Fluid and electrolyte balance Flashcards
summarise water balance
intake = 2000ml
turnover = 9000ml
excretion = 100ml
What are the sites of fluid loss?
Fluid loss is not just restricted to the GI tract (colon)
the kidney is also a site of fluid loss. Insensible loss
occurs through the skin
How is the movement of water driven?
Through movement of electrolytes
What is the difference between ion channels and transporters?
Ion channels are the fast means of exchanging electrolytes
Transporters are slower
o These are carriers for drugs
o Drugs target particular transporters
How does sodium enter the intestinal cell?
sodium co-transporters transporting amino acids, peptides, bile salts and vitamins in, with sodium.
How is sodium pumped out of the intestinal cell?
(active transport)
Sodium is pumped out of the cell and potassium enters
o A gradient forms as there is less sodium in the cell
o Transporters then use this gradient to transport sodium back into the cell via passive diffusion
o Active transport takes sodium out
How is water absorbed in the intestines?
NB: Water follows the ions that are entering the cell via passive diffusion
How does glucose influence sodium transport?
When glucose is available, then the transport of sodium can happen more efficiently
How does sodium drive nutrient absorption?
The sodium gradient (generated by active removal of sodium from cells) provides energy for the active transport of many minerals, vitamins and metabolits
How is ORS used to treat cholera?
patients with cholera did
not absorb sodium chloride when salt was provided orally.
However, if glucose was also given, it was fully absorbed and it enhanced sodium absorption.
Cholera causes huge loss of water and severe dehydration
ORS contain sodium, potassium and glucose
o When these were introduced, deaths from cholera dropped by millions
What is the Cystic fibrosis transmembrane conductance regulator
(CFTR) is a membrane protein and chloridechannelin
vertebrates that is encoded by theCFTRgene.
The CFTR channel is present in the intestines, airways and pancreatic duct
The CFTR gene is not part of cyclic AMP system and therefore can be affected by different substances that
affect the cAMP pathway
CFTR functions as an ATP-gated anion channel, increasing the conductance for certain anions (e.g. Cl−) to flow down their electrochemical gradient.
How does the Cystic fibrosis transmembrane conductance regulator usually work?
Normally, the protein moves chloride and thiocyanate ions (with a negative charge) out of an epithelial cell to
the covering mucus.
Positively charged sodium ions follow passively, increasing the total electrolyte concentration in the mucus,
resulting in the movement of water out of the cell via osmosis.
In the intestines and pancreatic ducts, this forms a layer of water close to the cells, which moistens the
epithelium and enables the pancreatic enzymes to function
What happens when there is a mutation in the Cystic fibrosis transmembrane conductance regulator?
A mutation in CFTR = CFTR not working properly (i.e. in cystic fibrosis) which causes problems with the
intestines and airways
What is Secretory diarrhoea?
High volume of fluid loss due to decreased absorption and increased secretion
What can cause Secretory diarrhoea?
o Acute infections o Failure of the bile salt absorption o Malabsorption of fat o Laxative abuse o Carcinoid syndrome o Zollinger-Ellison syndrome o Secreted Na+, Cl- K+ HCO3- o Endocrine is mostly secretory diarrhoea
What is Osmotic (malabsorptive) diarrhoea
Osmotically active substances accumulate in the lumen
There is a decrease intestinal absorption leading to a high volume of fluid loss
What causes Osmotic (malabsorptive) diarrhoea?
o Laxatives o Antacids o Acarbose (alpha-glucosidase inhibitor) o Orlistat (lipase inhibitor) o Digestive enzyme deficiencies (lactase) in the intestine and pancreas o Pancreatic insufficiency o Inflammatory disease o Short bowel syndrome
What is Inflammatory diarrhoea
Increased secretion and propulsive activity of the bowel leads to a low volume of fluid loss
This type of diarrhoea lasts longer than others
What can cause Inflammatory diarrhoea
o Inflammatory bowel disease o Crohn’s disease o Ulcerative colitis o Infectious disease > Shigella > Salmonella o Irritable colon
What are the types of diarrhoea found in children?
- acute watery
- bloody
- persistant
What is acute watery diarrhoea?
diarrhoea causing dehydration
o V. cholerae, E. coli, Rotavirus
What is bloody diarrhoea?
diarrhoea (dysentery) leading to intestinal damage and nutrient loss
o Shigella
What is persistant diarrhoea?
diarrhoea (>14 days) particularly in undernourished children or children with concominant diseases
o Campylobacter, Salmonella and Cryptosporidium protozoa (In HIV patients) are all causes of child-
death worldwide due to diarrhoea
How is diarrhoea treated in children?
