The GI Tract: Secretions, Motility & PONV

Dr James F PeerlessOctober 2013

ObjectivesAnnex 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 ControlFactor Gastric

MotilityGastric

EmptyingIntestinal 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 ToneIncreased Tone Decreased Tone

Cholinergic Stimulation Cholinergic InhibitionDopaminergic Inhibition Dopaminergic Stimulation

Histamine Oestrogenα-adrenergic Stimulation α-adrenergic Inhibition

β-adrenergic Blockade β-adrenergic StimulationGastrin CholecystokininMotilin 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 SecretionspH Volume (mL day-1)

Saliva 6-7 1000-1500

Gastric 1-3.5 1500-2500

Pancreatic 8 1000-1500

Bile 7-8 700-1200

Small bowel 7-8 1800

Large bowel 7-8 200

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

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

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