simple

task for the experienced clinician. Then, you can proceed to lunch.

Slide 7

So how did you score? No matter whether you got

all 5 right or maybe missed a few, I can guarantee that

watching this module and the future uid therapy modules

will help solidify your knowledge. So now lets jump into

Fluid Therapy Basics. Enjoy!

Slide 8

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The Hungarian Biochemist Albert Szent-Gyorgyi

once said, Water is lifes mater and matrix, mother and

medium. There is no life without water. In veterinary

medicine, a variety of diseases are accompanied by

the loss of body uids. Without adequate hydration,

normal body functions become impaired and perfusion

decreases, which ultimately can lead to death if thera-

peutic interventions are not implemented.

Slide 9

Perfusion refers to the process in which blood carries

oxygen and important nutrients to body tissues. Perfusion

depends on many body compensatory responses, but

also on the administration of appropriate uid volumes to

maintain vascular volume. A discussion of intravenous

uid administration begins with an understanding of total

body water and uid balance between the various com-

partments within the body, so thats where well start.

Slide 10

Water is a major contributor to an animals body

weight. In healthy animals, approximately 60% of body

weight is water. This value can change slightly depend-

ing on age, lean body mass, degree of leanness or

obesity, and gender. For example, neonatal puppies

and kittens have a relatively higher percentage of water

in their bodies than adults. Adipose tissue contains

more water than muscle and can contribute to a larger

percentage of water in obese animals.

Water is located in separate yet intertwined com-

partments within the body. Conceptually, the body

can be divided into the intracellular and extracellular

compartments.

Slide 11

Approximately two-thirds, or 66%, of total body water is

located within the intracellular uid compartment inside of

cells. Extracellular uid is the uid that is located outside of

cells. Approximately one-third, or 33%, of total body water

is in the extracellular uid compartment. The extracellular

uid compartment can be further subdivided into the intra-

vascular and interstitial compartments. The intravascular

space contains uid within blood vessels.

It is through these blood vessels that plasma water,

cellular components, proteins, and various electrolytes ow.

The interstitial extravascular compartment is the space

located outside of the blood vessels. Intravascular uid

contributes only 8% to 10% of total body water, whereas

interstitial uid contributes 24% to total body water. A very

small amount of uid is known as transcellular uid, and is

located within the gastrointestinal tract, joints, cartilage, and

cerebrospinal space.

Slide 12



It has been estimated that total body water is

approximately 534 to 660 ml/kg in a healthy dog.

Intravascular water volume has been estimated to be

approximately 90 ml/kg in dogs and 45 ml/kg in cats.

Slide 13

To fully understand the uid therapy needs of all

patients, we need to rst understand how uid move

throughout the body. Water in the bodys various com-

partments is in a constant state of ux and moves from

compartment to compartment. The capillary membrane

between various compartments is composed of a thin

membrane of endothelial cells that contain tight or gapjunctions through which uid and solutes can ow.

Solutes dissolved in uid can ow between compartments

by passive diffusion, where a solute moves from an area

of higher to lower concentration down its concentration

gradient. Other solutes can be moved from one compart-

ment to another by active transport mechanisms.

Slide 14

The rate of uid exchange largely depends on the

forces that favor uid retention within a compartment

versus the forces that favor uid movement or ltration

from a compartment. The colloid oncotic pressure of

a uid compartment is dictated by the concentration

of protein within that space. Albumin is a protein that

contributes approximately 80% to the colloid oncotic

pressure. The hydrostatic pressure is the pressuregenerated by the force of a uid within a compartment.

The colloid oncotic pressure inuences uid retention

within a compartment, while the hydrostatic pressure

inuences uid movement from that compartment.

Slide 15

Starling created an equation that predicts exchange of

uid between the bodys uid compartments. The equa-

tion is shown here. Kf equals the ltration coefcient that

varies from tissue to tissue within the body. Pc

and Piare the

hydrostatic pressure within the capillary (Pc) and interstitial

space (Pi). Sigma () is size of the pores in the capillary

membrane, and idescribes the effect of proteins such as

albumin that promote uid retention in the capillary (c) andthe interstitium (

i). Finally, Q

lymphdescribes the rate of lymph

ow from the interstitium.

When hydrostatic forces exceed colloid oncotic forces,

uid will leave one compartment and go to the other. Con-

versely, a relative increase in the colloid oncotic forces within

a compartment can retain uid within or may draw uid into

a compartment. So depending on whether hydrostatic forces

or colloid oncotic forces are stronger, uid will either leave

the blood vessel or enter it. This equation can have a num-

ber of important physiologic implications, especially when

disease conditions alter one or more of the variables.

Slide 16


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Insensible losses are those which cannot directly

by measured and are lost in the form of sweat, saliva,

and excessive panting. Sensible uid loss constitutes

approximately two-thirds of an animals daily uid

requirement. Since insensible losses account for the

other one-third of a patients daily uid requirement,

their importance to the total daily uid requirement

should not be underestimated. You should be sure to

include the insensible uid loss measurement in your

patients daily uid requirement calculations so that the

volume administered is adequate. Normally, insensible

losses are estimated to be 20 to 30 ml/kg/day.

