Anesthesiologists often use the word "sleep" to explain general anesthesia during preoperative visits, but are general anesthesia and natural sleep the same in reality? At first glance, both appear to be reversible states of loss of consciousness, which is also the most immediate sensation of the patient's surgery. Some patients even have dreams under general anesthesia that make them feel like they have indeed "slept" during the procedure.
But in reality, there is a fundamental difference between the two:
1. Different definitions: General anesthesia inhibits the central nervous system through intravenous, inhaled, intramuscular injection or rectal perfusion of anesthetic drugs, so that the patient loses consciousness, has no pain, forgets, and inhibits autonomic reflexes to achieve the best surgical state. Clinically, general anesthesia involves three phases: the induction period (from awakeness to anesthesia), the maintenance period (maintaining a certain depth of anesthesia), and the recovery period (stopping anesthetic drugs or using reversal agents to restore consciousness to patients).
Natural sleep, on the other hand, is a decrease in the physiological activity of animals caused by the regulation of brain physiological functions, which can be fully awakened by appropriate stimulation, which is an active neuromodulation process in the central nervous system.
2. The process is different: Natural sleep has a cyclic rhythm, including REM and non-REM periods, and each cycle is about 90 minutes.
There is no REM phase during general anaesthesia. During REM, the reticular structures of the brainstem send signals to the nerves in the spinal cord, causing the body to remain immobile and the eyes move rapidly, which can quickly wake the body up.
3. EEG characteristics are different: At the onset of natural sleep, frontal slow waves and δ waves increase, and the frontal area is synchronized earlier than other cortical regions. In clinical anesthesia, anesthesiologists use electroencephalography to classify anesthetic states, such as deep hypnosis, surgical anesthesia, and conscious sedation, so as to adjust the dose of general anesthetic drugs.
Figure 1 Sleep cycle and corresponding EEG (Source: Sleep Xiaobao)
Figure 2: EEG under anesthesia
4. Different responses to stimuli: In natural sleep, the body responds to loud sounds or strong tactile stimuli and wakes up quickly. In general anesthesia, the patient's auditory, tactile, and pain responses are all inhibited and are in a non-responsive state.
Figure 3 Depth staging of anesthesia (Source: Anesthesiology, 4th Edition)
5. Different effects on memory: Natural sleep plays an important role in memory because the storage brain works at this time. Whereas, after general anesthesia, the patient may lose memory for a period of time after anesthesia induction. In addition, studies have shown that the use of general anesthetics after learning tasks impairs memory consolidation and disrupts hippocampal activity.
6. Differences in the effects on the cerebral cortex: The human body's sleep-wake transition is an active neuromodulatory behavior, while general anesthesia is a broad inhibition of the central nervous system through drug induction. In natural sleep, loss of consciousness is mainly driven by active areas under the cortex, which inhibits the upstream arousal nucleus. In general anesthesia, almost all of the central nervous system is suppressed, especially the cerebral cortex.
In conclusion, although general anesthesia is sometimes described as "sleeping," it differs significantly from natural sleep in several ways. General anesthesia brings the patient into a pharmacological coma, closer to a deep coma than sleep in the traditional sense.
References:[1]Deng Daling,Zhao Shuai,Chen Xiangdong. Journal of Clinical Anesthesiology,2022,38(02):199-202.)
[2] Brown EN, Lydic R, Schiff ND. General anesthesia, sleep, and coma. N Engl J Med. 2010; 363(27):2638-2650. doi:10.1056/NEJMra0808281
Tang Yi, Department of Anesthesiology, Shenzhen Third People's Hospital
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