A large amount of oral and airway secretions during the awakening period is likely to put the patient at risk of asphyxiation and aspiration and greatly affect the patient's experience and comfort[7]. Cholinergic receptor antagonists, including atropine, scopolamine, and pentoxifylline hydrochloride, are commonly used in clinical practice to inhibit glandular secretion, but these drugs may have adverse effects such as increased cardiac burden, difficulty urinating, and dryness of the mouth and nose. Therefore, this experiment aimed to explore other methods that could effectively reduce glandular secretion. This study showed that the secretory activity of both salivary and respiratory glands was suppressed under anesthesia, and this effect was more pronounced as anesthesia deepened.
In the human respiratory tract, 5% of the respiratory mucus is secreted by cupped cells, the tips of which are rich in mucin-secreting granules, and the remaining 95% is released by submucosal glands located in the submucosal layer of the respiratory hypodermis. This mucus can resist the invasion of foreign particles, and the bactericidal enzymes in them can also reduce the risk of infection[8]. However, various stimulation of the respiratory tract leads to increased mucus secretion from the glands, which may be fatal for patients who need to complete surgery under general anesthesia, and large amounts of oral and airway secretions may cause violent choking, laryngospasm, or even respiratory obstruction during extubation, which will result in serious consequences if not detected in time. The secretion of respiratory glands is regulated by nerves, and the nerves at the airway are closely connected to mucus-secreting cells, which have corresponding neurotransmitter receptors, and stimulating the nerves will increase the rate of glandular secretion. There are three signaling pathways in the airway: adrenergic, cholinergic, non-adrenergic, non-cholinergic (non-adrenergic, non-cholinergic NANC) systems[9], and their regulatory mechanisms are shown in Table 5. In addition, smoking is a strong respiratory irritant[10], and some studies have shown that smoking can lead to epithelial and cupped cell hyperplasia and mucosal gland hypertrophy, which can cause respiratory glands to secrete large amounts of mucus and cause airflow obstruction[11]. Acute and chronic respiratory diseases such as bronchial asthma, chronic obstructive pulmonary disease, bronchiectasis, or lung infections can lead to airway hypersensitivity, increased inflammatory response, and increased mucus exudation[12, 13]. Therefore, patients should be advised to quit smoking and actively control lung infections before surgery so that they can maintain a good lung condition to undergo surgery and reduce the incidence of pulmonary complications.
Tab5 Neural regulation of airway gland secretion
Neural division
|
Neurotran-smitters
|
Nerves present near secretory cells
|
Receptors present on secretory cells
|
Effects on mucus secretion
|
Receptor agonists
|
Nerve stimulation
|
Parasym-pathetic
|
ACh
|
Yes
|
Yes
|
Increase
|
Increase (M3);
inhibit (M2)
|
Sympathetic
|
NA
|
Yes
|
Yes
|
Small increase
|
small increase (α/β receptors)
|
NANC
|
VIP
|
Yes
|
Yes
|
Variablea
|
inhibit
|
|
NO
|
Yes
|
Not applicable
|
Variable
|
inhibit
|
|
SP
|
Yes
|
Yes
|
Increase
|
Increase (NKA receptors)
|
|
NKA
|
Yes
|
Yes
|
Increase
|
Increase (NKA receptors)
|
|
CGRP
|
Yes
|
Few
|
Increase
|
inhibit
|
a Demonstration of effects on nerve-stimulated secetion either of endogenous transmitter (using apropriate receptor antagonists) or of exogenous administration of drug.
ACh, acetylcholine; NA, Norepinephrine; VIP, vasoactive intestinal peptide; NO, nitric oxide; SP, substance P; NKA neurokinin A; CGRP, calcitonin gene-related peptide.
