THIRD YEAR TBL
NUTRITION MODULE/ MEDICINE
Clinical Assessment and Management of
Dehydration, Calculation of Fluid
Replacement and Types of Intravenous Fluids
2018-2019
Learning objectives
At the end of this TBL; students will be able to:
1- Define the terms osmolality, oncotic pressure and osmosis
and be able to use these terms to explain the movement of
water between the different fluid compartments.
2- Describe water and sodium homeostasis.
3- Explain dehydration.
4- Analyze the clinical presentation of water balance disorders.
5- Recognize replacement therapy to treat dehydration
and describe types of i.v. fluids.
CASE SCENARIO
A 70- year old male patient was brought to the emergency
department with disturbed consciousness and his family gave
history of diarrhea and vomiting for the last 5 days, with anorexia
Delirium-hallucination NOT dementia. (Me).
and fever.
Food poisoning. (Doctor).
O/E; he was drowsy with sunken eye balls, dry tongue, cold
extremities and loss of skin turgor.
• PR 110 beat/min of small volume
• BP 90/50 mmHg
• RR 20 Cycle/min
• Temp 38o C
• Chest, heart and abdominal examination were within normal
TASKS
1. Describe the distribution of water and sodium in the
main fluid compartments.
2. Describe the physiology of renal sodium and water
handling.
3. Define dehydration.
4. How do you clinically assess a patient with dehydration?
5. Mention different types of IV fluids and how can you
calculate fluid loss.
Water and electrolyte distribution
• In an adult male, total body water (TBW) is approximately 60% of body weight
(somewhat more for infants and less for women).
• For an average individual:
40 L TBW - 25 L is in intracellular fluid or ICF.
Infants (70%), women (50%, due to fat) and
elderly (50%, due to fat). (MedMastery).
- 15 L is in the extracellular fluid (ECF) compartment:
12 L is in the interstitial fluid, which is within the tissues.
3 L is in the plasma compartment.
• In ICF The dominant
cation is potassium
anions is Phosphates and negatively charged proteins
• In ECF- The dominant cation is sodium
anion is chloride and bicarbonate.
[ONLY PLASMA CONTAINS SIGNIFICANT CONCENTRATIONS OF PROTEIN]
What do we mean by significant? (Me).
Normal distribution of body water and electrolytes
• The major force maintaining the difference in cation concentration
between the ICF and ECF is the sodium– potassium pump
(Na,K-activated ATPase), which is present in all cell membranes.
• Maintenance of the cation gradients across cell membranes is
essential for the excitability of conducting tissues such as nerve and
muscle.
GOLD!
• The difference in protein content between the plasma and the
interstitial fluid compartment is maintained by the impermeability
of the capillary wall to protein. This protein concentration gradient
(the colloid osmotic, or oncotic, pressure of the plasma) contributes
to the balance of forces across the capillary wall that favors fluid
retention within the plasma compartment.
Why can’t we just use moles/L?
Osmoles = only osmotically active molecules
Moles = all molecules. (Physiology lecture).
• Osmolality:
The number of osmoles of solute in a kilogram of solvent
(mOsm/kg)
• Osmolarity:
The number of osmoles of solute in a liter of solvent. (mOsm/L). (Me).
mg/dL -> mOsm/L (divide by molar weight). (Me).
Calculation of Osmolarity:
BUN x 2 = urea. (Me).
2 x [Na] + [glucose / 18] + [BUN / 2.8]
Normal osmolarity is 280 -300 mOsm/L
• Osmol is one mole of any dissociable substances. ?
• Osmosis is the movement of a solvent (such as water) through a
semipermeable membrane (as of a living cell) into a solution of
higher solute concentration that tends to equalize the
concentrations of solute on the two sides of the membrane.
Functional anatomy and physiology of renal sodium and water handling
- Total body sodium is a principal determinant of ECF volume and
regulation of Na excretion by the kidney is crucially important in
maintaining normal ECF volume, and plasma volume.
- The functional unit for renal excretion is the nephron.
- Blood undergoes ultrafiltration in the glomerulus, generating a
fluid which resembles plasma in its electrolyte composition.
