MAINTENANCE FLUID THERAPY

191.

MAINTENANCE FLUID THERAPY

S. C . Haskins University of California, Davis, California, U.S.A.

Introduction The total daily fluid requirement is the sum of the requirements for the existing deficits, the basal maintenance, and the expected ongoing losses. The patient's fluid needs should be calculated at the beginning of each day and should be reassessed several times during the day to make sure that therapy is keeping current with the disease process(es).

A l l plans should be constructed in consider-

ation of the patient's requirements for volume, packed cell volume o r haemoglobin concentration, total protein concentration, water requirements and sodium, potassium and chloride concentration, and acid-base balance (Table 1 ) . If parenteral fluid therapy is extended for more than a few days, consideration should be given to the patient's requirements for calories, vitamins, amino acids, fatty acids and other common minerals (Table 1 ) .

It is not the

purpose of this overview to discuss these nutritional aspects of maintenance fluid therapy nor is it t o give detailed descriptions of the physiology of sodium and water balance o r acid-base regulation. A general approach will

be presented with regard to the highest priorities in maintenance fluid therapy: volume,red cell and protein concen-

192.

trations, sodium, potassium and chloride concentration, and acid-base balance.

Guidelines for the construction of a

daily fluid plan for a variety of common disorders will be described.

There must be no doubt that inadequate fluid volume, irrespective of the balance of the constituent parts, is the single most common abnormality in critical patient care

and that its normalisation deserves first and foremost attention.

The signs of a fluid volume deficiency depend upon

the compartmental distribution of the loss.

Fluid volume

deficiency within the vascular compartment (hypovolaemia) is heralded by the familiar signs of shock: pale colour and prolonged capillary refill time, cold extremities and oliguria, tachycardia and weak pulse, low arterial and central venous blood pressure, reduced cardiac output, and absence of haemorrhage at sites of surgery or trauma.

Restitution

therapy is complete when all signs of hypovolaemia have been alleviated. Examples of fluid l o s s which are all or mostly derived from the vascular compartment include surgical or traumatic blood loss and third space proteinaceous fluid losses (pleuritis, peritonitis, fracture-induced muscle trauma, extensive surgery involving the thorax, abdomen o r extensive muscle exposure, glomerulonephritis or burns). Interstitial fluid volume deficiencies result in decreased skin turgor

or dehydration (pitting Edema repres-

ents an interstitial fluid excess).

Any fluid loss which is

high in sodium (vomition, diarrhoea, diuresis, and low

193.

protein third space accumulations into one of the body cavities or gut lumen) is derived to a great extent from the interstitial fluid compartment. Since there is no diffusion barrier between the vascular and interstitial compartments fluid losses of this nature come from both areas (extracellular fluid compartment losses).

Since

there is no way for an animal to lose interstitial fluid without coincidentalry losing vascular fluid, it is appropriate to expect that all dehydrated patients are also hypovolaemic (the opposite, of course, is not true). Intracellular fluid volume deficit (and excess) cause rather nonspecific clinical signs such as depression, disorientation, muscular twitching, seizures and coma.

Intra-

cellular water volume aberrations are more dependent upon

intracellular/extracellular osmotic gradients than they are on absolute volume changes in the body as a whole.

The

importance of severe disturbances in sodium concentration (as the major extracellular cation) and extracellular osmolality are primarily in the effect that they have on the intracellular/extracellular distribution of water.

If

the net l o s s of fluid from the body was absolutely isotonic then it would be possible to achieve a state of severe extracellular depletion in the face of a normal intracellular volume.

This is not quite possible however since

not all of the intracellular osmolar particles are fixed and nondiffusable, and since cell water is probably respons-

ive to interstitial hydrostatic pressure changes as well.

194.

Most fluid losses (except third space) are, however, low in sodium with respect to normal plasma sodium concentration and insensible losses (respiratory and skin) are completely devoid of sodium.

Intracellular water is invariably lost

in conjunction with all exogenous fluid losses. Anaemia and Hypoproteinaemia When whole blood or proteinaceous fluids are lost from the vascular compartment they may initially be replaced with a crystalloid sodium-containing replacement solution such as lactated Ringer's solution.

