Body mass and spread of spinal anesthesia with bupivacaine

The effect of body mass on spinal anesthesia with isobaric 0.5% bupivacaine was examined in 90 patients. The first 50 patients received 3 ml isobaric 0.5% bupivacaine. Another 40 patients, selected at random to receive 3 ml of either hyperbaric or isobaric bupivacaine 0.5%, were then studied. Levels of pin-prick analgesia and motor block were tested during induction, surgery, and recovery. The effects of high spinal anesthesia on peak expiratory flow (PEF) and arterial blood gas tensions were also examined in the last 40 patients. With isobaric bupivacaine, persons with higher than normal BMI [body mass index = weight (kg) divided by height-squared (m2)], or persons who were shorter than normal had higher cephalad spread of anesthesia. With hyperbaric bupivacaine only shorter individuals developed higher levels of anesthesia. Because of considerable interindividual variability, however, these observations are of only limited clinical value in predicting the spread of bupivacaine spinal analgesia. The higher spread of analgesia was associated with a decrease in PEF, whereas, the blood gas tensions remained undisturbed.