- In patients with acute respiratory distress syndrome, it can be challenging to balance the higher positive end-expiratory pressure (PEEP) values needed to minimize tidal recruitment and the lower values needed to avoid overdistension of less injured lung
- A recently published study of a porcine model of lung injury demonstrates that prone positioning has a homogenizing effect on PEEP-associated transpulmonary pressures
- An editorial accompanying the study explains the results and notes that several important clinical questions remain unresolved
Prone positioning has gained renewed attention during the COVID-19 pandemic because in the treatment of acute respiratory distress syndrome (ARDS), it usually improves oxygenation even when positive end-expiratory pressure (PEEP) falters. An animal model of lung injury, recently published in the American Journal of Respiratory and Clinical Care Medicine, demonstrates the physiologic basis of the benefit of proning during PEEP for ARDS.
In an accompanying editorial in the same journal, Corey Hardin, MD, PhD, physician in the Division of Pulmonary and Critical Care Medicine at Massachusetts General Hospital, and a colleague explain the purpose and findings of the study, along with clinical issues that remain unresolved.
Background on the Problem
Improved oxygenation is a key motivator for using PEEP. However, adjustments are most often made to find a balance between the higher PEEP values needed to minimize tidal recruitment in the injured lung and the lower values needed to avoid overdistension of less injured lung.
Finding this balance is a challenge in patients with ARDS. The injured lung exhibits an increase in the normal vertical gradient of pleural pressure, primarily due to the superimposed weight of the heart and mediastinum. The supine gradient of end-expiratory transpulmonary pressures is thereby increased, declining from ventral to dorsal regions. Any increment of applied airway pressure, such as PEEP, tends to expand disproportionately the more compliant non-dependent alveolar units.
Why Use Prone Positioning?
Dorsal portions of the chest wall are less compliant than the ventral portions, and the prone position partially offsets this disparity by buttressing the ventral surface. This changes the distribution of transpulmonary pressure, which tends to improve the uniformity of lung expansion.
In particular, in the prone position, the dorsal lung is relieved of the weight of the heart and mediastinal contents, which results in a more homogenous distribution of ventilation and transpulmonary pressures.
The Lung Injury Model
In the new study, researchers in Toronto confirmed the value of proning by evaluating pleural pressure gradient, ventilation distribution and regional compliance in dependent and non-dependent lung across PEEP values in mechanically ventilated pigs. They demonstrated that in the prone position, compared with the supine position, that:
- Oxygenation was improved and pleural pressure was increased, which decreased transpulmonary pressure for any PEEP and reduced the dorsal–ventral pleural pressure gradient at PEEP <10 cm H2O
- The distribution of ventilation was homogenized between the dependent and non-dependent lung and was less dependent on the PEEP value
- Tidal recruitment was more evenly distributed (dependent/non-dependent lung) and hyperinflation was lower
Thus, a single value of PEEP applied to the airway opening in the prone position may exert similar distending forces on both dependent and non-dependent lung units.
The animal study has clear implications for clinical practice, but several important questions remain unanswered:
- In whom is proning most likely to prove effective and helpful? The study suggests it can help prevent ventilator-induced lung injury, but clinical studies have demonstrated convincing benefit only for patients who already have severe lung injury
- Is proning indicated even when there is no benefit to oxygenation and ventilation pressures are modest? It's conceivable that proning could have adverse effects in such patients, particularly since it often requires sedation and neuromuscular blockade
- If proning becomes integral to a generalized lung-protective strategy, when is it safe to withdraw it? The mechanisms of oxygenation benefit may not always be tied to recruitment—a variety of vascular disturbances have long been reported in ARDS and have recently been well described in COVID-19
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