Technical report: Optimal quantity of saline for epidural pressure waveform analysis

Abstract

Purpose: Epidural waveform analysis (EPWA) is a simple test to confirm the optimal placement of an epidural catheter. It relies on the transmission of the thecal pressure waveform through the epidural catheter. Little is known about the optimal volume of catheter injectate needed to optimize the EPWA test. Methods:In a randomized controlled trial we compared three different catheters: stimulating, open-ended hole [S1]; stimulating, three-hole [S3]; non-stimulating three-hole [NS3]) in 60 patients in an unblinded pilot study. After catheter insertion using a loss of resistance to air technique, we recorded the dose response curve of EPWA and the presence of epidural pressure wave transmitted through the catheter to repeated injections of 2.5 mL saline boluses in the catheter to a total volume of 10 mL. All patients were followed by the acute pain service to confirm adequacy of postoperative epidural analgesia. Results:The S3 catheters were significantly more responsive (able to transmit a pulsatile waveform) compared to the S1 [P = 0.003 (S3) and P = 0.012 (NS3)] at low injectate volume, while there was no differences between the S3 catheters in their ability to transmit a waveform for EPWA (P = 0.25). There was no increase in responsiveness after 2.5 mL injected for S3 catheters and 5 mL for the S1 catheter. Three patients had soft catheters that did not transmit an epidural pulsative waveform.Conclusions: For EPWAs, S3 epidural catheters are superior to S1 catheters for determining pulsative waveform transmission at low injectate volume. The optimal initial quantity of saline for EPWA is 5 mL regardless of the catheter type used. [P = 0,003 (S3) et P = 0,012 (NS3)

Reliability of Waveform Analysis as an Adjunct to Loss of Resistance for Thoracic Epidural Blocks

“…1 Although the sensitivity of EWA through the needle exceeds that of the catheter, 9 most trials have focused on catheter EWA. [10][11][12] In 2006, Lennox et al 9 assessed pulsatile waveforms through the epidural needle after obtaining LOR. These authors reported 97.5% sensitivity, 100% specificity, 100% positive predictive value, and 50% negative predictive value.…”

“…We speculate that false-negative waveforms were due to the administration of suboptimal volumes of normal saline through the epidural needle before EWA. Our protocol used a 5-mL volume based on the findings of de Médicis et al 12 These authors advocated 5-mL injectates because of the minimal differences seen between 5, 7.5, and 10 mL; however, 5-mL volumes yielded positive waveforms in only 90% of their subjects. 12 Thus, we hypothesize that, in clinical practice, the rate of false-negative waveforms could be decreased with higher injectates of normal saline.…”

“…Our protocol used a 5-mL volume based on the findings of de Médicis et al 12 These authors advocated 5-mL injectates because of the minimal differences seen between 5, 7.5, and 10 mL; however, 5-mL volumes yielded positive waveforms in only 90% of their subjects. 12 Thus, we hypothesize that, in clinical practice, the rate of false-negative waveforms could be decreased with higher injectates of normal saline. We speculate that our false-positive waveforms stemmed from the definition of success (sensory block to ice at 10 minutes).…”

Reliability of pressure waveform analysis to determine correct epidural needle placement in labouring women

“…Our protocol employed a 5 ml saline injection through the epidural needle before waveform analysis, based on the findings of de M edicis et al [15]. These authors observed minimal differences between 5, 7.5 and 10 ml; however, 5-ml volumes yielded positive waveforms in only 90% of their subjects.…”

A Randomized Comparison Between Conventional and Waveform-Confirmed Loss of Resistance for Thoracic Epidural Blocks

“…9,[12][13][14][15] However, due to false-positive waveforms, block failure remains a possibility despite EWA. 9 Furthermore, falsenegative waveforms may lead to unnecessary attempts and a higher rate of dural puncture or vascular breach.…”