Follow-up lung ultrasound to monitor lung failure in COVID-19 ICU patients.

OBJECTIVES: Point-of-care lung ultrasound (LU) is an established tool in the first assessment of patients with coronavirus disease (COVID-19). To assess the progression or regression of respiratory failure in critically ill patients with COVID-19 on Intensive Care Unit (ICU) by using LU.
MATERIALS AND METHODS: We analyzed all patients admitted to Internal Intensive Care Unit, Ludwig-Maximilians-University (LMU) of Munich, from March 2020 to December 2020 suffering lung failure caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV2). LU was performed according to a standardized protocol at baseline and at follow up every other day for the first 15 days using a lung ultrasound score (LUSS). Ventilation data were collected simultaneously.
RESULTS: Our study included 42 patients. At admission to ICU, 19 of them (45%) were mechanically ventilated. Of the non-invasive ventilated ones (n = 23, 55%), eleven patients required invasive ventilation over the course. While LUS did not differ at admission to ICU between the invasive ventilated ones (at baseline or during ICU stay) compared to the non-invasive ventilated ones (12±4 vs 11±2 points, p = 0.2497), LUS was significantly lower at d7 for those, who had no need for invasive ventilation over the course (13±5 vs 7±4 points, p = 0.0046). Median time of invasive ventilation counted 18 days; the 90-day mortality was 24% (n = 10) in our cohort. In case of increasing LUS between day 1 (d1) and day 7 (d7), 92% (n = 12/13) required invasive ventilation, while it was 57% (n = 10/17) in case of decreasing LUS. At d7 we found significant correlation between LU and FiO2 (Pearson 0.591; p = 0.033), p/F ratio (Pearson -0.723; p = 0.005), PEEP (Pearson 0.495; p = 0.043), pplat (Pearson 0.617; p = 0.008) and compliance (Pearson -0.572; p = 0.016).
CONCLUSION: LUS can be a useful tool in monitoring of progression and regression of respiratory failure and in indicating intubation in patients with COVID-19 in the ICU.

Introduction

Since December 2019 the new coronavirus disease (COVID-19) has been keeping the world in suspense. The infection is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV2) and leads to viral pneumonia and other organ manifestations like renal and liver failure, thrombotic complications, myocardial dysfunction, acute coronary syndrome, and neurologic illnesses [1]. The main cause of ICU admission remains lung failure.

Point-of-care ultrasound is a useful tool to assess critically ill patients. Especially lung ultrasound (LU) is getting an important technique to diagnose and manage patients with lung failure as it can be used in detecting pneumothoraxes, atelectasis, and pleural effusions [2]. Some data support the validity and potential applicability of LU for disease monitoring of interstitial lung disease [3] or viral pneumonia in the newborn [4].

LU was also used to examine COVID-19 patients [5,6]. Lichter et al. presented that LU used in the emergency department can predict the clinical course and outcomes in COVID-19 patients [7]. Our study group was able to demonstrate that LU predicts the clinical course of COVID-19 ICU patients [8] using an adapted version of the CLUE protocol established by Manivel et al [9]. Vetrugno et al. showed that application of LU allowed identification of lung involvement and severity and might be useful in follow-up for progression or regression of disease [10].

As most of the established LUS protocols for patients with COVID-19 focus on the initial diagnostic value and the first assessment of symptomatic patients, we aimed to perform follow-up LU in patients on ICU to explore LU as a tool for monitoring lung failure caused by SARS-CoV2. Primarily we want to address two questions. First, whether the course of LUS collected on days 1, 7, and 15 of ICU stay helps to assess the progression or regression of respiratory failure. Second, is there a difference in LUS over the course between invasive and non-invasive ventilated patients that can be useful to help indicate intubation in patients with COVID-19 on ICU?

Materials and methods

Design

This was a retrospective, single-centre study at the Internal Intensive Care Unit, Ludwig-Maximilians-University (LMU) hospital Munich approved by the local ethics committee (No 20–0227). The need for written or verbal informed consent was waived because of the retrospective and non-interventional design of the investigation. All data were fully anonymized.

Study cohort

All patients admitted to ICU with proven diagnosis of COVID-19 between March 2020 and December 2020 were analyzed via chart review, there were no exclusion criteria. The diagnosis of COVID-19 was confirmed by polymerase chain reaction (PCR) analysis of nasal or throat swab samples in all cases. Admission to our ICU took place via the emergency department, ICUs of other hospitals or inside our hospital.

