This study observed and analyzed the contamination status of anesthesia machines in PACUs within two weeks post-disinfection. Prior related research primarily focused on anesthesia machines in operating rooms [5,6,7,8]. PACUs serves as the location for postoperative anesthesia recovery and tracheal extubation for the majority of surgical patients throughout the hospital except for those who cannot be safely extubated and need to go to the intensive care unit, making its contamination impact extensive and highlighting the paramount importance of disinfection of anesthesia machines [15]. Consequently, this study selected anesthesia machines in PACUs as the subject of investigation. During the patient’s awakening phase, there are significant fluctuations in airway pressure, whereas the operating room primarily serves as the site for anesthesia induction and surgical procedures for patients within this room. During the anesthesia maintenance phase, airway management is relatively stable, which may potentially lead to greater contamination of anesthesia machines in PACUs compared to those in operating rooms. Furthermore, earlier literature suggested a disinfection interval of 7 days for anesthesia machines in operating room without respiratory filters [6]. Subsequently, research has found that respiratory filters can effectively reduce contamination of anesthesia machines [7, 8]. In this study, both the external breathing circuits and respiratory filters of the anesthesia machines were used once per patient and discarded afterward. This practice ensures a higher level of sterility compared to previous studies conducted under the mode of non-use of respiratory filters or reuse of respiratory filters and external breathing circuits. These characteristics introduce unknown variations in the contamination of anesthesia machines in PACUs.

As indicated by the results of this study, within two weeks of use after disinfection, the positivity rate of bacteria in anesthesia machines in PACUs fluctuated between 10% and 20%. The positivity rate of bacteria did not significantly increase with increase in the number of days of use within two weeks of using the anesthesia machines in PACUs following disinfection in the internal circuits. In the same period, there was also no significant increase in the positivity rate between the 3rd and the 14th days of use. It was also observed that there were bacterial cultures turning from positive to negative in the anesthesia machines, with the positivity rate on the 7th and 12th days after disinfection being lower than the positivity rate on the 5th and 10th days. Hartmann et al. [8] also found that the bacterial positivity rate of anesthesia machines was lower at 72 h after disinfection than at 48 h, which is similar to the results of this study. Some studies have indicated that the disinfection interval for the breathing circuits in anesthesia machines in specific usage scenarios is 7 days [5, 6]. However, based on findings in this study, when a single external circuit and respiratory filter of the anesthesia machine is used for each individual, contamination of PACU did not signifcantly increase within two weeks. Moreover, 85% of the bacterial cultures in two weeks were negative, suggesting that re-disinfection was not necessary. Only 15% of the bacterial cultures were positive, and the total number of colonies was <20, suggesting that disinfection was not necessary unless there were pathogens. Therefore, unless the anesthesia machine was used by patients with recent respiratory illnesses or an infectious disease history, perhaps the disinfection interval of the anesthesia machine can be extended. This is also consistent with the research findings of Spertini et al. [7]

Using one external circuit and respiratory filter in anesthesia machines in PACUs exclusively for a specific individual significantly lowers the risk of bacterial contamination in the external circuits of anesthesia machines, thereby avoiding contamination in the internal circuits [16]. The bacterial contamination rate of the anesthesia machine on the 26th day after disinfection was only 2.17% when using a respiratory filter, as shown by the research results of Wang et al. [12] Dubler et al. indicated that there was no significant increase in the positivity rate of bacteria in the internal respiratory circuit of the anesthesia machine on the 7th day as compared to the 1st day when the external respiratory circuit and filter were reused [17].

This following is related to the working modes of PACUs: (1) Patients in PACUs need to undergo endotracheal and oral suctioning under brief deep anesthesia unless contraindicated, which greatly reduces the source of contamination in the anesthesia machines’ internal breathing circuits [18, 19]. (2) Patients in PACUs have a fast turnover in using anesthesia machines and a short duration of mechanical ventilation using endotracheal tubes. (3) High-flow ventilation is used in PACUs in conditions such as scavenging of anesthetic gases and unstable respiratory circulation in patients after extubation [20, 21]. Under high-flow ventilation, the essential temperature and humidity for bacteria in the internal circuits of anesthesia machines cannot be maintained [22, 23]. Furthermore, studies show that increasing the fresh air flow, decreasing the amount of carbon dioxide that needs to be filtered by soda lime, and decreasing the generation of calcium carbonate from soda lime can maintain the self-sterilizing effect and provide a strong filtering and sterilizing effect, lowering the bacterial levels in the breathing circuit [23,24,25]. As a result, in this investigation, the switch from positive to negative bacteria culture findings was found in some anesthesia machines.

