To the Editor:
We read with great interest the article by Jonkman and colleagues (1). The study aimed at determining the optimal positive end-expiratory pressure (PEEP) level in patients with COVID-19 and acute respiratory distress syndrome (ARDS) by assessing lung collapse and overdistention using chest electrical impedance tomography. The main goals of mechanical ventilation are to maintain adequate gas exchange and to allow appropriate oxygen delivery to the tissues. In clinical practice, identifying the optimal level of PEEP that limits lung collapse and overdistention is a challenging task (2), and the authors have to be congratulated for their study. Several methods have been proposed to set the PEEP, but there is no validated technique so far. In their study, the authors propose a new method based on chest electrical impedance tomography. They observed that oxygenation increased with incremental PEEP steps and that their method identified an optimal PEEP level higher than the PEEP level estimated by the highest compliance method. In addition, the authors report a negative correlation between oxygenation and overdistention at high levels of PEEP, attributing this finding to possible lung perfusion abnormalities caused by COVID-19–related endothelial dysfunction. We agree with the authors that overdistention could potentially reduce perfusion in the overdistended lung areas, leading to intrapulmonary shunting. However, we stress a detrimental effect of increasing PEEP in this setting. Increasing PEEP increases intrathoracic pressure and leads to challenge the cardiac performance, which may result in a drop in cardiac output (3, 4). Despite observing an increase in SaO2 during incremental PEEP, the drop in cardiac output may eventually induce a decrease in oxygen delivery (Figure 1). This effect has been notably reported in patients with COVID-19 (5, 6). Calculation of oxygen delivery requires the determination of cardiac output, hemoglobin concentration, and SaO2, which can be obtained easily at the bedside. Unfortunately, arterial pressure monitoring is not accurate to assess changes in cardiac output, which can be noninvasively measured with echocardiography. Therefore, assessing the impact of PEEP on hemodynamics and its influence on oxygen delivery to determine the optimal PEEP for patients with ARDS could complement the method proposed by Jonkman and colleagues. It would provide a more comprehensive understanding of the effects of PEEP and help optimize ventilatory strategies for patients with ARDS.
Figure 1.
Determinants of oxygen delivery. EELV = end-expiratory lung volume; EIT = electrical impedance tomography; PEEP = positive end-expiratory pressure; SaO2 = oxygen arterial saturation.
Footnotes
Originally Published in Press as DOI: 10.1164/rccm.202305-0891LE on July 25, 2023
Author disclosures are available with the text of this letter at www.atsjournals.org.
References
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