To the Editor:- We read with interest the article “Intrathecal Sufentanil for Labor Analgesia Does Not Cause a Sympathectomy,” by Riley et al. Using calf, toe, and core temperature analysis, the authors detected a predictably marked sympathectomy in patients receiving 12 mg intrathecal bupivicaine for elective cesarean section (BUP-CS group). The temperature changes included a decrease in core temperature and a decrease in the calf-to-toe temperature (C-T) index. Using the same technique the authors failed to observed any change in these temperature indices in laboring parturients receiving 10 [micro sign]g intrathecal sufentanil (ITS-LW group) and suggested that blood pressure reductions observed after ITS may be the result of pain relief and not the result of sympathectomy.
Skin temperature measurements are an indirect, qualitative method and do not necessarily reflect the true intensity of the sympathetic block. Matsukava et al., using changes in skin temperature as a marker of sympathetic block, found that epidural anesthesia (bilateral T10 sensory level in all cases) produced a bilateral sympathetic block in only 6 of 12 healthy volunteers. Although Riley et al. reported results in intrathecal and not in epidural anesthesia, Stevens et al. found no significant difference between the degree of sympathetic block after spinal or epidural anesthesia.
However, the central problem with this study is that the reference group with presumed sympathectomy (BUP-CS group) is markedly dissimilar to the study group (ITS-LW). The authors cite the decrease in the central temperature in the BUP-CS group as evidence of sympathectomy and conclude that the lack of core hypothermia in the ITS-LW group argues against the presence of sympathetic blockade in these patients. Sympathectomy and redistribution are not the only causes of hypothermia; net loss of heat to the environment (not measured in this study) is another important factor. In the BUP-CS group, as opposed to the ITS-LW group, the exposure of patients to typically cold operating room conditions (ambient temperature was not measured), twofold increase in cold intravenous fluid loading, skin preparation with cold solutions, and abdominal surgical exposure are all factors that increase heat loss to the environment.
Even assuming (as seems reasonable) that sympathectomy was present in the BUP-CS group, the thermal dissimilarities of the two groups prevent extrapolating the data and hence do not support the contention that sympathectomy was not present in the ITS-LW group. The authors argue that most of the thermal differences (together with prophylactic ephedrine administration in the BUP-CS group) will cause vasoconstriction in the BUP-CS group, thereby increasing the significance of the vasodilation observed in these patients. We disagree; vasoconstriction before the induction of sympathetic blockade amplifies the thermal markers of a subsequent sympathectomy.
Skin thermal testing used alone to diagnose the presence or absence of sympathectomy is a relatively gross assessment largely the result of the thermal variables involved. If choosing to use this method, greater care should be taken in study design to eliminate as many of these variables as possible.
Elyad Davidson, M.D.
Yehuda Ginosar, B.Sc., M.B.B.S.
Department of Anesthesiology and Critical Care Medicine; Hadassah University Hospital; Hebrew University; Jerusalem, Israel