Comparative effects of intrathecal bupivacaine and tetracaine on analgesia, cardiovascular function and plasma catecholamines.

Forty otherwise healthy male patients, scheduled for elective inguinal herniotomy, were randomly allocated to spinal anaesthesia with 3 ml 0.5% hyperbaric tetracaine or bupivacaine under double-blind conditions. The extent of blockade (pin-prick and cold sensation), blood pressure and heart rate and plasma catecholamines were measured before and 5, 10, 15, 20 and 30 min after injection, before skin incision. Cephalad spread of sensory and temperature analgesia was insignificantly higher after tetracaine. The mean time taken to reach maximal spread of analgesia was 22 min in both groups. Mean arterial pressure showed a more pronounced decrease in the tetracaine group (22 vs 12%, P less than 0.009). Heart rate fell slightly and similarly in both groups, while rate-pressure product was significantly lower in the tetracaine group. The correlation between the decrease in mean arterial pressure and the cephalad spread of sensory analgesia was similar in the two groups, suggesting that at identical levels of sensory analgesia changes in blood pressure were similar after tetracaine and bupivacaine. Plasma norepinephrine and epinephrine measurements before spinal puncture and at maximal decrease in mean arterial pressure showed a depressed response to fall in blood pressure in the tetracaine group. It is concluded that spinal anaesthesia with 3 ml hyperbaric 0.5% tetracaine is followed by a more pronounced fall in blood pressure compared to an identical dose of bupivacaine. The more pronounced sympathetic blockade, confirmed by plasma catecholamine measurements, following tetracaine is probably due to a higher cephalad spread of neurogenic blockade, rather than a differential effect on sympathetic nerve fibres.