Key results
We conducted a prospective observational study to investigate the current practice of intraoperative ventilation and to evaluate the associations between ventilator settings during OLV and PPCs in patients undergoing thoracic surgery. We found that FIO2 of ≥0.8, VT of approximately 6 ml/kg, and PEEP of approximately 4 cm H2O were common. Patients with PPCs received higher FIO2 during OLV, while they had lower minimum SpO2 than those without PPCs. However, in multivariate logistic regression analysis adjusting for ventilator settings, ARISCAT score, and minimum SpO2, only TWA FIO2 was associated with the occurrence of PPCs, and the adjusted OR per FIO2 increase of 0.1 was 1.30. Therefore, an increase in oxygen concentration of 10% was associated with approximately 30% increase in the risk of PPCs.
Interpretation
We found that V
T was around 6 ml/kg, and PEEP was set around 4 cm H
2O in most patients. These findings were consistent with recent studies or textbook oriented lung protective strategy [
15,
21,
22]. We also found that high F
IO
2 was frequently used during OLV. These findings, however, were inconsistent with recent recommended management [
22]. An F
IO
2 of 1.0 was classically a routine component of OLV [
15,
23]. However, the incidence of hypoxemia during OLV has been decreasing [
15,
22], and the harmful effects of high F
IO
2, including absorption atelectasis [
24‐
27], production of reactive oxygen species, and increased lung injury [
28,
29], have been reported. Therefore, this classic practice has been questioned and avoidance of excessive F
IO
2 has been proposed [
15]. The latest textbook suggests that F
IO
2 should be titrated to maintain a stable saturation level above 92-94% during OLV [
22]. However, some reports revealed that relatively high F
IO
2 was still applied as a common practice during both two-lung ventilation [
30,
31] and OLV [
13‐
16]. In our survey, intraoperative minimum SpO
2 was ≥95% in 111 patients (56%), with 83% of them receiving TWA F
IO
2 of ≥0.6 (Additional file
3). These findings indicated that almost half of the patients may have received excessive oxygen regardless of their SpO
2. There was low compliance with recommended standards to maintain a SpO
2 above 92-94% during OLV.
According to our results, high F
IO
2 during OLV was independently associated with the increasing incidence of PPCs, and patients with PPCs had a longer LOS in the hospital. Worse clinical outcomes due to high F
IO
2 were previously reported in critically ill adults, including patients with chronic obstructive pulmonary disease, myocardial infarction, cardiac arrest, stroke, and traumatic brain injury [
32‐
35]. Given the above concern, a conservative oxygenation strategy has been shown to be feasible, safe, and effective for mechanically ventilated patients in recent decades [
36,
37]. Notably, conservative oxygen therapy could be associated with decreased evidence of atelectasis as well as earlier weaning from mandatory ventilation in the ICU [
38]. Additionally, a recent randomized control trial of conservative oxygen therapy in ICU showed lower mortality [
39].
Only a few studies investigated the effect of intraoperative F
IO
2 on clinical outcomes in thoracic surgery with OLV. Yang et al. reported a lower incidence of postoperative lung dysfunction and satisfactory gas exchange was provided by the lung protective strategy using F
IO
2 of 0.5 compared to the conventional strategy using F
IO
2 of 1.0 during OLV [
40]. However, F
IO
2 was one of components in this lung protective strategy, because V
T, PEEP, and mode of mechanical ventilation were also different between the groups. Thus, it remains uncertain whether a conservative approach to oxygen therapy during OLV is beneficial or not. To our knowledge, this is the first study to demonstrate an association between high F
IO
2 during OLV and the occurrence of PPCs. To confirm and dissect these findings, additional studies should be performed in different settings. Moreover, our findings support the need for randomized control trials to evaluate the safety and feasibility of conservative oxygen therapy during OLV.
Limitations
There were several limitations in this study. First, because this was an observational study, causality was not determined. It should be noted that higher FIO2 might be confounded by the incidence of hypoxemia, which could cause PPCs. Thus, the role of FIO2 is difficult to differentiate between “unnecessary use” and “need for higher support.” However, after adjusting by ARISCAT score, minimum SpO2, ΔP, and PEEP to reduce potential confounding, only higher FIO2 remained statistically significant as an independent risk factor for PPCs. In subgroup analyses, FIO2 has been associated with the incidence of PPCs even in patients with comparatively lower risk for PPCs. Additionally, the present study indicated that patients might receive excessive oxygen during OLV. Therefore, we believe that intraoperative FIO2 could be titrated safely even during OLV.
Second, the incidence of PPCs could have heavily depended on our definition. There are various definitions of PPCs. For instance, pneumonia was diagnosed based on radiologic images, symptoms, laboratory findings, or antimicrobial treatment used. The diagnosis of atelectasis was based on images or bronchoscopy. In our study, we used definitions of PPCs from previous studies [
17,
20] and CDC guidelines [
19] as shown in Fig.
1. As a result, the incidence of PPCs in our study (25.9%) was similar to that of previous works [
17,
20].