Water/Electrolyte Balance and Diarrhoea Flashcards
In a healthy person how much water is absorbed through the GI tract every day?
9L
In a healthy person how much water is expelled in faeces every day?
100-200ml
What is the approximate value of daily insensible loss?
800ml
2L of the 9L of fluid that are absorbed every day by the GI tract are from dietary intake. Where does the remaining 7L come from?
Salivary, gastric, pancreatic, biliary and intestinal secretions
How does increase fluid lead to a self perpetuating cycle that presents as diarrhoea?
Increased fluid causes more rapid propulsion of the intestine which decreases the amount of absorption which can take place and leads to further fluid accumulation. This is a self perpetuating cycle which results in diarrhoea
Give example of drugs and conditions which can lead to osmotic diarrhoea?
Laxatives Antacids Acarbkse Orlistat Digestive enzyme deficiencies Pancreatic insufficiency Inflammatory disease Short bowel syndrome
What is osmotic diarrhoea?
Diarrhoea which results from either the accumulation of non absorbable solutes or a failure to digest or absorb nutrients
What molecules are absorbed in the large intestine?
Water
Electrolytes
Short chain fatty acids
Which transporter protein is responsible for the cotransport of sodium and glucose?
SGLT1
Once sodium and glucose have been absorbed into the enterocyte, how do they e it the enterocyte?
Glucose moves out via facilitated diffusion with a GLUT2 transporter in the basolateral membrane
Sodium is actively pumped out of the cell at the basolateral membrane via the sodium potassium pump
Why are low osmolality oral rehydration salts preferable than isoosmotc ORS in the treatment of diarrhoea?
These further reduce the need for IV fluid use as the low osmolality prevents any unwanted secretion of water into the intestinal lumen
List some of the causes of secretory diarrhoea?
Infections such as rotavirus, e.coli, shigella, campylobacter, salmonella and v cholerae
Carcinoid syndrome
Zollinger ellison syndrome
How can inflammation cause diarrhoea?
This can damage the epithelium and impair its ability to absorb water and electrolytes
Infection with which bacterium commonly causes bloody diarrhoea?
Shigella
Describe how diarrhoeal disease might be prevented
Vaccinations against organisms such as rotavirus and measles which can cause diarrhoea
Good hand washing
Improved sanitation
What are the differences between colloid and crystalloid IV fluids?
Colloids are large molecular weight molecules which act to increase blood volume
Crystalloids contain water and electrolytes and restore water homeostasis in tissues - these include saline and dextrose solutions
What percentage of saline solution will remain intravascular after infusion?
25%
What percentage of dextrose solution will remain intravascular after infusion?
10%
Describe the situations in which you would choose to use IV saline or IV dextrose solutions to treat diarrhoea
Saline is used when the patient has lost both water and electrolytes
Dextrose is used when the patient has only lost water
0.9% saline is most commonly used, why is this?
Because this is closest to the physiological level of sodium
Which IV fluid may be used to treat metabolic acidosis and why?
Ringer lactate solution as the lactate in this solution is readily metabolised and will produce bicarbonate ions
What ratio of saline:dextrose is most commonly given as a postoperative regimen?
2:1
What patient factors will influence the rate of fluid replacement?
Age Cardiovascular status Renal function Severity of dehydration Length of time it took to become rehydrated
What volume of fluid is contained in a standard IV fluid bag?
500ml
Why prescribing IV fluids, how is the dose commonly written?
As time of infusion of a standard 500ml bag e.g. 4hr bag
Why is it important to check the patients cardiovascular status before giving IV fluids?
Because if a patient has a weak CV system them given too much fluid too quickly will overload the heart leading to heart failure and pulmonary oedema
Potassium is often supplemented in IV fluids. What is the maximum rate of infusion of potassium and the maximum concentration per bag?
10mmol/h
20mmol
In diarrhoea it is common to lose electrolytes. Why is hypokalaemia so dangerous?
It can lead to arrythmias
What factor determines whether fluid and electrolytes will be absorbed paracellularly or cellularly?
The permeability of tight junctions in the intestinal epithelium
What water and electrolyte transporters are contained in the apical membrane of epithelial cells of the jejunum?
Sodium glucose cotransporters
Sodium amino acid cotransporters
Sodium hydrogen antiporters
In the jejunum, hydrogen is secreted into the lumen of the intestine in order to absorb sodium. Where does this hydrogen come from?
