Bronchoscopy was performed in 117 cases during the study period, with transbronchial lung biopsy performed in 100 cases and transbronchial lung cryobiopsy performed in 17 cases. There were four cases of measurement failure, including poor auricular placement (n = 2), prolonged dropout due to patient movement during the examination (n = 1), and poor calibration of the measurement device (n = 1). Therefore, only 113 cases could be measured continuously and were successfully analyzed.

The measurement success rate was 96.6%. Patients’ clinical and demographic data are presented in Table 1. Midazolam and fentanyl were administered as anesthesia during bronchoscopy in 46 cases, whereas midazolam alone was administered in 67 cases. Midazolam was additionally administered during the examination in 14 cases and fentanyl in 8 cases. Univariate and multivariate analysis for tcPCO2 and SpO2 on these parameters are presented in Table 2A, B.

Figure 2 show the dynamic trends of tcPCO2 and SpO2, respectively, in all cases. The mean bronchoscopy time was 38.5 min. The mean tcPCO2 and SpO2 were 42.7 ± 6.52 mmHg and 95.3 ± 3.11%, respectively.

Dynamic trend of tcPCO2/SpO2 of all cases during bronchoscopy. SpO2 saturation of percutaneous oxygen, tcPCO2 transcutaneous carbon dioxide partial pressure

The tcPCO2 (mmHg) trends in the midazolam plus fentanyl and midazolam alone groups are shown in Fig. 3A, B. The tcPCO2 in the midazolam plus fentanyl and midazolam alone groups was 44.8 ± 7.83 and 40.6 ± 4.10 mmHg, respectively. The tcPCO2 was significantly higher in the midazolam plus fentanyl group (p = 0.01). In most cases, tcPCO2 remained between 40 and 50 mmHg, and gradual CO2 sequestration during the examination was observed in many cases. Patients with high tcPCO2 before the examination exhibited high CO2 retention during the examination. Compared with the midazolam plus fentanyl group, the midazolam alone group showed no rapid increase in tcPCO2; however, CO2 slowly accumulated in many cases. The median Δ value was 8.10 and 4.00 mmHg in the midazolam plus fentanyl and midazolam alone groups, respectively, with the midazolam plus fentanyl group having a significantly larger value (p = 0.005). The tcPCO2 in the groups with and without additional midazolam was 40.5 ± 3.34 and 40.6 ± 4.02 mmHg, respectively, with no statistically significant difference (p = 0.45) (Table 2A). The median Δ value was 3.85 and 4.15 mmHg in the groups with and without additional midazolam, respectively, with no statistically significant difference (p = 0.32). The tcPCO2 in the groups with and without additional fentanyl was 46.8 ± 8.23 and 42.9 ± 5.54 mmHg, respectively, with no statistically significant difference (p = 0.26) (Table 2A). The median Δ value was 8.35 and 7.85 mmHg in the groups with and without additional fentanyl, respectively, with no statistically significant difference (p = 0.12).

tcPCO2/SpO2 over time in the midazolam + fentanyl group and the midazolam group. SpO2 saturation of percutaneous oxygen, tcPCO2 transcutaneous carbon dioxide partial pressure

The SpO2 (%) trends in the midazolam plus fentanyl and midazolam alone groups are shown in Fig. 3C, D. SpO2 fluctuation was greater in the midazolam plus fentanyl group, and SpO2 imbalance was observed even with oxygen administration at a high flow rate. SpO2 in the midazolam plus fentanyl and midazolam alone groups was 94.4 ± 3.37 and 96.2 ± 2.61%, respectively, with the midazolam alone group showing a significantly larger value (p = 0.01). The SpO2 in the groups with and without additional midazolam was 96.6 ± 3.02 and 95.8 ± 3.22%, respectively, with no statistically significant difference (p = 0.63) (Table 2B). The SpO2 in the groups with and without additional fentanyl was 93.7 ± 3.46% and 95.1 ± 3.08%, respectively, with no statistically significant difference (p = 0.21) (Table 2B).

Subgroup analysis according to smoking history and graphs over time are shown in Fig. 4. Among the patients, 49 (43.4%) were heavy smokers, whereas 64 (56.6%) were nonsmokers or light smokers. The tcPCO2 trends over time in the heavy smoker and nonsmoker or light smoker groups are shown in Fig. 4A, B. The tcPCO2 in the heavy smoker and nonsmoker or light smoker groups was 43.3 ± 7.87 and 42.1 ± 5.34 mmHg, respectively, with no statistically significant difference (p = 0.59). The median Δ value was 6.80 and 5.30 mmHg in the heavy smoker and nonsmoker or light smoker groups, respectively, with no statistically significant difference (p = 0.063).

tcPCO2/SpO2 over time in the heavy smoker group and the light/never smoker group. SpO2 saturation of percutaneous oxygen, tcPCO2 transcutaneous carbon dioxide partial pressure

The SpO2 trends over time in the heavy smoker and nonsmoker or light smoker groups are shown in Fig. 4C, D. In both groups, SpO2 fluctuated significantly during bronchoscopy. SpO2 in the heavy smoker and nonsmoker or light smoker groups was 95.3 ± 3.10 and 95.3 ± 3.14%, respectively, with no statistically significant difference (p = 0.57).

Subgroup analysis according to BMI and graphs over time are shown in Fig. 5. Among the patients, 28 (24.8%) had a BMI of ≥25, whereas 85 (75.2%) had a BMI of <25. The tcPCO2 trends over time in the BMI ≥ 25 and BMI < 25 groups are shown in Fig. 5A, B. The tcPCO2 in the BMI ≥ 25 and BMI < 25 groups was 44.2 ± 8.97 and 41.2 ± 5.23 mmHg, respectively, with the BMI ≥ 25 group having a significantly higher tcPCO2 (p = 0.05). The median Δ value was 7.40 and 4.70 mmHg in the BMI ≥ 25 and BMI < 25 groups, respectively, with no statistically significant difference (p = 0.19).

tcPCO2/SpO2 over time in the BMI ≥ 25 group and the BMI < 25 group. BMI body mass index, SpO2 saturation of percutaneous oxygen, tcPCO2 transcutaneous carbon dioxide partial pressure

The SpO2 trends over time in the BMI ≥ 25 and BMI < 25 groups are shown in Fig. 5C, D. SpO2 in the BMI ≥ 25 and BMI < 25 groups was 95.4 ± 2.94 and 95.3 ± 3.17%, respectively, with no statistically significant difference (p = 0.5).

Subgroup analysis according to sex and graphs over time are shown in Fig. 6. Among the patients, 65 (57.5%) were men, whereas 48 (42.5%) were women. The tcPCO2 trends over time in the male and female groups are shown in Fig. 6A, B. The tcPCO2 in the male and female groups was 42.8 ± 7.07 and 42.6 ± 5.43 mmHg, respectively, with no statistically significant difference (p = 0.86).

tcPCO2/SpO2 (%) over time in male and female group. SpO2 saturation of percutaneous oxygen, tcPCO2 transcutaneous carbon dioxide partial pressure

The median Δ value was 6.60 and 5.50 mmHg in the male and female groups, respectively, with no statistically significant difference (p = 0.08).

The SpO2 trends over time in the male and female groups are shown in Fig. 6C, D. SpO2 in the male and female groups was 95.1 ± 3.20 and 95.5 ± 2.99%, respectively, with no statistically significant difference (p = 0.58).