To the Editor:
We read with great interest the study published in CHEST (March 2023) by Dr Okin and colleagues1 examining whether prone positioning of extended duration (> 24 h) is associated with decreased mortality in mechanically ventilated COVID-19-related ARDS. In this study, patients who did not show clinical improvement after prone positioning of standard duration were left prone, sometimes for several consecutive days.
Because prone position contributes to lung protection during mechanical ventilation, prolonging its duration might further maintain a more homogenous distribution of the gas-tissue ratio, thus delaying the increase in overdistention observed when patients are in supine position.2 Reducing the number of turning-over maneuvers also might avoid or reduce potential derecruitment associated with these maneuvers. There are, however, issues in the article that we would like to highlight and comment on.
First, the main outcome of the study is 30-day all-cause mortality. However, the precise start of follow-up is not specified.
Second, the definition of the two groups seems unclear. They are indeed defined as follows: “Prolonged prone positioning ventilation (PPV) was defined as a prone duration lasting at least 24 hours before supination. Intermittent PPV was defined as daily pronation and supination events. Patients who underwent a single PPV event during their ICU stay, or who had prone sessions on nonconsecutive days, were classified as intermittent if the longest prone session length was less than 24 hours.”
As described, patients in the prolonged group may have been turned prone for a prolonged duration only after one or several prone positioning sessions of duration 24 hours or less. This definition of the exposure might induce an immortal time bias if the exposure assignment does not coincide with the start of follow-up.
Finally, among prone positioning sessions of duration 24 hours or longer, the exact duration varies widely, between 24 hours and more than 300 hours for a single session. The criteria used to stop an extended prone positioning session are not provided. Because several studies have been published on extended prone positioning,3, 4, 5 being able to compare the different protocols is of prime importance for designing future randomized control trial to the best benefit of patients.
Financial/Nonfinancial Disclosures
The authors have reported to CHEST the following: J. D. R. reports that Fisher & Paykel has covered travel expenses and provides high flow devices to a multicenter randomized controlled trial which J. D. R. is conducting on the use of nasal high flow in patients with acute hypercapnic respiratory failure. None declared (T. W., D. H.).
References
- 1.Okin D., Huang C.Y., Alba G.A., et al. Prolonged prone position ventilation is associated with reduced mortality in intubated COVID-19 patients. Chest. 2023;163(3):533–542. doi: 10.1016/j.chest.2022.10.034. [DOI] [PMC free article] [PubMed] [Google Scholar]
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