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

Abstract

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 (Nsp1) and two positive (Psp1 and Psp2) peaks were identified in all tracings except for two calves. There was a significant effect of isoflurane on the latency of Psp2 (P = 0.01). Inter-amplitude decreased significantly with PaO2, PaCO2 and temperature (P < 0.05). Psp2 latency decreased with PaO2 (P = 0.01). For c-SSEP: two positive (Pc1 and Pc2) and two negative (Nc1 and Nc2) peaks were identified. There were identifiable peaks for the analysis of Pc1 latencies only. There was a significant positive linear relation between end-tidal isoflurane partial pressure (ETiso) and Pc1 latency (P = 0.04). None of the co-variables had a significant effect on the latency of Pc1 (P > 0.1). Isoflurane has a major impact on the recording of c-SSEP. Recording should be done at the lowest ETiso as possible, and anesthesia parameters should be kept constant.

Figure 1

Experimentation protocol. SSEP — somatosensory-evoked potential. ET_iso — end-tidal isoflurane partial pressure. ET_iso 1 to 4 refers to the chronological sequence of the different ET_iso but not the real ET_iso tracings, which were randomized. During equilibrium, ET_iso was set at 1.3%.

Figure 2

Illustrated tracings showing latencies and peak-to-peak amplitudes of spinal and cortical recordings. A — Cortical somatosensoryevoked potentials (c-SSEPs). P_c1, N_c1, P_c2, and N_c2 identify the different peaks. 1 to 4: c-SSEP latency of P_c1, N_c1, P_c2, and N_c2 respectively. 5 to 7: P_c1 to N_c1, N_c1 to P_c2, P_c2 to N_c2 peak-to-peak amplitudes respectively. B — Spinal somatosensory-evoked potentials (s-SSEPs). P_sp1, N_sp1, and P_sp2 identify the different peaks. 1 to 3: s-SSEP latency of P_sp1, N_sp1, and P_sp2 respectively. 4 to 5: P_sp1 to N_sp1 and N_sp1 to P_sp2 peak-to-peak amplitudes.

Figure 3

Typical tracings from different individuals observed during the study. A to D — cortical somatosensory-evoked potentials (c-SSEPs). E to F — Spinal somatosensory-evoked potentials (s-SSEPs). A — Typical c-SSEP with 4 well defined peaks: P_c1, N_c1, P_c2, and N_c2. B — c-SSEP in which the different peaks were difficult to identify. C — c-SSEP in which only P_c1 could be identified. D — c-SSEP in which no peaks could be identified. E — A typical s-SSEP with its well defined 3 peaks: P_sp1, N_sp1, and P_sp2. F — s-SSEP in which movement artefacts were observed after the s-SSEP was recorded.