Spread of spinal anaesthesia for caesarean section in singleton and twin pregnancies.

We have compared the spread of spinal anaesthesia in parturients with singleton and those with twin pregnancies. Fifty-five unpremedicated patients with uncomplicated pregnancy scheduled for Caesarean section were allocated to two groups: group I = 35 singleton mothers; group II = 20 with twin pregnancy. Both groups received spinal anaesthesia with hyperbaric bupivacaine 10 mg (2 ml of 0.5%). Mean birthweight was 3290 (SD 452) g and 5008 (495) g in groups I and II (combined birthweights), respectively. We found a statistically significant difference in onset and maximal cephalad spread of spinal anaesthesia (group I median T5, range T8-T4; group II T3, range T6-T2). The mechanisms of higher cephalad spread of spinal anaesthesia in parturients may be a decrease in cerebrospinal fluid volume secondary to shunting of blood from the obstructed inferior vena cava to the extradural venous plexus and increased nerve sensitivity to local anaesthetics because of increased concentrations of progesterone. The twin pregnancy group had heavier, larger uteri and greater daily production of progesterone.