Patients’ data

Data of lung ultrasound, laboratory findings and mechanical ventilation was collected at baseline and follow up. Baseline was defined as the day of admission to our ICU and was named day 1 (d1). Further follow up took place every other day afterward for the first two weeks named day 3 (d3), day 5 (d5), day 7 (d7), day 9 (d9), day 11 (d11), day 13 (d13) and day 15 (d15). To provide clear results we focused our follow-up evaluation on days 7 (d7) and d15 (d15). Baseline data included age, sex, height, weight, and body mass index (BMI) as well as pre-existing comorbidities. The Sequential Organ Failure Assessment (SOFA) score and the Acute Physiology and Chronic Health Evaluation (APACHE) II score were calculated at baseline to grade the severity of illness. ARDS was defined and graded according to the Berlin Definition[11].

At baseline (d1), the following parameters were recorded: Arterial blood gas parameters such as partial pressures of oxygen (pO), carbon dioxide (pCO), ventilatory settings such as respiratory rate (RR), positive end-expiratory pressure (PEEP), plateau pressure (P), driving pressure (ΔP), tidal volume (Vt), compliance (C), ratio of partial pressure of oxygen (pO) and fraction of oxygen (FiO) named p/F ratio, and laboratory findings such as white blood cell count, ferritin, lactate dehydrogenase (LDH), interleukine-6 (Il-6) and C-reactive protein (CRP). At follow up (d7, d15), we collected pO, pCO, P, V, compliance, FiO, PEEP, and p/F ratio. Likewise, lung ultrasound examinations were executed according to the protocol outlined below.

Furthermore, treatment with prone position (pp) as well as the need for extracorporeal membrane oxygenation (ECMO) were analysed. Beyond the follow up of d15, length of stay, duration of invasive ventilation and individual outcome were documented. For those under invasive ventilation, we performed a subanalysis with parameters of ventilation (LUS, paO, paCO, FiO, p/F ratio, PEEP, Pplat, Vt, and compliance).

Lung ultrasound

Lung ultrasound score (LUS) is a valid tool to assess regional and global aeration in ARDS and can be used in COVID-19 ARDS as well. LUS was performed according to an adapted version of the CLUE protocol [12] as described at full length before [8]. This protocol recommends scanning the chest systematically in 12 zones, 6 zones for the right lung (R1-R6) and 6 zones for the left lung (L1-L6). Due to limited positioning options of our patients (mechanically ventilation, severe lung failure, hemodynamically unstable), we had to adapt the recently published CLUE-protocol. Instead of twelve we systemically scanned 8 zones. We defined 4 zones for the right lung (R1 to R4) and 4 zones for the left lung (L1 to L4), Fig 1. For each patient, LUS was recorded for the zones R1 to R4 and L1 to L4 and finally totaled. LUS examination was performed and analyzed by the ICU physician on duty supervised by one senior physician with expertise in LU recording and interpretation with the same equipment (Venue, GE Healthcare). A randomly selected number of ultrasound examinations were blinded and examined by another senior physician to obtain reliable results. The images were saved on an ultrasonic device and then transferred to the electronic patient file including the medical report with the corresponding LUS. The protocol was used in both, invasive and non-invasive ventilated patients, to achieve comparability.

Fig 1

8 zones (R1-R4, L1-L4) for LUS, adapted from CLUE protocol [12].

While performing lung ultrasound, sonographic signs of pneumothorax e.g., lack of lung sliding, lack of B lines, and barcode sign were considered throughout. X-ray of the thorax was amended in case of suspected pneumothorax by sonography.

At each of these 8 zones, LUSS ranges from 0 to 3 points, with higher points allocated to severe lung changes (Table 1).

Table 1

Lung ultrasound score according to CLUE protocol [12].

	LUS0 points	LUS1 point	LUS2 points	LUS3 points
A-lines	Yes	no	no	no
B-lines	1–2	>2	confluent	confluent
Pleural line	smooth, thin	irregular, thickened	irregular, thickened	irregular, thickened
Consolidation	No	no	yes, height < 1cm	yes, height > 1cm
Accessory				+/-air bronchogram+/- vascularity

Statistics

Binary data are presented as numbers (n) and percentage (%). Continuous data are presented as mean ± standard deviation (sd) if normally distributed, otherwise as median with range. To identify differences in continuous variables, paired t-test or Wilcoxon-test were used for paired variables while Mann-Whitney-U test or ANOVA were used for unpaired variables. Categorical data were compared with Fisher`s exact test. Correlation between data was examined using Pearson´s correlation. To compare inter-rater variability for LUS, Bland- Altman analysis was used. P-value < 0.05 was considered statistically significant. Statistical analysis was performed with GraphPad Prism 8 (GraphPad Software, San Diego, CA).

