Ultrasound guidance can lower anesthesia volume in knee surgery

May 9, 2007

Nerve blocks used for regional anesthesia in knee surgery can be made safer using ultrasound guidance for needle placement, lowering the necessary dose, according to a recent Italian study.

Nerve blocks used for regional anesthesia in knee surgery can be made safer using ultrasound guidance for needle placement, lowering the necessary dose, according to a recent Italian study.

Injecting anesthesia in the wrong place can cause local nerve damage, and even clinically acceptable doses of regional anesthetics have been reported to cause sudden acute cardiovascular toxicity and central nervous system toxicity to the point of coma and death. These risks are lessened as anesthesia volume drops.

Dr. Andrea Casati and colleagues in the anesthesia and pain therapy department at the University of Parma wondered if using ultrasound guidance to place the needle more precisely could allow them to use smaller doses of anesthetic.

Sixty patients undergoing arthroscopic knee surgery were given standard premedication and a sciatic nerve block. All subjects were given femoral nerve blocks using ropivacaine 0.5%. Thirty patients were injected using standard nerve stimulation for needle placement; needle placement for the other 30 patients was guided with ultrasound.

Researchers used 12 mL of anesthetic on the first patient in each group. Half an hour after injection, a blinded observer checked for sensation in the femoral nerve. If the patient felt a pinprick in two places, researchers increased the dose for the next patient by 3 mL. If there was no sensation half an hour after injection, the researchers decreased the next patient's dose by 3 mL. In this way they could fine-tune the range of effective anesthetic volume.

When results were examined, investigators found that the mean effective dose for the patients who had ultrasound guidance was 15 mL. The mean effective dose for those in the nerve stimulation group was 26 mL. Further statistical analysis supported these results. The researchers concluded that ultrasound guidance allowed them to reduce the volume of anesthetic by 42% compared with using nerve stimulation guidance.