Fluid and Electrolyte
Management of the Surgical Patient
Hashmi
ANATOMY OF BODY FLUIDS
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Total Body Water
Intracellular Fluid
Extracellular Fluid
Osmotic Pressure
Total Body Water
• constitutes 50-70 % of total body weight
• fat contains little water, the lean individual
has a greater proportion of water to total
body weight than the obese person
• total body water as a percentage of total
body weight decreases steadily and
significantly with increasing age
Total Body Water
Body Water
ICF
ECF
Intravascular
Interstitial
% of Body Weight
% of Total Body Water
60
40
20
4
16
100
67
33
8
25
Intracellular Fluid
• largest proportion in the skeletal muscle
• potassium and magnesium are the
principal cations
• phosphates and proteins the principal
anions
Extracellular Fluid
• interstitial fluid: two types
– functional component (90%) - rapidly
equilibrating
– nonfunctioning components (10%) - slowly
equilibrating
• connective tissue water and transcellular water
• called a “third space” or distributional change
• sodium is the principal cation
• chloride and bicarb the principal anions
Osmotic Pressure
• physiologic and chemical activity of electrolytes depend
on three factors:
– the number of particles present per unit volume
(moles or millimoles [mmol] per liter)
– the number of electric charges per unit volume
(equivalents or milliequivalents per liter)
– the number of osmotically active particles or ions per
unit volume (osmoles or milliosmoles [mOsm] per liter)
Terminology
• mole: molecular weight of that substance in grams mole
eg: sodium chloride is 58 g (Na–23, Cl–35)
• equivalent: chemical combining activity; atomic weight
expressed in grams divided by the valence
– divalent ions (calcium or magnesium) 1 mmol equals 2
mEq
• osmole: used when the actual number of osmotically
active particles present in solution is considered
• millimole of sodium chloride, which dissociates
nearly completely into sodium and chloride,
contributes 2 mOsm
NORMAL EXCHANGE OF FLUID AND
ELECTROLYTES
• Water Exchange
• Salt Gain & Losses
Water Exchange
• daily water gains
– normal individual consumes 2000 to
2500 mL water per day
– approximately 1500 mL taken by mouth
– rest is extracted from solid food, either
from the contents of the food or as the
product of oxidation
Water Exchange
• daily water losses
– 250 mL in stools, 800 - 1500 mL in urine, and 600 mL
as insensible loss
– total losses ~ 2.2 liters
– Insensible loss: skin (75%) and lungs (25%)
• increased by hypermetabolism, hyperventilation,
and fever
• 250 mL/day per degree of fever
• unhumidified tracheostomy with hyperventilation =
insensible loss up to 1.5 L/day
Water Exchange
Minimum of 500 to 800 mL urine
per day required to excrete the
products of catabolism
Salt Gain and Losses
• daily salt intake varies 3-5 gm as NaCl
• kidneys excretes excess salt: can vary
from < 1 to > 200 mEq/day
• Volume and composition of various types
of gastrointestinal secretions
• Gastrointestinal losses usually are isotonic
or slightly hypotonic
• should replace by isotonic salt solution
CLASSIFICATION OF BODY FLUID
CHANGES
• Volume Changes
• Concentration Changes
• Composition Changes
– Acid/Base Balance
– Potassium Abnormalities
– Calcium Abnormalities
– Magnesium Abnormalities
Volume Changes
• If isotonic salt solution is added to or lost from
the body fluids, only the volume of the ECF is
changed, ICF is relatively unaffected
• If water is added to or lost from the ECF, the
conc. of osmotically active particles changes
– Water will pass into the intracellular space until
osmolarity is again equal in the two
compartments
Volume Changes
• BUN level rises with an ECF deficit of sufficient
magnitude to reduce GFR
• creatinine level may not incr. proportionally in young
people with healthy kidneys
• hematocrit increases with an ECF deficit and
decreases with ECF excess
• sodium is not reliably related to the volume status of
ECF
– a severe volume deficit may exist with a normal,
low, or high serum level
Volume Deficit
• ECF volume deficit is most common fluid
loss in surgical patients
• most common causes of ECF volume
deficit are: GI losses from vomiting,
nasogastric suction,diarrhea, and fistular
drainage
• other common causes: soft-tissue injuries
and infections, peritonitis, obstruction,
and burns
Volume Deficit
• signs and symptoms of volume deficit:
• CNS: sleepy, apathy – stupor, coma
• GI: dec food consumption – N/V
• CVS: orthostatic, tachy, collapsed
veins - hypotension
• Tissue: dec skin turgor, small tongue
– sunken eyes, atonia
Volume Excess
• Iatrogenic or Secondary to renal
insufficiency, cirrhosis, or CHF
• signs & symptoms of volume excess:
– CNS: none
– GI: edema of bowel
– CVS: elevated CVP, venous distension
– pulmonary edema
– Tissue: pitting edema – anasarca
Concentration Changes
