GIT - Stomach And Vomiting Flashcards
List the functions of the stomach
- Storage of large meals - releasing ingested food slowly into the small intestine
- Secretion: Digestive enzymes (gastrin)
- Mixing: mix with enzymes and liquify into chyme
- Secretion gastric acid: (defence micro-organsims + absorption Vit C / Iron)
- Secretion Intrinsic Factor: for B12 absorption
- Endocrine: Hormones to control gastric emptying
What quantity of fluid is produced + secreted by the following organs over 24 hours:
- Salivary glands
- Stomach
- Bile
- Pancreas
- Intestines
- Total
- Salivary glands - 1.5 L
- Stomach - 2.5 L
- Bile - 0.25 L
- Pancreas - 0.75 L
- Intestines - 3 L
- Total: 8 - 9 L/day
List the substances secreted by the stomach
- HCl
- Pepsinogen
- Gastrin
- Intrinsic Factor
- Mucus
Describe the structures, mechanism and function of HCl secretion into the stomach
Structures - Parietal cells
Mechanism (Parietal cell function –> Draw cell)
Page 280 Chambers
BASOLATERAL
- . CO2 inward diffusion
- Na-K ATPase pump
- CL-/HCO3 antiport
- M receptor
- G receptor
- H2 receptor
ECL cell (enterochromaffin like cell)
- Release Histamine to stimulate H2
- Have somatostatin inhibitory receptors
CELL LUMEN
Bicarbonate equilibrium reaction with CA
LUMINAL
Proton pump = H+/K+ ATPase exchanger
Cl- channel (into GIT)
K+ channel (into GIT)
HCl reduces pH which:
- Converts pepsinogen to pepsin
- Destroys ingested bacteria
What is the concentration and equivalent pH of the Hcl secreted into the stomach
150mml/L
pH = 0.8
How is HCL secretion regulated
DIRECT STIMULATION on parietal cell
- Histamine (H2 receptors)
- Ach (M3 receptors)
- Gastrin (G receptors)
INDIRECT STIMULATION
1. Gastrin stimulates ECL cells to release histamine
INHIBITION
1. Somatostatin receptors on ECL cells are inibitory
How is protein catabolism initiated and where does this occur
In the stomach
Gastrin and Ach (PSNS) stimulate CHIEF cells to secrete pepsinogen which is converted to pepsin by the acidic environment
Which cells secrete Gastrin, when is it released and what is its function. How is gastrin secretion regulated.
G cells
Function of gastrin
- PARIETAL CELLS –> Secrete HCl
- CHIEF CELLS –> Secrete Pepsin
- GASTRIC MOTILITY
Regulation
Release:
- PSNS –> Ach
- Distension of stomach
- Presence of partially digested proteins in stomach
Inhibition
1. High levels of gastrin –> release of somatostatin from DELTA cells in the stomach. –> inhibits further gastrin release
Which cells secrete intrinsic factor and explain the nature and site of its release and function
Parietal cells
Intrinsic Factor is an important co-factor for the absorption of vitamin B12.
IF does not bind to B12 in the acid environment of the stomach
B12 is released from digested animal proteins in the stomach
Haptocorrin binds a protects B12 from the acid environment in the stomach
Trypsin digests haptocorrin in the small intestine re-releasing vitamin B12
IF now binds to B12
Receptors for IF-B12 complexes in the terminal ileum allow for the absorption of B12 here.
What is pernicious anaemia
Autoimmune destruction of parietal cells in the stomach with deficiency of IF –> low B12 –> megaloblastic anaemia
What cells secrete mucus in the stomach and what is the function
Mucus cells –> HCO3- rich mucus
Function
- protection of stomach mucosa from acid
- Lubrication stomach wall
Differentiate between particulate and non-particulate antacids and their mechanism of action
Both particulate and non-particulate antacids react directly with HCl to neutralise the gastric pH.
PARTICULATE ANTACIDS
- Aluminium Hydroxide
- Magnesium Hydroxide
- Calcium Carbonate
Particulate = Aluminium / Magnesium and Calcium Salts formed are non-absorbable and form particulate matter –> if this formed particulate matter is aspirated into the lungs, pulmonary damage will still occur
NON-PARTICULATE ANTACIDS
1. Sodium Citrate
Non-particulate
Rapid onset of action
Why do gastric acid inhibitors take longer than antacids to work
Antacids work immediately via an immediate reaction with HCl in the stomach.