If the red cell or

protein loss is severe or prolonged this exchange process will eventually lead to the depletion of these important blood components.

When the packed cell volume is decreased

to below 20% the oxygen carrying capacity of the blood may be jeopardized and if further volume therapy is indicated it should be in the form of whole blood. protein is decreased to below 3.0

When the total plasma

- 3.5 G/dl the colloid

oncotic pressure of the vascular compartment may be jeopardized and if further volume therapy is indicated it should be in the form of plasma or a colloidal solution such as

dextran 70. Deficits and Ongoing Losses

-

Volume

The fluid volume required to restore an existing deficit in a patient is difficult to quantitate unless: ( 1 ) it is specifically known because the patient was kept in a metabolic cage/calorimeter and all sensible and insensible losses were collected and measured; o r (2) current weight

195.

can be subtracted from a very recent predisease weight and most of the loss can be attributed to fluids. Fluid volume deficit repair in most clinical presentations is almost reduced to a flgive-some-and-letfs-see-what-happensflbasis. This does not represent a serious disadvantage since therapy is usually titrated to some particular end-point.

It is

convenient to be able to generate an initial estimate of the fluid volume deficit and this is often derived by the extent of the decrease in skin turgor.

A barely perceptible

decrease classically indicates a fluid deficit equivalent

to 5% of the body weight.

Skin which stands in catatonic

folds represents a 1296 deficit.

The accuracy of the tech-

nique is marred by the subjectivity of the evaluation, individual patient variation, emaciation (decreases turgor without dehydration), and obesity (obscures skin turgor changes in dehydration), but it is used because often it is the only available guide.

Solace must be sought in the

fact that this inaccuracy helps explain why the patient is still dehydrated after having given the total of what was thought to be necessary.

This excuse should not be used

for more than about one day, however. The quantity of the abnormal ongoing losses are predicted on the basis of yesterdays estimated or measured vol-

umes and the expected progress of the disease. Deficits and Ongoing Losses

- Electrolyte Concentrations

It is convenient to consider the deficit and ongoing l o s s needs of the patient together when the ongoing losses are assumed to have caused the deficit (the indicated fluid

196.

composition would be t h e same f o r b o t h ) . V i r t u a l l y a l l abnormal l o s s e s o f f l u i d from t h e body a r e hyponatraemic i n n a t u r e (60

-

1 2 0 mEq/L) with t h e excep-

t i o n o f whole blood o r plasma l o s s and u r i n a r y l o s s e s i n severe end-stage r e n a l d i s e a s e when t u b u l a r m o d i f i c a t i o n of t h e glomerular f i l t r a t e i s minimal.

Insensible losses

( r e s p i r a t o r y and s k i n ) a r e devoid o f e l e c t r o l y t e s and consequently t h e n e t d a i l y l o s s e s a r e i n v a r i a b l y low i n sodium.

Without exogenous water, dehydrated p a t i e n t s w i l l

always be hypernatraemic and w i l l be d e p l e t e d of water volume i n a l l body water compartments. Since t h e d i f f e r e n c e between sodium and water c o n t e n t (volume) and sodium c o n c e n t r a t i o n a r e o f t e n confusing, and s i n c e t h e concept i s important i n t h e c o n t e x t of f l u i d therapy, t h e f o l l o w i n g e x p l a n a t i o n i s provided.

The t o t a l

q u a n t i t y of sodium i n t h e e x t r a c e l l u l a r f l u i d compartment, along with i t s a s s o c i a t e d anion, i s p r i m a r i l y r e s p o n s i b l e f o r g e n e r a t i n g t h e osmotic f o r c e s which h o l d water i n t h e compartment.

When sodium i s l o s t , it i n v a r i a b l y drags

water w i t h i t , t h e volume of t h e e x t r a c e l l u l a r compartment

i s diminished and t h e p a t i e n t becomes dehydrated.

Dimini-

shed e x t r a c e l l u l a r sodium c o n t e n t , sodium and water d e f i c i ency, s a l i n e d e f i c i e n c y , and dehydratiun

a r e synonyms.

Oedema i s t h e o p p o s i t e of all of t h e above.

