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Prone Position in COVID-19 and -COVID-19 Acute Respiratory Distress Syndrome: An International Multicenter Observational Comparative Study*

OBJECTIVES: Prone position is used in acute respiratory distress syndrome and in coronavirus disease 2019 acute respiratory distress syndrome. However, it is unclear how responders may be identified and whether an oxygenation response improves outcome. The objective of this study was to quantify the response to prone position, describe the differences between coronavirus disease 2019 acute respiratory distress syndrome and acute respiratory distress syndrome, and explore variables associated with survival. DESIGN: Retrospective, observational, multicenter, international cohort study. SETTING: Seven ICUs in Italy, United Kingdom, and France. PATIENTS: Three hundred seventy-six adults (220 coronavirus disease 2019 acute respiratory distress syndrome and 156 acute respiratory distress syndrome). INTERVENTION: None. MEASUREMENTS AND MAIN RESULTS: Preproning, a greater proportion of coronavirus disease 2019 acute respiratory distress syndrome patients had severe disease (53% vs 40%), worse Pao2/Fio2 (13.0 kPa [interquartile range, 10.5–15.5 kPa] vs 14.1 kPa [interquartile range, 10.5–18.6 kPa]; p = 0.017) but greater compliance (38 mL/cm H2O [interquartile range, 27–53 mL/cm H2O] vs 31 mL/cm H2O [interquartile range, 21–37 mL/cm H2O]; p < 0.001). Patients with coronavirus disease 2019 acute respiratory distress syndrome had a longer median time from intubation to prone position (2.0 d [interquartile range, 0.7–5.0 d] vs 1.0 d [interquartile range, 0.5–2.9 d]; p = 0.03). The proportion of responders, defined by an increase in Pao2/Fio2 greater than or equal to 2.67 kPa (20 mm Hg), upon proning, was similar between acute respiratory distress syndrome and coronavirus disease 2019 acute respiratory distress syndrome (79% vs 76%; p = 0.5). Responders had earlier prone position (1.4 d [interquartile range, 0.7–4.2 d] vs 2.5 d [interquartile range, 0.8–6.2 d]; p = 0.06)]. Prone position less than 24 hours from intubation achieved greater improvement in oxygenation (11 kPa [interquartile range, 4–21 kPa] vs 7 kPa [interquartile range, 2–13 kPa]; p = 0.002). The variables independently associated with the “responder” category were Pao2/Fio2 preproning (odds ratio, 0.89 kPa–1 [95% CI, 0.85–0.93 kPa–1]; p < 0.001) and interval between intubation and proning (odds ratio, 0.94 d–1 [95% CI, 0.89–0.99 d–1]; p = 0.019). The overall mortality was 45%, with no significant difference observed between acute respiratory distress syndrome and coronavirus disease 2019 acute respiratory distress syndrome. Variables independently associated with mortality included age (odds ratio, 1.03 yr–1 [95% CI, 1.01–1.05 yr–1]; p < 0.001); interval between hospital admission and proning (odds ratio, 1.04 d–1 [95% CI, 1.002–1.084 d–1]; p = 0.047); and change in Pao2/Fio2 on proning (odds ratio, 0.97 kPa–1 [95% CI, 0.95–0.99 kPa–1]; p = 0.002). CONCLUSIONS: Prone position, particularly when delivered early, achieved a significant oxygenation response in ~80% of coronavirus disease 2019 acute respiratory distress syndrome, similar to acute respiratory distress syndrome. This response was independently associated with improved survival.

Ventilation in prone position (PP) improves oxygenation while achieving a more homogeneous distribution of the mechanical forces, therefore reducing injury (4,5). Following the demonstration of significant decrease in mortality in moderate-to-severe ARDS in the Proning Severe ARDS Patients (PROSEVA) trial (5), PP (for at least 12 hr) has become one of the most commonly used and effective strategies in ARDS (6,7).
However, it is unclear: 1) how the physiologic response to PP compares between C-ARDS and ARDS; 2) how responders may be identified and whether response might translate to improved outcomes; and 3) whether the outcome benefits are conferred equally to patients with differing compliance or recruitability, which may reflect the phenotypic characteristics described in C-ARDS (13)(14)(15). Specifically, it is possible that PP is more beneficial to earlier and to patients with greater recruitability.
The aim of this study was to quantify response to PP in C-ARDS, understand the differences in response between ARDS and C-ARDS, and explore the variables associated with outcome. Our hypothesis was that the oxygenation response to PP is greater in C-ARDS and if applied early. This hypothesis was based on the fact that in early C-ARDS, hypoxemia is determined by alteration in perfusion, lung edema, or atelectasis, rather than dense consolidation of fibrosis-which may be less modifiable with PP.

