Effects of isoflurane on somatosensory-evoked potentials in calves: A pilot study

Somatosensory evoked potentials (SSEP) are used to monitor sensory function and are often recorded under general anesthesia. The objective of the study was to evaluate the effects of isoflurane on SSEPs in calves as it has not been reported. Eight calves (mean age: 40 days), were included in the study. Calves were anesthetized with a randomized sequence of four different isoflurane partial pressures. Blood gas analysis was performed before each measurement. SSEP were induced by repeated stimulation of the common dorsal digital nerve III. SSEPs were recorded from the lumbo-sacral junction (s-SSEP) and the head (c-SSEP). Latency and inter-amplitude of each peak were measured. For s-SSEP: One negative (N_sp1) and two positive (P_sp1 and P_sp2) peaks were identified in all tracings except for two calves. There was a significant effect of isoflurane on the latency of P_sp2 (P = 0.01). Inter-amplitude decreased significantly with PaO₂, PaCO₂ and temperature (P < 0.05). P_sp2 latency decreased with PaO₂ (P = 0.01). For c-SSEP: two positive (P_c1 and P_c2) and two negative (N_c1 and N_c2) peaks were identified. There were identifiable peaks for the analysis of P_c1 latencies only. There was a significant positive linear relation between end-tidal isoflurane partial pressure (ET_iso) and P_c1 latency (P = 0.04). None of the co-variables had a significant effect on the latency of P_c1 (P > 0.1). Isoflurane has a major impact on the recording of c-SSEP. Recording should be done at the lowest ET_iso as possible, and anesthesia parameters should be kept constant.