It is uncertain whether the reuse of disinfection machines leads to cross-contamination or the growth of drug-resistant bacteria in the internal circuits of anesthesia machines. Disinfecting the internal circuits is time-consuming and labor-intensive, and repeating the disinfection when the bacterial positivity rate is low would squander medical resources and increase the department’s disinfectant and soda lime costs. Therefore, unless the anesthesia machine was used by patients with recent respiratory illnesses or an infectious disease history, our results might rather support an extension of re-disinfection intervals. when the practice of exclusive use of one external respiratory circuit and respiratory filter of the anesthesia machine for a specific individual is adopted in the PACU.

Age, laparoscopic surgery, surgical time, prone position, and lateral position are risk factors for contamination in the circuits of anesthesia machines in operating rooms when respiratory filters are not employed, according to research findings by Yang et al. [26] In this study we examined the effects of the number of patients using anesthesia machines in PACUs, the number of patients who were over 60 years old and used anesthesia machines in PACUs, the number of patients who underwent laparoscopic surgery and used anesthesia machines in PACUs, the number of patients who were in a lateral or prone position during surgery and used anesthesia machines in PACUs, as well as the effect of the total surgical duration on the 14-day bacterial positivity rate of anesthesia machines in PACUs. According to the findings of the multi-factor modified Poisson regression analysis, the surgery duration of patients and the number of elderly patients are risk factors for contamination of internal circuits of anesthesia machines in PACUs. Airway pressure will continue to increase during laparoscopic surgery and prone surgery. This will lead to restrictive ventilation dysfunction and mucosal damage in patients, resulting in lung immune system response, increased secretion, and sputum formation. The internal circuit of the anesthesia machine in the operation room is more vulnerable to contamination. However, both laparoscopic surgery and surgical posture occur only for a period of time during the operation, and the anesthesia nurse will suction sputum in time after the patient enters PACU. Therefore, the operation position and laparoscopic operation formula have a significant impact on the contamination of anesthesia machines in the operation rooms, but not in PACUs. Longer surgical duration and more elderly patients will significantly aggravate the contamination of anesthesia machines in PACUs and operation rooms. On the one hand, lengthy operations foster a more hospitable environment for bacterial development in the respiratory circuit, including temperature and humidity, which exacerbates the growth of germs in the external circuits of anesthesia machines in the operating rooms. On the other hand, patients bring external breathing circuits utilized in the operating rooms into PACUs. As the patients breathe in PACUs, airflow from the external circuits of the anesthesia machines enter their internal circuits. This exacerbates the contamination in the internal circuits of anesthesia machines in PACUs. At the same time, the organ functions of the elderly undergo degenerative changes, with impaired gag reflex, decreased mucociliary function, and increased secretions during surgery and the recovery period of anesthesia, which are more likely to contaminate the anesthesia machine [27]. Therefore, when disinfecting anesthesia machines in PACUs, it is advisable to prioritize disinfecting the internal circuits of anesthesia machines that have been used for a longer cumulative surgical duration or have been used by a higher number of elderly patients over 60 years old, as this can effectively reduce the incidence of contamination in the anesthesia machines.

This study has certain limitations. This study did not detect pathogenic bacteria and viruses, nor did it carry out the cultivation of anaerobic bacteria, which makes this study unable to give a specific disinfection time interval, and can only give exploratory disinfection suggestions. On the basis of this study, it is suggested that the precise effective period after each disinfection should be further studied by lengthier experimental cycles based on the detection of pathogenic bacteria and viruses. At the same time, it is advised that the bacterial communities that contaminated anesthesia machines and were linked to patient respiratory tract infections and hospital postoperative infections be investigated further. This will enable the selection of more effective disinfectants targeting the relevant bacterial communities. Thus, it can provide a set of disinfection guidance scheme that is both safe and efficient, while taking into account cost savings, labor reduction and time optimization.