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Fluids & Electrolytes

in

Surgical Patients

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Objectives

Review physiology controlling fluid/elec

balance

Appreciate differences in surgical patients

Be able to order fluid regime for surgical

patients

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Total Body Water

body wt% Total bodywater%

total 60 100intracellular 40 67

extracellular 20 33

intravas 5 8interstitial 15 25

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Composition of Fluidsplasma interstitial intracel lular

CationsNa 140 146 12K 4 4 150Ca 5 3 10

Mg 2 1 7

AnionsCl 103 104 3HCO 24 27 10

SO4 1 1 -HPO4 2 2 116Protein 16 5 40

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Osm or mOsm unit for number of particles

1 mol of NaCl - 2 osm

Osmolarity - mOsm/L

Osmolality - mOsm/Kg water

Osmolality defines concentration of solution Tonicity defines effect of fluid on cell volume

Osm or mOsm unit for number of particles

1 mol of NaCl - 2 osm

Osmolarity - mOsm/L

Osmolality - mOsm/Kg water

Osmolality defines concentration of solution Tonicity defines effect of fluid on cell volume

Osmotic Activity of Fluid

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Osmolality

Plasma osmolality Posm

- measure of body osmolality

Usually

Posm(mOsm/l) = 2x serum [Na]

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Osmolality

Posm(mOsm/l)

= 2x serum [Na] + glocose/18 + BUN/2.8

Look for osmolar gap

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Control of Volume

Effective circulating volume

Portion of ECF that perfuses organs

Usually equates to Intravascular volume

Third space loss

Abnormal shift of fluid for Intravascular to

tissues eg bowel obst, i/o, pancreatitis

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Volume Control

osmoreceptors - day to day control

baroreceptors - respond to pressure change

neural output

hormonal mediators

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Osmoregulation

osmolality 289 mOsm/kg H20

osmoreceptor cells in paraventricular/supraoptic nuclei

osmoreceptors control thirst and ADH

small changes in Posm - large response

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Osmoregulation

Excess free water (Posm 280)

thirst inhibited

ADH declines

urine dilutes to Uosm 100

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Osmoregulation

Decreased free water (Posm 295)

thirst increased

ADH increases

urine concentrates to Uosm 1200

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Osmoregulation

Change in uOsm= 95 x change in Posm

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Volume Control

osmoreceptors - day to day control

baroreceptors - respond to pressure change

neural output

hormonal mediators

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Neural mechanism

stretch - tachycardia and increased renal

blood flow and decreased Nareabsorption

Baroreceptors

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Baroreceptors

Hormonal mediators

renin

aldosterone

ANPdopamine

Hormonal effect

ECF Na and water reabsorption

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Renin-angiotensin

Renin secreted when

drop BP

drop Na delivery to kidney

increased sympathetic tone

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Angiotensin II

Increases vascular tone

increases catecolamine release

decrease renal blood flow

increases Na reabsorption

stimulates aldosterone release

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Aldosterone

Release stimutlated by

Angiotensin II

increased K

ACTH

Effect

Na and water absorption

in distal tubular segments

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Normal Water Exchange

Avg dai ly ml M in dai ly ml

Sensible

urine 800-1500 300

intestinal 0-250 0

sweat 0 0

Insensible

lungs/skin 600-900 600-900

8-10 mls/kg/D - 10%/ o rise in Temp

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Normal Intake of Water

2000mls - 1300 free water

700 bound to food

additional water comes from catabolism

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Water and Eletrolyte Exchan

Surgical patients prone to disruption

nil orally

anesthesia

trauma

sepsis

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Fluid and Electrolyte Therap

Surgical patients have

Maintenance volume requirements

On going losses

Volume excess/deficits

Maintenance electrolyte requirements

Electrolyte excess/deficits

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Maintenance Requirem

This includes: insensible

urinary

stool losses

Body weight Fluid required

0-10Kg 100ml/kg/d

next 10-20kg 50 ml/kg/d

subsequent 20 Kg 20ml/kg/d

15ml/Kg/d for elderly

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70 Kg Man Needs

10 x 100 = 1000

10 x 50 = 500

50 x 20 = 1000

2500 mls / d

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On Going Losses

NG

drains

fistulae

third space losses

Concentration is similar to plasma

Replace with isotonic fluids

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Volume Deficit - Acute

vital signs changes Blood pressure

Heart rate

CVP

tissue changes not obvious

urine output low

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Volume Deficit - Chronic

Decreased skin turgor

Sunken eyes

Oliguria

Orthostatic hypotension

High BUN/Creatine ratio

HCT increases 6-8 points per litre deficit

Plasma Na may be normal

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Volume Excess

Over hydration

Mobilisation of third space losses

Signsweight gain

pulmonary edema

peripheral edema

S3 gallop

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Fluid and Electrolyte Therap

Goal

normal haemodynamic parameters

normal electrolyte concentration

Method

replace normal maintenance requirements

ongoing lossesdeficits

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Fluid and Electrolyte Therap

Normal maintenance requirements use BW formula

On going losses

measure all losses in I/O chart estimate third space losses

Deficits

estimate using vital signs estimate using HCT

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Fluid and Electrolyte Therap

The best estimate of the volume required

is the patients response

After therapy started observe vital signs

Urine output (0.5mls/Kg/hr

Central venous pressure

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Time Frame for Replaceme

Usually correct over 24 hours

For ill patients calculate over

shorter period and reassess e.g. 12hours or 3 hours for e op cases

Deficits - correct half the amount

over the period and reassess

Fluid therapy in the ward

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Fluid therapy in the ward

Example 1

65 year old, 75 kg 2nd POD

urine out put 40 mls/hr

NG 1.5 L

drains 500 mls

Fluid therapy in the ward

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Fluid therapy in the ward

Example 2

65 year old, 75 kg 2nd POD

urine out put 40 mls/hr

NG 1.5 L

drains 500 mls

HCT 55

BP 90/60

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Maintenance Electrolyte Requireme

Na 1-2mEq/Kg/d

K 0.5 - 1 mEq/Kg/d

Usually no K given until after urine output isadequate and U/E done.

Always give K with care, in an infusion slowly

- never bolus Ca, PO4, Mg not required for short term

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Postoperative Fluid Therap

Check i/v regime ordered in op form

Assess for deficits by checking I/O chart andvital signs

Maintenance requirements calculated

Usually K not started

Monitor carefully vital signs and urine output

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Postoperative Fluid Thera

Urine specific gravity may be used(1.010 - 1.012)

CVP useful in difficult situations(5-15 cm H2)

Body weight measured in specialsituation e.g. burns

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Concentration Changes

changes in plasma Na are indicative ofabnormal TBW

losses in surgery are usually isotonic

hypoosmolar condition usually caused byreplacement with free water

Hyponatremia -

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Hyponatremia

Usually Excess Free Water

Free water replacement of isotonic losses

Increased ADH secretion

Low intravascular volume states like cirrhosis/low albumin

Excess solute e.g. glucose - intracellular watershifts to ECF

Hyponatremia -

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Features - depends on rapidity

acute drop below 120weaknessfatigueconfusioncrampsnausea/vomiting

headache/delirium/seizures/comapermanent CNS damage

Hyponatremia

Usually Excess Free Water

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Diagnosis of Hyponatremia

assess circulating volume

exclude hyperosmolar states

check for losses

check for excess free water replacement

In difficult situations measure urine Na

(>

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Treatment of Hyponatremia

replace volume deficits in dehydration

restrict free water in overload

Na required = [desired Na] - [actual Na] x (TBW)TBW = 0.6xWt

Correct half the deficit over 12 hours and

reassess

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UESTION

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