Isobaric bupivacaine

Intrathecal blockade with 0.5% isobaric bupivacaine 10 mg has been compared with 0.5% isobaric bupivacaine 5 mg combined with fentanyl 25 micrograms (diluted to 2 ml with isotonic saline) in 32 patients undergoing elective cesarean section (177). The bupivacaine + fentanyl combination was associated with significantly less hypotension than bupivacaine alone (31 versus 94%) and a near 10-fold reduction in the mean ephedrine requirement (2.8 versus 23.8 mg). There were also significant differences in the incidence of nausea (31 versus 69%) and the median time to peak block (8 versus 10 minutes) with bupivacaine plus fentanyl. The authors advised further large-scale studies to quantify the minimum dose of bupivacaine plus fentanyl for single-dose spinal anesthesia. 

Mahe V, Ecoffey C. Spinal anesthesia with isobaric bupivacaine in infants. Anesthesiology 1988 68(4) 601-3. 

Isobaric bupivacaine is associated with less cerebral oxygenation changes than hyperbaric bupivacaine in subjects undergoing spinal anesthesia for Caesarian section and therefore may be the preferred agent of choice for preventing maternal cerebral blood flow reduction after spinal anesthesia for Caesarian section [31. ... 

Kondo Y, Sakatani K, Hirose N, Maeda T, Kato J, Ogawa S, et al. Effect of spinal anesthesia for elective cesarean section on cerebral blood oxygenation changes comparison of hyperbaric and isobaric bupivacaine. Adv Exp Med Biol 2013 765 109-14. 

The high baricity of the local anesthetic solutions was thought to be chiefly responsible for transient radicular irritation. However, isobaric local anesthetics have also been implicated commonly the concentrations are 2% or greater. From comparisons of 2 and 5% hyperbaric and isobaric lidocaine and hyperbaric 0.5% bupivacaine, it seems more likely that high concentrations of local anesthetic solutions are responsible for transient radicular irritation rather than the osmolarity of the solutions (247-249). That the concentration of lidocaine is not a contributory factor to transient radicular irritation has been shown in 109 patients who received hyperbaric spinal lidocaine 50 mg, as a 2, 1, or 0.5% solution (250). The incidence of transient radicular irritation did not differ (16, 22, and 17% respectively). 

Imbelloni LE, Beato L, Gouveia MA, Cordeiro JA (2007) Low dose isobaric, hyperbaric, or hypobaric bupivacaine for unilateral spinal anesthesia. Rev Bras Anestesiol 57(3) 261-270... 

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