Fluid Balance and Administration Flashcards
What are the three main components of bodily fluid?
Water
Ions
Protein
What forces affect the net movement of fluid through the capillary membrane?
Osmotic pressure
Hydrostatic pressure
What percentage of humans is water?
Men - 60%
Women - 50% - due to higher body fat
Children - 75%
Definition of total body water?
Total volume of water in the body
Definition and components of extracellular fluid?
Total volume of fluid outside of cells - intravascular (plasma), interstital (in tissues), lymphatics and transcellular
Definition of intracellular fluid?
Total volume of fluid inside of cells (TBW - ECF)
Definition of plasma
Blood without cells - contains protein, water and electrolytes
Definition and examples of transcellular fluid?
The smallest component of ECF (mostly made up of interstital fluid and plasma). Fluid contained in epithelial lined spaces
- CSF
- intraocular fluid
- pleural
- synovial
- digestive secretions
What are third space losses?
Fluid ‘lost’ into the transcellular fluid spaces as it can’t be exchanged with rest of ECF
Breakdown of total body water in average 70kg man.
45L total
- 2/3 intracellular (30L)
- 1/3 extracellular (15L) - plasma 3.5L, lymph 1.5L, interstital 8.5L and transcellular 1.5L
What are the main ions in ECF?
Sodium 135 - 145mmol/l
Bicarbonate - 25mmol/l
What are the main ions in ICF?
Potassium - 150mmol/l
Magnesium - 40mmol/l
Phosphate - 100mmol/l
Protein - 8mmol/l
By what cellular pump are potassium levels maintained?
Na+/K+ ATPase pump
What are the 5 main ways that plasma potassium levels are controlled?
1) dietary intake
2) renal excretion
3) plasma pH
4) endocrine hormones
5) medications
Describe how renal excretion can affect plasma potassium levels.
Filtration at the glomerular apparatus depends on the plasma concentration
In the collecting duct, sodium reabsorption depends on exchange with potassium (controlled by aldosterone)
- aldosterone is produced in response to low BP, hyponatraemia and/or hyperkalaemia
Describe how plasma pH can affect plasma potassium levels.
Hydrogen ions move in and out of cells in exchange for potassium
- when hydrogen ion levels in plasma rise, they are exchanged with intra-cellular potassium, causing hyperkalaemia
- when hydrogen ion levels in plasma drop, intra-cellular hydrogen ions are are exchanged with extra-cellular potassium, causing hypokalaemia
Describe how endocrine hormones can affect plasma potassium levels.
e. g. insulin, adrenaline and aldosterone stimulate cellular uptake of potassium
- hyperaldosterinism causes hypokalaemia, and can be caused by cirrhosis, renal artery stenosis, heart failure and nephrotic syndrome
Describe how different medications can affect plasma potassium levels.
Hyperkalaemia - ACEI, ARBs, ciclosporin
Hypokalaemia - loop + thiazide diuretics, beta2-agonists, corticosteroids, amphotericin and theophylline
What effects does plasma sodium have on the body
Excess - water retention
Deficiency - neuromuscular dysfunction
How is sodium excreted?
Freely filtered through glomerular membrane
- around 65% freely reabsorbed in PCT
- remainder actively reabsorbed via sodium-potassium pumps in the ascending loop of Henle
ANP
- stimulated by fluid overload and decreases sodium reabsorption
ADH
- stimulated by increasing plasma osmolality and increases water reabsorption in DCT
Describe plasma protein movement within the body
Capillary beds are impermeable to proteins, so they remain within the capillary and generate osmotic pressure
Albumin 75% of this osmotic pressure
Describe the capillary bed and how this has a role in fluid movement.
This is the site of fluid and solute exchange between bloodstream and interstitum
Flow to capillary beds is controlled by pre-capillary sphincter
Capillary wall - single layer of endothelial cells surrounded by a basement membrane
- potential space between adjacent cells regulates permeability
Definition of osmotic pressure
Difference in concentration between particles in solution on either side of a semi-permeable membrane (can be proteins or ions)
- water is drawn by higher osmotic pressures
Definition of capillary osmotic pressure
Pressure generated by the plasma proteins in the capillaries
Definition of tissue osmotic pressure
Pressure generated by the interstitial fluid
- affected by interstitial protein concentration and capillary wall permeability to proteins
- the more permeable to capillary wall is to proteins, the higher the tissue osmotic pressure
Definition of hydrostatic pressure
The difference between capillary pressure and pressure of interstitial fluid within tissues
What factors affect the capillary and the tissue hydrostatic pressure?
