Loss of Resistance to Saline with a Bubble of Air to Identify the Epidural Space in Infants and Children: A Prospective Study

Abstract

Since 1986, epidural analgesia has been routinely used in our hospital to provide postoperative analgesia for children undergoing major surgery. Identification of the epidural space is accomplished by using the loss of resistance to saline with a bubble of air. This technique is used in our hospital to identify the epidural space in adults, and its use in children has already been suggested (1). As this technique has not yet been described in pediatric patients, we prospectively collected data to evaluate the incidence of technical problems at the time of identification of the epidural space and of subsequent insertion of an epidural catheter in infants and children. Methods All epidural catheters inserted in children <15 yr old, from October, 1995, to June, 1998, were included in the study. Epidural analgesia was used for major surgery with parental consent. Contraindications were local or general infection, congenital or acquired coagulation disorders, vertebral or skin abnormalities at the level of puncture, or lack of consent. All epidural catheters were inserted after the induction of general anesthesia. A 19-gauge Tuohy needle was used in children weighing <12 kg, and a 18-gauge needle was used in children weighing >12 kg. The epidural space was punctured at the dermatomal level involved by surgery. The patient was positioned in the lateral position with the legs slightly flexed. Most epidural blocks were performed by anesthesiologists in training, but experienced in adult epidural blockade and under the direct supervision of a consultant anesthesiologist. As a rule, a maximum of three attempts was allowed to the trainee. In case of failure to identify the epidural space after these three attempts, the consultant performed the block. Demographic and technical data were prospectively collected on a special data sheet. The median approach and “the loss of resistance to saline with a bubble of air” technique were used. This technique consists of using a 2- or 5-mL syringe filled with saline and a small bubble of air (±0.5 mL), taking care to maintain the bubble of air at some distance from the hub of the Tuohy needle (to avoid injecting air into the epidural space). The progression toward the epidural space is discontinuous, on a mm-per-mm basis, as if the loss of resistance to injection of air were used. As long as the tip of the epidural needle is located in the different ligaments between the skin and the epidural space, there is some resistance to injection, and the volume of the bubble of air diminishes when pressure is applied on the plunger of the syringe. But once the tip of the needle pierces the ligamentum flavum, a brisk loss of resistance to injection is felt, and the volume of the bubble of air does not change. However, if resistance to injection is felt but the volume of the bubble of air does not change, a dysfunction of the syringe plunger should be suspected, and another syringe should be used to perform the epidural block. The bevel of the Tuohy needle is directed cephalad from the start to avoid turning the needle once it is in the epidural space (2,3). The epidural catheter was threaded 3 or 4 cm into the epidural space. Absence of blood or cerebrospinal fluid reflux was carefully checked. The patient was then placed supine, and a test dose containing epinephrine 1/200 000 was slowly injected, followed by the total dose of local anesthetic solution. χ2 test and Yates correction, when appropriate, or Fischer’s exact test was used to assess statistical significance. Differences were considered statistically significant when the P value was <0.05. Results Four hundred epidural blocks were performed. Indications included abdominal surgery in 109 (27%) cases, urological surgery in 195 (48%) cases, orthopedic procedures in 81 (20%) cases, and other indications in 6 cases (e.g., terminal cancer pain, complex regional pain syndromes). Except for the two dural taps, no failure occurred. The epidural space was identified at first attempt in 60.4% (55 of 91) of cases in which the puncture was performed above T-10 level and in 77.6% (149 of 192) of cases (P = 0.042) in which it was performed at or under the L2 level. The number of attempts needed to identify the epidural space according to the child’s body weight are shown in Table 1. Overall, the epidural space was identified on the first attempt in >71.5% of cases. However, the epidural space was identified on the first attempt in only 50% of cases in the group weighing <5 kg (13 of 26 vs 271 of 371;P = 0.022). The distribution of the technical difficulties according to the children’s body weight is shown in Table 2. Identifying the epidural space was significantly more difficult in children weighing <20 kg (22% [62 of 278] vs 11% [14 of 121];P = 0.016). In the same way, threading the epidural catheter was significantly more difficult in children weighing <10 kg (15% [20 of 137] vs 3.7% [10 of 265];P = <0.001). There was no statistically significant difference among the groups regarding the presence of blood in the epidural catheter (P = 0.936). We recorded two dural taps in our series.Table 1: Number of Attempts Needed to Identify the Epidural Space According to the Child’s Body Weight (BW)Table 2: Distribution of the Technical Difficulties Encountered According to the Child’s Body Weight (BW)Discussion In our series, using the loss of resistance to saline with a bubble of air technique, we observed an incidence of dural tap of 0.5% (2 of 400). By comparison, in Dalens’ series (4), the incidence of dural puncture was 2.3% (5 of 216) when loss of resistance to air was used, 2% (6 of 293) when loss of resistance to carbon dioxide (CO2) was used, and 5.6% (9 of 141) when loss of resistance to saline was used. Similarly, Yamashita and Tsuji (5) observed an incidence of dural tap of 1.43% (5 of 350) when using a microdrip infusion set. Using a pressure-guided method, Suwa et al. (6) observed an incidence of dural tap of 0% (0 of 20) vs 4% in his control group (loss of resistance to saline). While the technique described appears safe, it has several other advantages. First, when loss of resistance to saline is used, a continuous pressure is applied to the plunger of the syringe, and a variable amount of saline is thus injected into the epidural space when it is entered. This may result in the dilution of the local anesthetic solution injected and a less extensive sensory block (7). Moreover, it can raise doubts about a dural effraction (or make it undiagnosed) when a backflow of clear fluid occurs. When the loss of resistance to saline with a bubble of air is used and discontinuous pressure is applied on the plunger of the syringe, the identification of the different ligaments between the skin and the epidural space is easier. Therefore, a smaller volume of saline is usually injected into the epidural space, thus reducing the risk of undiagnosed dural tap. Second, the loss of resistance to air is potentially dangerous because there is a risk of venous air embolism (1,8). There is also a possibility of compression of nervous roots or of maldistribution of analgesia if air accumulates into the epidural space (4,8). When the “saline with a bubble of air technique” is used properly, no air is actually injected into the epidural space. The loss of resistance to CO2 technique is an alternative to air, but is more cumbersome to use, despite the universal availability of CO2 cylinders for coelioscopic surgery. Last but not least, another advantage of the loss of resistance to saline with a bubble of air technique is educational: when the senior anesthesiologists supervise a trainee using this technique, they cannot feel the resistance to injection but are able to observe the deformation of the bubble of air in the syringe and thus to follow the progression of the Tuohy needle. Moreover, this technique can be used with glass, plastic, or special “low-resistance” syringes. The number of attempts required in our series to identify the epidural space is similar to those reported by Dalens (4), (i.e., one attempt in 64.4% and 79.7% of the cases when the median approach was used at the thoracic and lumbar levels, respectively). In our experience, the lower success rate in infants weighing less than 10 kilograms was mostly a result of difficulty identifying the narrow interspinous ligament and remaining in a median plane at the start of the puncture. In conclusion, we recommend using the loss of resistance to saline with a bubble of air technique to identify the epidural space in infants and children because of its low incidence of dural taps, even when used by anesthesiologists in training and because it is easy to teach and to learn.