• Na+ primarily responsible for ECF osmolarity
• Hyponatremia and hypernatremia s&s often occur if
changes are severe or occur rapidly
• The concentration of most ions within the ECF can be
altered without significant osmolality change, thus
producing only a compositional change
– Example: rise of potassium from 4 to 8 mEq/L would
significantly effect the myocardium, but not the effective
osmotic pressure of the ECF
Hyponatremia
(water intoxication)
• acute symptomatic hyponatremia (< 130)
• hypertension can occur & is probably induced by the rise
in intracranial pressure
• signs & symptoms:
– CNS: twitching, hyperactive reflexes – inc ICP,
convulsions, areflexia
– CVS: HTN/brady due to inc ICP
– Tissue: salivation, watery diarrhea
– Renal: oliguria - anuria
Hyponatremia
(water intoxication)
• Hyponatremia occurs when water is given to
replace losses of sodium-containing fluids or
when water administration consistently
exceeds water losses
• Hyperglycemia: glucose exerts an osmotic
force in the ECF and causes the transfer of
cellular water into the ECF, resulting in a
dilutional hyponatremia
Hypernatremia
(water deficit)
• The only state in which dry, sticky mucous membranes are
characteristic
• sign does not occur with pure ECF deficit alone
• signs & symptoms:
– CNS: restless, weak - delirium
– CVS: tachycardia - hypotension
– Tissue: dry/sticky muc membranes – swollen tongue
– Renal: oliguria
– Metabolic: fever – heat stroke
Composition Changes
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Acid/Base Balance
Potassium Abnormalities
Calcium Abnormalities
Magnesium Abnormalities
Acid-Base Balance
• large load of acid produced endogenously as a by-product
of body metabolism
• acids are neutralized efficiently by several buffer systems
and subsequently excreted by the lungs and kidneys
• Buffers:
– proteins and phosphates: primary role in maintaining
intracellular pH
– bicarbonate–carbonic acid system: operates principally
in ECF
Acid-Base Balance
• buffer systems consists of a weak acid or base
and the salt of that acid or base
• Henderson-Hasselbalch equation, which defines
the pH in terms of the ratio of the salt and acid:
– pH = pK + log BHCO3 / H2CO3 = 27 mEq/L /
1.33 mEq/L = 20 / 1 = 7.4
– As long as the 20:1 ratio is maintained,
regardless of the absolute values, the pH will
remain at 7.4
Acid-Base Balance
• Four types of acid-base disturbances
• combinations of respiratory and metabolic
changes may represent:
• compensation for the initial acid-base
disturbance or,
• two or more coexisting primary disorders
• 10-mmHg PaCO2 change yields a 0.08 pH
change
Respiratory Acidosis
• retention of CO2 secondary to decreased alveolar
ventilation
• management involves prompt correction of the
pulmonary defect, when feasible, and measures to
ensure adequate ventilation
• prevention: tracheobronchial hygiene during the
postoperative , humidified air, and avoiding oversedation
Respiratory Alkalosis
• PaCO2 should not be below 30 mmHg
• dangers of a severe respiratory alkalosis
are those related to potassium depletion
– hypokalemia is related to entry of potassium ions into
the cells in exchange for hydrogen and an excessive
urinary potassium loss in exchange for sodium
• shift of the oxyhemoglobin dissociation
curve to the left, which limits the ability of
hemoglobin to unload oxygen at tissues
Metabolic Acidosis
• Anion gap is a useful aid:
• normal value is 10 to 15 mEq/L
• unmeasured anions that account for the
“gap” are sulfate and phosphate plus
lactate and other organic anions
• measured ions are sodium, bicarb, and
chloride
Metabolic Acidosis
• treatment of metabolic acidosis should be
directed toward correction of the underlying
disorder
• sodium bicarbonate is discouraged, attempt to
treat underlying cause
• shifts the oxyhemoglobin dissociation curve left
• interference with O2 unloading at the tissue level
Metabolic Alkalosis
• common surgical patient has hypochloremic,
hypokalemic metabolic alkalosis resulting from
persistent vomiting or gastric suction in the
patient with pyloric obstruction
• unlike vomiting with an open pylorus, which
involves a combined loss of gastric, pancreatic,
biliary, and intestinal secretions
Pathophysiology of Paradoxic
Aciduria occurring with GOO
• GOO -> hypochloremic, hypokalemic, metabolic alkalosis
• urinary bicarb excretion to compensate for alkalosis
• volume deficit progresses  aldosterone-mediated
sodium resorption is accompanied by potassium excretion
• kidneys primary goal becomes volume preservation 
sodium resorption
• either K+ or H+ must be excreted to keep a
balanced
• due to already excessive hypokalemia, the kidney
excretes H+ in place of K+, producing paradoxic aciduria
Potassium Abnormalities
• normal daily dietary intake of K+ is approx. 50 to
100 mEq
• majority of K+ is excreted in the urine
• 98% of the potassium in the body is located in ICF