Gastric acid inhibitors prevent gastric acid from being secreted from parietal cells –> takes longer to work but there is a prolonged duration of action
What are the risks associated with chronic PPI use
Clostridium difficile infection Calcium deficiency and osteoporosis Magnesium deficiency B12 deficiency Iron deficiency anaemia
Name and describe the phases of gastric secretion
- Cephalic (30%)
- thought, smell, taste –> Vagus –> Ach –> Gastrin/HCl/Pepsinogen - Gastric (60%)
- Distension –> gastrin release - Intestinal
- chyme entry –> secretin from brush border –> HCO3- from pancreatic ductal cells and inhibits gastrin from G cells
What is secretin, where and when is it secreted and what is its function
As chyme enters the duodenum, secretin is secreted by the intestinal mucosa. Secretin then has the following effects:
- Stimulates pancreatic ductal cell secretion of HCO3- to neutralize acidic chyme
- Inhibits release of gastrin from G cells –> reduces HCL output from parietal cells
What is gastric dumping syndrome
Normally, chyme enters duodenum very slowly.
After gastric surgery –> rapid gastric emptying –> largely undigested chyme passes into the small intestine
- Duodenal distension (EARLY DUMPING SYNDROME)
- -> Large amounts of hyperosmolar chyme draws in water from ECF –> cramping pains/nausea and bloating. EARLY DUMPING (soon after meal ingestion) - Hypoglycaemia (LATE DUMPING SYNDROME)
–> Rapid absorption of large amounts of CHO –> triggers B cells of islets of Langerhans in pancreas to secrete large amounts of insulin.
–> Rapid blood glucose uptake
–> Slower insulin metabolism
Leading to hypoglycaemia
Occurs 1 - 3 hours after a meal (LATE)
What is the rate of gastric emptying depend on?
DEPENDS ON CHARACTERISTICS OF THE DUODENAL AND GASTRIC CONTENTS
DUODENAL CONTENTS
- Duodenal distension –>reflex inhibition of enteric nervous system –> closing pylorus.
- Acid –> low pH –> secretin release –> HCO3 secretion and inhibition of gastrin –> reduced gastrin –> slowed emptying
- Fat –> CCK released when fat enters duodenum –> increased pyloric sphinter tone –> allows intestine more time to digest fat
- Hyperosmolarity –> inhibits gastric emptying
GASTRIC CONTENTS
- Consistency of chyme - solids –> Constriction of pyloric sphincter. liquids pass through.
- Gastric Volume –> increased volume promotes emptying
- Content: Protein faster than CHO faster than fat (slow)
Summarise the intrinsic and extrinsic factors that reduce gastric emptying
INTRINSIC Gastric contents 1. Solids 2. Low Volume 3. Fat > CHO > Protein
Duodenal contents
- High volume chyme with duodenal distension
- Acidic (secretin release –> inhibit gastrin)
- Fat (CCK increases pyloric sphincter tone)
- Hyperosmolar chyme (complex mechanism)
EXTRINSIC
- SNS acitivity
- PAIN
- DRUGS (opioids)
- DISEASES: diabetic autonomic neuropathy, acut abdomen, ileus
Draw the graph the demonstrates the rate of gastric emptying which differs between ingestion of solids and liquids.
Solids –> 30 minute lag where remains 100%
then linear decline until about three hours. 30 minute lag is for mixing in stomach and action of pepsin
Liquids –> exponential decline
–> unless liquids contain fats or acidic –> may mimmic solids linear emptying line
What are the disadvantages of pre-operative fasting
- Patient discomfort
- Dehydration
- Electrolytes
- PONV increased
- Glycaemic disturbance in diabetics
- Increased body muscle catabolism following major surgery
Define vomiting
Involuntary, rapid, forceful expulsion of gastric contents through the mouth. Nausea often precedes vomiting and is an unpleasant upper abdominal sensation
Where is the vomiting center
The vomiting centre is an anatomically ill defined area in the medulla oblongata in close contact with three important structures
- Respiratory centre
- Nucleus tractus solitaris (receives afferents from various cranial nerves)
- Chemoreceptor trigger zone (CTZ) (floor of fourth ventricle of the medulla = area postrema)
What is significant about the chemoreceptor trigger zone
It is located outside of the blood brain barrier and receives systemic blood directly. This:
- Provides a faster vomiting response to emetic stimuli
- Antiemetic drugs do not have to cross BBB to reach their target receptors.