Results

From March 2020 to August 2020, 42 patients were admitted to the Internal Intensive Care Unit, LMU hospital Munich with the diagnosis COVID-19. Most of them were male (n = 29; 69%) with a mean age of 66 years. The average APACHE II score indicated 20 points. The most common comorbidities were essential hypertension (n = 34; 81%) and obesity (n = 24; 57%). About one-third of our cohort (n = 13; 31%) was under immunosuppression due to solid or hematological malignancy and therefore a deficiency in B- and/or T-cell response or due to medical immunosuppression in the context of solid organ transplantation. The mean initial score of lung ultrasound counted 12 points. Coherent the average required FiO2 at baseline was 0.70. According to the p/F-ratio eight patients met the criteria for mild ARDS, 19 patients for moderate ARDS, and five patients for severe ARDS at baseline. The remains were treated with a PEEP <5mbar and were so inaccessible to ARDS classification. Table 2 illustrates further baseline characteristics.

Table 2

Baseline characteristics.

Baseline characteristics(mean±sd; otherwise, median and range)
Age (years)	66±12
Male (n (%))	29 (69)
BMI (kg/m2)	29±5
APACHE II (points)	20±8
SOFA (points)	7±4
Comorbidities (n (%))
Essential hypertension	34 (81)
Coronary heart disease	9 (21)
Diabetes mellitus	14 (33)
Obesity	24 (57)
Solid malignancy	3 (7)
Haematological malignancy	6 (14)
Immunosuppression	13 (31)
COPD	3 (7)
Neurological disorder	5 (12)
Laboratory
Ferritin (ng/mL)	1533 (247;5111)
C-reactive protein (mg/dL)	10 (1;47)
Lactate dehydrogenase (U/L)	412±125
White blood cell count (G/L)	10±5
Interleukine-6 (pg/mL)	90 (4;580)
Gas exchange
paO2 (mmHg)	91 (53;227)
paCO2 (mmHg)	39 (25;83)
FiO2	0.7 (0.21;1.0)
RR (breaths/min)	24±6
Invasive Ventilation (n=21)
p/F-ratio (mmHg)	177±76
PEEP (mbar)	13±4
Pplat (mbar)	25±5
Driving pressure (mbar)	12±3
Vt (mL)	443±171
Compliance (mL/mbar)	39±15
Lung ultrasound (n (%))
LUS (points)	12±4
Pleural effusion	4 (10)
Homogeneous B-lines	0 (0)
Subpleural consolidations	23 (55)
Thickened pleural line	38 (90)

At baseline, 19 patients (45%) were under mechanical ventilation. The other ones (n = 23, 55%) were not mechanically ventilated at admission to ICU. Eleven of them suffered respiratory deterioration over the course with the need for intubation and invasive ventilation (Fig 2).

Fig 2

Course of invasive ventilation.

The length of stay on ICU was significantly longer for those patients, who required mechanical ventilation (at baseline or during ICU stay) compared to those patients, who required only non-invasive ventilation (26 (4;141) vs 12 (2;15) days, p<0.001). While LUS did not differ at admission to ICU between these groups, LUS was significantly lower at d7 for those, who had no need for invasive ventilation over the course (13±5 vs 7±4 points, p = 0.0046). There was no case of pneumothorax. During their stay on ICU 9 patients (21%) required ECMO therapy and 12 patients (29%) prone position. The median time of invasive ventilation counted 18 days. The 90-day mortality was 24% in our cohort, only patients under invasive ventilation died (33% vs 0%, p = 0.0400). Detailed data on the course of ICU stay are presented in Table 3.

Table 3

Course of ICU.

	All, n = 42	Invasive ventilation at ICU, n = 30	No invasive ventilation at ICU, n = 12	p-value
ECMO (n (%))	9 (21)	9 (21)	--
Duration of    ECMO (days)	9±4	9±4	--
PP (n (%))	12 (29)	12 (29)	--
Invasive ventilation (days)	18 (1;141)	18 (1;141)	--
Length of ICU stay (days)	16 (2;150)	26 (4;141)	12 (2;15)	<0.001
LUS (points)
d1	12±4	12±4	11±2	0.249
d7	11±5	13±5	7±4	0.005
d15	9±6	9±6	--
Outcome (n (%))
90-day mortality	10 (24)	10 (33)	0 (0)	0.040

Within the first 15 days of ICU, lung ultrasound score (LUSS) was collected eight times for each subject, unless premature death. Fig 3 shows a representative case to demonstrate the course of LUS exemplarily.

Fig 3

Course of LUS.