Just without protein. (Me). <— FALSE!;
TRUE —> contains 200 mg protein and
150 mg of protein appears in urine.
(Old Davidson NT MCQs).
- The glomerular filtration rate (GFR) is approximately 125 mL/min
(equivalent to 180 L/day) in a typical adult.
- This is then delivered into the renal tubules, where reabsorption
of water and various electrolytes occurs.
• Over 99% of this filtered fluid is reabsorbed into the blood in the
peritubular capillaries largely as a result of tubular reabsorption
of sodium.
Increased ADH -> Increased urine osmolarity
• Urine osmolality is affected by antidiuretic hormone (ADH),
which is released by the posterior pituitary gland.
• When water intake is restricted and plasma osmolality is high, or
in the presence of plasma volume depletion, ADH levels rise. This
causes water permeability of the collecting ducts to increase
through binding of ADH to the V2 receptor, which enhances
collecting duct water permeability.
Regulation of sodium transport
• Mechanisms serve to maintain whole body sodium balance and
hence ECF volume:
Superior vena cava and its draining veins. (Me).
1. Volume receptors in the cardiac atria and the intrathoracic veins.
not volume regulation!
2. Pressure receptors in the aortic arch and carotid sinus. Pressure
(Me).
apparatus
3. The afferent arterioles within the kidney. Juxtaglomerular
not afferent arteriole. (Me).
A further afferent signal is generated within the kidney itself; renin is
released from the juxtaglomerular apparatus.
Renin release is stimulated by:
- Reduced perfusion pressure in the afferent arteriole.
- Increased sympathetic nerve activity.
- Decreased NaCl concentration in the distal tubular fluid.
-Renin released into the circulation activates the
effector mechanisms for sodium retention, which are
components of the renin–angiotensin–aldosterone
(RAA) system.
Then molecule, substance on
which the enzyme acts. (Me).
-Renin acts on the peptide substrate,
angiotensinogen (manufactured in the liver),
producing angiotensin I in the circulation.
-This in turn is cleaved by angiotensin-converting
enzyme (ACE) into angiotensin II, largely in the
pulmonary capillary bed.
-Angiotensin II has multiple actions: it stimulates
proximal tubular sodium reabsorption and release of
aldosterone from the zona glomerulosa of the
adrenal gland. The superficial part of adrenal cortex. (Me).
Contraction alkalosis
DISORDERS OF WATER BALANCE
Wrong! Stool is alone, not with sweat
• Daily water intake: 500 mL to several liters a day.
and breath. (Surgery lectures).
• Insensible losses is the amount of water lost through the stool,
This is very important as alcohols
sweat and the respiratory tract: 800 mL/day.
(including methanol and ethynyl glycol)
• Water that is generated by oxidative metabolism and carbohydrates (teas and toast
syndrome) are metabolized to water
and CO2 -> dilution effect on plasma.
(‘metabolic water’): 400 mL/day.
(Me).
• The kidneys are chiefly responsible for adjusting water excretion
to maintain constancy of body water content and body fluid
osmolality (reference range 280–295 mmol/kg).
Osmolality. (Me).
Basic daily water, electrolyte and calories requirements
Minimum 500 mL.
(Me).
Les than 2g a day.
(Me).
Calories: 50-100g glucose in 24 hours to prevent starvation
ketosis.
Presenting problems in disorders of water balance
• Disturbances in body water balance, in the absence
of changes in sodium balance, alter plasma Na
concentration and hence plasma osmolality.
• When extracellular osmolality changes abruptly,
water flows rapidly across cell membranes with
resultant cell swelling (during hypo-osmolality) or
shrinkage (during hyper- osmolality).
• Cerebral function is very sensitive to such volume
changes, particularly brain swelling during hypoosmolality, which can lead to an increase in
intracerebral pressure and reduced cerebral
perfusion.
Dehydration
Just water
• Dehydration can be defined as an excessive loss of body water
when the loss of water and salts is more than is replacement.
• Dehydration can lead to any of the following problems:
Muscle cramps, Headache, Diarrhea, Fever, Vomiting,
low BP, tachypnea, pallor, loss of
Hallucinations, and Death. Tachycardia,
skin turgor, low urine output, visual aura. (Me).
