Expert Review of Respiratory Medicine Downloaded from informahealthcare.com by Washington University Library on 12/31/14 For personal use only.
Review
A practical approach to the use of prone therapy in acute respiratory distress syndrome Expert Rev. Respir. Med. 8(4), 453–463 (2014)
Krishna P Athota, D Millar, Richard D Branson* and Betty J Tsuei Department of Surgery, University of Cincinnati, Cincinnati, OH, USA *Author for correspondence: Tel.: +1 513 558 5661 Fax: +1 513 558 3136 [email protected]
In this article we propose a practical approach to the use of prone therapy for acute respiratory distress syndrome (ARDS). We have attempted to provide information to improve the understanding and implementation of prone therapy based on the literature available and our own experience. We review the basic physiology behind ARDS and the theoretical mechanism by which prone therapy can be of benefit. The findings of the most significant studies regarding prone therapy in ARDS as they pertain to its implementation are summarized. Also provided is a discussion of the nuances of utilizing prone therapy, including potential pitfalls, complications, and contraindications. The specific considerations of prone therapy in open abdomens and traumatic brain injuries are discussed as well. Finally, we supply suggested protocols for the implementation of prone therapy discussing criteria for initiation and cessation of therapy as well as addressing issues such as the use of neuromuscular blockade and nutritional supplementation. KEYWORDS: acute respiratory distress syndrome • critical care • mechanical ventilation • prone position • refractory hypoxemia
Acute respiratory distress syndrome (ARDS) was first coined in 1967 by Ashbaugh et al. and used to describe acute bilateral pulmonary abnormalities, although the clinical entity of pulmonary edema without heart failure was initially described by Laennec in 1821 [1,2]. ARDS is associated with a number of clinical diagnoses and has been shown to have a high mortality of almost 40% [3]. Recent improvements in critical care have focused on developing new methods of treating the severe hypoxia of ARDS and decreasing the incidence of this disease including lung protective strategies, novel modes of ventilation and prone positioning. Prone positioning for the treatment of ARDS was first described in 1977 by Douglas et al. In a series of six patients with acute respiratory failure, they reported improved oxygen requirements in four of the five mechanically ventilated patients when prone positioning was utilized [4]. Since that time, studies of prone positioning have included different patient populations, various criteria for initiation of prone positioning and a wide duration of treatment times, making it difficult to draw unified conclusions from these disparate studies. Furthermore, the
informahealthcare.com
10.1586/17476348.2014.918850
development of protocolized lung protective strategies, which have been shown to be beneficial, such as those proposed by ARDSnet, has only occurred within the last decade and was not incorporated into many of the early studies of prone positioning [5]. As such, while there is significant evidence that prone positioning can dramatically improve the respiratory status in selected patients, there has been few prospective randomized studies establishing its overall benefit, and the role of prone positioning in the treatment of ARDS has been widely debated. This article will summarize prior studies examining the use of prone positioning in the treatment of ARDS, with recommendations for criteria to initiate therapy, therapy protocol, indication for cessation of therapy and potential pitfalls and complications during the use of positional therapy. ARDS as an entity was formally defined by the American–European Consensus Conference (AECC) in 1994. The majority of research which has been done on the use of prone positioning in ARDS has utilized this AECC definition of ARDS. As time passed, it was apparent that this definition had limitations regarding
2014 Informa UK Ltd
ISSN 1747-6348
453
Expert Review of Respiratory Medicine Downloaded from informahealthcare.com by Washington University Library on 12/31/14 For personal use only.
