Perioperative Lung Protection Strategies in Cardiothoracic Anesthesia: Are They Useful?
Introduction
Patients are at risk for several types of lung injury in the perioperative period. These injuries include atelectasis, pneumonia, pneumothorax, bronchopleural fistula, ALI, and acute respiratory distress syndrome (ARDS). Anesthetic management can cause, exacerbate, or ameliorate most of these injuries. Lung-protective ventilation strategies using more physiologic Vts and appropriate levels of positive end-expiratory pressure (PEEP) can decrease the extent of this injury.1 This review discusses the effects of mechanical ventilation and its role in ventilator-induced lung injury (VILI) with specific reference to cardiothoracic anesthesia. The specific clinical scenarios of chronic obstructive pulmonary disease (COPD), one-lung ventilation (OLV), CPB, and transfusion-related acute lung injury (TRALI) are examined. Newer work addressing lung protection strategies, including the relevance of fluid restriction and inflammation, is discussed. The terms, ARDS and ALI, are used in this review. A recent recommendation by the ARDS Definition Task Force has suggested that the term, ALI, be replaced with mild ARDS (200 mm Hg < Pao2/fraction of inspired oxygen [Fio2] ≤ 300 mm Hg) and that ARDS be subdivided into moderate ARDS (100 mm Hg < Pao2/Fio2 ≤ 200 mm Hg) and severe ARDS (Pao2/Fio2 ≤100 mm Hg).2
Section snippets
Mechanical ventilation
Historically, anesthesiologists have been taught to ventilate patients in the perioperative period with large Vts. Volumes as high as 15 mL/kg−1 ideal body weight (IBW) have been suggested to avoid intraoperative atelectasis.3 This far exceeds the normal spontaneous Vts (6 mL/kg−1) common to most mammals. Recent studies have identified the use of large Vts as a major risk factor for development of lung injury in mechanically ventilated patients without ALI. Gajic and colleagues4 reported that
Ventilator-induced lung injury
The phenomenon of VILI is well recognized and can be particularly significant in surgical specialties that require large transfusions, CPB, and associated lung ischemia-reperfusion injury. The deleterious effects of mechanical ventilation may be mediated by localized inflammation and the systemic release of inflammatory cytokines (biotrauma). Mechanical stretch from cyclical alveolar opening and closing sets up an inflammatory response in the alveolar epithelial cells and the vascular
Perioperative surgical environment factors
There are many factors in the surgical environment that can contribute to lung injury; the most obvious is the surgical approach. Site of operation is an important predictor of pulmonary complications, with upper abdominal and thoracic incisions the most important (any surgery approaching the diaphragm).18 A decrease in respiratory complications has been documented if major cavity procedures can be done with minimally invasive versus open techniques.19, 20 Atelectasis occurs frequently after
Patients with chronic obstructive pulmonary disease
COPD patients are at an increased risk of lung injury in the perioperative period.30 Key concepts that are relevant to anesthetic management and lung protection include the following.
Physiotherapy
It has been shown that patients with COPD benefit from an intensive program of preoperative chest physiotherapy, with fewer postoperative pulmonary complications.41 It is possible to improve exercise tolerance in even the most severe COPD patients.42 Little improvement is seen before 1 month, however. Those COPD patients with excessive sputum production benefit the most from chest physiotherapy.43
Smoking Cessation
A preoperative smoking cessation program can significantly decrease the incidence of respiratory
One-lung ventilation
Anesthesiologists are faced with a heterogeneous patient group, in terms of underlying pathology and surgical procedure, requiring OLV. Both a patient's pathology and the surgical procedure can predispose to or cause ALI. ALI after pulmonary resection has been described since the beginning of OLV use for thoracic surgery. The most publicized report is a compilation of 10 pneumonectomy cases published in 1984,49 which focused on the role of intravenous overhydration as a cause of
Role of volatile anesthetic agents in lung protection
Volatile agents have immunomodulatory effects. Much work has been done, especially in the cardiac setting, on the role of volatiles in ischemia-reperfusion injury and in preconditioning and postconditioning. Recent studies in models of ALI, during OLV and in cases of lung ischemia-reperfusion,71 suggest that volatiles may act as preconditioning and postconditioning agents inducing lung protection by inhibition of the expression of proinflammatory mediators. Isoflurane pretreatment in an
Transfusion-related lung injury
TRALI jury has emerged as a leading cause of transfusion morbidity and mortality,77 with a disproportionate number of cases occurring in the perioperative period.78 Anesthesiologists are routinely involved in transfusion decisions and are well placed to both decrease the incidence and the morbidity and mortality of TRALI. Diagnostic criteria consist of hypoxia or bilateral pulmonary edema during or within 6 hours of transfusion, in the absence of circulatory overload.79 Difficulties lie in
Cardiopulmonary bypass
Pulmonary dysfunction post-CPB is a well described but poorly understood phenomenon.84 Although the incidence of ARDS post-CPB is low (<2%) the mortality associated with it is high (>50%).85 Although the systemic inflammatory response syndrome initiated by CPB plays a major role, the pulmonary insult is multifactorial and not all related to the bypass itself. Extra-CPB factors are general anesthesia, sternotomy, and breaching of the pleura. Intra-CPB factors include but are not limited to
Ultraprotective lung ventilation
On the continuum of lung-protective ventilation in ALI/ARDS is the concept of ultraprotective ventilation. This concept uses extracorporeal lung assist, such as the pumpless Novalung interventional lung-assist (iLA) device and near static ventilation. An iLA is a membrane ventilator that allows O2 and CO2 gas exchange via simple diffusion.88 The membranes are biocompatible and provide a nonthrombogenic surface. It is designed to work without a mechanical pump in an arteriovenous configuration (
Fluids, inflammation, and the glycocalyx
A retrospective cohort study of specifically intraoperative risk factors for ARDS in critically ill patients found that for patients receiving fluid resuscitation greater than 20 mL/kg−1/h−1 the odds of developing ARDS were 3 times greater than if less than 10 mL/kg−1/h−1 was given (odds ratio 3.1; 95% CI, 1.0–9.9; P = .05).96 Vt/IBW−1 (mL/kg−1) and number of blood products were not associated with ARDS in this study. The majority of patients were ventilated with a Vt/IBW−1 of 8 mL/kg−1 to 10
Other therapies for lung protection
Beyond those already discussed, there are several therapies that may play a future role in lung protection. Permissive hypercapnia's place in protective ventilation has been alluded to previously but, as found in the original ARDsnet data, may be protective in the presence of higher Vt.114 Hypercapnic acidosis is protective in a variety of models of ALI. Beneficial effects include attenuation of lung neutrophil recruitment, pulmonary and systemic cytokine concentrations, cell apoptosis, and
Summary
To summarize what is known: (1) nonphysiologic ventilation in healthy lungs induces ALI; (2) protective lung ventilation in patients with ALI/ARDS improves outcome; (3) protective lung ventilation in noninjured lungs and in the absence of a primary pulmonary insult may initiate subclinical VILI (as evidenced by inflammatory markers); (4) VILI has important implications for organs remote to the lungs and is associated with significant morbidity and mortality; (5) volatile anesthetics may have a
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