Barrier Pressure

• Barrier pressure = LOS pressure - intragastric pressure

Anatomy

• The LOS is a physiological sphincter, ie anatomically it is indistinguishable from the upper oesophagus
• Formed by the  lower 2-4cm segment of oesophagus
• Maintains tonic contraction of circular smooth muscle
• Its function as a sphincter is contributed to by the following
• ↑ number of nerve cells
• providing tonic contraction of circular smooth muscles in the internal layer of the lower oesophagus
• The pinch-cock mechanism from crural fibres of the diaphragm
• wraps the oesophagus and pinches it closed during muscular contraction.
• The flap-valve mechanism at the gastro-oesophageal junction
• from gastric mucosal folds closing the orifice when the stomach is distended. 
• Oblique entry of the oesophagus into the stomach
• prevents retrograde flow
• Intra-abdominal position of the gastro-oesophageal junction
• subjects the lower oesophagus to intra-abdominal pressure

Barrier Pressure

• Barrier pressure = LOS pressure - intragastric pressure
• ie the pressure gradient that opposes gastric reflux
• LOS maintains resting intraluminal pressure 25-35cmH2O
• Intragastric pressure = 10cmH2O at rest
• Therefore barrier pressure normally 15-25cmH2O
• Reflux can occur when barrier pressure drops below 13cmH2O

Factors Affecting LOS tone

Hormone Factors

• ↑ LOS tone
• gastrin
• CCK
• motilin
• α -adrenergic stimulation
• oestrogen
• ↓ LOS tone
  • progesterone
  • PGE2
  • glucagon
  • VIP
  • GIP
  • secretin

Drug Factors

• ↑ LOS tone
  • metoclopramide
  • anticholinesterases
  • histamine
  • suxamethonium
• ↓ LOS tone
  • anti-muscarinics
  • dopamine
  • opioids
  • ethanol
  • ganglion blockers
  • beta adrenergics

Physical Factos

• Cough/sneeze → ↑ intra-abdominal pressure → ↑ LOS tone
• Swallowing → transient ↓ LOS tone to allow passage of food bolus

Physiological Factors

• Obesity → ↓ LOS tone
• Pregnancy → ↓ LOS tone
• Hiatus hernia → ↓ LOS tone

Physiology of Swallowing

• Swallowing is a complex reflex controlled by the swallowing centre in the medulla
• There are 3 phases

Oral Phase (voluntary)

• Tongue forms a food bolus and pushes it into oropharynx by pushing up and against hard palate
• Sensory endings of CN IX stimulated → initiates the next 2 involuntary phases

Pharyngeal Phase (involuntary)

• Respiration is inhibited 1-2 secs, food bolus passes into upper oesophagus
• Nasopharynx is closed by the soft palate
• Laryngeal inlet is closed by adduction of vocal cords and aryepiglottis muscle
• Contraction of pharynx and opening of upper oesophageal sphincter → food pushed into oesophagus

Oesophageal Phase (involuntary)

• As soon as food enters upper oesophagus, upper sphincter closes and lower sphincter starts to relax
• Primary, slow peristaltic wave (2-4cm/sec) initiated by swallowing centre via CN X → pressure 20 - 60mmHg
• Gravity helps fluids to pass faster than solids
• Presence of food in oesophagus activates stretch receptors in wall → activate intrinsic enteric nervous system → secondary peristaltic wave
• Lower oesophageal sphincter is fully relaxed by the time food bolus gets there
• After food passes into stomach, LOS contracts to 1-15mmHg above resting tone for 10-15secs, before returning to baseline