Ultrasound can also be used to identify the muscle layers and ensure accurate placement of local anaesthetic. The ultrasound probe is placed transversely between the 12th rib and the iliac crest in the mid-axillary line. The TAP can easily be observed, and using a 10 cm short-bevelled needle with an ‘in-plane’ approach, local anaesthetic spread distending the plane between the transversus abdominis and internal oblique can be seen in real time.
The TAP block provides analgesia for the abdominal wall but not for the visceral contents, and it is ideally used as part of a multimodal approach to analgesia. Good post-operative analgesia and a decrease in morphine requirements have been demonstrated. Used bilaterally, it provides a simple alternative in patients for whom an epidural is not possible. It may not provide equivalent analgesia, but there may be a lower risk of systemic side effects.
You are performing a sciatic nerve block for a patient undergoing an Achilles tendon repair. You are using the posterior approach and a nerve stimulator technique. At a depth of 4 cm, you elicit twitching of the calf muscles, which gradually decreases as you reduce the current until there is no further motor response below 0.26 mA.
What is the most appropriate course of action?
a) Withdraw the needle completely and start the block again
b) Withdraw the needle slightly and slowly inject the local anaesthetic
c) Aspirate and, if negative, slowly inject the local anaesthetic
d) Slowly inject the local anaesthetic
e) Withdraw the needle slightly and recheck the minimum current required to obtain a motor response
When using a nerve stimulator technique to perform a nerve block, it is important to have a good understanding of the anatomy of the nerve that you are planning to block, and also how to use the nerve stimulator effectively and safely. With regard to the anatomy, the sciatic nerve is large and essentially comprises two distinct nerves, the tibial and the common peroneal, of which the tibial remains medial to the common peroneal throughout its course. This knowledge is important, as it helps identify the location of the needle tip. Tibial stimulation causes plantar flexion of the foot and inversion, whereas common peroneal stimulation causes dorsiflexion and eversion.
When using a nerve stimulator technique, you should start with a current impulse of 1–2 mA, a frequency of 2 Hz and a pulse width of 100 ms. The current should be reduced to the threshold level (the minimum current to obtain a motor response). Ideally, this current should be between 0.2 and 0.5 mA. If the threshold level is below 0.2 mA, you should be suspicious of intraneural placement and therefore not inject the local anaesthetic mixture but reposition the needle. In the case here, the calf twitch suggests that the sciatic nerve is being stimulated – remember you may not see plantar flexion with this if the Achilles tendon has ruptured. The minimum current to obtain a motor response in this case is 0.26 mA, and it is therefore in the ‘safe range’ – and so providing aspiration is negative (making intravascular injection unlikely), it would be safe to slowly inject the local anaesthetic agent.
It is important to remember, however, that if there is a large amount of resistance or the patient complains of pain on injection, you should stop injecting and reposition the needle before continuing. Although the first and last options presented are not incorrect, they would both result in further manipulation of the needle, resulting in more pain for the patient and increasing the likelihood of damaging surrounding structures – and therefore they are not the best answers.
Although a negative aspiration does not completely rule out intravascular needle placement, it will detect some cases, and it is therefore an important step to undertake prior to injecting the local anaesthetic.