Spinal anaesthesia (second part)

Advantages of spinal anaesthesia include rapid onset, reliability, low plasma levels of the drugs used, and the use of lower doses compared to epidural anaesthesia. Systemic absorption in the subarachnoid space is minimal, compared to the epidural space. Disadvantages include short duration of action, impossibility of titration to clinical effect, and the inability to provide extensive surgical blockade without side effects. Indications for spinal anaesthesia are surgical procedures involving rectum, prostate, sacral area, hind limbs and caudal abdomen, including obstetric procedures. As a general principle, the greater the extent of anaesthesia required (i.e. exploratory laparotomy), the lesser is regional anaesthesia indicated, however spinal anaesthesia may still be a valuable part of a balanced technique in operations above the level of the umbilicus.

Hypotension and bradycardia are well known side-effects of spinal anaesthesia: they may be prevented with careful dose titration, and they are treated with fluids, alpha-adrenergic drugs (e.g. phenylephrine and metaraminol), or mixed alpha and beta adrenergic agonists (e.g. ephedrine and epinephrine) administration. Severe complications are rare: sporadically reported cardiac arrest always follows severe untreated hypotension, and it is usually responsive to CPR (Casati A and Vinciguerra F, Curr Opin Anaesthesiol 2002). Selection of local anaesthetic  is usually based on the expected duration of surgery in order to ensure excellent surgical anaesthesia with quick functional recovery (Casati A and Vinciguerra F, Curr Opin Anaesthesiol 2002). However recent concerns about lidocaine toxicity have increased the interest in other spinal local anaesthetics, and the use of small doses of long-acting agents (Kuusniemi KS et al, Reg Anesth Pain Med 2001) and analgesic additives such as lipophilic opioids (Ben-David B et al, Anesth Analg 1997; Vaghadia et al, Can J Anesth 2001) or alpha-2 agonists has been investigated.

In veterinary spinal anaesthesia there is no  easy way to calculate the dose to be administered, and the reduction of the intended epidural dose does not consider the different distribution of intrathecally injected isobaric local anaesthetics compared to the epidural route. The dose should be chosen according to the crown-rump length, the body area to be blocked, the expected duration of surgery, and the expected surgical stimulation. As a result, a skilled surgeon limiting the surgical field and stimulation, along with careful positioning of the patient on the operating table, will allow to reduce the dose of local anaesthetic used, also reducing side effects and improving recovery (i.e. ability to walk and return of normal urination). This approach may be even more effective in patients requiring general anaesthesia or sedation to perform regional anaesthesia, as most of our patients are.