The d e c i s i o n

t o administer sodium s o l u t i o n s i s u s u a l l y based on t h e need f o r e x t r a c e l l u l a r volume.

Sodium c o n c e n t r a t i o n , on t h e o t h e r

hand, i s a measurement of c o n c e n t r a t i o n and makes no r e f erence t o volume.

I t speaks t o t h e i s s u e of t h e n a t u r e

( n o t t h e volume) o f t h e f l u i d l o s s and t h e r e p l e t i o n

197.

requirements.

Abnormalities in sodium concentration have

importance with regard to extracellular/intracellular water distribution.

Variations in sodium content and sodium

concentration can occur in any combination. Hyponatraemia does not indicate dehydration any more than a low packed cell volume indicates hypovolaemia. Hyponatraemia always indicates an excessive accumulation of water in the generation of the electrolyte disturbance since no disease causes the loss of hypernatraemic fluids.

Many patients would respond favourably if a standard isotonic sodium replacement solution (such as lactated Ringer's or an equivalent solution) containing an alkalinisin& anion (such as lactate, acetate or gluconate) was routinely administered for all deficits and ongoing losses without regard for the cause, as long as it was given in sufficient volumes and

appropriate attention was given to

red blood cell and plasma proteiii concentrations. Therapeutic results may be improved with appropriate attention

to the potassium needs of the patient. Most abnormal losses contain 5

-

25 mEq/L of potassium and replacement solutions

should reflect this loss.

Potassium (10

-

20 mEq/L) should

be added when the deficit was created by diarrhoea, vomition,

golyuria (except end-stage renal failure), or when it is unknown but is suspected to be due to one of the above. This is especially important when an animal has not been eating. Further improvements in response to therapy may be achieved by attention to the nature of the fluid loss (Table 2).

It should be noted that lactated Ringer's o r

an equivalent solution is the basic starting fluid for all

198.

types of fluid loss except for the patient that is dehydrated due to lack of access to water and food (enclosed in a room, caught in a trap, or a frozen water supply).

These

patients should receive a maintenance solution. Dehydration csuses thirst and increases water consumption.

The exchange process, wherein the patient loses high

electrolyte containing fluids and replaces them with nearly electrolyte-free fluids, causes a dilutional hyponatraemia. When it can be verified by history that the patient's water consumption has dramatically increased in association with the disease process it is likely that the patient is hyponatremic and that saline o r Ringer's solution instead of lactated Ringer's would be a more appropriate rehydrating solution. The nature of fluids lost via vomition are very variable and deserve special comment.

Gastric secretions con-

tain very high concentrations of hydrogen and chloride, and moderate concentrations of sodium (30 potassium (5

-

25 mEy/L).

- 90 mZq/L)

and

bhen it c a n be verified that the

Tromited solutions come exclusively from the stomach (i.e. pyloric obstruction, gastric suction), it should be assumed that the patient is alkalotic and that Ringer's or saline would be the rehydrating drug of choice.

If this patient

has not been drinking water, he probably will be hypernatraemic, and the above fluid should be further modified by diluting it with an equal volume of 5% dextrose in water

(D5W).

If he has been drinking a lot of water, he will

probzbly be hyponatzaemic and the straight Ringer's o r saline

299.

solution indicated above should not be modified.

More often

than not, however, vomition involves the regurgitation of duodenal fluids and then the nature of the net fluids lost changes dramatically and has a much higher concentration of sodium (80 - 120 mEq,/L) and bicarbonate (30

-

50 mEq/>).

Owners should be questioned closely with regard t o colour of the vomitus, since yellow or green discoloration indicates bile staining and duodenal regurgitation. Most vomit.ing patients fall into this category and are acidotic rather than alkalotic.

Potassium-supplemented lactated Ringer's

or an equivalent solution containing an alkalinising anion is a more appropriate choice than is saline for rehydration.

If it can be ascertained that the patient has been

drinking a lot of water or that he is hyponatraemic, saline supplemented with potassium and bicarbonate should be administered-

All of the foregoing discussion is predicated cm the assumption that the existing electrolyte disturbances are not known and cannot be measured.