Capillary hydrostatic pressure - blood pressure - pulse pressure Tissue hydrostatic pressure - interstitial fluid volume - tissue compliance
Describe the Starling hypothesis
Factors affecting the net flow of fluid/solutes across the capillary bed
- osmotic pressure is normally constant (25mmHg) as there are few proteins in the interstitial fluid and the capillary wall is normally impermeable
- tissue hydrostatic pressure is constant, so movement of fluid is determined by capillary hydrostatic pressure (so due to blood pressure; net flow of water out of capillary at arterial end, and net flow of water into capillary at venous end)
Starling’s equation
K = outward pressure - inward pressure
K - filtration constant for capillary membrane
OP - capillary hydrostatic pressure + tissue osmotic pressure
IP - tissue hydrostatic pressure + capillary osmotic pressure
Causes of oedema
Increased capillary hydrostatic pressure - fluid overload - venous obstruction Decreased capillary osmotic pressure - hypoproteinaemia (cirrhosis, nephrotic syndrome) Increased tissue osmotic pressure - increased capillary permeability (burns/inflammation) Decreased tissue hydrostatic pressure
Average daily water balance
Input - 2600mls - oral fluid - 1500mls - food - 750mls - metabolism - 350mls Output - 2600mls - urine- 1500mls - faeces - 100mls - lungs 400mls
Average daily electrolyte maintenance requirements (mmol/l)
Sodium - 70-140 Potassium - 70 Chloride - 70 Phosphate - 14 Calcium - 7.0 Magnesium - 7.0
How does fever affect daily water requirements
20% increase for each 1 degree of temperature increase
- 1L/24 hour extra
Average urine output.
0.5ml/kg/hr
Sources of excess fluid loss in surgical patients
Blood loss (trauma, surgery)
Plasma loss (burns)
Intra-abdominal inflammatory fluid loss (pancreatits)
Sepsis
Abnormal insensible loss (fever)
GI fluid loss (vomiting, bowel obstruction, diarrhoea, ilesotomy)
What should be taken into account when assessing fluid balance
Patient size and age Abnormal ongoing losses, pre-existing deficits/excesses, fluid shifts Renal and CV function Fluid balance charts Serum electrolytes
How does the body try to prevent fluid lost post-op?
Surgical trauma causes ADH and aldosterone release. This promotes water conservation via sodium retention and potassium excretion
How to assess fluid depletion
History - thirst, fluid loss, fluid balance chart
Examination - dry mucous membranes, decreased capillary refill time, sunken eyes, low skin elasticity, decreased urine output, increased heart rate, decreased blood pressure/pulse pressure and confusion)
Methods of fluid replacement
Enteral - preferred if GI tract functioning and there isn’t an excessive deficiency
Parenteral
- GI tract non-functioning
- needs rapid fluid replacement
Types of parenteral fluid replacement
Crystalloid (isotonic) - normal saline, 5% dextrose, Hartmann's and dextrose saline (0.18% saline and 4% dextrose) Colloid - human albumin - gelatins (haemaccel, gelofusion) Dextrans (40 or 70) Hetastarch (6% in saline) Blood
Describe an IV regimen to meet basal fluid requirements
3L fluid/24 hours - either 2 saline:1 dextrose, or 2 dextrose: 1 saline.
- plus 60mmol/l potassium
- doesn’t account for patients requiring fluid resuscitation
What to assess when correcting dehydration
Which compartment has lost fluid
- e/g/ bowel losses are from ECF
Extent of dehydration
Why should no potassium be given in the 24 hours post-op
Potassium rises during surgery due to
- cellular injury
- blood transfusions
- decreased renal potassiunm clearance due to transient renal impairment
- ‘stress hormones’ encourage potassium release from cells
