Epidural Infusion

To the Editor: The role of lumbar epidural analgesia for labor and delivery pain relief is well established because of excellent analgesia achieved with few side effects. There are presently two types of catheters in use: single-orifice and multiorifice. Logically, a multiorifice catheter should produce better sensory blockade than a single-orifice catheter, because the injected drug may have a better chance of spread. Segal et al. [1] and D'Angelo et al. [2] reported that multiorifice catheters are superior to single-orifice catheters. Furthermore, Michael et al. [3] observed that unilateral and missed segments were more common when a single-orifice catheter was used. However, Morrison and Buchanan [4] did not observe any differences in the quality of sensory analgesia between the two catheters. Magides et al. [5] used an epidurography technique to study the spread of injected iohexol, a nonionic, water-soluble, radioopaque contrast medium, in the epidural space. They found no difference between the single- and multiorifice catheters in the spread of the dye above and below the site of insertion. Therefore, the efficacy of the multiorifice catheter compared with the single-orifice catheter is still in dispute. One of the reasons for this unpredictable efficacy of multiorifice catheters may be the differential flow that can occur with this catheter. In differential flow [6], the flow appears first at the proximal, then middle, then the distal hole in the multiorifice catheters. Flow is also shown largest from the proximal hole, particularly at low injection pressures, and no flow was observed from the distal port. Hence, the multiorifice catheter behaved as a single-orifice catheter. We designed an experiment to compare the area of diffusion of the contrast agent from a multiorifice catheter under different infusion modalities (Figure 1). Methylene blue in isotonic sodium chloride solution was delivered via a continuous infusion or intermittent boluses. Using a 10.5-mL/h infusion, there was practically no flow through the distal hole, whereas with 3.5-mL boluses delivered over 1 min every 20 min, the dye solution flowed through all the pores. On placing the catheters on a white semiabsorbent paper, wider spread of the dye was observed during a 60-min study interval (0.26 vs 1.4 in. [2] for continuous infusion versus intermittent bolus) with the multiorifice catheter compared with the single-orifice catheter. The area of distribution was measured using the NIH.Image 1.61[registered sign] software (National Institutes of Health, Bethesda, MD).Figure 1: Area of diffusion of the contrast agent during continuous and intermittent infusions.In this experiment, we showed that intermittent bolusing of the epidural catheter has a wider spread, which probably contributes to the better quality of the block in the clinical setting. This hypothesis is currently being investigated at Brigham & Women's Hospital. A. Murat Kaynar, MD K. B.