A NEW TEST ACCURATELY IDENTIFIES THE THORACIC EPIDURAL SPACE

Abstract

S311 Introduction: The most common method used to identify the thoracic epidural (TE) space is the manual loss of resistance (LOR) to air or saline. [1] Occasionally, the LOR obtained may be equivocal, and the operator may be unsure whether the TE space has been identified. In such situations a test sensitive for the TE space would be of value. Since pressure drops as the epidural space is entered [1], a fluid column (FC) attached to a correctly placed TE needle should fall, especially with inspiration. Conversely, absence of a FC drop may suggest that the needle is located outside of the epidural space. We hypothesize that the FC drop technique is useful as a confirmatory test for the identification of TE space. The purpose of this study is to evaluate the clinical application of the FC drop test and determine its sensitivity and specificity as a marker of a correctly identified TE space. Methods: Following IRB approval and oral consent, 131 consecutive TE catheters (T2-L1) were placed in either the sitting (108) or lateral (23) positions via the midline (92) or paramedian (39) approach. After identification of the TE space by the LOR technique to air, a 50 cm extension tube filled with preservative free normal saline was attached to the 17 g Hustead epidural needle. We held the fluid filled tubing upright so that the full length of the tubing was above the level of the needle except at the point of attachment. A 10 cc filling syringe was then removed from the top of the extension tubing, and the fluid column meniscus was observed by the operator. The fluid column drop was assessed to be either spontaneous (immediately coinciding with syringe detachment), inspiratory (no spontaneous fall, but meniscus fell after patient instructed to take deep breath), or absent. A spontanous or inspiratory drop of the FC meniscus > 4 cm (two finger breaths) was considered a positive test by the operator. No movement or less than a 4 cm drop of the FC meniscus was considered a negative test. Catheters were inserted 4 cm beyond the needle regardless of the FC test result and were then dosed as clinically appropriate. Catheters were considered to be functional if a band of analgesia was present either pre or post operatively. Results: 132 catheters were placed in a total of 120 patients. 119 of the 121 functional catheters exhibited a positive FC test (119 true positives) with a FC drop ranging from 7-39 cm (mean drop 20 cm). Three functional catheters displayed a negative FC test (3 false negatives). Of the 11 non functional catheters, 8 had a negative FC test (8 true negatives), while three displayed a positive FC test (3 false positives). In the patients with a false positive test, one catheter was determined to be in a vein, while the other was identified by the surgeon during thoracotomy and appeared to have pierced the parietal pleura. The location of the third false positive catheter was not known. The extent of the FC drop (# of cm the meniscus fell from top of tubing) did not correlate with patient position, age, weight, midline or paramedian approach, or thoracic level. All false negatives and false positive tests were obtained in epidurals placed in the sitting patients via the midline approach but at various thoracic levels. The sensitivity (true positive/true positive + false negative) and specificity (true negative/true negative + false positive) of the FC test were determined to be 98% and 72% respectively. The test had a positive predictive value of 98% and a negative predictive value of 72%. The accuracy of the test was 96%. Discussion: The FC test has not been previously described, although pressure characteristics of the TE space have been extensively examined. [2,3] The high sensitivity (98%) indicates that a needle placed in the TE space will exhibit a positive FC test. Therefore, a negative FC test (no FC drop) indicates that the TE space has not been identified, and that the epidural needle needs to be repositioned. However, not all positive tests indicate correct needle placement (specificity = 72%). Intravenous and intrathoracic catheters are sources of false positives. Despite its lower specificity, the FC test is a fast, easy, inexpensive, and highly sensitive confirmatory test for the identification of the TE space by LOR technique.