FLUID AND ELECTROLYTE MANAGEMENT OF THE SURGICAL PATIENT

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DR SHEHU

LECTURER/PAEDIATRIC SURGEON

INTRODUCTION

• Fluid and electrolyte management is paramount to the care of the surgical patient
• Changes in both fluid volume and electrolyte composition occur preoperatively, intraoperatively, and postoperatively, as well as in response to trauma and sepsis
• The normal anatomy of body fluids, electrolyte composition and concentration abnormalities and treatments, common metabolic derangements, and alternative resuscitative fluids are essential in management

BODY FLUIDS

• Total Body Water
• Water constitutes approximately 60% of total body weight
• The relationship between total body weight and total body water (TBW) is relatively constant for an individual and is primarily a reflection of body fat
• Young, lean males have a higher proportion of body weight as water than elderly or obese individuals
• In an average young adult male 60% of total body weight is TBW, whereas in an average young adult female it is 50%

• The lower percentage of TBW in females correlates with a higher percentage of adipose tissue and lower percentage of muscle mass in most
• Estimates of percentage of TBW should be adjusted downward approximately 10 to 20% for obese individuals and upward by 10% for malnourished individuals
• The highest percentage of TBW is found in newborns, with approximately 80% of their total body weight comprised of water
• This decreases to approximately 65% by 1 year of age and thereafter remains fairly constant

Fluid Compartments

• TBW is divided into three functional fluid compartments: plasma, extravascular interstitial fluid, and intracellular fluid
• The extracellular fluids (ECF), plasma and interstitial fluid, together comprise about one third of the TBW and the intracellular compartment the remaining two thirds

20% of the total body weight

5% of body weight

15% of body weight

40% of an individual's total body weight

Composition of Fluid Compartments

• The ECF compartment is balanced between sodium, the principal cation, and chloride and bicarbonate, the principal anions
• The intracellular fluid compartment is comprised primarily of the cations potassium and magnesium, and the anions phosphate and proteins
• The concentration gradient between compartments is maintained by adenosine triphosphate–driven sodium-potassium pumps located with the cell membranes.
• The composition of the plasma and interstitial fluid differs only slightly in ionic composition

• Proteins add to the osmolality of the plasma and contribute to the balance of forces that determine fluid balance across the capillary endothelium
• Water is distributed evenly throughout all fluid compartments of the body
• Sodium-containing fluids are distributed throughout the ECF and add to the volume of both the intravascular and interstitial spaces

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Osmotic Pressure

• The concentration of electrolytes usually is expressed in terms of the chemical combining activity, or equivalents. An equivalent of an ion is its atomic weight expressed in grams divided by the valence:

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For univalent ions such as sodium, 1 mEq is the same as 1 mmol

For divalent ions such as magnesium, 1 mmol equals 2 mEq

• The principal determinants of osmolality are the concentrations of sodium, glucose, and urea (blood urea nitrogen, or BUN):

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• The osmolality of the intracellular and extracellular fluids is maintained between 290 and 310 mOsm in each compartment
• Because cell membranes are permeable to water, any change in osmotic pressure in one compartment is accompanied by a redistribution of water until the effective osmotic pressure between compartments is equal

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• If the ECF concentration of sodium increases, there will be a net movement of water from the intracellular to the extracellular compartment
• Conversely, if the ECF concentration of sodium decreases, water will move into the cells

BODY FLUID CHANGES

• Normal Exchange of Fluid and Electrolytes
• The healthy person consumes an average of 2000 mL of water per day, approximately 75% from oral intake and the rest extracted from solid foods

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• Daily water losses include 800 to 1200 mL in urine, 250 mL in stool, and 600 mL in insensible losses

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• Insensible losses of water occur through both the skin (75%) and lungs (25%), and can be increased by such factors as fever, hypermetabolism, and hyperventilation

• Sensible water losses such as sweating or pathologic loss of GI fluids vary widely, but these include the loss of electrolytes as well as water

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• To clear the products of metabolism, the kidneys must excrete a minimum of 500 to 800 mL of urine per day, regardless of the amount of oral intake

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Water Exchange (60- to 80-kg Man)

• The typical individual consumes 3 to 5 g of dietary salt per day, with the balance maintained by the kidneys

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• With hyponatremia or hypovolemia, sodium excretion can be reduced to as little as 1 mEq/d or maximized to as much as 5000 mEq/d to achieve balance except in people with salt-wasting kidneys

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• GI losses are isotonic to slightly hypotonic and contribute little to net gain or loss of free water when measured and appropriately replaced by isotonic salt solutions

Classification of Body Fluid Changes

• Disorders in fluid balance may be classified into three general categories:

disturbances in

a. volume

b. concentration

c. composition

• Isotonic gain or loss of salt solution results in extracellular volume changes, with little impact on intracellular fluid volume

• If free water is added or lost from the ECF, water will pass between the ECF and intracellular fluid until solute concentration or osmolarity is equalized between the compartments
• Unlike with sodium, the concentration of most other ions in the ECF can be altered without significant change in the total number of osmotically active particles, producing only a compositional change
• For instance, doubling the serum potassium concentration will profoundly alter myocardial function without significantly altering volume or concentration of the fluid spaces