Study Population
From seven international centers (Italy-Milan; United Kingdom-London; France-three in Lyon, one in Grenoble, and one in Roanne), we included 376 consecutive adult (>18 yr) ARDS patients who, while mechanically ventilated, received at least one session of PP lasting greater than or equal to 12 hours (Fig. 1). All patients fulfilled the Berlin definition of ARDS (16) prior to proning. The decision to initiate PP and the timing were at the discretion of the treating clinician; however, PP was recommended in patients with Pao 2 /Fio 2 ratio less than 20 kPa (150 mm Hg) and Fio 2 greater than 60%, despite optimization of positive end-expiratory pressure (PEEP) (5). Among these patients, 220 were admitted to the ICU and received invasive mechanical ventilation between February 2020 and May 2020 with documented COVID-19-positive reverse transcriptasepolymerase chain reaction test results from either upper airway swab or bronchoalveolar lavage. Another 156 patients with non-COVID-19 ARDS were admitted to the ICU and ventilated between December 2017 and May 2020. Approval for data collection was obtained at each participating institution (Comité d' éthique du Centre Hospitalier Universitaire de Lyon Number of approval 20-42-France; Approval number 10796-London, and Comitato Etico Milano Area I-17263/2020-2020/ ST/095), consent was waived, and data were anonymized before collating into the central database.

Measurements
Data were collected retrospectively from clinical documentation. Raw data were collated immediately after intubation and for the first proning session, where we extracted paired measurements in supine position prior (<2 hr) to the prone positioning and immediately (<2 hr) before the patient was returned to supine position. We hypothesized that the response to the first proning session would be the most important prognostic indicator and allow comparison of the entire patient cohort.
Data included hemodynamics, gas exchange, ventilatory parameters, and respiratory mechanics, in addition to demographic and anthropometric variables. We derived values for Pao 2 /Fio 2 ratio, minute volume, tidal volume per predicted body weight, ventilatory ratio (17), corrected minute volumes (18), driving pressure (19), respiratory system compliance, and mechanical power (20,21). Predicted mortality was calculated from severity scores Acute Physiology and Chronic Health Evaluation (APACHE) II and Simplified Acute Physiology Score (SAPS) II scores.

Statistical Analysis
In addition to the main etiology, we prespecified the following two groupings: 1) response to PP-we defined a positive response to PP as an absolute increase in Pao 2 /Fio 2 greater than or equal to 2.67 kPa (20 mm Hg); 2) "higher or lower" compliance-based on the median value of compliance in the population. Comparisons within groupings were made with Mann-Whitney U significance tests. The difference in paired measurements between the prone and supine positions (prone minus supine or its relative change) was examined with Wilcoxon signed-rank tests. Between-group differences of categorical data were performed using Pearson χ 2 test.
Multivariable logistic regression models were constructed with variables that demonstrated significance in univariable logistic regression. This process was used to inform expert domain knowledge employed in variable selection for the reported multivariable logistic regression model. Data were assumed to be missing at random with no imputation or interpolation of missing values employed. Categorical data were reported as counts and percentages, and continuous data as median and interquartile ranges (IQRs) or 95% CIs, as appropriate. A value of α = 0.05 was used for all significance tests. All analysis was performed using R version 4.0.3 (Foundation for Statistical Computing, Vienna, Austria).

Baseline Characteristics
The flowchart of patients' inclusion in the study is presented in Figure 1. The characteristics of patients after intubation are presented in Table E1 (http://links.lww. com/CCM/G870). In the ARDS cohort, 1.3% of patients (2/156) versus 32.1% (72/220) in the C-ARDS group had received noninvasive ventilation (NIV) preintubation. The C-ARDS cohort had a greater percentage of men, lower Sequential Organ Failure Assessment (SOFA) scores, higher Pao 2 /Fio 2 ratio, greater compliance, and PEEP compared with the ARDS cohort. The majority of the C-ARDS patients had moderate ARDS (16). Baseline organ failure is reported in Table E2 (http://links.lww.com/CCM/G870).

