Fluid, Electrolytes and Acid-base Disturbances

7/30/2019 Fluid, Electrolytes and Acid-base Disturbances

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FLUID, ELECTROLYTESAND ACID-BASEDISTURBANCES IN

SURGERY


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Body water

accounts for 50%-70%of body weight. There is a higher

percentage of water in youngpeople, thin people, and men and

a lower percentage of water in

older people, obese people, andwomen.


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Compartments

Intracellular Intravascular

Interstitial


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Intracellular

This compartment accounts for 30% - 40%of body weight (65% of total body water).Most of the bodys intracellular water is

contained in skeletal muscle cells; verylittle water is contained in adipose cells,which accounts for the lower percentageof water in obese and older people.


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Intravascular

This compartment accounts forapproximately 5% of body weight (10%of total body water).

The body water in this compartmentsupplies the blood.Maintenance of the intravascular

compartment is essential to survival andshould be the primary consideration influid and electrolyte management.


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Interstitial

This compartment accounts forapproximately 15 % of body weight(approximately 25 % of total body

water).This compartment is equilibratedrapidly with the intravascularcompartment to maintain theintravascular compartment close toor within normal parameters.


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Electrolyte composition Electrolytes determine the amount of water that exists in any one space atany time.Electrolyte concentrations in theintracellular space differ comparedwith the extravascular spaces.Water follows electrolytes across cellmembranes to equilibrate osmolality.


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Compartments

Intracellular Intravascular Interstitial


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Intracellular

This compartment containsapproximately 200 mEq/L of osmotically active particles.

This compartment has the highestosmotic pressure, which accounts

for cell turgidity.


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Intravascular

This compartment containsapproximately 154 mEq/L of osmotically active particles.

The intravascular compartment has aslightly higher osmotic pressure thanthe interstitial compartment, so that

the contents of the intravascularcompartment do not diffuse into theinterstitial space.


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Interstitial

This compartment is similar to theintravascular compartment andcontains approximately

153 mEq/L of osmotically activeparticles.


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NB!

Change in osmotic pressure in one

compartment causes water toredistribute from the othercompartments until equilibrium

is returned.


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Blood volume Knowing how to calculate theapproximate blood volume for apatient is important.Using the two-thirds rule,approximately 7 % of body weight isblood volume. This calculation is based on lean bodymass for a 70-kg man, and it varieswidely depending on the patients age,gender, and body


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Water

The amount of water required by aperson depends on the persons weight,age, gender, and illness.

Most healthy adults are able tocompensate for most inappropriatelyadministered fluids. Therefore, any of the frequently used methods of calculating water requirements areusually satisfactory.


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Methods of calculatingwater requirements

The utilization of the body water can beused as a guide for the amount of watermaintenance needed.The amount of water required formaintenance can be based on thepatients weight.A given amount of water per kg of body

weight.A given amount of fluid can be used,regardless of body weight.


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The major water loss from the body is throughurine production.

Generally, 0.5 mI/kg/hr is sufficient urineoutput to excrete the daily solute load.

The next highest daily water loss is insensible

loss (i.e., sweat, respiration, stool), which isestimated as 500 ml/24 hr. In a 70-kg man,water maintenance would be:

(70 kg x 0.5 mI/kg/hr X 24 hr)+ 500 ml/24 hr


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This method usually is used for

pediatric patients, because theirbody weights vary widely.Estimations are 100 mI/kg for the first

10 kg of body weight, 50 mI/kg forthe second 10 kg of body weight, and20 mI/kg for each additional kg of

body weight (i.e., a 35-kg childwould require 75 mI/hr).


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A given amount of water per kg

of body weight can be used todetermine water maintenance

requirements.The value used for this method is

generally 35 ml/kg/24 hr.


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Adults frequently have fluidsadministered at 125 ml/hr. This

amount may be well tolerated by

a healthy 70-kg man but maylead to water excess in a small,elderly patient.


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Because water requirements vary, the mostimportant aspect of water maintenance

administration is evaluating the adequacyfor the patient in question.

Fever, environmental temperature, and

respiratory rate can increase waterrequirements.

The important factors are urine output, blood

urea nitroge, and creatinine levels, heartrate, blood pressure, skin turgor, andperipheral edema.


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Urine output variations

If urine output is high (> 0.5ml/kg/hr), then less water may berequired.

If urine output is low, more watermay be required or anotheraction may be necessary.


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As people age, muscle mass and thenumber of glomeruli decrease.

The elderly should not be expected toproduce as much urine as youngerpeople.

Elderly patients can be pushed intocongestive heart failure by continuedwater administration for a low urineoutput, when the urine output isalready adequate for their size.


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The natural response in patients stressedby injury, illness, or surgery is to retainsodium and water.

The physician must consider the urineoutput variations .

If the patients overall condition andcreatinine are stable, low urine output (

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Fluid, Electrolytes and Acid-base Disturbances