Prone positioning as an emerging tool in the care provided to patients infected with COVID-19: a scoping review

Objective: to describe scientific evidence regarding the use of prone positioning in the care provided to patients with acute respiratory failure caused by COVID-19. Method: this is a scoping review. PRISMA Extension for Scoping Reviews was used to support the writing of this study. The search was conducted in seven databases and resulted in 2,441 studies, 12 of which compose the sample. Descriptive statistics, such as relative and absolute frequencies, was used to analyze data. Results: prone positioning was mainly adopted in Intensive Care Units, lasted from a minimum of 12 up to 16 hours, and its prescription was based on specific criteria, such as PaO2/FiO2 ratio, oxygen saturation, and respiratory rate. The most prevalent complications were: accidental extubation, pressure ulcer, and facial edema. Decreased hypoxemia and mortality rates were the main outcomes reported. Conclusion: positive outcomes outweighed complications. Various cycles of prone positioning are needed, which may cause potential work overload for the health staff. Therefore, an appropriate number of trained workers is necessary, in addition to specific institutional protocols to ensure patient safety in this context.

additional therapy to treat severe hypoxemia caused by ARDS (8) .
The Prone Positioning in Severe Acute Respiratory Distress Syndrome (PROSEVA) (9) trial provided scientific evidence of the effectiveness of the PP in the treatment of ARDS. The results of the randomized clinical trial conducted with 466 participants indicate that its early adoption (between 12 and 24 hours after the diagnosis of ARDS) for prolonged periods significantly decreased mortality in the intervention group. The rate of mortality in 28 days was 16% in the prone group and 32.8% in the control group (p<0.001), and 23.6% and 41.0%, respectively, in a 90-day period (p<0.001) (9) .
Considering that ARDS is the most severe complication of COVID-19, accounting for considerably high mortality rates, this study's objective is to describe scientific evidence of the use of PP in the care provided to patients with acute respiratory failure caused by

Method
This is a scoping review, which is characterized by the objective of mapping the main concepts of a field of knowledge, in this case, the Nursing field, and examining the extent, scope, and nature, in addition to summarizing and disseminating the results of studies, and identifying existing research gaps (10) .

The recommendations of the Joanna Briggs Institute
Reviewer's Manual (11) were adopted. Additionally, the instrument titled PRISMA Extension for Scoping Reviews (PRISMA-ScR) was used in the elaboration of this study.

This instrument is divided into seven domains and 22
items, providing recommendations regarding the title, abstract, introduction, method, results, discussion, conclusion, and financial support.   Hence, the following question was established: "What is the evidence available concerning the use of prone positioning in the care provided to patients with acute respiratory failure caused by COVID-19?".

The following descriptors indexed in Medical
Subject Headings (MeSH) were used in the search: After determining the descriptors and establishing the aforementioned strategy, the search in the databases/repositories was initiated. The databases were accessed through the CAPES periodicals portal, using the CAFe platform, a service that facilitates digital access through the use of a login registered at the university. An external search was also conducted in the gray literature, as recommended by the Reviewer's Manual (11) .
After defining the sample, a protocol was adapted from the Cochrane Data collection form to extract data. The form addressed the following: country, year of publication, study objective, study design, eligibility criteria, institution where the intervention was conducted, population, methods used to implement the intervention, measures adopted to assess the intervention, outcomes, and complications accruing from the intervention.
The following information was extracted from the selected studies to answer the study question: 1) institution where the prone intervention was adopted; 2) criteria to adopt the PP; 3) PP duration; 4) main outcome and secondary outcome, and;

5) complications.
Note that the studies were classified in terms of involve human subjects, it did not require submission and approval by an Institutional Review Board.
Additionally, there is minimum risk involved, as it is not experimental. Law No. 9,610/98 was fully complied with, intending to preserve and respect the ideas, concepts, and definitions adopted by the authors of the primary studies included in this review.

Results
Twelve of the 2,441 studies assessed were included in the final sample, as presented in Figure 1. As for the method adopted, reviews (42%) and expert consensus (42%) predominated.

predominance of the United States is likely explained by
its current status as the epicenter of the pandemic, with 4,932,510 cases up to August 7 th , 2020 (2) . Additionally, the USA ranks first in the number of scientific papers published from 2013 to 2018, which characterizes it as the world scientific hub (24) .

As for the studies' level of evidence in relation to
the methods adopted, reviews provide robust evidence regarding a given topic. Additionally, they are original studies that do not require approval by an Institutional Review Board, which speeds up the process of writing and publishing papers (25) . In turn, when there is a lack of experimental studies or even reviews, consensuses written by renowned experts or experts with confirmed experience can be used to support and provide evidencebased practice (26) .

