Monday, 14 March 2016

Cutaneous sensory block area, muscle relaxing effect and block duration of the transversus abdominis plane block. RAPM August 2015.

The purpose of this randomised, blinded and placebo controlled study of the TAP block was to:
1. Characterise the cutaneous sensory area by sensory mapping
2. Using ultrasound investigate the abdominal wall muscle relaxing effect
3. Use self reporting to determine the duration of sensory and motor block

16 healthy volunteers were randomised to receive an ultrasound guided unilateral TAP block with 20mls 0.75% ropivicaine and placebo on the contralateral side. Exclusion criteria included: age < 18, ASA 3 or >, body weight < 50kg, BMI 35 or greater, analgesics 24 hrs prior to examination, prior surgery to the abdominal wall and allergy to study drugs. All study volunteers and investigators were blinded to treatment allocations.

Volunteers were assessed before the TAP blocks (baseline) and 90 minutes after the end of injection. Primary outcome measure was cutaneous sensory block area 90 min after the end of the injection. Secondary outcomes measures were differences between baseline and T90 in muscle thickness of the lateral abdominal wall, waist circumference, mechanical detection threshold and mechanical pain threshold and subjective reported duration of skin numbness and abdominal wall muscle relaxing effect.

As there was no cutaneous effect on the control side at T90, the cutaneous sensory block area did not cross the midline in any volunteers. Both the mechanical detection and pain threshold increased significantly on the blocked side. There was also a significant increase in abdominal circumference from baseline to T90 in the relaxed state. Lateral abdominal wall muscle thickness decreased significantly from baseline to T90 on the blocked side. In terms or reported sensory and motor blockade, the average sensory blockade was 570min and the motor was 609min.


The study found that TAP block resulted in a non dermatomal cutaneous sensory block area. In previous study regarding TAP block there has been a lack of precise description of the assessment of the cutaneous effect. All volunteers had a consistent and highly significant effect on all 3 muscle layers. The effect of this clinically, is unknown but may contribute to the analgesic effect. There was a large variation in the duration of the sensory and motor blockade, but on average was approximately 10hrs for both.

The limitations of the study included using only ultrasound for imagining. MRI or fluoroscopy would have given additional information about the spread of local anaesthetic.

MK

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