Disturbances in Fluid Balance

• Extracellular volume deficit is the most common fluid disorder in surgical patients and can be either acute or chronic
• Acute volume deficit is associated with cardiovascular and central nervous system signs, whereas chronic deficits display tissue signs, such as a decrease in skin turgor and sunken eyes, in addition to cardiovascular and central nervous system signs
• Laboratory investigation may reveal an elevated blood urea nitrogen level if the deficit is severe enough to reduce glomerular filtration and hemoconcentration

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• Urine osmolality usually will be higher than serum osmolality, and urine sodium will be low, typically <20 mEq/L
• Serum sodium concentration does not necessarily reflect volume status and therefore may be high, normal, or low when a volume deficit is present
• The most common cause of volume deficit in surgical patients is a loss of GI fluids from nasogastric suction, vomiting, diarrhea, or enterocutaneous fistula
• In addition, sequestration secondary to soft tissue injuries, burns, and intra-abdominal processes such as peritonitis, obstruction, or prolonged surgery can also lead to massive volume deficits

Signs and Symptoms of Volume Disturbances

Composition of GI Secretions

• Extracellular volume excess may be iatrogenic or secondary to renal dysfunction, congestive heart failure, or cirrhosis
• Both plasma and interstitial volumes usually are increased. Symptoms are primarily pulmonary and cardiovascular
• In fit patients, edema and hyperdynamic circulation are common and well tolerated. However, the elderly and patients with cardiac disease may quickly develop congestive heart failure and pulmonary edema in response to only a moderate volume excess

Volume Control

• Volume changes are sensed by both osmoreceptors and baroreceptors
• Osmoreceptors are specialized sensors that detect even small changes in fluid osmolality and drive changes in thirst and diuresis through the kidneys
• Baroreceptors also modulate volume in response to changes in pressure and circulating volume through specialized pressure sensors located in the aortic arch and carotid sinuses.

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Electrolyte Concentration Changes

HYPONATREMIA

• A low serum sodium level occurs when there is an excess of extracellular water relative to sodium.
• In most cases of hyponatremia, sodium concentration is decreased as a consequence of either sodium depletion or dilution.
• Dilutional hyponatremia frequently results from excess extracellular water and therefore is associated with a high extracellular volume status

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• Excessive oral water intake or iatrogenic IV excess free water administration can cause hyponatremia
• Postoperative patients are particularly prone to increased secretion of antidiuretic hormone (ADH), which increases reabsorption of free water from the kidneys with subsequent volume expansion and hyponatremia
• Causes include decreased sodium intake, such as consumption of a low sodium diet or use of enteral feeds, which are typically low in sodium ; GI losses from vomiting, prolonged nasogastric suctioning, or diarrhea; and renal losses due to diuretic use or primary renal disease

HYPERNATREMIA

• Hypernatremia results from either a loss of free water or a gain of sodium in excess of water
• Hypervolemic hypernatremia usually is caused either by iatrogenic administration of sodium-containing fluids, including sodium bicarbonate, or mineralo corticoid excess as seen in hyperaldosteronism, Cushing's syndrome, and congenital adrenal hyperplasia
• Normovolemic hypernatremia can result from renal causes, including diabetes insipidus, diuretic use, and renal disease, or from nonrenal water loss from the GI tract or skin, although the same conditions can result in hypovolemic hypernatremia

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Requirements in The Tropics�

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• 3L of water
• 1molecule of water holds 6-8 molecules of water
• Sodium 130mmol/day
• Potassium 50 – 60 mmol/d
• Chloride ion 100mmol/d
• Energy 100g/day
• Vitamin C
• Vitamin B complex

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• 5% dextrose → 100mls →5g glucose, 1L → 50g glucose
• So minimum of 2L of 5% dextrose needed. This must be given to spared the glycogen, protein and fat stores from gluconeogenesis

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Normal values  

• Extracellular (plasma)
• Na 135 -140mmol/l
• K 3.5- 5.5mmol/l
• HCO₃^- 21-31mmol/l
• Ca 2.1 -2.5mmol/l
• Urea 2.5-6mmol/l

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Intracellular

• K 140mmol/l
• Na 4-5mmol/l
• Mg 9mmol/l
• PO₄^2- 20mmol/l
• Proteins 9mmol/l

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Common fluids used in surgical practice�

• Water not used alone (hypoosmolar)
• Normal saline 0.9%; Na=154mmol/l, Cl= 154mmol/l
• Ringer’s lactate solution 1L contains, Na (130mmol), Ca (4mmol), K (4mmol), Cl (111mmol), HCO₃^- (27mmol). DO NOT USE in metabolic alkalosis (GOO)
• Darrow’s solution contains Na (124mmol), K (36mmol), Cl (104mmol), HCO₃^- (56mmol)
• Child of 8-12yrs needs half what an adult needs (1.5L/24hrs →15dpm)
• Adult →3L/24hrs →30dpm

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