A series of papers published in the British Journal of Anaesthesia detail the differences in the way common anaesthetics affect the brains of older patients and children, findings that could lead to ways of improving monitoring technology and the safety of general anaesthesia for such patients.
Recent investigations into the neurobiology underlying the effects of general anaesthesia have begun to reveal the ways different anaesthetic agents alter specific aspects of the brain’s electrical signals, reflected by electroencephalogram (EEG) signatures.
While those studies have provided information that may lead to improved techniques for monitoring the consciousness of patients receiving general anaesthesia, until now they have been conducted in relatively young adult patients.
“Anaesthesiologists know well that the management of patients aged 60 years or older requires different approaches than for younger patients,” said Emery Brown, MD, Massachusetts General Hospital, Boston, Massachusetts. “The doses required to achieve the same anaesthetic state in older patients can be as little as half what is needed for younger patients. Explanations for that difference have focused on age-related declines in cardiovascular, respiratory, liver and kidney function, but the primary sites of anaesthetic effects are the brain and central nervous system.”
“We know even less about how anaesthetic drugs influence brain activity in children, and the current standard of care for assessing the brain state of children under anaesthesia calls only for monitoring vital signs like heart rate and blood pressure,” added Patrick Purdon, PhD, Massachusetts General Hospital. “This lack of knowledge is especially troubling, given recent studies suggesting an association between early childhood surgery requiring general anaesthesia and later cognitive problems.”
The researchers investigated EEG signatures indicating when patients lose and regain consciousness and the EEG oscillations produced by specific drugs while patients are unconscious. In young adults, anaesthesia-induced unconsciousness is associated with medium frequency (around 10 Hz) EEG oscillations called frontal alpha waves that are highly synchronised between the cerebral cortex and thalamus, a pattern that is believed to block communication between those brain structures.
The 2 studies are the first to take a detailed look at anaesthesia-induced brain changes in older patients. Dr. Purdon and Dr. Brown are co-corresponding authors of one study that analysed detailed EEG recordings of 155 patients aged 18 to 90 years receiving either propofol or sevoflurane.
That study found that the EEG oscillations of older patients were 2 to 3 times smaller than those of younger adults with reduced occurrence of frontal alpha waves. The synchronisation between the cortex and thalamus occurred at slightly lower frequencies in older patients, who were more likely than younger patients to experience a state called burst suppression that reflects profoundly deep anaesthesia at lower doses.
In the other study, Dr. Purdon and Oluwaseun Akeju, MD, Massachusetts General Hospital analysed EEG patterns of 54 patients ranging from infancy through age 28 years during anaesthesia with sevoflurane. They found that anaesthesia-induced EEG signals tripled in power from infancy until around age 6 and then dropped off to the typical young adult level at around age 20. Frontal alpha waves were not observed in children under the age of 1, suggesting that the brain circuits required for cortical/thalamic synchronisation had not yet developed.
“It appears as though the structure of anaesthesia-induced brain dynamics mirrors brain development in children, with different brain wave patterns ‘turning on’ at ages that coincide with known developmental milestones,” said Dr. Purdon. “In older patients we see a similar effect but in reverse, with certain brain waves decaying in a manner consistent with brain aging. It’s been known that commercially available EEG-based anaesthesia monitors were developed for young adults, and while they are limited for that population – reducing brain activity to a single number – they are even more inaccurate for children and the elderly. These studies illustrate why this is the case and suggest a new, age-specific monitoring paradigm that — along with monitors that track a broader range of EEG signals — could help avoid both anaesthesia-induced neurotoxicity in children and post-operative delirium and cognitive dysfunction in elderly patients.”
“Understanding how the brain’s responses to anaesthesia change with age allows us to provide personalised, patient-specific strategies for monitoring the brain and dosing the anaesthetics, thereby moving us closer to side-effect free anaesthesia care,” added Dr. Brown.