(A) and (B): d1, LUS 12 points. (A) d1, position L4. (B) d1, position R4. (C) and (D): d7, LUS 16 points. (C) d7, position L4. (D) d7, position R4. (E) and (F): d15, LUS 7 points. (E) d15, position L4. (F) d15, position R4.

Considering the difference of LUSS of d1 and d7 (Δd1/d7) respectively d15 (Δd1/d15), our cohort showed interesting results.

In case of a positive Δd1/d7 (n = 13), indicating an increasing LUS between d1 and d7 and therefore indicating a deterioration of LUS, 12 patients (92%) were mechanically ventilated at their stay on ICU and 6 patients (46%) died within the first 90 days of ICU, while in case of a negative Δd1/d7 (n = 17), indicating a decreasing LUS between d1 and d7 and therefore indicating an improvement of LUS, only 10 patients (57%) required mechanical ventilation and none of them died within the first 90 days of ICU (Table 4). For those patients with a positive Δd1/d15 (n = 5), 3 patients (60%) died while only one of them (7%) with a negative Δd1/d15 (n = 15) died in this period (Table 5).

Table 4

LUS Δd1/d7.

	Δd1/d7 positiven = 13	Δd1/d7 negativen = 17	p-value
Need for mechanical ventilation (n (%))	12 (92)	10 (57)	0.092
Death within first 90 days of ICU (n (%))	6 (46)	0 (0)	0.003

Table 5

LUS Δd1/d15.

	Δd1/d15 positiven = 5	Δd1/d15 negativen = 15	p-value
Death within first 90 days of ICU (n (%))	3 (60)	1 (7)	0.032

For patients under invasive ventilation (n = 30), the course of respiration and mechanical ventilation is outlined in Table 6 and data of respiration and ventilation in correlation with LUS are outlined in S1 Table. While there is no significant correlation between LUS and any data of respiration or ventilation at d1 of invasive ventilation, our data identified a significant correlation at d7 of invasive ventilation for the following data: LUS and compliance (Pearson -0.572; p = 0.016), LUS and PEEP (Pearson 0.495; p = 0.043), LUS and Pplat (Pearson 0.617; p = 0.008), LUS and FiO2 (Pearson 0.591; p = 0.033) and LUS and p/F ratio (Pearson -0.723; p = 0.005). At d15 of invasive ventilation, there was a significant correlation for LUS and paCO2 (Pearson 0.834; p = 0.001), LUS and FiO2 (Pearson 0.827; p = 0.002) and LUS and p/F ratio (Pearson -0.861, p = 0.001).

Table 6

Course of respiration and mechanical ventilation.

	d1 of mechanical ventilation	d7 of mechanical ventilation	d15 of mechanical ventilation
LUS (points)	12.2±3.8n = 22	11.9±5.4n = 17	8.7±6.6n = 12
p a O 2 (mmHg)	108.7±50.4n = 22	75.6±13.2n = 22	73.9±18.9n = 14
paCO2 (mmHg)	51.9±11.6n = 22	52.4±14.8n = 22	51.2±18.8n = 14
FiO 2	0.71±0.26n = 20	0.44±0.18n = 16	0.42±0.24n = 12
p/F ratio (mmHg)	169.3±73.2n = 20	169.9±55.4n = 16	214.3±112.3n = 12
PEEP (mbar)	12.4±3.8n = 22	10.5±3.3n = 21	10.5±4.1n = 11
P plat (mbar)	23.3±3.7n = 22	21.9±4.3n = 21	22.9±6.4n = 11
Vt (mL)	392.3±108.0n = 22	404.9±128.0n = 21	444.1±90.9n = 11
Compliance (mL/mbar)	38.2±13.9n = 22	37.7±16.0n = 21	37.1 ±9.4n = 11

Comparison of inter-rater variability for LUS showed good agreement between measurements: mean difference -0.04 ± 0.53 points, r = 0.997, p<0.001. The Bland-Altman plot showed a random scatter of points around 0, indicating no systematic bias or measurement error proportional to the measurement value.

Discussion

The infection with SARS-CoV2 leads to interstitial pneumonia, therefore using LUS to assess severity, predict the course of illness and assist in treatment decisions is obvious and was early discussed in the pandemic [13]. This descriptive study aimed to perform follow-up LUS in non-invasive and invasive ventilated patients with COVID-19 on the ICU to explore LUS as a tool for monitoring lung failure. The first point we wanted to address was whether the course of LUS collected on days 1, 7, and 15 of ICU stay helps to assess the progression or regression of respiratory failure.