The breakdown of carbon dioxide and water to hydrogen ions and bicarbonate ions by carbonic anhydrase
When should handwashing be carried out when dealing with patients with diarrhoea, disease?
After handling the patient, their bedding or clothing or sick room equipment and before preparing or serving food
What is diarrhoea defined as?
Frequency if stool passage of more than three times a day with decreased stool consistency and a stool weight of greater than 250g per day
What is dysmotility diarrhoea?
Diarrhoea resulting fro abnormal gut motility where decreased transit times allow insufficient time for fluid absorption
Use of which antibiotics is particularly associated with c.difficile infection?
Clindamycin
Celhalosporins
Ampicillin
Amoxicillin
Describe how toxins from vibrio cholerae can result in secretory diarrhoea?
Chloride ions enter enterocytes from the bloodstream via cotransport with sodium or potassium and cause an increase in intracellular cAMP activity by activating adenylyl cyclase. This activates cystic fibrosis transmembrane conductance regulator (CFTR) to pump out the chloride ions into the lumen of the small intestine. This secretion of negative ions into the lumen draws sodium ions into the lumen through tight junctions between enterocytes. This leads to the movement of water into the intestinal lumen by osmosis. This is the normal physiology. When infected with vibrio cholerae, toxins from the bacteria cause an inappropriate increase in intracellular cAMP, which activates CFTR to secrete chloride ions and therefore water into the lumen.
What is the result of the loss of bicarbonate ions due to diarrhoea?
Metabolic acidosis
What is the result of the loss of potassium ions due to diarrhoea?
Arrythmias
Which serum blood test is often used to assess diarrhoea?
Urea and electrolytes test
An increase in urea and creatinine serum concentration indicates clinical dehydration. Why do these molecules increase in concentration in patients with diarrhoea?
Urea is the waste product of protein metabolism and creatinine is a muscle waste product - these are both excreted in urine by the kidneys. Diarrhoea causes a large loss of fluid leading to decreased blood volume and this causes slower movement of blood through the kidneys. This leads to a decreased glomerular filtration rate which allows for increased reabsorption of urea and creatinine and therefore explains the rise in their serum concentrations
When prescribing IV fluid how is the fluid required calculated?
Fluid required = sum of previous days measured losses + insensible loss + previous days deficit (previous days measured losses + insensible loss - previous days intake)
What fluids can be measured to calculate the measured losses for the day in a patient with diarrhoea?
Urine output
Facal output
Vomit
Fluid from surgical drains
All strains of E.coli are pathogenic. T/F?
False - some E.coli strains exist as commensal organisms
Cryptosporidium and giardia are protozoa which can cause diarrhoea and are endemic in Scotland. T/F?
True
What age group is particularly affected by norovirus?
Elderly people
What host factors can determine the likelihood of enteric infection?
Age Genotype Personal hygiene Gastric acidity Intestinal motility Enteric microflora Immunity Intestinal receptors
What microbial factors can determine the likelihood of enteric infection?
Toxins
Attachment
Invasiveness
Infectious doses
Which four antibiotics can particularly affect the bowel flora to cause c.difficle infection?
clindomycin
coamoxyclav
cephalosporins
ciprofloxacin
What is tenesmus?
The feeling of incomplete emptying after defecation
Why is a follow up stool sample required when a patient has been infected with salmonella typhi?
Because they could still be carrying the bacteria in the dormant state and this has implications on their occupation if, for example, the work handling food or in healthcare
What is meant by growing an organism on selective culture media?
The organism is grown on an agar plate that is designed to only be able to grow one specific bacterium
What is meant by growing an organism on a differential culture media?
The organism is grown on an agar plate which contains specific chemicals which cause specific bacterial strains to exhibit a specific colour change reaction in order to identify the organism
What common test is used to look for specific viruses in a stool sample?
PCR
Which part of the GI tract absorbs the most water?
Small intestine
Which part of the GI tract has the most net absorption of water?
Large intestine
In the colon, sodium can be absorbed through a sodium channel. Which hormone stimulates this?
Aldosterone
What is the name for the combination of crypt cell secretion and villous cells absorption that occurs in the small intestine?
Fluid circuit hypothesis