The use of PP among patients hospitalized in ICUs
is explained by the severity of their conditions (A1, A2, A3, A4, A5, A6, A8, A9, A10, A11, A12), with a low PaO 2 /FiO 2 ratio, revealing respiratory distress and negatively affecting noble organs such as brain, heart, and kidneys. ARDS of viral etiology stands out due to its high mortality, equal to about 50% of the cases, and is characterized by pulmonary edema of cardiogenic origin, causing hypoxemia and the need for invasive ventilatory support (27) .
Regarding the duration of prone positioning, recommendations vary but most studies recommend a minimum from 12 to 16 continuous hours (A2, A3, A4, A8, A9, A10, A11). The American Association of Critical-Care Nurses (28)  Regarding complications, even though prone positioning was found to decrease pressure on bony prominences commonly injured in the supine or lateral positions (30) , PP exerts pressure on the frontalis and orbicularis muscles, chin, humerus, thorax, pelvis, and knees, causing several related adverse events (31) .
Additionally, such pressure causes a heterogeneous distribution of blood and lymph flow in the face, as well as tissue ischemia and consequent necrosis, which results in the undesirable outcomes "pressure ulcers" and "facial edema", identified in five of the studies in the sample (A2, A8, A9, A11, A12).
Direct pressure on the orbits, together with vascular changes, cause extraocular muscle impact, with the potential to culminate in conjunctival edema, bleeding, and even corneal injury, one of the complications that stands out in the sample (A9, A11, A12). A clinical trial reports that after ten minutes in the prone position, patients presented high intraocular pressure, as well as a greater risk of corneal ulceration. This type of damage may compromise eye function and require lifelong eye care, even though evidence shows that corneal abrasion and scleral wounds caused by prone positioning are generally self-limiting (32) .
Additionally, PP may cause traction on the humerus, either on its flexion or extension, leading to increased intraneural venous pressure, local edema, and impairment in the axoplasmic transmission of the elements that compose the brachial plexus (33) .
A case study of a patient placed in the prone position for a surgical procedure verified that, after five hours in this position, he developed brachial plexopathy. Studies suggest adopting measures such as cushions to reduce pressure on the pectoral muscles and prevent them from being pushed into the axillary fossa, pressing the plexus, as well as palpating the tendon of the pectoralis major muscle to monitor its tension (34)(35) .
Among complications, six studies report the leading to extubation (36) .
Another study, the objective of which was to report experiences in a COVID-19 ICU, also reports accidental extubation as one of the potential complications in prone positioning used to treat ARDS (37) . Therefore, studies (38)(39) recommend constant and vigilant monitoring of OTT and timely action when this problem occurs, as it may aggravate an already critical condition, imposing even greater risks on patients (38) .
Likewise, PP presents some peculiar hemodynamic challenges. One observational prospective study verified that compression of the abdomen during PP may restrict blood flow from the inferior vena cava, causing venous engorgement and consequent decrease in the cardiac output (40) . In the context of a patient with severe acute respiratory failure caused by COVID-19, this may be a desired outcome to achieve decreased myocardial work and prevent cardiac factors associated with respiratory failure. The combination of arterial hypotension, increased intra-abdominal pressure, and hypovolemia in impact caused by increased lung weight, caused by the edema, under important regions, enabling improved oxygenation (47) , as discussed in studies A1, A3, A4, A5, A6, A7, A9, A10, A11, and A12. Additionally, studies (48)(49) show an increase in tidal volume to be responsible for improved oxygenation in PP, which is in line with study A8.
Improved oxygenation is the effect most frequently expected and discussed in studies when PP is adopted. In addition to what we discussed previously, this effect also takes place due to a decrease in the various factors that contribute to alveolar collapse, such as redistribution of alveolar ventilation, reordering of perfusion, and reducing dorsal lung compression (47,50) . Note that improved pulmonary perfusion is an outcome reported in studies A1, A4, A5, and A10.
The case study (12) of a patient with severe acute respiratory failure caused by COVID-19 reports that, after 12 hours in the prone position, the patient progressed from the initial environmental O 2 saturation of 85% to 95% at rest and 90% when walking. Clinical trials report that oxygenation is improved in the prone group when compared to a group of patients in the supine position, with an increased PaO 2 /FiO 2 ratio (9,51) .
In contrast, a study (52) that analyzed the oxygenation of ten critical patients who tested positive Another important aspect is the role that the position of the rib cage plays in transpulmonary pressure. In PP, the rib cage presents a rectangular format, which results in decreased alveolar collapse, according to study A8.
Additionally, it is known that the cardiac muscle plays an important role in the lungs when in normal physiological conditions. This effect in a patient with respiratory failure due to COVID-19 may be even more important due to an increase in the right cardiac chamber, secondary to pulmonary hypertension, and hypoxic vasoconstriction, which results in increased pulmonary vascular resistance (A2, A7, and A8).
Likewise, two studies show that PP promotes the mobilization of secretions, which may improve the oxygenation of patients (A1 and A2). The reason is that PP enables improved drainage of secretion from the airways, which further promotes a reduction in the risk of respiratory infection associated with mechanical ventilation.
One study (53) reports that PP greatly impacts the cardiopulmonary physiology, being a useful maneuver accessible for most ICUs.
PP improves pulmonary mechanics and gas exchange and guidelines currently recommended it (54-55) .

Conclusion
This study's objective was to describe scientific evidence concerning the use of PP in the care provided to patients with acute respiratory failure caused by COVID-19. The sample was composed of 12 studies, which showed the use of PP, mainly in ICUs, with a duration from 12 to 16 hours.
The criteria used by the health staff to implement PP include the PAO 2 /FiO 2 ratio, oxygen saturation, and respiratory rate. Complications caused by PP were also identified: accidental extubation, pressure ulcers, and facial edema were the most frequently reported.
The positive outcomes outweighed the complication though. Thus, considering the evident decrease in hypoxemia and mortality rates, its use is recommended for patients with respiratory failure caused by SARS-

CoV-2.
That said, sustained improvement in oxygenation requires various cycles of pronation, a factor that may potentially overload the work of the health staff. Indeed, the studies suggest that having an appropriate number of trained workers and specific institutional protocols to ensure patient safety in this context is required.