LUS was assessed with an 8-zone method (4 zones for each lung) according to an adapted version of the CLUE protocol [9]. The adaption was necessary because our patients had limited positioning options (mechanically ventilation, severe lung failure, hemodynamically unstable), considering the need for a universal and comparable tool that fits all patients. One weakness of this method is that it omits the dorsal parts of the lungs that are potentially even more affected by COVID-19. In our experience, lung ultrasound findings at points R4 and L4 provide as an indicator for alterations in the posterior regions like atelectasis and pleural effusions, especially, in the bedridden patient. Different methods including 6-zone, 8-zone, 12-zone, and 14-zone protocols have been successfully used in the critical care setting (mostly in the emergency department) to monitor lung failure patients including COVID-19 patients [13]. In 2012, an international guideline recommended either the use of 8 or 28 zones for the assessment of LUS [14]. Deng et al. showed that LUS correlated well with CT findings and could effectively distinguish critical-type patients from severe-type patients [15]. They used an 8-zone protocol to determine LUS similar to our protocol which supports the idea that this protocol is effectively applicable. Another crucial point is the duration of an examination. First, shorter examination times lead to reduced virus exposure for the examiner. Second, in the pandemic with temporarily extremely high numbers of patients, time is a relevant point considering good medical care and pressure of time. It remains to be investigated which method performs best to explore COVID-19 lung failure.

We could show that in case of an increasing LUS between d1 and d7 most of the patients (all but one) needed invasive ventilation and nearly half of them died on ICU. In case of a decreasing LUS between d1 and d7, only 57% required mechanical ventilation and none of them died within the first 90 days. This underlines that not only LUS assessed at admission, as shown by several authors [7,8,16], can predict clinical course but follow-up assessed LUS can help predict the progression and regression of respiratory failure. This is in line with the experience of an Italian study group [10] and with the data published by Hoffmann et al. [17]. They showed that follow-up lung ultrasound on days 1, 3, and 5 of hospital stay could predict ICU admission and reasoned that lung ultrasound can indicate impending development of severe disease in COVID-19 patients. The need for mechanical ventilation is clearly different between the increasing LUS group (97%) and the decreasing LUS group (57%). However, this rate of 57% despite decreasing LUS is much higher than expected. One reason might be that the LUS increased earlier and explains therefore the deterioration with need of mechanical ventilation. Further analyses of LUS in the early course of lung failure must be performed to clear this point.

For patients with an increasing LUS between d1 and d15, 60% died while only 7% with a decreasing LUS between d1 and d15 died. These results appear plausible because the progression of lung failure as indicated by increasing LUS leads to death [18]. But these results should be viewed with caution. The group with increasing LUS between d1 and d15 was very small (only 5 patients), for that reason larger cohorts may lead to others results.

The second point we addressed was whether there is a difference in LUS over the course between invasive and non-invasive ventilated patients. Comparing the groups invasive ventilation and no invasive ventilation over the course of ICU stay, we found a significant difference in the LOS, death within the first 90 days of ICU stay and the LUS at d7, while LUS at d1 did not differ. This means, that there is no difference in LUS between the non-invasively ventilated ones and those under invasive ventilation at admission to ICU, but a significantly higher LUS at d7 for those under invasive ventilation compared to those under non-invasive ventilation, demonstrating the severity of illness. In line with these findings, patients with a positive Δd1/d7, indicating a deterioration of LUS, required invasive ventilation in 92% and had a 90-day mortality of 46%, while patients with a negative Δd1/d7, indicating an improvement of LUS, required invasive ventilation in only 57% and had a 90-day mortality of 0%. These results strengthen several other studies, that LU can be a valuable monitoring tool to assess the course of lunge failure and can detect clinical deterioration early [7,10,17,19]. In a previous work we could show that LUS significantly increased in case of respiratory deterioration with the need for invasive ventilation [8].

Furthermore, we analyzed parameters of invasive ventilation in relation to LUS. At d7 of invasive ventilation, we found a significant correlation between LUS and FiO2, LUS and PEEP, LUS and pplat and a significant inverse correlation between LUS and p/F ratio and LUS and compliance. At d15 of invasive ventilation, we found a significant correlation between LUS and paCO2, LUS and FiO2 and a significant inverse correlation between LUS and p/F ratio. So, in our cohort LUS might also be a valuable tool in the course of lung failure because it correlates with parameters of invasive ventilation. This supports the findings of Lichter et al. [7]. They showed that worsening of LUS was significantly associated with worsening of ventilation parameters. In their study, seven invasive ventilated patients received repeated LU due to respiratory deterioration besides baseline LU. They found a significant positive correlation between the change in LUS and the change in PEEP requirements. However, the reservation must be made, the correlation coefficients especially for the correlations on d7 are in the lower “high degree” and “moderate degree”. This might be due to the small cohort.