IN FACT, DEHYDRATION IS THE LEADING CAUSE IN DEATHS OF
INFANTS
Is This Patient Dehydrated?
• The best measure of dehydration is the percentage loss of body weight.
• Classification of patients with dehydration into subgroups with:
➢
No or minimal dehydration
➢
Mild – Less than 5% weight loss
How do I know his/her
original body weight? (Me).
➢
Moderate – 5-10% weight loss
➢
Severe – 10-15% weight loss
Severe dehydration requires immediate medical attention and can lead
to death
GUIDANCE FOR INTRAVENOUSFLUID AND
ELECTROLYTE PRESCRIPTION IN ADULTS
• Fluid prescriptions are very important.
• Prescribing the wrong type or amount of fluid can do serious harm.
• Assessment of fluid requirements needs care and attention, with
adjustment for the individual patient. One way is through ultrasonography. (MedMastery).
• This is as important as safe drug prescribing – fluids are drugs.
• Try to prescribe fluids during daytime ward rounds for patients you
know rather than leaving it to the night teams.
• However, complex patients need review of fluid requirements more
than once a day.
1500 + [(Body Weight - 20) x 20] = Amount of fluid given in a day.
Then divide by 24 to get the amount of fluid given in an hour. (?).
Maintenance requirement: 30ml/kg/24hrs of water
• It is vital that sick patients receive
THE RIGHT AMOUNT OF THE RIGHT FLUID AT THE RIGHT
TIME.
• Questions to ask before prescribing fluid:
1. Is the patient euvolemic, hypovolemic or hypervolemic?
2. Does the patient need IV fluid? Why?
3. How much?
4. What type(s) of fluid does my patient need?
1. Assess the patient
• Euvolemic: veins are well filled, extremities are warm, blood
pressure and heart rate are normal.
• Hypovolemic: Patient may have cool peripheries, respiratory
rate>20, systolic BP<100mmHg, HR>90bpm, postural
hypotension, oliguria and confusion. History of fluid loss or low
intake.
• Hypervolemic: Patient is edematous, may have inspiratory
crackles, high JVP and history/charts showing fluid overload.
Clinical features of hypovolemia and hypervolemia
15-20 mmHg. (Doctor).
Low BP. (Me).
Pallor.
(Me).
Tachypnea. (Me).
= pulmonary edema.
(Me).
Causes of sodium and water depletion
2. Does my patient need IV fluid?
• NO:
• He may be - drinking adequately
- receiving adequate fluid via NG feed or TPN
- receiving large volumes with drugs or drug infusions
- Hypervolemic: need fluid restriction or gentle diuresis
• YES:
• not drinking, has lost, or is losing fluid
• ALLOW PATIENTS TO EAT AND DRINK IF POSSIBLE.
So WHY does the patient need IV fluid?
Maintenance fluid only:
• Patient does not have excess losses above insensible loss/urine.
• If no other intake; he needs approximately 30ml/kg/24hrs.
• He may only need part of this if receiving other fluid.
• Patients having to fast for over 8 hours should be started on IV maintenance fluid.
Replacement of losses:
If losses are predicted it is best to replace these later rather than give extra fluid in
anticipation of losses which may not occur. This fluid is in addition to maintenance
fluid.
Resuscitation:
• The patient is hypovolemic and requires urgent correction of intravascular
depletion to correct the deficit as a result of:
-Dehydration
-Blood loss
-Sepsis Shock
3. How much fluid does my patient need?
• Obtain weight (estimate if required). Maintenance fluid requirement
approximately 30ml/kg/24hours.
• For the frail elderly, patients with renal impairment or cardiac failure consider
giving less fluid: 20-25ml/kg/day. Dilutional anemia? (Me).
• Review recent urea and electrolytes and Hb.
• Recent history – e.g. fasting, input/output, sepsis, operations, fluid overload.
• Check fluid balance charts.
• Calculate how much loss has to be replaced and work out which type of fluid
has been lost: e.g. gastro-intestinal (GI) secretions, blood, inflammatory losses.
• Average vomit equal or greater than 200ml
• Average diarrhea equal or greater than 300ml
• Urine does not need to be replaced unless excessive.
