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
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
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 pHVolume (mL day -1 ) Saliva6-71000-1500 Gastric1-3.51500-2500 Pancreatic81000-1500 Bile7-8700-1200 Small bowel7-81800 Large bowel7-8200
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
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
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.
Process of Vomiting Pre-ejection PhaseEjection 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 Anaesthetic Patient Surgical
Risk Factors - Patient Female Non-smoker Previous PONV H x motion sickness
Risk Factors - Anaesthetic N 2 O 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
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