Our study showed that among patients undergoing VATS for lung cancer resection, ultrasound-guided RMs significantly reduced the incidence of atelectasis and improved lung aeration at the end of surgery compared with conventional RMs. The difference in lung aeration between the groups remained after spontaneous breath before discharged from the PACU, as evaluated by the LUS scores. However, the ultrasound-guided RMs failed to further improve arterial oxygenation at that time. In addition, we noticed a higher incidence of atelectasis in the dependent lungs than in the independent lungs after surgery and in the PACU.
With continuous advancements in surgical techniques and perioperative anaesthetic management, lung resection with VATS has been widely accepted for its less invasive and faster recovery compared with the traditional thoracotomy [15]. However, perioperative lung complications, including atelectasis, remain a major concern [16]. The RMs have been demonstrated to be effective at reducing perioperative atelectasis and lung infections, improving oxygenation and lung compliance, and reducing the need for high FiO2 in patients undergoing thoracic surgery [17, 18]. However, there is no consensus on the optimal recruitment strategy for individual patient clinically [16]. Therefore, individualized and tailored RM strategies are needed for surgical patients that take into account their physiologic differences.
Recent studies have suggested that ultrasound-guided RMs reduce atelectasis in children or adult laparoscopic surgical patients [7, 8, 12]. In our study, ultrasound-guided RMs was found to be more effective at improving lung aeration as well as reducing perioperative atelectasis in patients undergoing thoracic surgery compared with Group CR. The LUS can be applied to evaluate the extent of atelectasis and the patients’ response to lung recruitment individually and dynamically; thus, it may be helpful to perform optimal lung recruitment in surgical patients [12].
During VATS procedures, the lungs undergo a variety of stress factors, including OLV, compression and injury of the lung, positive pressure ventilation, and surgical trauma-induced inflammatory response [19]. These factors may contribute to increase the development of atelectasis, hypoxemia, and lung dysfunction during the perioperative period [20]. In the present study, we found that ultrasound-guided RMs improved SpO2 30 min after patients entering the PACU, which may be due to reduced atelectasis and improved lung aeration. However, ultrasound-guided RMs did not significantly improve the PaO2/FiO2 ratio in the early postoperative period, suggesting that some other mechanisms, such as postoperative pain, sputum excretion, and a reduction in lung volume might also influence oxygenation.
Since RMs also predispose patients to cycles of derecruitment/recruitment, clinicians should be careful when applying RMs. Insufficient RMs produce suboptimal lung aeration, while excessive high-pressure RMs may contribute to alveolar overdistention and barotrauma, leading to decrease in cardiac output, hypotension, and ventilator-induced lung injury [21]. From the view of risk-benefit, RMs guided with real-time LUS at the bedside may be a good strategy.
A previous study demonstrated that atelectasis caused by surgery and anaesthesia rapidly resolved after extubation in the PACU [11]. Similarly, our study showed that the incidence of atelectasis decreased significantly after extubation in both groups, which may be due to lung recruitment by the cough reflex. However, our results showed that a certain degree of lung aeration deterioration still existed after extubation in Group CR, as evidenced by high LUS scores at T3, indicating that aeration loss in several lung areas remained even after the neuromuscular blockade had been completely reversed. These findings further demonstrate the advantages of LUS-guided RMs in terms of lung aeration improvement.
We also noticed a higher incidence of atelectasis in the dependent lungs than in the independent lungs in both groups after surgery and in the PACU, which may be related to the lateral decubitus position and the fact that RMs of the surgical lung were performed directly under surgeon supervision [22]. During OLV in the lateral decubitus position, the expansion of the dependent lung is usually hindered by the overlying compression of the mediastinum and abdominal organs, the raised paralysed diaphragm, and the pressure and noncompliance of the chest wall [20]. Therefore, atelectasis may readily appear in the dependent lung, resulting in a lung surface with lower ventilation and oxygenation [23]. A previous study demonstrated that the incidence of atelectasis was higher in the dependent anterior chest in the prone position after surgery. In another study, worst LUS scores were detected in the inferoposterior quadrant in the supine position during gynaecological surgery [7, 24]. In our study, atelectasis occurred mainly at the posterior lung regions of the dependent lungs, which is consistent with the idea that atelectasis primarily exists in gravity-dependent lung regions [25].
A high oxygen concentration is often used to maintain adequate oxygenation during OLV [26]. In our study, an 80% FiO2 was used during OLV, and no patient developed dangerous hypoxemia during surgery. Exposure to 100% O2 can precipitate absorption atelectasis and an overproduction of radical oxygen species and proinflammatory cytokines, ultimately aggravating lung injury during OLV [26]. A previous study suggested that if a low FiO2 can be tolerated during OLV, lung injury such as pulmonary oedema and alveolar thickening can be minimized compared with a higher FiO2 [27].
Our study had several limitations. First, while reduced atelectasis was observed with the ultrasound-guided recruitment strategy, other clinically relevant data, such as incidences of postoperative pulmonary complications and lung mechanics, were not evaluated or followed up. Therefore, the clinical implications of ultrasound-guided RMs may be limited, and these endpoints should be investigated in further studies. Second, we excluded patients with morbid obesity or potential lung diseases to minimize confounding factors. Further research is needed to assess the effects of ultrasound-guided RMs in those specific patients. Third, hyperinflation that may have occurred during our RMs were not detected on LUS. However, the peak inspiratory pressure in our study was similar to or lower than that reported previously to avoid lung hyperinflation [7].