Transcript The GI Tract: Secretions, Motility & PONV

The GI Tract: Secretions, Motility
& PONV
Dr James F Peerless
October 2013
Objectives
Annex B
• Physiology & Biochemistry: Gastrointestinal
– PB_BK_80 Gastric function; secretions, nausea and vomiting
– PB_BK_81 Gut motility, sphincters and reflex control –
neurohumoral integration
– PB_BK_82 Digestive functions; composition of secretions;
digestion of carbohydrates, lipids, proteins, vitamins, minerals
Annex C
• Applied Physiology & Biochemistry: Gastrointestinal Tract
– PB_IK_30 Nausea and vomiting
Gut Motility
The GI Tract
• Series of organs with specialised functions and
characteristic properties
• Digestion of ingested food
• Absorption of
– Water
– Nutrients
– Electrolytes
– Vitamins
• Excretion of indigestible and waste products
Gut Motility
• Circular and longitudinal muscle
• Smooth muscle cells with gap junctions allows
for a functional syncytium
– Relaxation: rhythmic depolarisation/repolarisation
with slow-wave activity
– Contraction: spike-burst activity as
transmembrane threshold is reached
• Frequency and amplitude controlled by nervous and
chemical mediators
Nervous Control
• Intrinsic and extrinsic control
• Short and long reflexes
• Somatic NS
– Pharynx & anus
• Autonomic NS
– PNS
• Vagus – oe  prox. colon
• S2,3,4 – dist. colon, rectum, anus
– SNS
• Sympathetic chain
Nervous Control
• Local enteric NS
– Latticework of plexuses and ganglia within the
bowel wall
– Auerbach’s (myenteric) plexus
• Between long. and circular muscle layers
– Neurones classified:
• Cholinergic (stim.)
• Adrenergic (inh.)
• NANC (inh.)
– NO, VIP
Humoral Control
Factor
Gastric
Motility
Gastric
Emptying
Intestinal
Motility
Gallbladder
Emptying
Gastrin



-
Cholecystokinin




Secretin (augments
CCK)




Gastric Inhibitory
Peptide


-
-
Motilin



-
Somatostatin




Control of the Lower Oe Junction
• Functional zone of high pressure (15-25
mmHg) 2-4 cm of lower oesophagus
• Histologically indistinct
• Prevents reflux of gastric contents into Oe
• Barrier pressure is the pressure difference
between LOS and intragastric pressure
– Any decrease in LOS or increase in intragastric
pressure  increased risk of reflux
Factors Affecting Lower Oe Tone
Increased Tone
Decreased Tone
Cholinergic Stimulation
Dopaminergic Inhibition
Histamine
α-adrenergic Stimulation
Cholinergic Inhibition
Dopaminergic Stimulation
Oestrogen
α-adrenergic Inhibition
β-adrenergic Blockade
Gastrin
Motilin
β-adrenergic Stimulation
Cholecystokinin
Secretin
PGF2
PGE1
Secretions
Secretions
• Main gastrointestinal secretions
– mucus and digestive enzymes
• Specialised secretory cells throughout the
gastrointestinal tract, plus liver and pancreas
as specialised glands
• Secretion stimulated by presence of food in
the GI tract, as well as PNS and the intrinsic
neuronal control
Summary of Gut Secretions
pH
Volume (mL day-1)
Saliva
6-7
1000-1500
Gastric
1-3.5
1500-2500
8
1000-1500
Bile
7-8
700-1200
Small bowel
7-8
1800
Large bowel
7-8
200
Pancreatic
Saliva
• Multifunctional:
–
–
–
–
–
Salivary amylase digests starch
Salivary lipase
Buffering and diluting irritants
Aids swallowing, speech and lubricant
Antibacterial: lysozymes, IgA
• Serous and mucous mixture: 1000-1500mL
day-1
– Parotid - serous
– Sublingual and submandibular – both
– Buccal – mucus
• Regulation
–
–
–
–
PNS – superior/inf. salivary nuclei
Triggered by taste and touch sensors
Appetite
Reflex salivation by GI irritation
Gastric Secretion
Mucus
Pyloric
G Cells
Chief
STOMACH
Oxyntic
Parietal
Mucus
Phases of Gastric Secretion
• Cephalic Phase
– Thought, sight, smell of food
– Vagal stimulation of oxyntic glands and G-cells
• Gastric Phase
– Vago-vagal reflexes and local enteric reflexes upon food entering the stomach
– Release of gastrin
– Secretion of acid to pH 2
• Intestinal Phase
– Chyme enters small intestine and gastric secretion declines
• Lack of stimulation
• Inhibitory factors:
– Duodenal distension, presence of acid in the duodenum
– CCK release
Acid Production in Parietal Cells
Modulation of Gastric Acid Production
Post-operative Nausea &
Vomiting
Definitions
Nausea is the sensation of the need to vomit
Vomiting is the involuntary, forceful expulsion of
gastric contents through the mouth
Postoperative nausea and vomiting (PONV) is any
nausea, retching, or vomiting occurring during
the first 24 – 48h after surgery.
PONV
• one of the most common causes of patient
dissatisfaction after anaesthesia
– reported incidences of 30% in all post-surgical
patients
– up to 80% in high-risk patients
• regularly rated in preoperative surveys as the
anaesthesia outcome the patient would most
like to avoid.
Physiology of Vomiting
Physiology of Vomiting
• Vomiting centre in medulla
• CTZ – area postrema (floor of fourth ventricle)
– Outside BBB
– Multiple receptors (e.g. H1, D2, 5-HT3)
•
•
•
•
•
•
Labyrinth (CN VIII)
Higher cortical centres (fear, sight, smell, memory)
Baroreceptors (CN X)
Pain pathways
GIT chemo- and mechanoreceptors (CN X)
Limbic
Physiology of Vomiting
Process of Vomiting
Pre-ejection Phase
Ejection Phase
Process of Vomiting
• Pre-ejection Phase
– Nausea
– SNS stimulation: HR, RR, sweating
– PNS stimulation: salivation, relaxation of upper &
lower oesophageal sphincters
– Retrograde contraction
Process of Vomiting
• Ejection Phase
– Respiratory pause mid-inspiration
– Hyoid and larynx raised to open crico-oesophageal
sphincter
– Glottis closes
– Soft palate elevates to close nasopharynx
– abdominal pressure
• diaphragm and abdominal muscles contract
– Gastro-oesophageal sphincter opens
– Ejection of contents
PONV is multifactorial
Surgical
Patient
Anaesthetic
Risk Factors - Patient
•
•
•
•
Female
Non-smoker
Previous PONV
Hx motion sickness
Risk Factors - Anaesthetic
•
•
•
•
•
N2O
Opioids
Etomidate
Neostigmine
Hypotension
– both regional and GA
Risk Factors - Surgical
• Middle ear surgery
• Ophthalmic surgery (esp. strabismus)
• Gynaecological procedures
Complications
• Unpleasant
• Delayed discharge from POCU
• Increased length of stay
•
•
•
•
•
•
Suture dehiscence
Aspiration of gastric contents
Oesophageal rupture
Raised intraocular & intracranial pressure
Electrolyte imbalance
Dehydration
Management
• Predict the at-risk patient
• Multimodal approach
– Anaesthetic technique, e.g. TIVA
– Local/regional technique
– Minimise baseline risk factors, where possible
• Minimise peri-operative opioid use
• Combine antiemetic use for additive effects