Mechanical ventilation is necessary during modern surgery, given the increased complexity of surgical procedures, the need for airway protection and the use of nerve blockers and intravenous anesthetics.1 General anesthesia has a variety of acute effects on the respiratory system, which mechanical ventilation aims to combat.1 However, mechanical ventilation is also associated with lung injury.2 Lung-protective ventilation strategies aim to reduce postoperative pulmonary complications and respiratory dysfunction, which are common after cardiothoracic surgery.2 In order to provide the best care, anesthesiology professionals should have knowledge about the effects of general anesthesia on the respiratory system, lung-protective ventilation strategies and the efficacy of such strategies.
General anesthesia affects the respiratory system by changing a patient’s breathing and activity of respiratory muscles; surgical positioning of the patient can further affect ventilation.1 General anesthetics also reduce muscular tone and alter diaphragmatic position, leading to reduction in lung volume, alteration in ventilation/perfusion ratio and lung atelectasis (i.e., alveolar collapse).2 Mechanical ventilation is used to combat these side effects by providing extrinsic respiratory functions to the patient.2 It is especially necessary and complex during cardiothoracic surgery, which involves substantial hemodynamic and pulmonary manipulation due to displacement of thoracic organs.1,3 The anesthesia provider must oversee fluid exchange and pharmacological alteration of vascular resistance during cardiothoracic surgery.3 However, mechanical ventilation can also be injurious, and it is associated with a variety of pulmonary complications.2 Thus, researchers have created lung-protective ventilation techniques to spare lungs from stress-induced damage from surgery, general anesthesia and mechanical ventilation.3
Lung-protective ventilation strategies may be helpful in reducing the harms of general anesthesia and mechanical ventilation.3 These techniques include low tidal volume ventilation (LTVV), positive end-expiratory pressure (PEEP) and recruitment maneuvers.4 LTVV uses ventilation with lower than traditional tidal volume (i.e., the volume of air moved into and out of the lungs during each ventilation cycle)5 to reduce ventilator-induced lung injury.4 Though LTVV has traditionally been used for patients with acute lung injury or acute respiratory distress syndrome (ARDS),6 it has also become common in lung-protective ventilation for patients without ARDS.7 Meanwhile, PEEP, which is a mechanical way to keep a patient’s airway pressure greater than atmospheric pressure,8 aims to prevent airway collapse and increase functional residual capacity (FRC; i.e., the volume of air in the lungs at the end of passive expiration).9,10 Increased FRC and open airways can prevent atelectasis. Finally, recruitment maneuvers reopen the alveoli after collapse and must be administered carefully, as overinflation may occur.11 When mechanical ventilation strategies use LTVV and adequate PEEP and recruitment maneuvers, they may reduce pulmonary stress and strain that normally occurs during cardiothoracic surgery.4
Though lung-protective ventilation strategies such as LTVV, PEEP and recruitment maneuvers have been recommended for non-ARDS patients undergoing general anesthesia, it remains unclear if they are necessary. Romagnoli and Ricci emphasize the lack of evidence on lung-protective ventilation’s role in preventing postoperative pulmonary complications.2 Coppola et al. argue that lung-protective ventilation should be considered for patients who are at high risk, who have pulmonary disease or who are undergoing long or high-risk surgical procedures.4 However, they write that the cardiovascular risks associated with lung-protective ventilation—such as reduction of venous return and cardiac output and the need for fluids and vasopressors—may make such ventilation unnecessary in other patients.4 A meta-analysis by Schreiber et al. found that the positive effects of lung-protective ventilation are likely short-lived and may not help a patient in the long term.12 Also, Wrigge et al. found that LTVV was not clinically beneficial for healthy patients in uncomplicated surgeries.13 On the other hand, Gu et al.’s and Serpa Neto et al.’s meta-analyses showed that LTVV reduced risk of lung injury and pulmonary infection14 and improved overall clinical outcomes7 when compared to higher tidal volumes. Furthermore, Zamani et al. found that lung-protective ventilation consisting of LTVV and adequate PEEP reduced pulmonary infection after coronary artery bypass grafting surgery.15 Evidently, the data on lung-protective ventilation are mixed, and more research is needed to assess its benefits and disadvantages.
General anesthesia and mechanical ventilation are necessary for cardiothoracic surgery, but they can also be injurious to the respiratory system and thoracic organs. Lung-protective ventilation, which is used in patients who have lung injuries or respiratory distress, may be helpful in reducing pulmonary stress during surgery. There is some evidence that strategies such as LTVV, PEEP and recruitment maneuvers can reduce pulmonary complications in patients undergoing cardiothoracic procedures, though data on lung-protective ventilation are lacking.3 Future researchers should search for a balance between LTVV, PEEP and recruitment maneuvers to optimize lung protection and reduce cardiovascular side effects. Also, longitudinal studies are necessary to assess the effects of lung-protective ventilation on a patient’s long-term outcomes.
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14. Gu W-J, Wang F, Liu J-C. Effect of lung-protective ventilation with lower tidal volumes on clinical outcomes among patients undergoing surgery: A meta-analysis of randomized controlled trials. Canadian Medical Association Journal. 2015;187(3):E101–E109.
15. Zamani MM, Najafi A, Sehat S, et al. The effect of intraoperative lung protective ventilation vs conventional ventilation, on postoperative pulmonary complications after cardiopulmonary bypass. Journal of Cardiovascular and Thoracic Research. 2017;9(4):221–228.