Summarise the inputs into the vomiting centre that can trigger vomiting
CTZ
- Dopamine (D2)
- Serotonin (5-HT3)
- ACh
- Opioid
- Substance P (NK-1)
CN 8 (Vestibulocochlear) - Vestibular: Muscarinic and H1 receptors
CN 9 (Glossopharyngeal) - Afferent for the gag reflex
CN 10 (Vagus) - Distension, Infection, chemotherapy, radiotherapy --> stimulation serotonin 5-HT3 receptors in GIT --> vomiting
Higher centres
- Limbic system –> anxiety / extreme emotional states
Describe the three phases and what happens in each of the phases during vomiting
Pre-ejection
- nausea
- decreased gastric motility
- Reverse peristalsis duodenum
- HCO3 rich saliva (PSNS)
- Sweating + tachycardia (SNS)
Retching phase
- Deep inspiration –> closure of glottis
- Rhythmic contraction of intercostal muscles, diaphragm, abdominal muscles against a closed glottis.
- Alkaline contents of SI are vigorously mixed with gastric contents increasing pH.
- Increased intrathoracic pressure compresses the oesophagus, preventing reflux of stomach contents
Ejection phase
- Continuation of glottic closure
- Contraction pylorus forcing gastric contents into body and fundus of stomach
- Relaxation LOS and oesophagus
- Sudden dramatic increased intra-abdominal pressure (contraction of abdominal muscles and descent of the diaphragm)
- Soft palate occludes nasopharynx
- Reverse peristalsis in the oesophagus rapidly expels contents upwards and out of the mouth
Describe the anaesthetic factors contributing toward PONV
Volatiles
N2O
Opioids
Anticholinesterases (Neostigmine)
Describe the surgical factors that contribute towards PONV
Longer duration of surgery
Middle-ear surgery
Laparoscopic surgery
Neurosurgery
Describe the patient factors that contribute toward PONV
Female Child PONV history Motion sickness history Non-smoker
What anaesthetic techniques can be implemented to avoid PONV
- Avoid opioids / N2O / Volatiles
- -> use regional / TIVA - IV fluid therapy
- Antiemetic drugs
How long can PONV persist after surgery
up to 5 days
Define nausea
A feeling of sickness with an inclination or an impending desire to vomit
Define retching
Laboured, spasmodic, rhythmic contractions of the respiratory muscles without expulsion of gastric contents
Define regurgitation
The effortless expulsion of food from the stomach or return of food to the mouth
Define vomiting
Forceful, involuntary and rapid ejection of gastric contents from the mouth
Draw a simplified diagram of the various afferents into the vomiting centre. List these afferents
- CTZ (Area postrema of the 4th ventricle)
2. GIT (Mechanoreceptors and serotonin release –> CTZ)
What receptors does the CTZ contain and what is the function of the CTZ in health
Receptors
- 5 HT3
- D2
- H1
- u (MOP)
- Substance P (NK-1)
Function
Blood Pressure regulation
Sleep
Food intake
Describe the afferents to the vomiting centre in the Medulla Oblongata
- Chemoreceptor Trigger Zone in the area postrema of the 4th ventricle: H1/D2/5HT3/MOP/NK-1
- GIT Receptors (CN 10)
- Mechanoreceptors
- Chemoreceptors (5HT3, NK-1 ——-> Vagal —-> VC) - Peripheral pain receptors (trauma)
- Nucleus solitaris (CN 9 –> gag reflex)
- Vestibular system (CN 8 –> CTZ–>motion sickness)
- Ocular (CN 5 –> opthalmic branch)
- Cerebral Cortex (Smell/physiological stress)
- Limbic system (emotional stress/anxiety)
Memory aid below:
BRAIN
CTZ (D2/5HT3/H1/MOP/NK-1)
Cerebral cortex (Physiological stress)
Limbic (Anxiety and emotional stress)
CRANIAL NERVES
10 - Vagus (GIT mechanoreceptors and chemoreceptors)
9 - Glossopharyngeal (gag reflex - nucleus solitarius)
8 - Vestibulocochlear (Motion sickness)
5 - Trigeminal (ophthalmic) - ocular pressur/pain
1 - Olfactory (smells)
2 - Optic (Visual stimuli)
7,9,10 - taste stimuli
PERIPHERAL NERVES
Peripheral pain receptors (vomiting after trauma)
Summarise the process of vomiting
PROTECTION of lungs and nasopharynx Deep inspiration Hyoid bone rises Glottis closes Soft palate lifted
PROTECTION FROM pH Reversed peristalsis into stomach from SI Increased salivation (HCO3)
RELAXATION
LOS + Oesophagus
CONTRACTION Abdominal wall Thoracic muscles Diaphragm --> RIAP ---> ejection
How can the risk factors for PONV be classified
Patient
Medical
Surgical
Anaesthetic
What is the APFEL score
1 point - Female
1 point - Non-smoker
1 point - History PONV or motion sickness
1 point - Postop opioids
