Abstract ID: A04

Abstract Title: Peak Injection Pressure Is A Better Monitor Of Intraneural Injection Than The Rate Of Pressure Rise

Poster Type: Either

ABSTRACT BODY

Introduction: Abnormal resistance to injection (Ён20psi) may indicate intraneural needle placement and increase the risk of neurologic injury. Because during peripheral nerve block (PNB), the peak pressure cannot be determined until it has passed, we sought to determine whether the rate of rise of injection pressure (IP) is superior to peak IP in predicting the development of neurologic injury. Our hypothesis is that real-time evaluation of momentary rate of rise of IP provides a window of opportunity during PNB to detect intraneural injection and abort the injection.

Materials: The study was conducted in accordance with the principles of laboratory animal care. Fifteen dogs of mixed breed (16-21kg) were given general anesthesia, and the sciatic nerves (n=30) were exposed bilaterally. Under direct microscopic guidance, a 25G needle was placed either perineurally (n=10) or intraneurally (n=20) and 4mL of preservative-free lidocaine 2% was injected using an automated infusion pump (4ml/min). IP data were acquired using an in-line manometer coupled to a computer via an analog-digital conversion board. After nerve block injection, the animals were awakened and subjected to serial neurologic examinations. The earliest injection time at which the peri- and intraneural groups diverged was assessed graphically with means and confidence limits. Discriminant analysis was used to determine whether the rate of rise of IP was superior to peak IP to segregate injections into peri- and intraneural injections. The co-linearity among the neurological measures was assessed with correlation analysis to determine if there was redundancy.

Results: All perineural injections resulted in pressures Ём5psi, and 8 of the intraneural injections resulted in high pressures (20-38psi) at the beginning of the PNB. Motor function returned within 3hrs with all perineural injections. Neurologic deficits persisted throughout the study period with all 8 intraneural injections resulting in high IP (>20psi). A discriminant analysis to determine which aspect of the IP recording provided the best segregation of peri- and intraneural injections showed peak pressure and slope-to-peak both provided 100% sensitivity and specificity. Examination of means and 95% confidence limits of the peri- and intraneural group showed that the earliest time at which the lowest pressure of the intraneural group exceeded the highest pressure of the perineural group was 72 seconds (Figure 1).

Discussion: Since high IP (>20psi) during PNB predicts the development of neurologic injury, we attempted to identify whether real-time evaluation of the build-up of nerve block injection pressure could provide a basis for action to prevent intrafascicular injury. In discriminant analyses both peak pressure and average slope-to-peak well discriminated the peri- and intraneural preparations. However, since the average time-to-peak in the intraneural group was 80 secs, rate of pressure rise did not offer a meaningful informational advantage over simple monitoring of absolute pressure.

ATTACHED FILES

Reg Anesth Pain Med 2004; 29(2):A04