Background
Traditionally, anaesthesiologists have applied ventilation with tidal volumes (V
T) between 10 and 15 mL/kg of body weight, and without positive end-expiratory pressure (PEEP), to prevent atelectasis [
1,
2]. However, the concept of lung-protective ventilation (LPV) has recently emerged, based on previous studies that demonstrated the significant benefit of low V
T with appropriate PEEP on mortality in patients with acute respiratory distress syndrome (ARDS) [
3]. Although the level of PEEP that balances alveolar recruitment against over-distension should be selected and titrated for individual patients [
4‐
7], currently, LPV with low V
T (6 mL/kg of ideal body weight [IBW]), limited plateau pressure (< 28–30 cm H
2O), and appropriate PEEP is generally accepted for ventilation in patients with ARDS.
Several studies have suggested the benefits of LPV during surgery [
8,
9]. During laparoscopic surgery, LPV is associated with a relatively low incidence of pulmonary complications and better oxygenation [
10‐
12]. The benefits of LPV have also been demonstrated in obese patients [
13,
14]. In addition, Xiong et al. have reported that intraoperative LPV reduces barotrauma and lung inflammation in patients undergoing spinal surgery in the prone position [
15,
16]. Overall, these findings highlight the advantage of using intraoperative LPV. There is an increasing amount of literature on intraoperative LPV patterns and trends. Although the traditional method of ventilation is still used [
17‐
19], implementation of intraoperative LPV has increased [
20]. According to a recent study, education and feedback decreased the average intraoperative tidal volume and improved the rate of LPV use [
21].
In this questionnaire-based survey of anaesthesiologists, we focused on the effect of cognizance of intraoperative LPV strategies on the practical implementation of LPV in cases requiring general anaesthesia. In addition, by means of a retrospective study in a single university hospital in South Korea, we identified the factors that had influenced the changes in ventilation strategy over the past decade.
Discussion
The main finding of this study was that anaesthesiologists with cognizance of LPV applied LPV more often during general anaesthesia than those without cognizance of LPV. These results suggest that the knowledge of LPV strategy is directly related to its implementation, and can explain the pattern of increased LPV use in general anaesthesia. Therefore, this study provide further supportive evidence for the effect of education and feedback that decreased the average intraoperative tidal volume and improved the rate of LPV use [
21].
We conducted a questionnaire survey on the assumption that knowledge of LPV among anaesthesiologists would affect implementation of LPV in the operating room. The majority of anaesthesiologists surveyed in 2016 responded that they knew about LPV. Most of the respondents who replied that they did not know about LPV were first- and second-year residents and senior anaesthesiologists. The conceptual and non-conceptual groups exhibited significant differences in VT and PEEP settings in laparoscopic surgery. The 2 groups also differed in terms of routine application of PEEP; however, there were no statistically significant differences in VT and PEEP settings during non-laparoscopic surgery and in cases of obese patients.
Because V
T in LPV is based on IBW [
22‐
24], and IBW is determined by sex and height, respondents who chose ‘height’ to question 5 (a multiple-choice question) could be considered to understand the concept of IBW. The percentage of correct answers for this question was high (63/82 [76.85%]), and senior anaesthesiologists had the lowest percentage of correct answers for this multiple-choice question (5/13 [38.5%]). The correct answer group and incorrect answer group (in terms of this question) also exhibited significant differences in V
T/kgIBW settings in laparoscopic surgery and in cases of obese patients. However, there were no statistically significant differences in V
T/kgIBW settings during non-laparoscopic surgery.
Overweight and obese patients are more often exposed to greater V
T than patients with normal body weight. Therefore, a greater awareness for appropriate selection of V
T on the basis of IBW is highly recommended in such patients [
13]. The ‘obese patient’ in our questionnaire was an extremely obese patient with a BMI of 41.52, and the correct answer group applied a relatively low V
T, even in this patient. Several studies have reported that LPV might also reduce pulmonary complications in laparoscopic surgery [
10,
14,
29]. It has been shown that LPV can reduce barotrauma and lung inflammation and improve postoperative oxygenation even for patients operated on in the prone position [
15,
30]. Both the conceptual group and the correct answer group applied lower V
T than the non-conceptual group and the incorrect answer group in cases of laparoscopic surgery. In contrast, the patient undergoing non-laparoscopic surgery in our questionnaire was a thin patient with a BMI of 18.12, and thus there was no significant difference in V
T/kgIBW according to cognizance of LPV strategy. These results suggest that the knowledge of LPV strategy is directly related to the application of LPV in general anaesthesia. Lack of education and knowledge might be obstacles to the application of LPV in practice.
Anaesthesiology residency training at our institution includes an intensive care unit (ICU) course. In the ICU, LPV is the standard treatment for ARDS; this strategy is also widely applied in patients with conditions other than ARDS. Residents are repeatedly instructed to set a V
T of 6–8 mL/kgIBW during mechanical ventilation in critically ill patients. Additionally, each ventilator carries a chart with pre-calculated values of V
T/kgIBW, which has been reported to be very useful in preventing high-V
T ventilation [
31]. It is presumed that this training process would have affected the ventilation practice in the operating room. However, most of the attending anaesthesiologists in our institution are not attending physicians at the ICU, but are dedicated to the operating room. Consequently, they might not have had the opportunity to gain knowledge regarding LPV.