Score (Risk PONV) 0 (10) 1 (20) 2 (40) 3 (60) 4 (80)
What are the medical risk factors for PONV
GIT disease (hiatus hernia / GORD) Metabolic disease (DM, Uraemia, Electrolyte) Pregnancy Pre-operative Anxiety The underlying surgical problem Intracranial stimulation (tumour) Acute abdomen, intestinal obstruction Chemotherapy or radiation
Which types of surgery are associated with a higher risk of PONV
ENT Dental Breast Orthopedic shoulder surgery Laparoscopy Ophthalmology (strabismus) Varicose vein stripping)
How does duration of surgery affect risk of PONV
Longer –> increase risk PONV
Adults > 60 minutes –> increased risk
Kids > 35 minutes –> increased risk
For every 30 minutes after these times the risk increases by 60%
Describe anaesthetic factors that increase and decrease risk of PONV
DECREASE
- Benzodiazepines (Premed anxiolysis)
- Regional anaesthesia/TIVA (vs. GA) - avoid all of below
- PROPOFOL TIVA/TCI - avoid all of below
- Higher FiO2 (reduces bowel distension/ischaemia)
- Atropine (crosses BBB unlike glycopyrrolate)
- IV Fluids
INCREASE
- N2O
- Volatiles
- Opioids
- Neostigmine
- Dehydration
- Hypotension (>35% decrease SBP)
Classify and list the dopamine receptor antagonists
Phenothiazines
- Chlorpromazine
- Promethazine
Butyrophenones
1. Droperidol
Benzamides
1. Metoclopramide
Compare phenergan to chlorpromazine including classification, MOA and side effects
Phenergan = Promethazine = phenothiazine Dose 12.5 - 25 mg 4 - 6 hrly MOA 1. D2 receptor antagonist CTZ 2. H1 receptor antagonist 3. M receptor antagonist 4. Alpha adrenergic antagonist - reduces hormone release from hypothalamus/Pituitary 5. Reduces stimuli to brainstem/RAS
Chlorpromazine = phenothiazine
Dose: 10 - 25 mg IM 4 - 6 hourly
MOA
- D2 receptor antagonist CTZ
- M receptor antagonist
- Alpha adrenergic antagonist - reduces hormone release from hypothalamus/Pituitary
- Reduces stimuli to brainstem/RAS
Adverse effects (both)
- Drowsiness
- Extra-pyramidal side effects
- PROLONG QT (more significant in chlorpromzine)
What are two examples of a butyrophenones. What is the Dose, MOA and adverse effects. Which agent is preferred and why
Droperidol
Dose:
PONV Rx: Max initial: 2.5 mg IM/IV
PONV Prevent: 1.25 IM/IV ±30 min before end of surgery (then 6hrly)
MOA
- D2 receptor antagonist
- Alpha adrenergic antagonist (low)
- H1 receptor antagonist (low)
- 5HT 1 + 2 antagonist (low)
Haloperidol
Dose: 2mg 30 minutes before end of surgert
Same MOA
Haloperidol has been associated with higher frequency of adverse effects:
- Drowsiness
- Dysphoria
- Extra-pyramidal side effects
- Prolonged QT
What are the benzamides? MOA,
Metoclopramide
MOA CTZ 1. D2 antagaonist CTZ 2. 5HT3 antagonist (at high doses) Also GIT 1. Increase LOS tone 2. Facilitation gastric emptying into small intestine
Side effects
- Drowsiness
- Extra-pyramidal SE
- Prolonged QT
What is benadryl: MOA, dose and clinical use
Diphenhydramine
Dose: 20mg ± 30 minutes before end of surgery
MOA:
1. H1 receptor antagonist
2. M receptor antagonsit
3. alpha adrenergic antagonist
Inhibition of integrative functioning of the vestibular nuclei in the inner ear by decreasing vestibular and visual input
Clinical use
- Motion sickness
- Inner ear disease
Give an example of a muscarinic receptor antagonist used for nausea and vomiting
In which perioperative setting is this usually used
What is the MOA and Side effects
Scopolamine transdermal patch
Used Middle Ear Surgery –> disruption of vestibular apparatus
MOA: Blocks transmission to the medulla (vomiting center) of impulses arising from overstimulation of the vestibular apparatus
Apply patch before induction of anaesthesia
Takes 2 - 4 hours to work
Adverse effects
- Dry mouth
- Dizziness
- Problems with accommodation
- Delirium
Are the serotonin antagonists more effective at treating nausea or vomiting
Vomiting
Name 4 serotonin receptor antagonists
Which of these agents is said to be effective when other ‘setron’s’ fail to be effective and why is this the case
Ondansetron
Granisetron
Tropisetron
Dolasetron
Granisetron is often effective despite failed efficacy of another setron. This is because, unlike the other drugs in this group, Granisetron is not dependent on CYP2D6 for metabolism, it depends on CYP3A4. Genetic polymorphism for CYP2D^ is very common. Therefore, Granisetron may be effective when the other three drugs fail for rescue PONV.