Review
Athota, Millar, Branson & Tsuei
the timing of onset, defined hypoxemia and variability in the radiographic findings, and a panel of experts was convened in 2011 to revise the original AECC definition of ARDS, creating the Berlin definition [6]. For the purposes of this paper, in order to summarize the most recent proning studies, which were undertaken largely before the development of the Berlin criteria, ARDS will refer to AECC criteria (now Berlin moderate or severe ARDS) unless otherwise specified. Management of acute hypoxia is a common goal of critical care medicine. One of the variables that can be manipulated to improve hypoxia clearly is oxygen concentration. While there continues to be debate on the significance of oxygen toxicity, there are limited experimental studies in humans. In the numerous animal studies that have been performed, high oxygen concentration with long duration of exposure results in pulmonary pathology similar to ARDS [7,8]. Additional animal studies have shown that exposure to less than 60% FiO2 did not result in lung injury, regardless of the duration of exposure [9]. A recent prospective observational study in humans also showed findings consistent with oxygen toxicity in a time and dose-dependent fashion [10]. As these results suggest that oxygen levels above 60% cause lung injury, treatment of the ARDS patient should include maneuvers to decrease supplemental oxygen to this level as rapidly as possible by using advanced lung protective ventilator strategies as well as adjuncts such as prone positioning. The initial report by Douglas et al. regarding improved oxygenation after prone positioning was followed by two additional observational studies that confirmed this finding [11,12]. During this period, Gattinoni and colleagues demonstrated that, in contrast to the diffuse alveolar disease seen on plain radiograph, computed tomography indicated that ARDS was actually an inhomogeneous pulmonary process, with affected areas primarily occurring in the dependent portion of the lung parenchyma [13]. Subsequently, a number of mechanisms were proposed to explain the beneficial effects of prone positioning including increased functional residual capacity, alterations of diaphragmatic motion, gravitational changes in lung perfusion and improved secretion management. However, in 1994, Lamm and colleagues dispelled most of these theories by using an animal model in which prone positioning was shown to be effective primarily by improving ventilation and increasing transpulmonary pressure, thus decreasing ventilation–perfusion mismatch in the dorsal lung [14]. Human studies have supported these findings, revealing improved lung volumes and recruitment, and increased ventilation when prone positioning is utilized [15,16]. Prone positioning redistributes pleural pressures and can result in more homogeneous transpleural pressures. This allows for more equal distribution of lung ventilation, decreasing mechanical overexpansion and lung strain to the ventral portion of the lung [17,18]. As such, prone positioning not only improves respiratory mechanics and gas exchange, but also is lung protective. While the use of proning may minimize lung injury resulting from other methods of oxygenation and ventilation, the 454
inciting process that caused the development of ARDS must be addressed as well. Despite the fact that prone positioning in patients with ARDS has been shown to improve oxygenation, early trials failed to demonstrate a significant survival advantage. Initial trials included a spectrum of ARDS patients and utilized relatively short proning times (6–8 h) over 4–10 days and showed no mortality benefit when this adjunct was utilized [19,20]. However, post hoc analysis in the Gattinoni study revealed that patients with severe ARDS (PaO2/FiO2 ratios
Review
A practical approach to the use of prone therapy in acute respiratory distress syndrome Expert Rev. Respir. Med. 8(4), 453–463 (2014)
Krishna P Athota, D Millar, Richard D Branson* and Betty J Tsuei Department of Surgery, University of Cincinnati, Cincinnati, OH, USA *Author for correspondence: Tel.: +1 513 558 5661 Fax: +1 513 558 3136 [email protected]
In this article we propose a practical approach to the use of prone therapy for acute respiratory distress syndrome (ARDS). We have attempted to provide information to improve the understanding and implementation of prone therapy based on the literature available and our own experience. We review the basic physiology behind ARDS and the theoretical mechanism by which prone therapy can be of benefit. The findings of the most significant studies regarding prone therapy in ARDS as they pertain to its implementation are summarized. Also provided is a discussion of the nuances of utilizing prone therapy, including potential pitfalls, complications, and contraindications. The specific considerations of prone therapy in open abdomens and traumatic brain injuries are discussed as well. Finally, we supply suggested protocols for the implementation of prone therapy discussing criteria for initiation and cessation of therapy as well as addressing issues such as the use of neuromuscular blockade and nutritional supplementation. KEYWORDS: acute respiratory distress syndrome • critical care • mechanical ventilation • prone position • refractory hypoxemia
Acute respiratory distress syndrome (ARDS) was first coined in 1967 by Ashbaugh et al. and used to describe acute bilateral pulmonary abnormalities, although the clinical entity of pulmonary edema without heart failure was initially described by Laennec in 1821 [1,2]. ARDS is associated with a number of clinical diagnoses and has been shown to have a high mortality of almost 40% [3]. Recent improvements in critical care have focused on developing new methods of treating the severe hypoxia of ARDS and decreasing the incidence of this disease including lung protective strategies, novel modes of ventilation and prone positioning. Prone positioning for the treatment of ARDS was first described in 1977 by Douglas et al. In a series of six patients with acute respiratory failure, they reported improved oxygen requirements in four of the five mechanically ventilated patients when prone positioning was utilized [4]. Since that time, studies of prone positioning have included different patient populations, various criteria for initiation of prone positioning and a wide duration of treatment times, making it difficult to draw unified conclusions from these disparate studies. Furthermore, the
informahealthcare.com
10.1586/17476348.2014.918850
development of protocolized lung protective strategies, which have been shown to be beneficial, such as those proposed by ARDSnet, has only occurred within the last decade and was not incorporated into many of the early studies of prone positioning [5]. As such, while there is significant evidence that prone positioning can dramatically improve the respiratory status in selected patients, there has been few prospective randomized studies establishing its overall benefit, and the role of prone positioning in the treatment of ARDS has been widely debated. This article will summarize prior studies examining the use of prone positioning in the treatment of ARDS, with recommendations for criteria to initiate therapy, therapy protocol, indication for cessation of therapy and potential pitfalls and complications during the use of positional therapy. ARDS as an entity was formally defined by the American–European Consensus Conference (AECC) in 1994. The majority of research which has been done on the use of prone positioning in ARDS has utilized this AECC definition of ARDS. As time passed, it was apparent that this definition had limitations regarding
2014 Informa UK Ltd
ISSN 1747-6348
453
Expert Review of Respiratory Medicine Downloaded from informahealthcare.com by Washington University Library on 12/31/14 For personal use only.