Response to Proning
The median and proportional change in the physiologic parameters between supine and PP are shown in , which equated to a 60% (IQR, 23-121%) increase from supine position (Fig. 2). Following PP, the ventilatory ratio decreased by 7% (IQR, -20 to 5) in ARDS, whereas increased by 2% (IQR, -11 to 17) in C-ARDS with wide variation in individual response. There was an improvement in the compliance of the respiratory system (16% [IQR, -5 to 31] vs 2% [IQR, -10 to 24]; p = 0.013, and driving pressure in the ARDS population -12% (IQR, -25 to 0). Overall, 77% of patients could be classified as responders, with an increase in Pao 2 /Fio 2 of greater than or equal to 2.7 kPa (20 mm Hg) upon proning, with similar prevalence or responders in the both cohorts (ARDS 78% vs C-ARDS 76%; p = 0.7). Responder characteristics prior to proning-stratified by COVID-19 status-are reported in Tables E3 and E4 (http://links.lww.com/CCM/G870). Responders had worse oxygenation at baseline in both cohorts. C-ARDS responders had significant deterioration in Pao 2 /Fio 2 from baseline with a median change in the Pao 2 /Fio 2 of 1.8 kPa (IQR, -7.00 to 0.84 kPa) vs 0 (IQR, -4.2 to 3.6 kPa) in ARDS (p = 0.0015). Patients with C-ARDS -who responded to PP had lower corrected minute ventilation. Only the responders in the C-ARDS cohort showed an improvement in compliance and decrease in Paco 2 (Tables E3 and E4, http://links.lww.com/CCM/ G870), whereas both nonresponders and responders in the ARDS population had a comparable increase in compliance and reduction in Paco 2 . Overall, responders had a shorter time interval between intubation and first proning session 1.4 days (IQR, 0.7-4.2 d) versus 2.5 days (IQR, 0.8-6.2 d) (p = 0.06).

Prediction of Pao 2 /Fio 2 Response to Proning
Lower supine Pao 2 /Fio 2 , shorter duration of intubation preproning (i.e., earlier proning), and non-COVID-19 ARDS were associated with a greater increase in Pao 2 / Fio 2 following PP. Although later PP was negatively associated with oxygenation response, we found this effect was near constant after the first 24 hours, and therefore, the timing of PP was analyzed as a dichotomous variable  (≤ 24 or > 24 hr) (Fig. E2,  The point estimate for OR related to COVID-19 status was 0.67 but with wide 95% CI [0. 38-1.17], therefore not significantly associated with response when timing of proning was also included in the model.

Response to Prone Position in the Low and High Compliance Groups
We further analyzed the two cohorts based on the supine respiratory system compliance dichotomized based on the median supine preproning value of 34.6 mL/cm H 2 O.
There was no difference in the proportion of responders based on the compliance category (77% in ARDS and 69% in C-ARDS, for the low compliance category; p = 0.3, and 72% in ARDS and 78% in C-ARDS, for the high compliance category; p = 0.5). Equally, the proportion of responders was similar in high and low compliance for the C-ARDS (78% vs 69%; p = 0.2) and ARDS (72% vs 78%; p = 0.6).
The survival was similar for the two cohorts regardless of compliance category (66% in ARDS and 56% in C-ARDS, for the low compliance category; p = 0.3, and 49% in ARDS and 49% in C-ARDS, for the low compliance category; p = 0.2). However, there was a trend for greater survival in the C-ARDS cohort with higher compliance (61% vs 46%; p = 0.07).

