Comparison of EEG responses to anaesthesia in birds and mammals

The EEG responses to anaesthesia are qualitatively similar between the chickens in the current study and mammals in previous studies. As was found in the chickens, in mammals, increasing the concentration of an anaesthetic generally causes a decrease in EEG frequencies and, with some anaesthetics, an increase in the amount of burst suppression present in the EEG (Johnson et al., 1994;

Johnson and Taylor, 1998; Schwender et al., 1998; Tsushima et al., 1998; Antunes et al., 2003; Orth et al., 2006; Murrell et al., 2008; Otto, 2008). However, the magnitude of the change in the FFT variables and the onset of burst suppression sometimes appear to differ between chickens and mammals.

Below, the similarities and differences of chicken and mammalian EEG responses are discussed separately for each anaesthetic. The changes in FFT variables are difficult to compare directly with previous work because the multiples of MAC

that were used were different. In addition, some of the results from previous mammalian studies are inconsistent with each other, particularly with regard to the way specific EEG variables respond to anaesthesia.

Halothane

In mammals, an increase in halothane concentration generally results in more low frequency activity and less high frequency activity in the EEG and also causes an increase in amplitude. Therefore, as depth of anaesthesia increases there are decreases in both median frequency and 95% spectral edge frequency (Johnson et al., 1994; Tsushima et al., 1998; Antunes et al., 2003).

Consistent with this, the effect of halothane on the median frequency of the chicken’s EEG was similar to that observed in mammals in terms of the trend in EEG changes, although the magnitude of the changes sometimes differed. Generally the EEG changes in the chickens were qualitatively smaller in magnitude than they were in mammals.

However, in contrast to some of these previous mammalian studies, the EEG median frequency of rats anaesthetised with halothane increased as the

halothane concentration was increased between 1.25 and 1.75 MAC (Murrell et al., 2008). The median frequency of the chicken’s EEG decreased as the

concentration of halothane increased between 1 and 2 MAC. In rats the total power decreased with increasing anaesthetic concentration (Murrell et al., 2008), whereas in the current chickens total power increased. It is unclear what the reasons are for the differing responses to anaesthetics (i.e. increases or decreases in FFT variables); the differences could be related to differences in how avian and

mammalian nervous systems respond to anaesthesia. Alternatively it may be that the EEG responses to anaesthesia are inherently variable.

The change in the 95% spectral edge frequency also differed between previous mammalian studies and the current chickens. In horses there was a decrease in 95% spectral edge frequency as halothane concentration was increased from 0.8 to 1.2 MAC (Johnson and Taylor, 1998). In the chickens anaesthetised with halothane, the spectral edge frequency trend was not linear. As the halothane concentration increased from 1 to 1.5 MAC the mean spectral edge frequency decreased. When the concentration was increased to 2 MAC, the spectral edge frequency increased (Figure 2.8). It is unknown why the trend in 95% spectral edge frequency reversed as the halothane concentration increased.

Burst suppression has not been reported during halothane anaesthesia in mammals (Antunes et al., 2003; Orth et al., 2006; Murrell et al., 2008). In agreement with this, in the chickens, burst suppression was minimal. This indicates that the suppressive effects of halothane on the EEG are not enough to cause burst suppression.

The changes observed in the rat EEG variables during halothane anaesthesia were described as being small and potentially of little biological significance, although they were statistically significant (Murrell et al., 2008). It was suggested that the dose-response curve for these EEG variables was comparatively flat over the halothane concentrations tested (Murrell et al., 2008). In view of the

relatively small changes in EEG variables during halothane anaesthesia in current experiment, there is evidence to suggest that the changes in the chicken EEG are also of little biological significance.

Methoxyflurane

Responses to methoxyflurane appear to be similar for the mammalian and chicken EEG. In horses, increasing the depth of methoxyflurane caused a decrease in median and 95% spectral edge frequencies. Burst suppression did not occur in concentrations up to 1.3 MAC (Johnson and Taylor, 1998). Similar trends in the median frequency and spectral edge frequency were seen for chickens in the current experiment and burst suppression was also absent or minimal.

Isoflurane

EEG responses to isoflurane anaesthesia in mammals are variable. In horses, there was an increase in 95% spectral edge frequency as the isoflurane

concentration increased between 1.5 and 1.8 MAC, but no other significant changes in the EEG variables (Johnson and Taylor, 1998). Likewise, studies in rats reveal an increase in 95% spectral edge frequency with increased isoflurane concentrations, but no change in the EEG median frequency (Antunes et al., 2003). In contrast, studies in humans have found a decrease in the 90% spectral edge frequency with increasing isoflurane concentrations (Schwender et al., 1998). Thus, as with halothane, these EEG variables appear to show inconsistent responses to isoflurane anaesthesia. As stated for halothane, these differences may indicate a species difference in the neural response to anaesthesia, or may reflect the variable character of EEG responses to anaesthesia.

Burst suppression due to isoflurane anaesthesia has been consistently observed during mammalian studies. The BSR increased at greater isoflurane

2008). In chickens, isoflurane caused burst suppression to such a degree that it prevented frequency analysis of the EEG at concentrations of 1.5 and 2 MAC. It is possible that the gradual change in the EEG waveform with increasing

anaesthetic concentration, of the type seen with halothane and methoxyflurane (Figures 2.7-2.12), occur in chickens at lower concentrations of isoflurane than were used in this experiment (Johnson and Taylor, 1998; Antunes et al., 2003). However, in another study of chickens, burst suppression (ratio not reported) was found at isoflurane concentrations of 0.75 MAC (Martin-Jurado et al., 2008).

In rats, the burst suppression ratio was greater than 95% at isoflurane

concentrations of 1.25 MAC (Murrell et al., 2008), whereas in the chicken, BSR was only 69% at an isoflurane concentration of 1.5 MAC. The reason for the difference is unknown.

Sevoflurane

There are similarities between mammals and birds in the EEG responses to sevoflurane. Increasing sevoflurane concentration in humans caused a decrease in 90% spectral edge frequency (Schwender et al., 1998). This indicates a

decrease in EEG frequency at higher concentrations of sevoflurane anaesthesia. As with isoflurane, sevoflurane caused burst suppression to such a degree that it prevented frequency analysis of the EEG. Sevoflurane seems to have a similar effect on BSRs in both bird and mammalian EEGs (Schwender et al., 1998; Tsushima et al., 1998). As in the chickens, in rats, increasing sevoflurane

concentration caused increased levels of burst suppression (Murrell et al., 2008). The BSRs for sevoflurane were similar for the rats studied previously (Murrell et al., 2008) and the chickens studied here.

2.5.3 General comparison between birds and mammals

In general, it appears that anaesthetics affect the avian EEG in a similar way to the mammalian EEG. That is, greater concentrations of anaesthetics cause an increase in low frequency activity and an increase in burst suppression in the EEG (when using anaesthetic agents that cause burst suppression). This suggests that the overall mechanisms of anaesthesia in the nervous system are similar between birds and mammals. There are a few relatively minor differences in the responses to anaesthesia in terms of trends and magnitudes of EEG changes, but on closer inspection these differences are no greater than those that exist between different mammalian species. Reasons for these slight differences in EEG

responses to anaesthesia between birds and mammals are not obvious, but may relate to differences in brain anatomy; this will be discussed in more detail in Chapter 4.