To the Editor:—
The article by Heller et al. 1was informative and adds precision to the effect of temperature on the baricity of local anesthetics. They provide interesting new data on the temperature at which local anesthetics used for spinal anesthesia are isobaric (the “isobaric temperature”). Clearly, the next question is, what is the clinical relevance of this added precision? The authors themselves state, “Whether this concept in fact improves patient safety in terms of hemodynamic stability or even allows dose reductions of local anesthetics must be confirmed in further clinical studies.”
In 1989, Beardsworth and I published a simple study comparing the injection of 3 ml plain 0.5% bupivacaine at room temperature to an identical solution adjusted to 37°C (very close to but not precisely within the limits [34.3°–35.8°C] of the so-called isobaric temperature).2The injection was performed with the patients in the lateral decubitus position, and they were then immediately turned to the supine horizontal position. For the same reasons indicated by Heller et al. , we hypothesized that increasing the temperature of the bupivacaine would make it more isobaric and limit its spread. We found no difference in the extent of pinprick analgesia. However, the 37°C solution produced a more prolonged block, which we suggested was due to a decrease in pKa associated with the increased temperature.
Beardsworth's study compared but one dose of bupivacaine and one position after its injection. Other doses and patient positions will likely produce different results.
Heller's and Beardsworth's studies beg the question as to whether it is possible (with the exception of using a very hyperbaric solution for saddle or thoracic levels of block) to precisely control the level of spinal anesthesia. Although the temperature effect on the baricity of local anesthetics used for spinal anesthesia reported by Heller has achieved this pinnacle of precision, this effect will have to overcome the manifold factors that control the level of spinal anesthesia3to significantly impact clinical practice. Whether this can be accomplished will only be determined through clinical trials that will undoubtedly derive from Heller's publication. However, after 25 yr of studying, practicing, and watching spinal anesthesia, I suspect that the precise control of the level of spinal anesthesia will require more than simply adjusting the temperature of the injected local anesthetic.