Assessing the best PEEP in ARDS ventilation is a challenging task. This also applies for COVID-19 ARDS. A small case series by Grasso et al showed in a comparison of a low and a high PEEP protocol that using a higher PEEP protocol led to improved oxygenation and lung aeration [20]. Oppositional, the results of Bonny et al preferred a low PEEP in handling those patients. They also compared two levels of PEEP regarding effects on lung mechanics and found an increased cardiac index and significantly higher lung compliance in PEEP decremental without significant changes in gas exchange [21]. Bouhemad et al concluded that PEEP-induced lung recruitment can be adequately estimated with bedside LU. But they also warned against using LU as the sole method for PEEP titration because PEEP-induced lung hyperinflation cannot be assessed by LU [22]. In our cohort, we did not define best-PEEP by evaluating LU and we did not use LU to evaluate whether it can predict best PEEP, which are limitations of this study. The best way to find best PEEP in COVID-19 ARDS remains unclear.

Conclusion

In our retrospective and descriptive study of 42 patients with COVID-19, LU seems to be a useful tool to assess the progression or regression of respiratory failure in non-invasively and invasively ventilated patients and in indicating intubation in these patients. Further studies with larger cohorts are needed to confirm these promising results.

Respiration and ventilation in correlation with LUS.

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18 Apr 2022

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Reviewer #1: Overall the study is clear but the hypothesis of the study is not new and the design of study is poor. The authors perform follow-up Lung ultrasound in patients on ICU to explore LU as a tool for monitoring lung failure caused by SARS-CoV2. furthermore, they evaluate the difference in LUS over the course between invasive and non-invasive ventilated patients. In my opinion the study did not add relevant information to the current knowledge in this specific ield of research and it could be limited interest by the readers. Overall confirmed results previously published by other group of research. The authors should clarify the novelty of the study.

methodologically; please clarify who pwerformed the analysis of US and perform analysis in terms of inter and intra variability. furthermore should be clarified how do you set the mechanical ventilation in particular regarding the PEEP selection. How do you manage the LU evaluation in case of pneumothorax during lenght of stay.

Reviewer #2: The problem described in the paper is actual. The diagnostic tools for monitoring the patients with coronavirus disease are in demand. The point-of-care lung ultrasound is promising because it is radiation free, comparatively available. Authors provided an adapted protocol for severe patients due to their limited positioning options. The research shown that the lang ultrasound can be a useful tool to monitor lung failure in COVID-19 ICU patients. The dynamics of lung ultrasound score is correlated with progression and regression of respiratory failure.

The manuscript is structured, written in clear English.

Raw data is not provided in manuscript and supporting materials, only mean values and standard deviations are given.

Statistically computations have been performed appropriately.

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Reviewer #2: No

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30 Apr 2022

Dear Editor, Dear Editorial Board Members, Dear Reviewers,

Thank you for handling and reviewing our manuscript. We found the reviewers´ comments very helpful and adapted the manuscript accordingly. Please find a point-by-point response below.

Best regards,

Michaela Barnikel and Stephanie-Susanne Stecher (on behalf of all co-authors)

Reviewer 1

Overall the study is clear but the hypothesis of the study is not new and the design of study is poor. The authors perform follow-up Lung ultrasound in patients on ICU to explore LU as a tool for monitoring lung failure caused by SARS-CoV2. Furthermore, they evaluate the difference in LUS over the course between invasive and non-invasive ventilated patients. In my opinion the study did not add relevant information to the current knowledge in this specific field of research and it could be limited interest by the readers. Overall confirmed results previously published by other group of research. The authors should clarify the novelty of the study.

Response: Thank you for this remark. We agree that numerous studies deal with lung ultrasound, especially during the pandemic. However, while performing our survey in 2020, lung ultrasound was just established in COVID-19 ICU patients. And as far as we know there are still only a few papers dealing with follow-up lung ultrasound in mechanical ventilated COVID-19 patients. Most of them handle lung ultrasound as an initial assessment of lung failure but not in follow-up. And so, we feel confident that our results are of interest to the readers of the PLOS ONE Journal. Exemplary, we listed some of the topical papers:

Vetrugno L, Bove T, Orso D, Barbariol F, Bassi F, Boero E, et al. Our Italian experience using lung ultrasound for identification, grading and serial follow-up of severity of lung involvement for management of patients with COVID-19. Echocardiography. 2020;37(4):625-7.