According to our retrospective study, even in 2014, the LPV strategy was not fully implemented in the operating room at our institution. However, application of LPV has definitely increased [
20]. There is accumulating evidence regarding the effectiveness of LPV during general anaesthesia [
9,
32]. Recent meta-analyses of randomised controlled trials demonstrated that, relative to surgery without LPV (high V
T [> 10 mL/kg] and no PEEP), intraoperative LPV strategies involving low V
T (6–8 mL/kgIBW), high PEEP (> 5 cmH
2O), and intermittent recruitment manoeuvres were associated with a statistically significant reduction in the incidence of postoperative atelectasis, lung infection, and acute lung injury [
1,
26,
33]. Because such knowledge is becoming universal, the adoption of an LPV strategy has increased.
There is typically a delay in dissemination of knowledge from the time of discovery of new evidence to its implementation in clinical practice [
34]. The current level of education and knowledge could be a contributing factor to this gap between the theoretical best practice and its practical application. There has been a lack of rapid and widespread adoption of the LPV strategy in ARDS treatment, where previous studies have demonstrated variations in practice with experience, knowledge, and position of the clinician [
35‐
37].
In fact, previous studies have reported that anaesthetic induction skills—including tracheal intubation and arterial and central line catheterisation—could be improved by gaining experience and education through workshops [
38‐
40]. Additionally, in LPV strategies, a knowledge deficit regarding the use of low-V
T for ARDS is common and varies according to the type and experience of the caregiver. A survey-based study on low-V
T ventilation in patients with ARDS reported lower perception of barriers and higher knowledge-test scores among fellows and attending physicians than among interns and residents [
37]. Previous studies have also demonstrated that usage of a low-V
T strategy increases after feedback and education involving presentation of actual ventilation settings and discussion on potential reasons for not using low-V
T [
36,
41,
42]. In a recent study, as in the ICU, education and feedback was found to be necessary for adoption of LPV in general anaesthesia [
21].
In our retrospective study, the percentage of cases involving intraoperative LPV (V
T < 10 mL/kgIBW and PEEP ≥5 cmH
2O) had significantly increased over a span of 10 years. These results correspond with those of earlier studies. In a 5-year retrospective study, Hess et al. reported a reduction in the percentage of patients receiving ventilation with V
T > 10 mL/kgIBW and without PEEP during general anaesthesia [
18].
In the present study, the mean VT/kgIBW among obese patients (BMI > 30 kg/m2) had significantly reduced between 2004 and 2014, while the usage of PEEP in this subgroup had significantly increased. The results of regression analysis revealed a significant difference in the factors affecting VT settings between the 2 study periods. In 2004, VT was associated with laparoscopic surgery as well as sex, height, and weight; in contrast, in 2014, only sex, height, and weight were associated with VT. Prone position was not associated with VT.
In 2004, anaesthesiologists tended to set lower V
T during laparoscopic surgery than during other open surgeries (530.5 [69.3] vs. 553.6 [69.6] mL;
p = 0.010). A possible explanation for this trend is that the V
T was inevitably set low for laparoscopic surgery, where the peak inspiratory pressure increases markedly [
43,
44]. In contrast, in 2014, there was no difference in V
T between laparoscopic and open abdominal surgery (482.3 [62.8] vs. 486.4 [64.0] mL;
p = 0.660). The knowledge of LPV has been accepted by anaesthesiologists, and the LPV strategy has been applied more frequently in open surgery. The variation in ventilator settings during open surgery seems to have reduced with the increase in number of and familiarity with laparoscopic surgeries over a span of 10 years [
45]. These trends may be interpreted as reflecting an improved cognizance of LPV in general anaesthesia.
The present study has some limitations. First, the questionnaires were given to predominantly junior anaesthesiologists, of which most would have trained in the era of LPV. The answers given to the questions and the actual practice may differ [
37], in that the decision on the patient management would depend on senior anaesthesiologists. Secondly, the definition of LPV (V
T < 10 mL/kgIBW and PEEP ≥5 cmH
2O) in our study—although based on previous studies [
20,
25,
26] in patients without acute lung injury undergoing general anaesthesia—is arbitrary and differs from the standard ARDS treatment guidelines. Thirdly, this retrospective study involves many uncontrolled co-factors—including fluid intake, operation time, blood products, and type of surgery and intravenous fluid—which cannot be controlled in this type of study. Therefore, we only used the initial ventilator settings for analysis. Finally, this study only involved a single centre in South Korea. Consequently, respondents in this questionnaire study probably do not represent the larger population of anaesthesiologists. In this retrospective study, it is not possible to determine whether our results are applicable to another institution in South Korea. Nevertheless, this questionnaire study is meaningful in that we achieved complete enumeration of the majority of anaesthesiologists who have been in charge of anaesthesia for more than a decade participated in the survey, and the responses of anaesthesiologists on all levels were used in the analysis. It is also important to note that there have been few studies on the relationship of cognizance of LPV and adoption of LPV strategy in general anaesthesia. The present results provide clues for understanding the changes in anaesthetic methods, including LPV, during general anaesthesia in South Korea.