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Description of intervention(s) / exposure:
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Ultrasound guided transversus abdominis plane (TAP) block with local anaesthetic. At the end of the caesarean section, a TAP block will be performed with either local anaesthetic or placebo.
TAP block technique
Equipment
Ultrasound – 38mm linear array probe 13-6MHz
22G 150mm Stimuplex needle
Study drug – 40ml of 0.5% ropivacaine / saline
Betadine antiseptic preparation for skin preparation
Pulse oximetry to measure heart rate
Availability of full monitoring and resuscitation equipment
Identification of landmarks
Ultrasound positioned in the mid axillary line in the axial plane half way between the iliac crest and the costal margin.
Structures to be identified: subcutaneous fat, external oblique muscle, internal oblique muscle, transversus abdominis muscle, peritoneum and intraperitoneal structures. As a means of confirmation, the transversus abdominis tends to be darker and less echogenic.
If these are not readily identified, the probe may be moved posteriorly to identify the latissimus dorsi muscle which is external to the fascial extension of these muscles. The probe may also be moved anteriorly to the rectus sheath and the fascial planes followed back out laterally. The final position of the probe is to be no further anterior than the anterior axillary line. Oblique angulation may assist with identifying the planes.
Landmarks are to be classified as 'unsatisfactory' if any of 1. external oblique, 2. internal oblique, 3. transversus abdominis, or 4. intraperitoneal structures cannot be identified posterior to the anterior axillary line. The block may still be performed if the investigator is satisfied that the transversus abdominis muscle can be differentiated. Views will be considered ‘excellent’ if quadratus lumborum can be seen in addition to the above four structures.
If landmark identification is impossible, the block will not be performed.
Needle technique
The needle is inserted 'in plane' and directed to lie between the internal oblique and transversus abdominis under real time ultrasound guidance.
To confirm location, 2ml of local anaesthetic can be injected.
Once position is satisfactory, local anaesthetic is then injected in 5ml aliquots with aspiration between injection to a total of 20ml each side. Appropriate placement will be judged as production of an echolucent plane between the two muscles. Visible separation of muscle bundles of the internal oblique considered superficial placement. Separation of muscle bundles of the transversus abdominis indicates deep placement. Superficial or deep placement requires the needle to be repositioned.
Intervention drug
The active treatment arm will receive 20ml of 0.5% ropivacaine on each side (total 200mg ropivacaine). The placebo arm will receive 20ml saline on each side. The local anaesthetic duration of action is expected to be between 12 and 24 hours.
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