For a unilateral procedure, your alternatives are to either A) use a hypobaric solution (ex. with the operative hip up) or B) use a hyperbaric/supine combination provided that the patient is able to lie with the operated side down for at least six minutes prior to rolling supine [Martin-Salvaj G et al. Anesth Analg 79: 1107, 1994]. Note that at some point, as the analgesic agents become diluted out of dextrose, the anesthetic becomes "fixed" and position no longer matters. Studies by Pvery et al [45, 46] and Bodily et al [51] however, have shown that this can take over 60 minutes to occur. Also note that there are no true isobaric solutions, all of them are slightly hypobaric. Additionally, "isobaric" solutions are some of the most widely variable in terms of distribution but in general, isobaric solutions offer lower blocks than hyperbaric solutions, decreasing the overall risk of cardiovascular compromise.
Baricity and patient position overwhelmingly affect the block height. Minor contributors such as concentration, dose, and volume, are of almost trivial consequence. Injection site seems to matter for isobaric solutions [74] but not for hyperbaric solutions [70]. Morphologic (ex. height) and various descriptive CSF variables (ex. CSF volume) have been shown to be statistically significant but are clinically irrelevant as they are far too variable and many are difficult to assess.
Spinal blocks wear off in a cephalad to caudad direction, thus sacral levels will last longer than thoracic. Higher blocks usually wear off faster than lower blocks. Adrenergic agonists can prolong a spinal block, with the maximum does of PHE (5 mg) usually providing longer duration than the maximal dose of EPI (0.5 mg). Clonidine can also prolong blocks, even when given orally [96-98] but has been associated with increased hypotension in some studies. Agonists are most effective in tetracaine blocks, less-so with bupivacaine
http://openanesthesia.org/index.php?title=Spinal_Anesthesia#Spinal_Anesthesia
