Subarachnoid spread of isobaric tetracaine in adolescents.

Sir,—In a recent report, Hirabayashi and colleagues [1] showed that subarachnoid administration of hyperbaric amethocaine (tetracaine) produced an unexpectedly higher level of analgesia in adolescents than in adults. The authors postulated that when adolescents lie in the supine horizontal position, the thoracic kyphotic curvature decreases because of the flexibility of their vertebral columns which become straighter at the upper part of the thoracic vertebrae. In our recent investigation [2], we studied 50 patients (17 adolescents, aged 13–16 yr and 33 adult patients aged 17–68 yr) who received isobaric tetracaine 10 mg in 2 ml. The anaesthetic solution was prepared from 1.0 % tetracaine solution with CSF. As in the study of Hirabayashi and colleagues, the drug was administered with the patient in the lateral position, at the L3–4 interspace, at a speed of 0.2 ml s 1, with the patient remaining in the supine horizontal position during the study. The extent of analgesia was assessed by loss of sensation to pinprick, and motor block according to the Bromage scale at 2-min intervals for 20 min and then every 15–20 min until full recovery. There were no differences in physical characteristics between the adolescent and adult groups; the median peak/highest sensory levels were T7 (range T11–3) and T7 (T10–3), respectively. This difference was not statistically significant (P 0.5). We also used 2 ml of anaesthetic solution, as did Hirabayashi and colleagues but the tetracaine dose that we used was 2 mg greater. When a patient is in the supine horizontal position, a hyperbaric solution spreads cephalad because of gravity and is therefore affected by the anatomical configuration and flexibility of the vertebral column. Adolescent patients have greater flexibility and thus a flatter thoracic curvature, which leads to a higher anaesthetic level. As gravity has little, if any, effect on the spread of isobaric solutions, the peak/highest levels of anaesthesia in our study did not differ significantly. The difference in anaesthetic levels between adolescent and adult patients observed by Hirabayashi and co-workers did not occur in our patients with isobaric tetracaine. A review of the literature has shown some correlation between patient age and subarachnoid spread of isobaric anaesthetic solution [3, 4]. However, these studies did not compare adolescents with adults, but young ( 30 yr) with elderly adults ( 70 yr). In addition, they were performed with 0.5 % plain bupivacaine solution which is slightly hypobaric at body temperature [5] and the results were poorly predictable [4, 5]. Although not a formal study, our data support the results of Hirabayashi and colleagues who concluded that the higher anaesthesia level in adolescent patients with hyperbaric spinal anaesthesia was probably caused by a flatter thoracic curvature. This problem does not arise with an isobaric anaesthetic solution which may therefore be preferable in adolescent patients.