Age was shown to have an effect on the potency of intrathecal bupivacaine. (Image source: Thinkstock)
Age was shown to have an effect on the potency of intrathecal bupivacaine. (Image source: Thinkstock)

You have a patient who is to undergo surgery that’s appropriate for spinal anesthesia.  How would you dose the spinal?  Most would adjust the dose and spread of anesthetic according to baricity, volume, and concentration of the local anesthetic, yet what about using patient age as a factor in dosing?

Dr. Mingquan Chen and colleagues from the Department of Anesthesiology, The First College Of Clinical Medical Science, China Three Gorges University, Yichang, China, determined the relationship between patient age and dose required for motor block after spinal anesthesia in patients undergoing transurethral or lower limb surgery.  The results of their prospective, randomized clinical trial are published in this month’s issue of Anesthesia & Analgesia in the article “Effect of age on the median effect dose (ED50) for motor block of intrathecally-administered plain bupivacaine.”

Patients received combined spinal-epidural anesthesia.  The epidural was placed in case analgesic supplementation was needed and/or for postoperative analgesia.  Bupivacaine 0.75% was used. The authors studied 129 patients.  Dose was varied according to the up-and-down method and response (hip, knee and foot motor function) was measured every minute for the first 5 minutes and at 10 minutes after the end of intrathecal injection of bupivacaine.  If there was not a motor block, the authors increased the bupivacaine dose by 0.75 mg for the next patient in the same group.  If motor response was weakened, the authors decreased the bupivacaine dose by 0.75 mg.  Patients for each group in the study were enrolled until six crossovers were obtained.

The ED50 for motor block of intrathecal bupivacaine was 10.2 mg for patients 20-30 years of age, 9.5 mg for patients 31-40 years of age, 8.4 mg for patients 41-50 years of age, 7.3 mg for patients 51-60 years of age, 6.6 mg for patients 61-70 years of age, and 5.8 mg for patients 71-80 years of age.  The number of patients who needed supplemental epidural analgesia did not differ according to patient age.

The authors did not standardize the solution volume in this study.  However, the conclusion that age affects the ED50 for motor block of intrathecal plain bupivacaine is novel.  Given the results of the authors’ research, it would seem wise to reduce the dose of spinal anesthetics in elderly subjects. Failure to reduce the dose may produce undesired side effects such as hypotension and urinary retention.