DISCUSSION
This study shows that PP, particularly when delivered early, resulted in a significant oxygenation response in almost 80% of patients with C-ARDS and non-COVID-19 ARDS, and this response was independently associated with improved survival. The C-ARDS cohort had greater respiratory compliance despite a worse oxygenation compared with the ARDS cohort. These characteristics are consistent with the greater lung gas volumes observed in C-ARDS (1, 2) and the fact that the relatively higher compliance coexists with severe oxygenation defect (3).
The majority of patients showed an improvement in the Pao 2 /Fio 2 ratio following the first episode of PP (22,23) with similar proportions in both cohorts, and their oxygenation response was not affected by the respiratory compliance.
This study also shows that greater oxygenation response to PP was observed in patients proned within 24 hours of intubation. Longer time from intubation to PP decreased the chance of oxygenation response by 6%/d and increased the risk of death by 6%/d. The relationship between timing of proning and outcome is consistent with the results of large observational study that showed a 16% relative risk reduction of death in patients who were proned within 2 days of ventilation (24) and a more recent U.K. study (25).
The other factor associated with greater oxygenation response to PP was a lower Pao 2 /Fio 2 ratio preproning (i.e., more severe disease had greater response), consistent with ARDS (4-6, 26) and C-ARDS (27) data that demonstrate higher likelihood of response to PP in the moderate-to-severe category.
When time-to-proning and change in Pao 2 /Fio 2 were included in the same model, the oxygenation response was a strong predictor of mortality particularly in C-ARDS. The analysis of the relationship between the time to PP and the deterioration in Pao 2 /Fio 2 from intubation to the time of proning in the two groups shows that C-ARDS had a higher Pao 2 /Fio 2 ratio at baseline but greater deterioration, that is, lower value preproning. This finding is in agreement with the data showing that improvement in oxygenation after the first PP was a predictor of survival (23,28). The relationship between timing of proning in relationship to hospital admission and severity of Pao 2 /Fio 2 may be explained by the changing pathophysiology of hypoxemia that characterizes the different phases of disease (e.g., alteration of perfusion, worsening edema, and atelectasis; vs the development of denser consolidation or fibrosis) (29). These findings-together with the higher compliance seen in C-ARDS-may suggest that the main mechanism for improvement in oxygenation may be the redistribution of pulmonary perfusion from lower to higher ventilation/perfusion ratio areas, rather than that significant alveolar recruitment, given that the reduction in Paco 2 following PP, was significantly smaller in C-ARDS in agreement with what reported in other studies (30). According to the model by Riley and Cournand (31), arterial Paco 2 is a weighted average of the Pco 2 in the shunted blood (mixed venous blood perfusing nonaerated lung regions) and Pco 2 in the capillary blood in equilibrium with the alveolar Pco 2 . Therefore, alveolar recruitment should lead to a reduction in dead space. This phenomenon was seen in the ARDS cohort consistently with previous studies (32). The higher compliance and, therefore, gas volume (1) and the higher PEEP used in C-ARDS may have worsened dead space ventilation regardless of disease severity (33) and are consistent with the reported dissociation between improvement in oxygenation and worsening indices of dead space with higher PEEP in COVID-19 (34). Prior systematic reviews and meta-analyses (7,35) demonstrated that the combination of low tidal volumes and prone positioning improved survival (36). Differently from meta-analysis of earlier trials, which included patients ventilated with tidal volumes greater than 8 mL/kg PBW, our patients were ventilated with lower tidal volumes (6-6.5 mL/kg PBW) and driving pressures, and this might explain why tidal volume was not independently associated with outcome.
The mortality predictions based on SOFA, SAPS II, and APACHE II score underestimated the mortality in COVID-19, similar to that reported in a U.S. cohort of mechanically ventilated patients based on the preintubation SOFA (37). Given that a lower proportion of patients with severe C-ARDS received vasopressors or renal replacement therapy, the excess mortality likely derives from respiratory mortality and possibly the effect of mechanical ventilatory support. Indeed, the mechanical power delivered to these patients was higher (27-29 J/min) than the threshold reported to be associated with mortality when mechanical power was higher than 17.0 J/min regardless of driving pressure and low tidal volumes (38).
This study has several strengths that derive from being a large multicenter physiologic study and, therefore, possessing greater external validity, and the fact that it compares the C-ARDS with ARDS highlighting the determinants of response to PP and their potential differences. However, this study presents several limitations due to its retrospective, observational design. Specifically, data included in the database were collected for clinical purposes and may occur at different time intervals before and after PP. Some important data-including the total duration of symptoms prior to hospital admission-have not been routinely collected. In addition, there was no prespecified sample size for the cohort enrolled, despite the fact that all COVID-19 patients who had a documented proning session were included in the study. Additional limitations relate to potential inclusion bias, as the initiation of prone positioning may ultimately reflect the practice of the treating clinical team. Furthermore, differences in patient selection among centers reflect potentially different resources at a time of pandemics. The issues of staffing, burden of patients, and ICU occupancy may have affected the timeliness of delivery of PP and the use of NIV prior to intubation. This leads to another potential limitation of the study, which is understanding to what extent the difference noted is intrinsic to C-ARDS or reflects the constraints of a healthcare system engaged in dealing with a pandemic. Finally, the comparisons refer only to the first proning session. In this cohort, the median number of PP session was two (ranging from one to five). Therefore, to ensure homogeneity of the data, we concentrated on the first session as this will allow to compare the entire cohort of patients.

CONCLUSIONS
PP, particularly when delivered early, achieves a significant oxygenation response in almost 80% of patients with severe C-ARDS. The response is similar to ARDS. Response to PP was independently associated with improved survival.