O5 Dose mean opening pressure predominantly assess internal anal sphincter function

We have suggested that mean Opening Pressure (Op) recorded during Anal Acoustic Reflectometry (AAR) predominantly represents IAS function however, the extent remains unknown. The aim was to investigate this by excluding the external anal sphincter during general anaesthetic (GA) with confirmed neuromuscular blockade (NMB). Method Patients undergoing elective abdominal surgery requiring GA+NMB were approached. Patients had pre-operative (awake) and during GA + NMB (asleep) AAR measurements performed. The rectoanal inhibitory reflex (RAIR) was assessed permitting the Op value to also be recorded when the IAS was inhibited. Op was recorded at prerectal distension and then after 100 mls of air was inflated within a balloon in the rectum (post-rectal distension). Result 19 patients were included. The values of Op (cmH20) and the reductions observed during the RAIR when awake/asleep are as follows: Awake Op: prerectal distension (64.94) and post-rectal distension (35.35) therefore mean change 29.59 cmH2O i.e. 44.6% reduction Asleep Op: prerectal distension (37.64) and post-rectal distension (15.55) therefore mean change 22.1 i.e. 55.3% reduction The contribution of the IAS to Op is calculated as follows: (Mean change Op awake x 100)/% reduction in RAIR asleep = IAS contribution awake (29.59cmH20 x 100)/55.3 = 53.51cmH20 Total mean Op awake—IAS contribution awake = EAS contribution awake 64.94cmH20–53.51cmH20 (82.4%) = 11.43cmH20 (17.6%) Conclusion The IAS accounts for 82.4% of Op at rest and it remains our hypothesis that Op primarily represents IAS function. Take-home Message Opening pressure primarily represents internal anal sphincter function.