The distribution and duration of the sensory block after a standard and refined ultrasound guided transversus abdominis plane (TAP) block.

The aims of the study are to evaluate the duration and distribution of the sensory block after an ultrasound guided TAP block using a standard and refined needling technique. Twenty participants undergoing lower abdominal surgery through lower midline incision will be studied prospectively. Each participant will receive a bilateral TAP block using the refined needling technique on one side of the abdomen and the standard needling technique on the other side. Randomisation will determine which side receives which technique. The standard technique requires the needle to be inserted medial and parallel to the ultrasound probe. With ultrasound guidance it is directed ‘in plane’ to ensure the tip lies between the internal oblique and transversus abdominis 1-2cm anterior to where the transversus abdominus muscle becomes a fascial extension. To confirm location, 2ml of normal saline can be injected. Once position is satisfactory, 0.5% Ropivacaine is injected in 5ml aliquots with aspiration between injection to a total of 0.75mg/kg. The time required to perform this procedure is 5 to 10 minutes. The refined technique requires the needle to be inserted more medially compared with the standard technique and directed slightly laterally towards the ultrasound probe. Using ultrasound 'in plane' guidance the needle is positioned in the transversus abdominis plane at its anterior origin and advanced along the plane to ensure at least 5cm of needle is contained within this plane. The tip should still lie 1-2cm anterior to where the transversus abdominus muscle becomes a fascial extension. Hydrodissection with saline may be used to assist needle positioning. Once position is satisfactory, 0.5% Ropivacaine is then injected in 5ml aliquots with aspiration between injection to a total of 0.75mg/kg. Local anaesthetic should form a echolucent band tracking back along the needle separating the internal oblique from the transversus abdominis. The time required to perform this procedure is 10 to 15 minutes. The refined and standard needling procedures will be performed on the participants at the end of surgery while they are still anaesthetised. They will be only performed once on each participant. Abdominal sensory mapping (assessment of loss of sensation to ice and light touch) will be performed by a blinded researcher every two hours until full sensation has returned. Assessment of pain on each side of the abdomen will be recorded using visual analogue scores.