To the Editor:-We read with interest the study of Biboulet et al., which shows that the orientation of the catheter tip has a major impact on the spread of local anesthetics (LA) irrespective of their baricity. 
These results are consistent with data from other studies using 28-gauge microcatheters and isobaric bupivacaine [2,3]and show convincingly that caudally positioned catheter tips are associated with a higher requirement of LA to reach the respective sensory level than cranially directed catheter tips. The results suggest that caudally extended catheter tip levels cause maldistribution of the LA and that cranially oriented catheter tips should be obtained in CSA.
However, the authors give no information how a cranial catheter tip position can be obtained in CSA. They do not mention the respective literature that relates to this issue. Data from a study of Ata et al. suggest that the paramedian lumbar approach may facilitate the spinal catheter passage into the subarachnoid space and possibly may provide more cranially directed catheters.* Yurino et al. described a preformed, coiled-tipped microcatheter that could easily be placed in the subarachnoid space within 2 min and provide adequate blocks in 94% of patients, suggesting that these catheters remained at the level of the puncture site or took a cranial direction. In one study we investigated the impact of different needle designs, directional and nondirectional, on the intrathecal catheter tip position. The use of a nondirectional Quincke needle was associated with a 35% incidence of caudad catheter tip positions, and the directional Sprotte needle provided no caudally positioned tips of the 28-gauge catheters.
The results of the three studies suggest that technical modifications can help to increase the percentage of spinal catheters with the intended cranial tip position.
Second, we would like to stress the different injection speeds of micro- and macrocatheters. The fact that maldistribution was not significantly related to the baricity of the applied LA in Biboulet's study may give the impression that hyperbaric LAs can be used in combination with microspinal catheters in the same way.
Rigler and Drasner clearly demonstrated in 1991 that a low injection speed produces inadequate distribution of dye-colored hyperbaric lidocaine. The mean injection time of 1.0 ml is 50 s for 32-gauge and 28 s for 28-gauge microcatheters compared with 3 s for 20-gauge, large-bore catheters.
Therefore, the reduced injection speed through microcatheters appears to be an additional factor, besides the orientation of the catheter tip, which may significantly enhance the risk for maldistribution of hyperbaric LA. The combination of the reduced injection speed through microcatheters, high doses of hyperbaric LA, and caudally oriented catheter tips have produced significant maldistribution in models of the spinal canal and probably have caused the well-known cauda equina syndrome as a result of toxic concentrations of the hyperbaric LA at the dorsal sacral spinal nerves. [7-9]
Although the results of the spinal canal models cannot completely be transferred on patients in whom lower concentrations of hyperbaric LA may occur because of diffusion and vascular uptake, and although neurologic sequelae have also been reported after macrocatheter CSA, the conclusion in Biboulet's article that “hyperbaric solutions do not appear to be a clinical factor in the development of limited block” should be taken with caution. In our opinion, this conclusion is only justified for the use of large-bore catheters and must not be transferred to microcatheter CSA.
As a consequence we would like to recommend techniques that facilitate cranial catheter tip placement and advise against the combination of microcatheters and hyperbaric LA for CSA. In light of these aspects we agree with the statement of the authors that we should not discourage the use of microcatheters for CSA.
Thomas G. Standl, M.D.
Associate Professor of Anesthesiology; Department of Anesthesiology; University Hospital Eppendorf; Hamburg, Germany;firstname.lastname@example.org
* Ata S, Shulman MS: Causes for the difficulty with placement of continuous subarachnoid catheters. Anesthesiology 1991; 75:A1092
(Accepted for publication November 23, 1998.)