• Post-op; high urine output may be due to excess fluid and low urine output is
common and may be normal due to ADH release.
• Assess fully before giving extra fluid.
4. What type of fluid does the patient need?
MAINTENANCE FLUID
• IV fluid should be given via volumetric pump if a patient is on fluids for over 6
hours or if the fluid contains potassium.
• Always prescribe as ml/hour.
• Never give maintenance fluids at more than 100ml/hour.
Preferred maintenance fluids:
• 0.18% NaCl/ 4% glucose with or without added potassium (20 mmol) in 1L.
• This fluid if given at the correct rate provides all water and Na+/K+
requirements until the patient can eat and drink or be fed.
• Excess volumes of this fluid (or any fluid) may cause hyponatremia..
Intravenous fluid therapy in hypotensive patient
• If fluid containing neither sodium nor protein is given, it will
distribute in the body fluid compartments in proportion to the
normal distribution of total body water.
8%
• Thus, giving 1 L of 5% dextrose will contribute relatively little
(approximately 3/40 of the infused volume) towards expansion
of the plasma volume. This makes 5% dextrose ineffective at
restoring the circulation and perfusion of vital organs.
• Intravenous infusion of an isotonic (normal) saline solution
results in more effective expansion of the extracellular fluid,
although a minority of the infused volume (some 3/15) will
contribute to plasma volume.
20%
Basic Electrolyte levels
• Sodium (Na+)
 Range 135- 145 mEq/L in serum
 Total body volume estimated at 40 mEq/kg
• Potassium (K+)
 Range 3.5 - 5.0 mEq/L in serum
 Total body volume estimated at 50 mEq/kg
REPLACEMENT FLUID
❖Fluid losses may be due to:
• Diarrhea, vomiting, fistulae, drain output, bile leaks.
• Blood loss, excess sweating or excess urine.
• Inflammatory losses (‘redistribution’) in the tissues are hard to
quantify and are common in pancreatitis, sepsis, burns and
abdominal emergencies.
❖It is vital to replace GI loss, otherwise patient may develop
severe metabolic derangement with acidosis or alkalosis and
hypokalemia.
❖Check blood gases in these patients and request chloride with
Urea& electrolytes.
❖Hyponatremia is common: in the absence of large GI losses,
causes are too much fluid, SIADH or chronic diuretic use.
Small cell lung carcinoma
Potassium maintenance and replacement:
• A normal potassium level does not mean that there is no total
body potassium deficit.
• Give potassium in maintenance fluid.
Composition of some isotonic intravenous fluids
In 1 liter
Converted to
bicarbonate in the liver
RESUSCITATION FLUID
• For severe dehydration, sepsis or hemorrhage leading to
hypovolemia and hypotension.
• Give Albumin only in severe sepsis. Lactated Rinegr? (Me).
• For severe blood loss initially use colloid until blood/clotting
factors arrive.
Falling rapidly and in copious quantities. (Me).
• Use O Negative blood for torrential bleeding.
• Severely septic patients with circulatory collapse may need
inotropic support in a critical care area. Their blood pressure may
not respond to large volumes of fluid; excessive volumes (many
liters) may be detrimental.
Oral rehydration salts
Converted to
bicarbonate in the liver?
SUMMERY
• TBW = 60% of Body Wt.= 40 L: 25 L/ ICF, 15 L/ ECF: (12 L/ IS, 3 L/ plasma)
• Only plasma contains significant concentrations of protein.
• The volume of body fluid and concentration of electrolyte (mainly Na &
K) at different compartments are regulated by the nephrons.
• Dehydrated patient should be assessed for the:
- severity of dehydration.
- the need for oral or parental replacement of fluid.
- the amount and the type fluid to be replaced.
• FLUID DESCRPTION SHOULD BE CONSIDERED AS DRUG PRESCRIPTION.
REFERENCES
• Davidson’s Principles& Practice of Medicine 22nd Edition.
• Comprehensive clinical nephrology 5th edition.
• Southampton Fluid Guidance 2009.
• NICE Intravenous Therapy in Adults in Hospital , Guideline
174 Dec 2013.