Subarachnoid anaesthesia is commonly known as spinal anaesthesia, despite the latter term strictly encompassing both subarachnoid and epidural anaesthesia.
The patient is placed in the left lateral position and the chosen area - usually the L3/L4 vertebral space - is sterilised. Full aseptic technique is required. The skin is infiltrated with local anaesthetic. A 25G spinal needle is advanced into the interspace until CSF is seen emanating from its end. A hyperbaric solution of local anaesthetic is then infused into the CSF. The distribution of the agent thus varies with gravity and so the level of the block can be controlled by changing the patient's position - tilting the anaesthetic table.
Subarachnoid anaesthesia is limited in so much as a single dose produces a one-off effect that is difficult to titrate. One way around this is to site a catheter for intermittent boluses or continuous infusion of drug.
Vital signs must be carefully observed as there is a risk of apnoea and hypotension.
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