Intrathecal anesthesia with ketamine.

BACKGROUND AND OBJECTIVES

Intrathecal anesthesia is the method of choice for transurethral prostate resection in the United Kingdom, despite its associated hypotension. Intrathecal ketamine with epinephrine has been used for lower limb surgery with minimal cardiovascular disturbance. Because cardiovascular stability might be advantageous in this group of elderly patients, we undertook this study to determine if intrathecal ketamine was a suitable intrathecal anesthetic agent for transurethral prostate surgery. Because of the high incidence of side effects and inadequate analgesia, the study was terminated after only ten patients had been recruited.

METHODS

Ten ASA I to III males were studied. Spinal anesthesia was performed with a 25-gauge Whitacre needle at the L2-L3 or L3-L4 space. The dose of ketamine was determined by a sequential allocation technique after initial empirical doses of 0.5 and 0.75 mg/kg. Sensory and motor block were monitored using pinprick and modified Bromage score, respectively. If spinal anesthesia was inadequate at any time, a general anesthetic was administered.

RESULTS

At doses higher than 0.7 mg/kg, intrathecal ketamine produced both motor and sensory block. The onset of motor block was within 2-3 minutes, peaked in 5-10 minutes, and lasted 30-60 minutes. Sensory block took 5-20 minutes to reach its maximal height. Maximum sensory block height varied from L1 to T7. Despite adequate block to pinprick, half the patients sensed the diathermy and were given a general anesthetic. The incidence of severe psychotomimetic side effects was 30%.

CONCLUSIONS

Intrathecal ketamine at doses of 0.7-0.95 mg/kg produces sensory and motor block. The frequency of psychomimetic disturbance, inadequate analgesia, and short duration of action preclude its use as a sole anesthetic agent.