Influence of hemorrhage and subsequent fluid resuscitation on transcranial motor-evoked potentials under desflurane anesthesia in a swine model.

PURPOSE Hemorrhage increases the effect of propofol and could contribute to false-positive transcranial motor-evoked potential (TcMEP) responses under total intravenous anesthesia (TIVA). We investigated the influence of hemorrhage and subsequent fluid resuscitation on TcMEPs under desflurane anesthesia. METHODS Sixteen swine (25.4 ± 0.4 kg) were anesthetized with a 4% end-tidal desflurane concentration (EtDes), which was incrementally increased to 6%, 8%, and 10% and then returned to 4% every 15 min. This procedure was repeated twice (baseline). After baseline measurements, animals were allocated to either the hemorrhage (n = 12) or control (n = 4) group. In the hemorrhage group, 600 ml of blood was removed and the EtDes protocol described above was applied. Hypovolemia was resuscitated using 600 ml of hydroxyethyl starch and the EtDes protocol was applied again. TcMEPs were measured at each EtDes. In the control group, measurements were performed without hemorrhage or fluid infusion. RESULTS TcMEP responses were observed in all conditions in all limbs with 4% EtDes (0.4 MAC). TcMEP amplitudes decreased according to the EtDes to a greater degree in the lower limbs compared with the upper limbs. Hemorrhage enhanced the effect of desflurane on TcMEP amplitudes, and decreased TcMEP by 41 ± 12% in upper limbs and 63 ± 17% in lower limbs compared with baseline. Subsequent fluid resuscitation did not reverse TcMEP amplitudes. CONCLUSIONS TcMEP amplitudes decrease during hemorrhage under desflurane anesthesia. This phenomenon might result from an enhanced effect of desflurane on the spinal motor pathway without increasing the desflurane concentration.