Weve talked about uid loss. Now lets talk about

uid balance, since the goal of uid therapy is to ensure

that the patient has an adequate amount of uid distrib-

uted appropriately in the body.

Slide 21

Fluid balance is largely governed by sodium concentration

within the body. Sodium and chloride concentrations in the

vascular space trigger osmoreceptors in the hypothalamus to

sense osmolality, or particle concentration. Other molecules

that contribute to osmolality include potassium, bicarbonate,

urea, and glucose. As serum sodium rises, usually due to

increased sodium intake or uid loss in excess of solute,

serum osmolality also rises. This rise in osmolality triggers the

hypothalamic release of arginine vasopressin, also known as

antidiuretic hormone, or ADH, into the peripheral circulation.

ADH stimulates the opening of water channels in the renal

collecting duct and causes the reabsorption of water. Reten-

tion of water within the vascular space dilutes the sodium

concentration and decreases osmolality. Once the animals

osmolality has been normalized and its major contributors of

osmolality have been diluted, the hypothalamus stops releas-

ing ADH, there is less re-absorption of water by the kidney,

and uid balance is normalized through this self-regulation

Slide 22

A knowledge of uid therapy would be not be complete

without a discussion of the other electrolytes and buffers

that are contained within the intracellular and extracellular

compartments. So lets take a moment to cover that next.

This is important in choosing the appropriate uid for your

compromised patients. Sodium, chloride, and bicarbonateare found in much higher concentrations in the extracellular,

intravascular compartment than in the intracellular compart-

ment. Conversely, magnesium and phosphate are found in

higher concentrations within cells, rather than within the inter-

stitial and intravascular spaces. The concentrations of these

important ions often become deranged in a variety of critical

illnesses because of gain or loss, or due to exchange second-

ary to changes in the bodys pH and buffering capacity.

It is important to understand the various electrolytes

and what can cause abnormalities so you can choose the

right uid for a dehydrated or hypovolemic animal. A more

thorough discussion of the electrolyte content of crystalloid

uids will be discussed in future uid therapy modules.

Slide 23

OK, so now its time to take this to the practical level.

Lets say this dog came into your hospital. Do you know

if she is experiencing hypovolemic shock or dehydration?

Would you be able to tell just by looking at her? Unfortu-

nately, no, you wouldnt be able to tell.

One of the most common misnomers in uid therapy is

when hypovolemia is referred to as dehydration. Dehy-

dration refers to a decrease in total body water, whereas

hypovolemia largely refers to inadequate circulating

intravascular uid volume. Dehydration refers to loss of

uid in the intracellular and interstitial uid compartments.

Since hypovolemia can be a much more serious condition

and may need to be treated much more aggressively than

dehydration, it is important that both the veterinarian and

technician be able to tell the difference between the two.

Slide 24



To get a rough estimate of an animals degree of

dehydration, you can use subjective guidelines of skin

tenting and mucous membrane dryness. For example,

historical vomiting or diarrhea without a measurable

increase in skin tenting or mucous membrane dryness

can be categorized as less than 5% dehydration. As

the volume of uid loss increases, external evidence

of dehydration becomes more readily apparent and

become more progressive and severe. At 7% dehydra-

tion, mild tachycardia could also be present. At 10%,

the patient might also have a decreased pulse pres-

sure. Then at 12% dehydration and higher you might

see sunken eyes and dry corneas and, in severe cases,

alterations of consciousness. Evaluating these physical

parameters and determining the percent of dehydra-

tion is a component of calculating the replacement uid

volume needed for your patient.

Slide 25

To recap what weve discussed so far, weve talked

about how uid loss leads to hypovolemia, which leads

to decreased perfusion, which can lead to morbidity and

sometimes death.

So how does the body respond to uid loss? It

responds in a number of ways. First, it pulls uid into the

intravascular space from the interstitial space in order

to maintain circulating blood volume. When the intersti-

tial space can no longer replenish intravascular volume

depletion, clinical signs of hypovolemia result.

Hypovolemia refers to inadequate circulating intra-

vascular volume. Hypovolemia can result in hypovolemic

shock from excessive hemorrhage, such as that observed

with a bleeding abdominal mass, vitamin K antagonist

rodenticide intoxication, or an arterial laceration.

Slide 26

Hypovolemia also can occur due to severe uid loss

and in end-stage dehydration, such as that observed

in a puppy with parvoviral enteritis or an elderly cat in

end-stage renal failure. Parameters used to determine

an animals hydration status should not be used to

determine its intravascular volume status.Intravascular uid volume and cardiac output are

major determinants of organ perfusion. In the peripheral

tissues, parameters of perfusion include capillary rell

time, mucous membrane color, blood pressure, and

heart rate. In a normal animal, the mucous membranes

should be pink and moist, with a capillary rell time

of less than two seconds. Pale pink to whitish gray

mucous membranes with a capillary rell time thats

prolonged, tachycardia, and hypotension can be found

with either hypovolemic or cardiogenic shock.