@ 150 mEq/L and it is the major cation of
intracellular water
• intracellular K+ is released into the extracellular
space in response to severe injury or surgical
stress, acidosis, and the catabolic state
Hyperkalemia
• signs & symptoms:
– CVS: peaked T waves, widened QRS
complex, and depressed ST segments
 Disappearance of T waves, heart
block, and diastolic cardiac arrest
– GI: nausea, vomiting, diarrhea
(hyperfunctional bowel)
Hypokalemia
• K+ has an important role in the regulation
of acid-base balance
• alkalosis causes increased renal K+/H+ excretion
• signs & symptoms:
– CVS: flatten T waves, depressed ST segments
– GI: paralytic ileus
– Muscular: weakness - flaccid paralysis,
diminished to absent tendon reflexes
Calcium Abnormalities
• majority of the 1000 to 1200g of calcium in the
average-sized adult is found in the bone
• Normal daily intake of calcium is 1 to 3 gm
• Most is excreted via the GI tract
• half is non-ionized and bound to proteins
• ionized portion is responsible for neuromuscular
stability
Hypocalcemia
• signs & symptoms (serum level < 8):
– numbness and tingling of the circumoral
region and the tips of the fingers and toes
– hyperactive tendon reflexes, positive
Chvostek's sign, muscle and abdominal
cramps, tetany with carpopedal spasm,
convulsions (with severe deficit), and
prolongation of the Q-T interval on the ECG
Hypocalcemia
• causes:
• acute pancreatitis, massive soft-tissue
infections (necrotizing fasciitis), acute
and chronic renal failure, pancreatic
and small-bowel fistulas, and
hypoparathyroidism
Hypercalcemia
• signs & symptoms:
– CNS: easy fatigue, weakness, stupor,
and coma
– GI: anorexia, nausea, vomiting, and
weight loss, thirst, polydipsia, and
polyuria
Hypercalcemia
• two major causes:
• hyperparathyroidism and cancer
– bone mets
– PTH-like peptide in malignancies
Magnesium Abnormalities
• total body content of magnesium 2000 mEq
• about half of which is incorporated in bone
• distribution of Mg similar to K+, the major portion
being intracellular
• normal daily dietary intake of magnesium is
approximately 240 mg
• most is excreted in the feces and the remainder
in the urine
Magnesium Deficiency
• causes:
– starvation, malabsorption syndromes, GI
losses, prolonged IV or TPN with
magnesium-free solutions
• signs & symptoms:
– similar to those of calcium deficiency
Magnesium Excess
• Symptomatic hypermagnesemia, although
rare, is most commonly seen with severe
renal insufficiency
• signs & symptoms:
– CNS: lethargy and weakness with progressive
loss of DTR’s – somnolence, coma, death
– CVS: increased P-R interval, widened QRS
complex, and elevated T waves (resemble
hyperkalemia) – cardiac arrest
Secretions
FLUID AND ELECTROLYTE THERAPY
• Preoperative Fluid Therapy
• Intraoperative Fluid Therapy
• Postoperative Fluid Therapy
Preoperative Fluid Therapy
• Correction of Volume Changes: Volume deficits result
from external loss of fluids or from an internal
redistribution of ECF into a nonfunctional compartment
– nonfunctional because it is no longer able to participate in the
normal function of the ECF and may just as well have been lost
externally
• Correction of Concentration Changes: If severe
symptomatic hypo or hypernatremia complicates the
volume loss, prompt correction of the concentration
abnormality to the extent that symptoms are relieved is
necessary
Postoperative Fluid Management
• replace losses & supply a maintenance:
– open abdomen losses: 8 cc/kg/hr
– NGT & urine output
– Blood loss x 3
– Replace with isotonic salt solution (LR or NS)
– unwise to administer potassium during the
first 24 h, until adequate urine output has
been established even a small quantity of
potassium may be detrimental because of
fluid shifts
Fluid Composition
Fluid Replacement Status
Acute Renal Failure
• Classified according to its cause:
• Prerenal: hypotension, hypovolemia, renal artery
occlusion/stenosis, cardiac failure
• Renal: trauma, toxins (contrast, endotoxin),
drugs (NSAIDS, aminoglycosides), pigment
(myoglobin, hemaglobin)
• Postrenal: ureteral obstruction, bladder dysfxn
(anesthesia, meds, nerve injury), urethral
obstruction, foley obstruction
Laboratory Studies
• Urinalysis: blood or myoglobin is a positive
diagnostic test - can test via Hemoccult
card
• Urinary lytes: urine sodium, creatinine,
urea, osmolality, and specific gravity help
classify type of renal failure using Renal
failure indices
Renal Indices
Indices
U Osm
U/P osm
U/P urea
U/P cr
Urine Na
Prerenal
> 500
>1.25
>8
> 40
< 20
Renal
< 350
<1.1
<3
< 20
> 40
Postrenal
Varies
Varies
Varies
< 20
> 40
FENa
< 1%
> 3%
> 3%
Indications for use of Dialysis in
Acute Renal Failure
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Severe acidosis
Electrolyte abnormalities
Inability to clear toxins
Volume overload
Uremic signs and symptoms
(encephalopathy, BUN > 100)