Apart from 5HT3 receptors what other receptors do Ondansetron and Tropisetron bind to
alpha adrenergic
MOP
Unknown clinical relevance
Why and how do the setrons prolong QT
Granisetron blocks sodium channels in the heart
Ondansetron blocks potassium channels in the heart
What is apomorphine, what is it used for and what happens when it is combined with one of the setrons
D1 and D2 agonist used in Parkinsons
Combined with setron: significant hypotension and loss of consciousness
Differentiate Ondansetron and Granisetron
Both effective in treating established PONV rather than preventing it.
Ondansetron Less selective T1/2 = 4 hours Dose: Adults 4 - 8 mg. Paeds: 0.1 mg/kg Side effects 1. HA, dizziness, transient transaminitis, 2. GIT: Warm epigastric sensation and constipation 3. Prolong QT
Granisetron
More selective
T1/2 = 9 hours
Dose: 3mg Paeds: 0.02 mg/kg
What is Aprepitant
Neurokinin-1 (NK-1) receptor antagonist
Substance P (Visceral afferent stimuli e.g. gastric distension) is a natural ligand for the NK-1 receptor
Aprepitant is said to be superior to ondansetron in preventing PONV in first 24 - 48 hours
What is the mechanism of action whereby steroids reduce PONV
Unknown
Maybe: inhibition of PG synthesis / stimulation of endorphin release / decreased central uptake serotonin
How long does dexamethasone take to elicit its antiemetic effect?
2 hours
Repeated doses not recommended
What is the minimum effective dose of dexamethasone to prevent PONV in adults and children
5 mg
Kids: 0.15 mg/kg
How does midazolam elicit its anti-emetic effect
Potentiation of GABA
–> decreases dopaminergic neuronal activity and 5HT release in the CTZ
Also reduces catecholamine levels
Does propofol elicit an anti-emetic effect subsequent to bolus administration at induction of anaesthesia
NO –> continuous sub-hypnotic propofol infusion required.
Describe the effective plasma concentration of propofol for the following indications:
- Antiemesis
- Sedation
- Anaesthesia
TCI Schnider: Cpl
- Antiemesis: 0.35 ug/mL
- Sedation: 0.9 - 3 ug/mL
- Anaesthesia: 3 - 10 ug/mL
How do alpha 2 adrenergic agonists possibly exhibit antiemetic effects
- Reduced SNS outflow
- Analgaesic properties (reduced opioids needed)
- Sedation (reduced volatile needed)
BUT sometimes dexmedetomidine causes hypotension which causes nausea and vomiting
How do cannabinoids reduce vomiting
CB - 1 receptor within the vagal nucleus complex –> mediation of serotonin and substance P
How does inhaled isopropyl alcohol reduce nausea and vomiting
Unclear mechanism
Possibly: Depressant effect on CNS
Ideal anaesthesia for patient with previous severe PONV
- Dexamethasone
- Droperidol
- Ondansetron
- TIVA/TCA (Propofol/Remifentanyl)
- Aggressive IVF 25 ml/kg
- No N2O
- No neuromuscular blockade
- Analgaesia: Paracetamol / ketorolac
