Here’s an oldie and not necessarily a goodie: Sequential Measurements of the Median Nerve Somatosensory Evoked Potential During Isoflurane Anaesthesia in Children.
During anesthesia when the volatile gas is increased there is a lag before the alveolar gas, arterial blood and brain gas concentrations respectively equilibrate. Until the brain reaches steady-state, the SSEP responses will be in flux.
Ten normal children (0.8-6 yrs) had their median nerve (MN) SSEPs tested. SSEP baselines were first obtained with 70% nitrous oxide. Then the isoflurane was added. The time from isoflurane increase until the SSEP responses stopped changing was measured. SSEPs were recorded over a 15-minute period at 0.25, 0.5 and 0.75 MAC of isoflurane. The MAC was corrected for the age of the child but not the presence of nitrous oxide.
The abstract states that it takes 5:16-7:37 minutes for SSEPs to stabilize, but the results section presents data that shows it took between 2:55-12:16 for MN SSEPs to equilibrate . There was great variability not only between subjects but also within subjects at the different MAC level.
SSEPs are in a state of flux for some minutes after increasing the gas and should be treated with caution during this time as they have not yet reached steady state.
This paper had so much potential. In fact, if you read just the abstract it sounds great – SSEPs take between 5:16 – 7:37 minutes to stabilize after an increase in gas. Fabulous! We have a nice concrete time to predict when our SSEPs will stabilize. Sadly, this time range is an illusion. These numbers are not in the results or discussion section and the data they publish show a variability of 2:55-12:16 minutes. With such a small sample size (10 children), variability this large means the only conclusions possible are that:
- In young children there is a lag between gas change and SSEP stabilization
- the time it takes SSEPs to stabilize varies tremendously from child to child and from gas-increase to gas-increase.
The ages of the children in this study also limits generalizability. Out of the 10 children, 4 were under the age of two meaning their nervous systems were not fully myelinated. Also, young children do not respond to anesthesia as adults do and so it may be that their SSEPs also do not respond the same to anesthesia as older children or adults.
Furthermore, the period of time it takes for gas to equilibrate in the brain depends on the solubility of the anesthetic agent and the metabolic rate. Children have a greater metabolic rate than adults so their equilibrium and SSEP stabilization may happen faster than adults.
Iso versus Des/Sevo?
While this paper sought to give a time frame to expect SSEP changes, it only looked at isoflurane. As isoflurane has a higher blood-gas coefficient than Sevoflurane or Desflurane it has slower onset. Being stored by fat cells it also has slower emergence. So theoretically we could expect these two other halogenated gasses to have faster SSEP changes and stabilization.
An obvious extension of this would to be to repeat this study comparing different halogenated gases in adults. An ideal sample pool would be late-adolescent idiopathic scoliosis patients. They have normal neurological and adult-like nervous systems but without all the co-morbidities and effects that come with aging.
Next time anesthesia asks 30 seconds after turning the gas down if your signals have improved, you can say “Based on Mason et al’s work it may take up to 12 minutes for the SSEPs to fully adjust. Mind you, that research was on children [isoflurane], so it may be different in adults [with different gases].”
What is the longest period you’ve experienced between a halogenated gas change and an SSEP stabilization? Let us know in the comments!