Hoffmann T, Bulla P, Jodicke L, Klein C, Bott SM, Keller R, et al. Can follow up lung ultrasound in Coronavirus Disease-19 patients indicate clinical outcome? PLoS One. 2021;16(8):e0256359.

Methodologically; please clarify who performed the analysis of US and perform analysis in terms of inter and intra variability.

Response: We have now clarified in our manuscript that only one senior physician with expertise in lung ultrasound supervised the performance of lung ultrasound and scored the images. A randomly selected number of ultrasound examinations were blinded and examined by another senior physician to obtain reliable results. We have now added a Bland Altman analysis for performing analysis in terms of inter variability.

Furthermore should be clarified how do you set the mechanical ventilation in particular regarding the PEEP selection.

Response: We thank the reviewer for broaching this issue. In our study cohort, best-PEEP selection was handled by performing PEEP trials by arterial blood gas analysis at different PEEP levels. We did not define best-PEEP by evaluating lung ultrasound when conducting this study.

How do you manage the LU evaluation in case of pneumothorax during length of stay.

Response: Thank you for this mindful remark. While performing lung ultrasound, sonographic signs of pneumothorax, e.g., lack of lung sliding, lack of B lines, and barcode sign, were considered throughout. X-ray of the thorax was amended in case of suspected pneumothorax by sonography. However, there was no case of pneumothorax. We have now added this information to our manuscript.

Reviewer 2

The problem described in the paper is actual. The diagnostic tools for monitoring the patients with coronavirus disease are in demand. The point-of-care lung ultrasound is promising because it is radiation free, comparatively available. Authors provided an adapted protocol for severe patients due to their limited positioning options. The research shown that the lang ultrasound can be a useful tool to monitor lung failure in COVID-19 ICU patients. The dynamics of lung ultrasound score is correlated with progression and regression of respiratory failure. The manuscript is structured, written in clear English. Raw data is not provided in manuscript and supporting materials, only mean values and standard deviations are given. Statistically computations have been performed appropriately.

Response: We thank the reviewer for dealing with our manuscript. We have now provided raw data in an excel-sheet. Please take heed of the attachment file.

Submitted filename: Response to Reviewers.rtf

Click here for additional data file.

10 May 2022

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). You should upload this letter as a separate file labeled 'Response to Reviewers'.

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Robert Jeenchen Chen, MD, MPH

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: (No Response)

Reviewer #3: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Partly

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: No

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: Yes

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: The authors satisfied partially my previous comments and in my opinion the novelty of the study remain poor. However, I appreciate the efforts of the authors in orders to improve the quality of the paper.

The authors reported that the novelty of the study regarding the follow-up lung ultrasound in mechanical ventilated COVID-19 patients. In my opinion, this is not clearly express in the text and I suggest to avoid confusion about the terms follow-up and the tool useful “to assess the progression or regression of respiratory failure.” Furthermore, the second aim is related to the first outcome. I strongly recommend to revise carefully it in according to the design of the study. In other words, I think that the study would like to evaluate the progression of the severity of disease and consequently assess if the LUS score could be useful to help in the decision of intubate or not intubate. This is the major point that should be addressed by the authors.

I appreciate the analysis performed to evaluate the inter – variability. I am not sure that this test is appropriate to test it. Please revise this aspect with an expert statistician before the publication of the paper.

The authors reported that “In our study cohort, best-PEEP selection was handled by performing PEEP trials by arterial blood gas analysis at different PEEP levels”. This approach probably reflect the clinical practice but not reflect a good scientific sound. This aspect “We did not define best-PEEP by evaluating lung ultrasound when conducting this study” should be included in the main limitation of the study and please discuss the optimal approach taking into account the following papers in your discussion and add appropriate references ( doi: 10.1186/s13054-020-03311-9; doi: 10.1097/CCM.0000000000004640; doi: 10.1186/s40560-020-00499-4.)

Reviewer #3: The authors reponse well, however, I still have one minor suggestions.

Please delete the second paragraph in the discussion section because this is not the focus of this study. The discussion about lung ultrasound should be written first.

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

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Reviewer #1: No

Reviewer #3: No

[NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.]

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step.

15 May 2022

Dear Editor, Dear Editorial Board Members, Dear Reviewers,

Thank you for handling and reviewing our manuscript again. We found the reviewers´ comments very helpful and adapted the manuscript accordingly. Please find a point-by-point response below.