Fluid replacement to prevent dehydration (ORS)
Zinc supplements (decrease the severity and duration)
Continue feeding
Use appropriate fluids available at home and increased fluids in general
Why is zinc supplementation important in treating children with diarrhoea
vital micronutrient essential for protein synthesis, cell growth and differentiation, immune function,
and intestinal transport of water and electrolytes
What is the public health implication of diarrhoea?
88% of worldwide deaths from diarrhoea are attributable to unsafe water, poor sanitation and poor hygiene.
What preventative measures are in place to lessen the public health burden of diarrhoea?
- Rotavirus and measles vaccinations
- Early breastfeeding and vitamin A supplementation
- Handwashing with soap
- Improved water quality
- Community-wide sanitation promotion
What key steps should be taken in managing diarrhoea?
Assess the patients clinical state. Are they severely ill or is it a mild issue? (Unconscious/too severely dehydrated, people cannot take fluids orally)
Fluid balance chart - assess what the patient took in and what came out
Check for and be aware of fluid shifts
Modify treatment to the condition of patient and renal function
What factors influence the speed of fluid replacement?
- age
- renal function
- cardiovascular status
- severity of existing dehydration
- time it took to develop
Describe the influence low osmolality has on water movement
If there is too much salt in the gut/circulation, the water will come out the cell, causing cell dehydration.
This means the cell will shrink
Describe the influence high osmolality has on water movement
Fluid enters the cell and the cell expands
Brain expanding cells is not good
o Hyponatraemia leads to brain oedema
What influences osmolality?
Less sodium, less chloride - less osmolality
What is the difference is osmolality in the gut?
Gut has lower osmolality than the enterocyte, so more water will enter the cell and cause cells to expand
o This is good in diarrhoea
What effect does reduced osmolality ORS have?
- decreases stool output and dehydration by 20-30% compared to the original formula
= decreases need for TV therapy
What are the different types of intravenous solutions?
- colloids
- crystalloids
What are colloids?
large molecular weight, e.g:
- albumin
- hydroxyethyl starch (HES)
- Haemaccel
What is the effect of colloids?
should theoretically preferentially increase the intravascular volume
What are crystalloids
water plus elctrolytes, e.g:
- saline
- dextrose
- ringer’s-lactate (Hartmanns)
What is the effect of crystaloids
should theoretically increase the intravascular volume, interstitial volume and intracellular volume.
What is isosmotic?
having the same osmotic pressure
An iso-osmotic solution is the preferred solution given, as you do not want to cause too much of a shift
Which fluids are isosmotic?
- saline 0.9%
- dextrose 5%
- sodium bicarbonate 1.96%
Which fluids are hypoosmostic
- saline 0.45%
Which fluids are hyperosmotic
sodium bircarbonate 8.4% (extremely hyperosmotic)
When should hyperosmotic fluids be given?
Acidosis Emergency (caridac arrest) Use sodium bicarbonate 8.4%
When should colloids be used?
used in acute circulatory shock when wanting to restore the volume as Colloids are given IV and they stay in the IV bed
What is the benefit of saline as a universal hydration solution?
Only 25% will stay in the vascular bed
Used as a universal rehydration solution (middle of the road)
o Also rehydrates tissues and stays in vascular bed
What is the effect of dextrose on hydration?
o Very quickly goes into cells
o Glucose and dextrose are similar
o Glucose will be quickly metabolised by the cells and therefore water will follow and enter the cells
What is the post-operative fluid regimen of a normal patient
involves saline and dextrose 2:1
Why do some crystalloids contain lactate?
substantial amount of bicarbonate is produced during lactate metabolism
This is good to use in those with chronic acidosis i.e. those who have lost bicarbonate.
How is rate of fluid replacement measured?
Based on time of infusion of a standard 500mL bag
What is the use of 2-hourly infusion and how much fluid would be infused over 24 hours
emergency rehydration
6 litres
What is the use of 4-hourly infusion and how much fluid would be infused over 24 hours
3 litres
What is the use of 6-hourly infusion and how much fluid would be infused over 24 hours
standard regimen
2 litres
What is the use of 8-hourly infusion and how much fluid would be infused over 24 hours
slow rehydration
1.5 litres
Why does potassium need to be tightly regulated?
It is extremely dangerous when potassium levels are too high/too low
o Low is around 2.5-3mmol/L
o High is around 6mmol/L
What is the maximum concentration of potassium for peripheral administration?
40mmol/L
What is the maximum rate of potassium administration?
10mmol/h:
faster only if cardiac monitoring/ central line are available (up to 20 mmol/h)
Describe the cardiac monitoring required during potassium supplementation
Baseline ECG required if K <3 mmol/l
Cardiac monitoring if K <2.5 mmol/L
Or if K is given faster than 10 mmol/h
