POST is a common complication after general anesthesia with tracheal intubation, and its severity is mainly dictated by the pressure on the tracheal mucosa and the perfusion pressure of the tracheal mucosa itself [12]. Monitoring of the pressure on the tracheal mucosa is complicated; therefore, it cannot be measured directly in clinical practice. However, this pressure is relatively well correlated with the endotracheal tube cuff pressure. Therefore, the cuff pressure can indirectly reflect the pressure on the tracheal mucosa. The MOP is the minimum pressure that effectively seals the gap between the cuff and the trachea [5]. Li et al. suggested that the MOP management mode could ensure effective ventilation of patients and could reduce the occurrence of complications [8]. Sole et al. recommended an ETTc pressure of 15–25 mmHg (20–33 cmH2O) [9]. However, it is unscientific to evaluate the cuff pressure based on clinical experience only [13], as it may easily lead to excessively high cuff pressure, causing damage to the tracheal mucosa. In this study, muscle relaxants were administered continuously using a micropump, to exclude POST caused by swallowing due to insufficient muscle relaxants, which would affect the accuracy of the results. Our results confirmed that tracheal intubation per se under general anesthesia did indeed cause POST.
The trachea is an organ that can be contracted by the action of breathing, but the extent of contraction possible is limited. When the pressure of the endotracheal tube cuff exceeds the bearing capacity of the trachea, the tracheal mucosa can be damaged [14]. During mechanical ventilation, if the cuff pressure of the endotracheal tube exceeds 10 mmHg (13 cmH2O, interstitial fluid colloid osmotic pressure), or even reaches or exceeds 20 mmHg (27 cmH2O), local edema of the compressed tracheal mucosa is unavoidable [15]. Xu wrote that, when the endotracheal tube cuff pressure reached 2.942 kPa (22 cmH2O), the blood flow in the tracheal mucosa began to decrease. When it reached 3.923 kPa (29 cmH2O), the blood supply could be completely blocked, leading to ischemic injury of the tracheal mucosa. When it exceeded 4.904 kPa (37 cmH2O), columnar necrosis and even serious complications, such as perforation and rupture of the airway wall, occurred [16]. Therefore, we consider that the endotracheal tube cuff inflating and deflating constitutes a process of ischemia–reperfusion. Dong et al. suggested that ischemia–reperfusion injury was an inflammatory reaction [17]. Various cytokines are expressed and inflammatory cells infiltrate in the ischemia–reperfusion injury area, which forms the basis of the transformation from ischemia–reperfusion injury to inflammatory injury. Puyo et al. found that the number of multinucleate cells increased 10-fold, and tumor necrosis factor interleukin-6 (IL-6), IL-1β, and C5a were all significantly increased after 3 h of endotracheal intubation [18]. The occurrence and development of pain is closely related to the inflammatory response caused by injurious stimuli [19]. Zhang et al. compared the pathological changes in the tracheal mucosa when the cuff pressure was 10 mmHg (13 cmH2O) or 20 mmHg (27 cmH2O). They found slight pathological changes in the low-pressure group, while the main pathological changes were the infiltration of the tracheal mucosa by inflammatory cells, leading to hyperemia and edema. The damage in the high-pressure group was significantly greater than that in the low-pressure group [20]. Therefore, we speculate that POST is related to the inflammatory response caused by ischemia–reperfusion injury.
According to the Prince Henry Hospital Scale scoring criteria, the incidence of POST in the MOP, ETTc, and Con groups were 20%, 37.5%, and 62.5%, respectively. The results suggested that different cuff pressure management modes have an effect on the incidence of POST. In the results of this study, there was no significant difference in incision pain among the three groups, as assessed by a visual analogue scale; thus, the effect of the postoperative analgesia pump on POST could be excluded, further suggesting that different tube cuff pressure management modes have an impact on the incidence of POST.
We then conducted a correlation analysis to determine the relationship between the incidence of POST and the cuff pressure. The results indicated that the incidence of POST increased with the increase in cuff pressure, and the relationship between the incidence of POST and the cuff pressure was strongest at 24 h after the operation. The results of the POST pain score also showed that the degree of POST-related pain was different with different tube cuff pressures, and this was most significant at 24 h after surgery. The incidence of POST at 24 h after surgery and the degree of POST were closely related to the tube cuff pressure. The results reported by Zhang et al. supported our conclusion [26]. The POST pain score also showed that with a cuff pressure > 10 cmH2O, the degree of POST was statistically significantly different at the three observation time points, which indirectly suggests that high cuff pressure caused more severe POST, and that POST gradually eased over time.
In conclusion, our results indicate that the MOP management mode is the ideal intraoperative cuff pressure management mode. MOP not only seals the airway effectively, but also reduces the incidence of POST.