Middle-latency auditory evoked potentials during induction of thiopentone anaesthesia in pigs

Abstract

A method is described for measuring middle-latency auditory evoked potentials (MLAEP) in consciously awake, non-sedated pigs during the induction of thiopentone anaesthesia (0.6 ml/kg, 2.5% thiopentone solution). It was done by using autoregressive modelling with an exogenous input (ARX). The ability to perceive pain during the induction was compared with (1) the changes in latencies and amplitudes of the MLAEP, (2) the change in a depth of anaesthesia index based on the ARX-model and (3) the change in the 95% spectral edge frequency. The pre-induction MLAEP was easily recordable and looked much like the one in man, dogs and rats. The temporal resolution in the ARX method was sufficiently high to describe the fast changes occurring during induction of thiopentone anaesthesia. As previously reported from studies in man, dogs and rats, induction of thiopentone anaesthesia resulted in significantly increased latencies and decreased amplitudes of the MLAEP trace as well as in a significantly reduced depth of anaesthesia index and spectral edge frequency. None of the changes, however, related well to the ability to react to a painful stimulus. Whether an ARX-based depth of anaesthesia index designed especially for pigs might be better than the present index (designed for man) for assessing depth of anaesthesia must await the results of further studies.

Assessment of unconsciousness in pigs during exposure to nitrogen and carbon dioxide mixtures

“…This delay in the onset of unconsciousness in the results of Raj (1999) and Martoft et al (2001) was also found by Rodriguez et al (2008) who monitored the brain activity using AEP and concluded that the onset of unconsciousness occurred after 60 s of exposure to 90 CO 2 . Raj (1999) and Martoft et al (2001) obtained different results, but in both studies pigs were hung in a hammock and rapidly immersed in a box that contained the modified atmosphere. However, in our experiment, as well as in that by Rodriguez et al (2008), the methodology of the gas exposure simulated commercial conditions, where pigs are immersed gradually to the required concentration in the base of a pit.…”

“…Statistical analysis Data were analysed with the Statistical Analysis System (SAS 9.2, SAS Institute Inc., Cary, NC, 1999-2001. Latency measures (such as time to the first gasping, loss of balance, vocalization, onset and duration of muscular excitation, the time to reappear pain sensitivity, rhythmic breathing and corneal reflex), IoC and ESR data were analysed using a mixed model ANOVA (PROC MIXED) with a covariance structure of compounds symmetric with 'gas mixture' and 'time of exposure' as fixed effects.…”

“…They stated that pigs that were exposed to 80% CO 2 lost brain responsiveness later (21 s) than those exposed to 30% CO 2 and 60% of argon in air (17 s) and to 90% argon (15 s). Martoft et al (2001), using the AEP, concluded that pigs exposed to 90% CO 2 lost brain responsiveness on average at 14 s.…”

“…Numerical indexes, based on the objective assessment of the electroencephalography (EEG), have been used by several authors to assess unconsciousness during stunning in pigs (Raj et al, 1997;Martoft et al, 2001;Rodriguez et al, 2008). They provide an on-time index of the brain activity that reports, by means of a numerical scale, the state of consciousness of the individual.…”

Inflation lung mechanics deteriorates markedly after saline instillation and open endotracheal suctioning in mechanically ventilated healthy piglets

“…The trachea was orally intubated (4.0 mm) and anesthesia was maintained with continuous infusions of fentanyl (30 mg Á kg À1 Á hr À1 ), midazolam (0.4 mg Á kg À1 Á hr À1 ), and propofol (3 mg Á kg À1 Á hr À1 ). The depth of anesthesia was monitored by auditory evoked potential (A-Line AEP monitor, Danmeter, Odense, Denmark) as described by Martoft,7 and the AAI-index kept below 30. The piglet was ventilated with volumecontrolled mechanical ventilation (Servo 900 C, Solna, Sweden) with FiO 2 0.7, tidal volume 8 ml Á kg À1 , PEEP 5 cmH 2 O, and the respiratory rate was adjusted to keep PaCO 2 between 5.5-6.5 kPa.…”