Best regards,

Michaela Barnikel and Stephanie-Susanne Stecher (on behalf of all co-authors)

Reviewer 1

The authors satisfied partially my previous comments and in my opinion the novelty of the study remain poor. However, I appreciate the efforts of the authors in orders to improve the quality of the paper. The authors reported that the novelty of the study regarding the follow-up lung ultrasound in mechanical ventilated COVID-19 patients. In my opinion, this is not clearly express in the text and I suggest to avoid confusion about the terms follow-up and the tool useful “to assess the progression or regression of respiratory failure.” Furthermore, the second aim is related to the first outcome. I strongly recommend to revise carefully it in according to the design of the study. In other words, I think that the study would like to evaluate the progression of the severity of disease and consequently assess if the LUS score could be useful to help in the decision of intubate or not intubate. This is the major point that should be addressed by the authors.

Response: We thank the reviewer for revealing this remarkable issue. We have now adapted our manuscript accordingly to enhance its quality.

I appreciate the analysis performed to evaluate the inter – variability. I am not sure that this test is appropriate to test it. Please revise this aspect with an expert statistician before the publication of the paper.

Response: We have now revised this aspect with the expert statistician of our department (Dr. Ujjwal Mukund Mahajan, PhD) to make sure our inter-variability test is valid. He confirmed our analysis, we have now mentioned him in our acknowledgment.

The authors reported that “In our study cohort, best-PEEP selection was handled by performing PEEP trials by arterial blood gas analysis at different PEEP levels”. This approach probably reflect the clinical practice but not reflect a good scientific sound. This aspect “We did not define best-PEEP by evaluating lung ultrasound when conducting this study” should be included in the main limitation of the study and please discuss the optimal approach taking into account the following papers in your discussion and add appropriate references ( doi: 10.1186/s13054-020-03311-9; doi: 10.1097/CCM.0000000000004640; doi: 10.1186/s40560-020-00499-4.)

Response: We have now added this information in the discussion of our manuscript as a main limitation of our study. Furthermore, we added a paragraph to discuss the optimal approach for defining best-PEEP, including your suggested references.

Reviewer 3

The authors reponse well, however, I still have one minor suggestions.

Please delete the second paragraph in the discussion section because this is not the focus of this study. The discussion about lung ultrasound should be written first.

Response: Thank you for this remark. We have now deleted the second paragraph in the discussion section and modified the order of the paragraphs.

Submitted filename: Response to Reviewers .rtf

Click here for additional data file.

20 May 2022

2 Jun 2022

Dear Editor, Dear Editorial Board Members, Dear Reviewers,

We thank the Reviewers for their input and have now adapted the manuscript accordingly. Please find a point-by-point response below.

Best regards,

Michaela Barnikel and Stephanie-Susanne Stecher (on behalf of all co-authors)

Reviewer 1

The authors satisfied my previous comments. I suggest to improve the quality of the figure and i suggest to include a representative case. Please revise the figure in according with the guidelines of the journal. (delete the black)

I would like to congratulate with the authors for the efforts done to improve the quality of the paper.

Response: Thank you for your previous comments and your efforts to improve our manuscript. We have now included a representative case to demonstrate the course of LUS and increased the resolution of our figures to improve their quality and to be in line with the guidelines of the journal.

Reviewer 3

The authors response well, so I have no more comment. I recommend that the manuscript can be accepted now.

Response: We thank the reviewer for his/her support.

Submitted filename: Response to Reviewers.rtf

Click here for additional data file.

30 Jun 2022

Follow-Up lung ultrasound to monitor lung failure in COVID-19 ICU patients

PONE-D-22-00632R3

Dear Dr. Barnikel,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org.

Kind regards,

Robert Jeenchen Chen, MD, MPH

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation.

Reviewer #1: All comments have been addressed

Reviewer #3: All comments have been addressed

**********

2. Is the manuscript technically sound, and do the data support the conclusions?

The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented.

Reviewer #1: Yes

Reviewer #3: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #1: Yes

Reviewer #3: Yes

**********

4. Have the authors made all data underlying the findings in their manuscript fully available?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified.

Reviewer #1: No

Reviewer #3: Yes

**********

5. Is the manuscript presented in an intelligible fashion and written in standard English?

PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here.

Reviewer #1: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: i have no further comments. the reviewers satisfied my previous comments. Congratulation for this nice paper

Reviewer #3: The authors response well, so I have no more suggetsion. The present form can be recommended as accept

**********

7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #3: No

**********

6 Jul 2022

PONE-D-22-00632R3

Follow-Up lung ultrasound to monitor lung failure in COVID-19 ICU patients

Dear Dr. Barnikel:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org.

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Thank you for submitting your work to PLOS ONE and supporting open access.

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on behalf of

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PLOS ONE