General anesthesia is a complex state in which the anesthesiologist uses drugs to bring the patient to a state of a reversible loss of consciousness, including memory loss, loss of generalized pain, inhibition of stress response, weakened glandular secretion, and relaxation of skeletal muscles[14]. At different depths of anesthesia (DoA), the patient's organ functions are inhibited differently, and the signs and symptoms displayed clinically vary. However, the lack of significant intraoperative arousal is one of the common problems in modern anesthesia, which may have serious negative effects on the patient's postoperative recovery. Therefore, the monitoring of anesthesia depth is particularly important[15]. Bispectral Index (BIS) and NT are the more mature methods of anesthesia depth monitoring used in clinical practice today[16, 17]. NI was developed by Hannover University School of Medicine, Germany, which divides the depth of anesthesia into six stages. A for the awake state, B and C for the sedated state, D and E for the anesthetized state, and F for the burst suppression. In this study, we chose to classify patients into conventional anesthesia (D0-D2) and deep anesthesia (E0-E1) groups and did not include the depth of anesthesia E2 level in the selected criteria because it is not within the recommended DoA, and a recent meta-analysis showed that too deep anesthesia might increase mortality in patients more than 90 days after surgery[18], and intraoperative too deep anesthesia or even eruptive suppression should be avoided, so careful consideration of patient safety factors ruled out this depth.
The discovery of the role of ether anesthesia made possible surgical procedures, which initially required only the absence of pain, without any expectation of a loss of consciousness. It was not until Guedel proposed the classical staging of ether anesthesia during World War II that people gradually began to focus on the concept of depth of anesthesia. The depth of anesthesia for ether anesthesia can be roughly divided into four stages and four levels, with general surgery performed in the second and third levels of the third stage (surgical anesthesia stage). In the early days, anesthesiologists could only judge the depth of anesthesia by all the clinical signs they could observe. Nowadays, with all kinds of advanced monitors, anesthesiologists can quickly obtain all the physiological data of patients, and primary conditions such as pupils, urine volume, mucosal color, and glandular secretion are often overlooked. Glandular secretion is mainly regulated by parasympathetic nerves, and the neurotransmitters released by them, such as Acetylcholine(Ach), can cause the glands to secrete mucus[19]. Anticholinergic drugs are effective in reducing the secretion of these glands, but there may be a risk of causing other side effects[20]. In this study, valacyclovir hydrochloride was used as a positive control group, and the results of the study showed that the respiratory gland secretion and salivary secretion were less in group H than in groups L and M, while the postoperative dry mouth VAS score was higher than in the other two groups, and the patients' postoperative dry mouth was more pronounced, and their comfort during the awakening period was significantly reduced. Compared with the basal values, salivary secretion was reduced in both groups L and M. The salivary glands are innervated by sympathetic and parasympathetic nerves, both of which can lead to increased salivary secretion after stimulation. As for the respiratory glands, the secretion in group M was less than that in group L. This indicates that the secretion of salivary glands and respiratory glands was inhibited to some extent under anesthesia, and the inhibitory effect of anesthetic drugs may have achieved the effect in the study, which became more pronounced with the deepening of anesthesia.
Muscarinic acetylcholine receptors on the airway are G protein-coupled receptors that regulate the release of various neurotransmitters, including gamma-aminobutyric acid (GABA), glutamate, and acetylcholine. One study reported that propofol could inhibit M1-type acetylcholine receptors on GABAergic neurons in the brain and induce anesthetic effects[21]. The DoA of propofol anesthesia was correlated with the GABA content for the canine brain, and the GABA concentrations in all brain regions were significantly higher in the deep anesthesia group than in the light anesthesia group. Another study showed that propofol and midazolam significantly inhibited cortical Ach release at sedative doses and that Ach returned to normal levels when the infusion was stopped[22]. Therefore, under anesthesia, various anesthetic drugs may inhibit the secretory effects of various glands by suppressing autonomic tone, enhancing or inhibiting certain neurotransmitters and corresponding receptors. However, more animal experiments are still needed to prove this idea.
There are some limitations to this study. First, the type of surgery selected for this study lasted of moderate duration, and it cannot be determined whether maintaining deep anesthesia for a more extended period would affect the patient's hemodynamics and other adverse effects; second, this study has some reference the significance for surgical patients without significant pulmonary disease, but for some specific positions and surgical procedures, the anesthesiologist should weigh the need for anticholinergic drugs based on the patient's actual condition.