All such averaging

techniques are decidedly inferior to knowledge of the actual electrolyte aberration. There are s3 many potential outcomes from the complicated interactions between disease processes and patient compensation that there is really no substitute for actual measurement of the electrolyte concentrations.

It is not currently a standard of practice in

veterinary medicine to measure electrolyte concentrations on every patient, but they should be measured when the patient is debilitated by multiple disease processes and when the

200.

patient has not responded favourably to initial therapy. When the electrolyte concentrations are known then the replenishment fluids can be tailored to the specific needs

of the patient. A standard replacement solution should be used if all electrclyte distributions a e known or assumed If "he sodium concentration ia known and is

tc be normal.

low ( I 6 0 niEl/L) equal parts of saline or lactated Ringer's

sclution and D W should be used.

5

If the potassium concen-

tration is known 2nd is high ( > 5.5 mEl/L) potassium-free fluid should be administered; if low (< 3 - 5 mEI/L) an enriched potassium solution should be given.

Up to a l l of

the fluids given for the day should contaiii ? i if the plasma]'K[

[K+]

is 2.5

-

is 3.0

- 3.5

mEq/L, 60 mEq/'L if the

7.3 mEq/L, and 80 mEq/L if the

< 2.5 mEq/L. These estimates assume a minimal ane and minimal muscular wasting. measured

A [K'J

at 40 mEci/L

c K 9 while

is p9 disturb-

Acidosis increases the

alkalos.:s decreases the measured cK+l.

of 3.0 mEq/L l-r conjunction with acidaemia represents

a much greater potassium deficit than the same [K'] normal pH.

with a

If either alkalosis o r muscle wasting exist, the

potassium concentrations suggested above should be reduced. Trouble may be encomtered in two ways when administering potassium: by administering a little of it too rapidly

(a 5 mEq bolus to a 15 K g dog will cause severe arrhythmias o r cardiac arrest) or by administering trJo much of it. A single loading dose of as much as 2 mEq/Kg can be adninist-

201.

ered in a concentration of less than 100 mEq/L in a period of 1

-

2 hours; or up to 0.5 mEq/Xg/hour can be given

the entire day (if rer,al funct.ion is normal).

for

All patients

subjected to serious potassium loading ( > 40 mEq/L) should be monitored electrocardiographically

continuously o r at

regular intervals for signs of potassium overload. The bicarbonate needs can be quantified by base deficit o r bicarbonate deficit calculations and the formula: mEq

bicarbonate to administer

x 0.3 x KgBw.

=

base o r bicarbonate deficit

Rapid administration of m y alkalinising

a.gent should be avoided (hypotension, vomition, and cardiac arrest have been observed).

The above calculated bioarbonate

dose can be safely administered in 70 to 60 minutes. Without base dePicit or bicarbonate deficit estimates the endpoint of the bicarbonate titration is rather difficult to Effective treatment of the underlying disease

determine.

process is always the preferred approach to correcting a metabolic acidosis. therapy ( 1

-

Without measurements bicarbonate

4 mEq/Kg) should be considered only when the

patient's involvement with the disease process is extensive and it is suspected that the acidosis is compromising

patient homeostasis, or when therapy cannot be rapidly effective. Maintenance Requirements The diiily volume of maintenance requirements is dependent upon the daily caloric expenditure of th.e patient.

The

202.

d a i l y c a l o r i c expenditure v a r i e s with p a t i e n t s i z e and metabolic r a t e and i s g e n e r a l l y h i g h e r p e r u n i t weight i n smaller pa tie n t s .

When t h e water of o x i d a t i o n i s included,

t h e d a i l y volume o f f l u i d (mls) t h a t a p a t i e n t r e q u i r e s i s approximately e q u i v a l e n t t o c a l o r i c (Kcal) requirements. Table 3 i t e m i z e s t h e d a i l y and h c u r l y f l u i d and c a l o r i c requirements f o r dogs over a wide range of body weights. N o r m a l u r i n e has a sodium c o n c e n t r a t i o n o f about, 60 80 mEq/L and a potassium c o n c e n t r a t i o n o f about 30

-

- 50

mEq/L, although t h e c o n c e n t r a t i o n s may v a r y widely under

I t i s assumed t h a t one-half o f t h e

c e r t a i n circumstances.

d a i l y l o s s e s a r e i n t h e f o r m o r u r i n e and t h e o t h e r h a l f i n t'ne form of i n s e n s i b l e pure water l o s s e s .