Slide 27

So what else happens in the body with hypovolemia?

Lets take a quick look at a ow chart. Baroreceptors are

located in the carotid body and aortic arch that sense the

stretch of the vessel walls, depending on how much uid is

circulating in the intravascular space. In healthy, euvolemic

animals, stimulation of the stretch receptors triggers thevagus nerve to automatically slow heart rate. When an

animal becomes hypovolemic, the stretch receptors sense

a decrease in wall tension and decrease ring of vagal

stimuli to the brain. This allows the sympathetic nervous

system to manifest itself, and epinephrine and norepineph-

rine are released from the adrenal glands. The release

of these hormones results in vasoconstriction, improved

cardiac contractility, and an increase in heart rate. These

effects are intended to compensate for decreased intravas-

cular uid volume by improving cardiac output and, thereby,

maintaining systemic blood pressure.

Slide 28



At the beginning of this module we dened perfusion,

and now well explore it in more depth. Oxygen delivery is

an important part of the perfusion discussion. It is depen-

dent on the animals cardiac output, which is represented in

this graph, and the amount of oxygen carried in the blood-

stream. Cardiac output is a function of heart rate and stroke

volume, or the amount of blood that the heart pumps in oneheartbeat. Factors that affect stroke volume are cardiac or

ventricular preload, ventricular afterload, and contractility.

Cardiac preload, by Starlings law of the heart, is affected

by the amount of blood that enters the ventricle and causes

the ventricular wall to stretch. The amount of wall stretch is

directly proportional to the force of contraction. If the heart

muscle is healthy, and if there is adequate circulating cardiac

preload because there is adequate intravascular circulat-

ing volume, and, thus, adequate wall stretch, the force of

contraction will be adequate. In an animal with insufcient

circulating blood volume, cardiac preload will be diminished,

thus decreasing the force of ventricular contraction. Intrave-

nous uid therapy can affect cardiac preload by replenishing

intravascular uid volume in a hypovolemic animal.

Slide 29

That brings us to another common misconception: that

uids instilled into the subcutaneous space can be used

to replenish the intravascular blood volume. In fact, uids

instilled into the subcutaneous space will be absorbed very

slowly, if at all, in a patient in hypovolemic shock.

Remember, hypovolemia refers to the intravascular

space, and dehydration refers to the interstitial and intra-cellular compartments. The only methods of accurately

replenishing intravascular uid decits are to place either

an intravenous or an intraosseous catheter. Fluids can be

administered at high rates and large volumes through these

catheters to replenish the intravascular pool.

Slide 30

There are two types of uids, crystalloids and

colloids. Which you choose depends on where you are

trying to replenish uids. Crystalloids are composed of

smaller molecules; therefore, approximately 80% of the

uid infused will leave the intravascular space within

the hour. Colloids are made of larger molecules, which

means they stay in the intravascular space longer.

There are times you might use both. An additional

module will be devoted to this discussion.

Slide 31

During compensatory shock, the heart rate and blood

pressure may be normal, or there may be tachycardia with

mild hypotension. As hypovolemic shock progresses, the

sympathetic output can become exhausted and no longer

allow an increase in cardiac output.

Slide 32


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Now weve set the stage for future modules with basics

of uid therapy. Weve covered body water composition and

how uid moves between the different cell compartments.

We reviewed animals maintenance uid requirements and

discussed how important it is to determine both a patients

sensible and insensible uid losses. Then we determined the

importance of properly diagnosing a patient with dehydration

versus hypovolemic shock.

The future modules will present practical informationfor use in your practice: the various types of crystalloid and

colloid solutions, uid administration and intravenous cath-

eter techniques, strategies for uid administration as well as

potential complications of uid therapy, and uid therapy for

specic disease states. So stay tuned!

Slide 35

During late decompensatory shock, severe hypovo-

lemia coexists with dramatic worsening of the perfusion

parameters, which are manifested as bradycardia,

prolonged capillary rell time, hypotension, pale pink

to whitish gray or cyanotic mucous membranes,

hypothermia, decreased central venous pressure, and

decreased urine output.In late decompensatory shock, rapid and aggressive

intravenous uid resuscitation is necessary to save the

animals life.

Slide 33

In conclusion, youve learned that without adequate

hydration, hypovolemia results, leading to decreased

perfusion, morbidity, and possibly death. Perfusion

depends on many body compensatory responses, but

also on the administration of appropriate uid volumes to

maintain vascular volume.

Youve also learned that administration of intravenousuids requires an understanding of the type of uid loss,

the presence of underlying disease processes, the animals

acid-base and electrolyte status, the animals ability to

retain uid within the vasculature, as well as determinations

of resuscitation end-points.

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Fluid Therapy Basics

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