In patients undergoing laparoscopic surgery in the Trendelenburg position, pulmonary function is affected by the movement of the CO2 pneumoperitoneum and abdominal contents against the diaphragm. Subsequent displacement of the diaphragm and mediastinal structures towards the head leads to a decrease in functional residual capacity (FRC) and lung compliance, and an increase in peak inspiratory and plateau pressures. Changes in lung physiology are more affected by pneumoperitoneum than by trendelenburg position[11, 12], so to reduce the interference of intraoperative factors, we fixed the pneumoperitoneum pressure and position angle on the premise of ensuring the surgical field of vision. In VCV, the ventilator provides a constant preset tidal volume, but airway pressure is influenced by lung compliance, patient's intraoperative lung compliance decreases may accompany with the tidal volume vary, especially during the transition from the supine to the Trendelenburg position and during pneumoperitoneum establishment with CO2. It also carries the risk of hypoventilation or hyperventilation[13]. The PCV-VG provides a target tidal volume by decelerating flow, similar to PCV. It compares the Cdyn measured with each breath and adjusts inspiratory pressure to achieve the set tidal volume. Intraoperative depth of anesthesia, muscle relaxation, and surgical maneuvers also alter compliance and resistance[14]. The PCV-VG effectively combines the volume control and pressure limitation ,its lower inspiratory pressure and decelerated flow rate reduce the degree of PIP elevation, which prevents lung injury (Barotraumas) and improves the distribution of inhaled air by minimizing pulmonary atelectasis[15] .
In previous studies focusing on one-lung ventilation in thoracic surgery, special patients such as obese patients, elderly patients, and special positions such as prone, lateral posterior laparoscopy, and Trendelenburg laparoscopy with pneumoperitoneum duration less than 3h, it has been demonstrated that PCV-VG provides lower Ppeak, Pplat and improved Cdyn compared to VCV. There were no significant differences in blood gas analysis parameters between the prone, lateral and Trendelenburg groups[16, 17]. PCV-VG in posterior laparoscopic surgery also reduced dead space ventilation, facilitated CO2 evacuation and shortened postoperative hospital stay[18]. In elderly single lung ventilation, PCV-VG mode was found to reduce airway pressure in open-chest patients compared to VCV, and also reduced neutrophil release, reducing inflammatory response and lung injury[19]. It has also been discovered that at each time point after pneumoperitoneum in the Trendelenburg position, the mean PaO2 levels were significantly higher in the PCV-VG group than in the VCV group[20]. In terms of respiratory mechanics, this experiment did not differ significantly from previous studies. PCV-VG provided lower Ppeak, Pplat and higher Cdyn and Raw. Ppeak reflects the dynamic compliance of the respiratory system and depends on factors such as VT, inspiratory time, endotracheal size and airway resistance[21], and Pplat correlates with static lung compliance[22] .
Our study included patients with pneumoperitoneum duration longer than 3h. There are no similar previous studies in Chinese and foreign studies. It was concluded that PCV-VG did not show the advantages in the first 2h of pneumoperitoneum, but provided a higher oxygenation index, less intrapulmonary shunt, respiratory index and arterial oxygen partial pressure difference compared to VCV after pneumoperitoneum over 2h, however, the differences in these parameters decreased or even did not differ when transferred to the horizontal position. Probably because it was the prior exclusion of patients with a history of pulmonary disease and pulmonary surgery from this experiment, the patients' own physiological conditions were still sufficient to compensate for the adverse physiological changes caused by pneumoperitoneum and position at the beginning of the surgery, and the two groups did not show a large difference. As the duration of surgery increases, this difference slowly becomes apparent, and the advantages of PCV-VG are gradually amplified. However, once these adverse interventions are withdrawn, there is a slow return to a comparatively normal physiological state.
Intraoperative hemodynamics were fairly stable in both groups, The increase in MAP when shifting to a head-down, foot-up position may be related to the redistribution of blood in the circulation due to increased resistance of the body circulation and compression of intra-abdominal organs by intra-abdominal pressure.
PCV-VG had a better quality of postoperative 24h recovery than VCV in terms of postoperative. We did a correlation analysis between QOR40 score and patients' age, gender, and pneumoperitoneum time and found that women had lower quality of recovery at 24h postoperatively than men, and the longer the pneumoperitoneum time, the lower the QOR40 score. No significant differences were found for the remaining complications. This suggests that the benefit of PCV-VG in this group of patients may be limited only to the intraoperative period and that it is challenging to assess and interpret the long-term effects of PCV-VG or VCV on patients. 8 of the 17 publications mentioned multiple postoperative outcome parameters. Only one study reported that PCV-VG reduced patients' ICU and hospital length of stay[23]. Our study included the quality of recovery at 24h postoperatively, yielding a better quality of recovery at 24h postoperatively for PCV-VG. As far as the assessed endpoints are concerned, PCV-VG appears to be a safe technique for ventilation, with no postoperative-related disadvantages so far .
Limitations of this paper.
1. Our study excluded patients who were obese and had poor preoperative lung function. The study found that the return to a flat position in such patients could make it more difficult to counteract hypercapnia with ventilation and oxygenation[24], assuming that these patient groups would benefit from PCV-VG, then the effect of PCV-VG may be underestimated.
2. In this paper, the patient's position was fixed in the Trendelenburg position at 30 degrees and the pneumoperitoneum pressure was fixed at 12 mmHg only due to the exclusion of interfering factors, but in clinical work, different pneumoperitoneum pressures and intraoperative changes in position are sometimes required due to different patient body types. The results of different angular positions and pneumoperitoneum pressures are often not the same.
Overall, intraoperative ventilation using the PCV-VG technique appears to be beneficial, although valid data on the long-term outcome parameters of the different ventilation modes remain to be determined. PCV-VG appears to offer unquestionable clinical advantages. Whether this modality can meet our high expectations for the perioperative patients’ lung protection remains to be determined in future studies.