The normal n e t

d a i l y f l u i d l o s s e s t h e r e f o r e have an average sodiixm concen-

t r a t i o r ? o f about 40 mSq/L and a potaasium c o n c e n t r a t i o n of about 20 mEq/L. The a d m i n i s t r a t i o n of replacement s o l u t i o n s w i t h sodium c o n c e n t r a t i o n s o f 130

-

155 m3q/L t o a p a t i e n t losing f l u i d s

wj.th s o d i u m concenxrations o f 40 mEq/L w i l l r e s u l t i n hypernatraemia i f t h e p a t i e n t cannot e x c r e t e t h e excess sodium. Sodium c o n c e n t r a t i o n can be i n c r e a s e d t o 170 mEq/L by u s i n g l a c t a t e d R i n g e r ' s 3 s a maintenance solutiori and t h i s p r a c t i c e must c e r t a i n l y be discouraged.

The a d m i n i s t r a t i o n of r e -

placement s o l t l t i o n s witl, a pota-ssium c o n c e n t r a t i o n of 4

-

mEq/L t o a p a t i e r i t l o s i n g f l u i d s with a potassium concent r a t i o n o f 20 mEq/L w i l l i n v a r i a b l y r e s u l t i n hypokslaemia i f t h e p a t i e n t i s iiot e a t i n g .

5

203.

The outstanding exception to the routine use of maintenance solution for maintenance is the patient with endstage renal disease where the average urine electrolyte concentrations are similar, and may be identical, to plasma concentrations. In this case a replacement solution tempered with D W for the insensible losses more closely 5 approximates the needs of the patient. If the patient is oliguric or anwic the total fluid volume indicate on Table

3 should be divided in half and given as D,W

, to cover the

insensible losses of the patient.

Urinary losses should

then be equalled with a potassium-free solution such as saline. Fluids specifically designed for maintenGnce are commercially available o r czn be constructed wit?. a mixture of 1 part lactated Ringer's solution and 2 parts of D5V

dilute the sodium)

plus 20 mEq/L of potassium.

(to

This com-

bination could be given alternately in series (a bottle o f

LR, then a bottle of D,W) o r preferrably mixed in a separate burrette or bottle and administrred cojointly. Formulating a Fluid Therapy Plan The following outline could be used for formulation o f the fluid therapy requirements of a patient. 1.

Determine the deficit volume requirexents of the patient by charge in weight r-easuremonts or estimations tased on skin turgor.

?.

Estimate the ongoing 10;s volume requirements.

204.

3.

Determine t h e n a t u r e o f t h e f l u i d s which w i l l be used f o r replacement of t h e d e f i c i t and ongoing l o s s e s with r e g a r d t o sodium.

4.

Determine t h e potassium requirements.

5.

Determine t h e b i c a r b o n a t e requirements.

6.

Determine t h e maintenance volume requirements of t h e patient.

A f t e r t h e f i r s t one o r two days, volume

d e f i c i t s and e l e c t r o l y t e d i s t u c b a n c e s w i l l be c o r r e c t e d as w i l l t h e d i s e a s e p r o c e s s r e s p o n s i b l e

for t h e ongoing l o s s e s ( h o p e f u l l y ) and maintenance requirements w i l l be a l l t h a t need t o be calculated.

7.

Determine t h e n a t u r e o f t h e f l u i d s which w i l l be used f o r maintenance.

8.

Add a B v i t a m i n complex t o a l l f l u i d s (1 m l p e r

litre).

9.

Glucose may be added t o a l l s o l u t i o n s i f t h e p a t i e n t

i s not eating.

It would t a k e approximately a 30%

s t r e n g t h s o l u t i o n of d e x t r o s e t o supply a l l a p a t i e n t ' s c a l o r i c requirements w i t h i n a d a y ' s normal volume of water.

