DIARRHOEAL DISEASES Flashcards
What is the result of diarrhoea on electrolytes?
Diarrhoea can result in the excessive loss of sodium, chloride, water, potassium, bicarbonate and
magnesium.
Explain why urea rises in diarrhoea
Loss of sodium chloride and water can result in dehydration and thus volume depletion.
This leads to hypotension
o NB: to determine that hypotension is due to volume depletion you must check both lying and
standing blood pressure.
o A drop >20 systolic will be observed on standing
Patient also becomes tachycardic (pulse >100(])
As a consequence of volume depletion, the glomerular filtration rate and tubular luminal flow rate are
reduced.
This reduced luminal flow rate allows increased tubular urea reabsorption.
Thus serum urea rises.
o NB: There is a proportionately bigger increase in urea than creatinine
Explain the effect of diarrhoea on creatinine
Creatinineis a break-down product ofcreatinephosphate in muscle, and is usually produced at a fairly
constant rate by the body (depending on muscle mass).
Creatinineand its clearance are the main measures of kidney function
When a significant reduction in extracellular fluid volume occurs and glomerular filtration is reduced,
serum creatinine can rise, but to a lesser extent than serum urea.
The pattern of an increase in the ratio of urea to creatinine is consistent with dehydration.
Explain the effect of diarrhoea on potassium
Loss of potassium can result in hypokalaemia.
Increased faecal loss of potassium with diarrhoea
Only 2% of total body potassium is in the extracellular compartment, yet this amount is very important as
potassium ions in plasma determine the resting membrane potential of cells, such as the muscle and
nerve cells of the heart.
Thus very high or low potassium levels can precipitate life-threatening cardiac arrhythmias such as
ventricular fibrillation and asystole.
Extracellular K + levels are therefore very tightly controlled
Explain the effect of diarrhoea on bicarbonate
Loss of bicarbonate can result in a metabolic acidosis.
Explain the effect of diarrhoea on magnesium
Loss of magnesium can cause low potassium and low calcium level
Magnesium functions as a co-factor for many biological reactions, including:
o Na + -K + -ATPase in the kidneys => Mg 2+ is required for K + reabsorption
o PTH release => affects Ca 2+ levels
What factors should be considered before prescribing/calculating fluids?
always ask the patient if they have a history of cardiac disease
- Measured losses (ML)
- Insensible losses (IL)
- Previous day’s deficit (PDD)
What are measured losses?
Measured losses include urine, vomit, diarrhoea and surgical drains, fistulae and stomas.
These volumes are recorded by the nurses on the patient’s fluid balance chart.
The previous day’s measured losses are used as a guide to estimate the predicted losses for the next day.
NB: normal urine output is ~0.5ml/kg/hour (30-35 ml/hour in the average adult)
What are insensible losses?
These include fluid loss via the skin and respiratory tract, around 800 ml daily in normal individuals.
These losses cannot be measured routinely.
Insensible losses are increased due to increased sweating from any cause and in patients being
ventilated.
Also increased in patients with:
o Burns
o Sepsis
What is the previous days deficit?
This is calculated by subtracting the previous day’s fluid intake (oral and IV) from the sum of measured
losses (total output) and insensible losses.
PDD = (previous day’s ML + IL) – previous day’s intake (oral and IV)
NB: if the patient has cardiac disease (or may have cardiac disease - elderly) the PDD does not have to be
replaced over the next 24 hours, it can be replaced over 2 to 3 days.
How are fluids calculated?
Fluid required = ML + IL + PDD
perform regular clinical examinations measuring blood pressure, pulse rate and checking for signs of cardiac failure and take into account repeat u&e results.
When is saline used?
o used when there is sodium and water depletion.
When is dextrose used?
o Used when only water replacement is required (when there are no electrolyte losses)
o Water loss without electrolyte loss may occur in fever, hyperthyroidism, high blood calcium, or
diabetes insipidus
What are the main reasons for the upper gut being almost sterile
The upper gut has very low populations of bacteria due to a range of factors including:
o gastric acidity
o Propulsive motility - prevents bacteria adhering to the epithelium
o Pancreatic enzymes
What are the main reasons for the large intestine having a luxuriant bacterial flora?
The large intestine has:
o very stagnant motility, with retropulsive contractions. This keeps the contents in the proximal
colon for long periods.
o The pH of the colon is buffered by bicarbonate secretion. This allows a large and complex
bacterial ecosystem to develop.
Most of the contents of the colon are actually bacteria.
There are approx. 10 12 cfu/g (colony-forming unit) which is 75% of the wet weight.
There are up to 400 different species in the colon and the vast majority (99.9%) are strict anaerobes.
What are the main actions of short chain fatty acids in relation to preventing diarrhoea?
acetate (2C), propionate (3C), and butyrate (4C), are absorbed readily by the colon, stimulating water and
sodium uptake at the same time
This prevents osmotic diarrhoea
What would happen to the normal gut microbiota if a patient was on large doses of broad spectrum
antibiotics?
Antibiotics may kill some commensal bacteria
Can impact colonic water reabsorption due to decreased fermentation of SCFA
o Can cause diarrhoea in the absence of infection
Increases the risk of pathogenic bacteria becoming established within the gut (e.g. C. difficile)
Probiotic:
a live microbial food supplement that beneficially affects the host animal by improving its
intestinal microbial balance
Prebiotic:
non-digestible food ingredient that beneficially affects the host by stimulating the growth
and/or activity in one or a limited number of bacteria in the colon that can improve human health
o Resistant starch, galacto-oligosaccharides, and fructo-oligosaccharides
Synbiotic:
mixture of prebiotics and probiotics that have synergistic effects
How may some probiotics influence diarrhoea?
Can reduce the duration of diarrhoea (although not by much)
This may be significant for very vulnerable patients, who cannot tolerate diarrhoeal losses
Faecal transplants are used to treat C. difficile
Define diarrhoea
3 or more loose or watery stools per day Occurs by two basic mechanisms: too much secretion or not enough absorption o Secretory- does not stop when patient fasts o Osmotic (malabsorptive)- diarrhoea stops when patient fasts
Secretory diarrhoea
Does not stop when the patient fasts Caused by: o Enterotoxin o Laxatives o Congenital defects
Osmotic (Malabsorptive) diarrhoea
Improves when patient fasts Caused by: o Lactase deficiency o Pancreatic enzyme deficiency o Loss of enterocytes o Bacterial overgrowth o Ingestion of nutrient binding substances o Lymphatic obstruction
Other causes of diarrhora
Motility disorders o Diabetes mellitus o Post-surgical Inflammatory exudation o IBD, infections (shigellosis) Diarrhoeal disease results from more fluid being presented to the colon by the small intestine than the colon can cope with.
How does Clostridium difficile cause fluid entry into small intestine?
produce enterotoxins
o can cause epithelial cells to fall off the mucosa, leaving spaces through which fluid can be extruded.
How does E. coli cause fluid entry into small intestine?
produce heat stable (STa) enterotoxin
o inhibit the enterocyte fluid absorption mechanism
How does Vibrio cholerae cause fluid entry into small intestine?
interrupt the fluid uptake mechanism in the same way that E.coli does but also produce an enterotoxin (zona occludens toxin or ZOT)
o ZOT loosens the tight junctions between enterocytes and causes intestinal vasodilatation.
o the catastrophically large fluid losses in cholera are mainly due to a combination of:
vasodilatation, leading to increased interstitial fluid pressure
increased passive water permeability because of enhanced conductivity between epithelial cells.
