Shock/sepsis/trauma/critical careInfluences of pleural effusion on respiratory mechanics, gas exchange, hemodynamics, and recruitment effects in acute respiratory distress syndrome
Introduction
Acute lung injury and acute respiratory distress syndrome (ALI/ARDS) cause substantial morbidity and mortality despite improved understanding of lung injury and advances in treatment [1]. Several approaches are referred to as “rescue” therapies for severe hypoxemia, including lung recruitment maneuver (RM), ventilation modes, prone positioning, inhaled vasodilator therapy, and the use of extracorporeal membrane oxygenation [1]. However, the only intervention found to reduce mortality from ALI/ARDS is mechanical ventilation that uses relatively small tidal volumes (VT) and low airway pressures [2].
Loss of aerated lung volume is the cardinal feature of ARDS as demonstrated by numerous studies [3]. In an effort to reduce the degree of lung collapse, RM has been proposed as an adjunct to mechanical ventilation [4]. The procedure of RM consists of high sustained airway pressures to open atelectatic alveoli, followed by the application of high positive end-expiratory pressure (PEEP) to maintain alveolar patency [5]. In addition to opening atelectatic alveoli, recruiting the lung by RM is also considered a ventilatory strategy that can prevent ventilator-induced lung injury [6]. The benefits are suggested to come from many mechanisms. The first benefit is an increase in the aerated lung mass, which contributes to minimizing lung heterogeneity and increasing the size of lung [6]. The second benefit is the prevention of repeated opening and closing of the alveoli [6]. Besides, RM has been advocated to reduce shunting by promoting alveolar recruitment and improving pulmonary gas exchanges [1], [7].
In clinical practice, RM is usually considered a useful strategy to improve oxygenation [2], [4]. Villar et al. even showed an improvement in both oxygenation and reducing mortality in patients with ARDS [8]. Thus, RM has been the subject of many investigations over the last two decades, and its use has become much more prevalent in patients with ALI/ARDS [5].
Although RM is frequently used in the treatment of ARDS, there are conflicting results regarding the effects of RM [2]. Timing (early versus late), underlying etiology of ALI/ARDS (pulmonary versus extrapulmonary), and methods of RM have been reported to be important determinants of response [9], [10], [11]. One study suggests that the etiology of ARDS may influence response to RM, such that extrapulmonary ARDS is more responsive to RM [12]. Kloot et al. suggest that RM may be of no benefit if the lung has already been optimally recruited by PEEP [13]. Lim et al. have shown that the methods of RM (sustained inflation versus incremental PEEP) may influence the durability of RM-induced oxygenation [11]. The application of higher levels of PEEP after RM has also been shown to affect sustainability of the effects [9]. Therefore, the effects of RM in different conditions are controversial and further studies are still necessary.
Gattinoni et al. suggested that the responses of PEEP in ALI/ARDS may depend on the respiratory mechanics [12]. Pleural effusion (PLE) has been reported to be present in 66% of patients with ALI/ARDS [14], and its accumulation has important effects on respiratory mechanics [15], which influence lung and chest wall distention. Distending pressures are critically dependent on the relevant pressure of the pleural space [15] and PLE causes increased pleural pressure, which results in an alteration of the distending pressure [15]. Thus, PLE causes increased respiratory system elastance and decreased compliance and resistance because of alteration in the dynamic properties of the lung [15]. Because PLE influences respiratory mechanics, it may also influence responses to RM in ALI/ARDS. However, studies on the influence of PLE on gas exchanges, hemodynamics, and respiratory mechanics in ALI/ARDS are limited.
Although sustained inflation in RM is frequently performed in patients with ALI/ARDS, the improvements in oxygenation as beneficial effects of RM warrant further investigation when ALI/ARDS is accompanied by PLE. Because PLE influences lung and chest wall distention, it can be hypothesized that the response to RM will vary among ARDS with or without PLE. Using a porcine model, this study aimed to determine the influence of PLE on the response to RM in ALI/ARDS by measuring arterial blood gas analysis, pulmonary and systemic hemodynamics, and respiratory mechanics.
Section snippets
Animal preparation
The study protocol was approved by the National Science Council and Animal Review Committee of the National Defense Medical Center (Taipei, Taiwan). A total of 24 pigs (mean age 70–80 d; body weight, 21–28 kg) were used. Anesthesia was induced via intramuscular injections of 1 mg/kg of Zoletil (tiletamine-zolezepam 1:1; Virbac Laboratories, France). General anesthesia was then maintained by infusion of pentobarbital and fentanyl. The anesthetic dose was titrated to ensure optimal anesthesia for
Baseline measurements
There were no significant differences in baseline values of gas exchanges, ventilatory, and hemodynamic parameters among the groups.
Gas exchanges and the influence of PLE on the responses to RM
The gas exchange values (Fig. 1) showed that in group A, the partial pressure of oxygen in arterial blood (PaO2), pH, alveolar–arterial O2 tension difference (PA-aO2), and PaCO2 had no significant changes during the experimental periods. In group B, the PaO2 decreased without changes in PaCO2 and pH after PLE (P < 0.05). There were significant decreases in PaO2 and
Discussion
Lung injury is often quantified by oxygenation, respiratory compliance, and lung water [18], whereas “successful” recruitment is currently assessed by improvements in oxygenation [19]. The present study has several important findings. First, the porcine model shows the typical presentations of ALI/ARDS, including poor oxygenation, lower lung compliance, and increased EVLWI. Second, RM results in improved oxygenation without negative consequences on the systemic and pulmonary hemodynamics or an
Conclusions
ALI/ARDS remain a challenge despite advances in treatment. In this study, RM improves oxygenation without negative consequences on systemic and pulmonary hemodynamics, or an exacerbation of lung injury. RM is often considered a useful strategy to improve oxygenation clinically, but many factors may influence response. This study suggests that PLE attenuates responses to RM in ALI/ARDS.
Acknowledgment
This study was supported by National Science Council of Taiwan grants NSC 96-2321-B-016-001, NSC101-2314-B-016-003 and NSC 97-2314-B-016-031-MY3, Tri-Service General Hospital grant TSGH-C101-088, and Buddhist Tzu-Chi General Hospital grant TCRD-TPE-101-26.
The authors have no financial or other potential conflicts of interest.
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