In the present study we found that in patients with SARS-CoV-2 pneumonia the prevalence of atelectasis was 24%. Among them, 19% of the patients showed small atelectasis and 5% of the patients showed large atelectasis. Patients with larger compared to smaller atelectasis showed less SatO
2/FiO
2 ratio, higher rate of ICU admission and longer length of in-hospital stay. Among the few published studies investigating atelectasis in chest CT in SARS-CoV-2 pneumonia patients, our study includes the largest number of patients (237 patients screened, of whom 155 patients met all entry criteria). In our study, the prevalence of atelectasis was 24% which is higher than the previously reported (around 5%) [
12]. However, previous studies did not focus on atelectasis but generally describing the most frequently patterns at chest CT scans [
13,
14]. Furthermore, the greatest prevalence of atelectasis was mostly composed by mild, segmental or subsegmental atelectasis. No previous study separated smaller and larger atelectasis associated with the clinical outcome. We made this distinction based on proposed pathophysiology of the SARS-CoV-2 pneumonia and its relationship with oxygenation: it has been suggested that the progression of the disease would be associated with more severe hypoxia [
15]. Currently and from anatomopathological studies carried out on patients with ARDS, we know that in addition to diffuse alveolar injury and hyaline membrane formation (typical of all ARDS and VILI), there are areas of dead space that worsen gas exchange even more [
16]. Moreover, the hypoxic pulmonary vasoconstriction reflex (Euler-Liljestrand mechanism [
17]) may further worsen ischemic and thrombotic phenomena in the lung. Patients with large atelectasis showed a tendency to a greater involvement of lung tissue by SARS-CoV-2 pneumonia than in those without and small atelectasis. In fact, they required a higher ICU admission rate, with greater need of intubation and invasive mechanical ventilation. In addition, patients with larger atelectasis showed worse oxygenation ratios (Sat O
2/FiO
2) during admission and had a longer in-hospital stay (nearly the double than in patients with small or no atelectasis). Previous studies reported a lower incidence of atelectasis in patients with COVID-19 pneumonia [
3]. Our data suggest that the presence of large atelectasis may be a factor affecting progressive evolution to the ARDS [
16]. The Sat O
2/FiO
2 ratio and atelectasis detection by chest CT might be two valuable tools for the lung function assessment of these patients and hence might be helpful to predict those patients who will need mechanical ventilation, ICU admission and prolonged length of in-hospital stay. In-hospital mortality was not different among patients with no, small or large atelectasis. This can be explained by the fact that mortality rate of SARS-CoV-2 pneumonia is not only related to pulmonary injury, but also to development of multiple organ failures [
18]. The development of significant atelectasis favors more chances of respiratory complications and a higher morbidity with greater risk of ICU admission and need for invasive mechanical ventilation. The establishment of an early treatment with non-invasive respiratory ventilation to reverse atelectasis or prevent their progression, together with intensive surveillance or in intermediate care, could prevent a progression of lung injury with a worse outcome in patients in which this condition is detected.
Our study has several limitations to be addressed. First, this is a retrospective study, but in a situation of healthcare collapse, it was impossible to carry out prospective studies which need more time for approval and organization. However, our hospital has all the computerized medical history data, making data collection very reliable. We do recognize that a number of patients that were included in the study did not have a specific medical record (it was even, in a number of cases, the first time to come to the hospital). Second, we selected all the chest CT scans in the period with the highest incidence of SARS-CoV-2 pneumonia in our hospital, with high clinical suspicion of PE. Additionally, the scale we used to measure the severity of the pulmonary affection in chest CT ([
8], shown in Additional file
1), based on the findings of Wong KT et al. [
9,
10], takes into account the percentage of lobar infiltration and even pleural effusion, which could be logically associated with worst outcome. However, there were no differences between the large atelectasis group and the other groups in this scale. Regarding atelectasis, the same radiologist carried out a visual quantification by lobes for every scan in order to reduce heterogeneity between various radiologists. Third, the group without atelectasis might not be considered as a standard control group but a group of patients with significant oxygenation impairment due to several causes (including PE), hence introducing a possible selection bias as patients without atelectasis and mild infection would not be represented. However, this makes our study's findings even more relevant in relation to the differences between the group of large atelectasis (which indeed is composed by a reduced number of patients, 8) and its relationships with outcome compared with the other groups. We did not evaluate complications occurring during the hospital stay, but we do know that there are several differences between the groups regarding oxygenation, oxygen-therapy needs, ICU admission and days of hospitalization. Whether if these differences are associated directly or indirectly to atelectasis has to be determined on further studies.