Review
Athota, Millar, Branson & Tsuei
the timing of onset, defined hypoxemia and variability in the radiographic findings, and a panel of experts was convened in 2011 to revise the original AECC definition of ARDS, creating the Berlin definition [6]. For the purposes of this paper, in order to summarize the most recent proning studies, which were undertaken largely before the development of the Berlin criteria, ARDS will refer to AECC criteria (now Berlin moderate or severe ARDS) unless otherwise specified. Management of acute hypoxia is a common goal of critical care medicine. One of the variables that can be manipulated to improve hypoxia clearly is oxygen concentration. While there continues to be debate on the significance of oxygen toxicity, there are limited experimental studies in humans. In the numerous animal studies that have been performed, high oxygen concentration with long duration of exposure results in pulmonary pathology similar to ARDS [7,8]. Additional animal studies have shown that exposure to less than 60% FiO2 did not result in lung injury, regardless of the duration of exposure [9]. A recent prospective observational study in humans also showed findings consistent with oxygen toxicity in a time and dose-dependent fashion [10]. As these results suggest that oxygen levels above 60% cause lung injury, treatment of the ARDS patient should include maneuvers to decrease supplemental oxygen to this level as rapidly as possible by using advanced lung protective ventilator strategies as well as adjuncts such as prone positioning. The initial report by Douglas et al. regarding improved oxygenation after prone positioning was followed by two additional observational studies that confirmed this finding [11,12]. During this period, Gattinoni and colleagues demonstrated that, in contrast to the diffuse alveolar disease seen on plain radiograph, computed tomography indicated that ARDS was actually an inhomogeneous pulmonary process, with affected areas primarily occurring in the dependent portion of the lung parenchyma [13]. Subsequently, a number of mechanisms were proposed to explain the beneficial effects of prone positioning including increased functional residual capacity, alterations of diaphragmatic motion, gravitational changes in lung perfusion and improved secretion management. However, in 1994, Lamm and colleagues dispelled most of these theories by using an animal model in which prone positioning was shown to be effective primarily by improving ventilation and increasing transpulmonary pressure, thus decreasing ventilation–perfusion mismatch in the dorsal lung [14]. Human studies have supported these findings, revealing improved lung volumes and recruitment, and increased ventilation when prone positioning is utilized [15,16]. Prone positioning redistributes pleural pressures and can result in more homogeneous transpleural pressures. This allows for more equal distribution of lung ventilation, decreasing mechanical overexpansion and lung strain to the ventral portion of the lung [17,18]. As such, prone positioning not only improves respiratory mechanics and gas exchange, but also is lung protective. While the use of proning may minimize lung injury resulting from other methods of oxygenation and ventilation, the 454
inciting process that caused the development of ARDS must be addressed as well. Despite the fact that prone positioning in patients with ARDS has been shown to improve oxygenation, early trials failed to demonstrate a significant survival advantage. Initial trials included a spectrum of ARDS patients and utilized relatively short proning times (6–8 h) over 4–10 days and showed no mortality benefit when this adjunct was utilized [19,20]. However, post hoc analysis in the Gattinoni study revealed that patients with severe ARDS (PaO2/FiO2 ratios