The i n f u s i o n of t h i s

amount of glucose i s a s s o c i a t e d w i t h a number of

problems, i . e .

t h e p o t e n t i a l f o r s e v e r e hyper-

glycaemia and hyperosmolar coma, f o r moderate hyperglycaemia and a n osmotic d i u r e s i s , and f o r p h l e b i t i s and thrombosis, and t h e r e f o r e p r e s e n t s a management problem of some magnitude.

It is

205.

current convention, therefore to administer partial caloric requirements in the form of 10% glucose (which avoids all of the problems except phlebitis) and then hope to get the patient back on a more complete diet as soon as possible.

If fluid

administration is through a peripheral vein, no more than a 5% dextrose in electrolyte solution should be infused to avoid the phlebitis problem.

10. All of the above fluid requirements are added together (on paper) and divided by the number of hours available f o r the fluid administration to determine the hourly rate. Understandable fluid orders should be written. The first bottle is mixed as per the fluid orders, completely labelled and the administration started.

The progress of the patient

is re-evaluated with regard to any necessary changes

in the fluid plan at regular intervals throughout the day.

206.

1.

Volume

2.

Packed cell volume, total protein

3.

Sodium, osmolality, potassium

4.

Bicarbonate, chloride

5.

Calories, water-soluble vitamins

6.

Amino acids

7.

Fatty acids, minerals (calcium, phosphate, magnesium, manganese, zinc)

Priority of concern of fluid and electrolyte

Table 1.

therapy considerations.

Priorities a r e based upon the relative importance with regard to rapidity of onset, propensity for causing harm to the patient, and the relative frequency with which each would occur in a random population of critically ill patients.

It does not exclude the possibility that

a

patient would be normal in all respects for numbers 1 to 6 and yet have a serious disturbance In calcium metabolism.

Body Weight (kg)

Basal energy, (Kcaljday) or water (mls/day)

70 35

1.

2

120

2.

2.5

140

3 3.5

160 180 200

1

1.5

4 5

235 270

6

7

300 330

8

9 10 15

360

20

660

25

785 900 1000 1100 1300

390 535

30

35 40 50

*

3.

Determine basal energy or water requirements. Multiply by: A. Activity factor Cage rest 1.2 Ambulatory 1.4 B. Injury factor Minor surgery 1.2 Skeletal trauma 1.4 Major sepsis 1.6 Obtain Kcal o r water required per day.

Body weight (kg)3’4

Table 3.

Example: a 7 kg ambulatory dog following femur fracture repair. 300 x 1.4 x 1.4 = 588 kcal and mls of water needed per day for maintenance.

x 70.

Daily calorie and water requirements for dogs.

207.

1.

Diarrhoea - Lactated Binger's (or equivalent solution)

+ 10 - 20 mEq/L K+.

If there has been a lot

of water drinking: Ringer's or saline + 10 20 mEq/L 'K 2.

Polyruia

-

-

+ 30 mEq/L HCOJ-.

Lactated Ringer's + 10 - 20 mEq/L K+.

Omit

K+ if polyuria is due to end-stage renal disease.

3.

Vomition

-

Lactated Ringer's

If fluids

+

10

-

20 mEq/L K+.

lost are known to be entirely

gastric in nzture and the patient has been drinking a lot o f water: saline + 10 - 20 rnEq/L K+; if there has been a lack of water intake: & saline or lactated Ringer's/+ D5W + 10

-

20 mEq/L K'.

If fluids lost are bile stained and patient has been drinking a lot of water: saline + 10

-

20 mEq/L K+ + 30 mEq/L HC03-; if it cannot be

verified that the water intake has increased dramatically: lactated Ringer's + 10 - 20 mEq/L .'K

4.

Haemorrhage

-

5.

Third space

- Lactated Ringer's + plasma.

6.

Maintenance losses secondary to lack of access to water

Lactated Ringer's

+

whole blood.

and food - a maintenance solution (Na+ mEq/L; 7.

40

K+ 20 mEq/L)

When the nature of the losses are unknown - lactated Ringer's (or an equivalent solution) + 10

-

20 mEq/L K+.

Table 2.

Fluids of choice for deficits and ongoing losses when the existing electrolyte abnormalities are unknown