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In Reply:-Drs. Parker and Behringer correctly point out that in our review 1 on the potential toxic effects of nitrous oxide we did not cover its contribution to the greenhouse effect; this omission was predicated by considerations of space, direct relevance to patient care, and a lack of relevant data that is less than 20 yr old. Correctly, Drs. Parker and Behringer indicate that nitrous oxide contributes to the greenhouse effect (Ϯ0.05%); however, as 99% of the nitrous oxide in the atmosphere originates from industrial and agricultural sources, the total elimination of medical use of nitrous oxide will likely have a negligible effect on the greenhouse effect. What also needs to be considered is the effect of increased use of another greenhouse gas, namely halogenated anesthetics, as a possible replacement for nitrous oxide. Furthermore, we need to understand the totality of the environmental impact of the volatile gases, and that includes its manufacture, transport, and storage. In absence of reliable data on these issues, it would be too simplistic to state that there would be an environmental benefit were we to abandon the anesthetic use of nitrous oxide. Lumbar Plexus or Lumbar Paravertebral Blocks?To the Editor:-We read with interest the report of Gadsden et al. 1implicating the role of high-pressure injection during the performance of lumbar plexus blocks in producing contralateral and epidural spread of local anesthetic in more than 50% of their patients. We wish to make three points regarding this report.1. Does one or more of the authors have any financial interest in the device used in the study? If so, it would have been proper to disclose this. 2. It is important to emphasize that both injection under a higher pressure and a large volume of injectate (35 ml in this study) constitute "necessary but insufficient conditions" for epidural/ contralateral spread of local anesthetic. That is, high pressure alone with a small volume injectate will likely not lead to epidural/contralateral spread of the local anesthetic. Likewise, as the authors showed, one can inject substantial amounts of local anesthetic under low pressure without significant risk of this complication. In our practice nearly all lumbar plexus blocks involve placement of a continuous catheter, and it has been our experience that even large-volume injection through these catheters does not lead to bilateral blockade. Of course, it is impossible to generate high pressures with such an injection because of the high resistance offered by the catheter, thereby obviating the need for an injection pressure monitoring device. 3. A lumbar plexus block is not a procedure with a consistently defined anatomic end point and really consists of two separate blocks-the psoas sheath block and the psoas compartment block-either of which result in blockade of the lumbar plexus. To add to the confusion, these terms are often used incorrectly and interchangeably. The former involves injection within the psoas sheath and into the body of the psoas muscle. The ...

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