PONE-D-21-37763Effects of supplemental oxygen on systemic and cerebral hemodynamics in experimental hypovolemia: Protocol for a randomized, double blinded crossover studyPLOS ONE

Dear Dr. Lindvåg Lie,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

1. It is not clear how hyperoxia would alter variables influencing cardiac output during LBNP and need a mechanistic rationale to justify the primary outcome variable.
2. Two potential sources of error for the primary variable mentioned by reviewer need to be confirmed and discussed.
3. Need a hypothesis statement.

Please submit your revised manuscript by Apr 08 2022 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file.

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Kind regards,

Quan Jiang, Ph,D.

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does the manuscript provide a valid rationale for the proposed study, with clearly identified and justified research questions?

The research question outlined is expected to address a valid academic problem or topic and contribute to the base of knowledge in the field.

Reviewer #1: No

Reviewer #2: Yes

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2. Is the protocol technically sound and planned in a manner that will lead to a meaningful outcome and allow testing the stated hypotheses?

The manuscript should describe the methods in sufficient detail to prevent undisclosed flexibility in the experimental procedure or analysis pipeline, including sufficient outcome-neutral conditions (e.g. necessary controls, absence of floor or ceiling effects) to test the proposed hypotheses and a statistical power analysis where applicable. As there may be aspects of the methodology and analysis which can only be refined once the work is undertaken, authors should outline potential assumptions and explicitly describe what aspects of the proposed analyses, if any, are exploratory.

Reviewer #1: No

Reviewer #2: Yes

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3. Is the methodology feasible and described in sufficient detail to allow the work to be replicable?

Descriptions of methods and materials in the protocol should be reported in sufficient detail for another researcher to reproduce all experiments and analyses. The protocol should describe the appropriate controls, sample size calculations, and replication needed to ensure that the data are robust and reproducible.

Reviewer #1: No

Reviewer #2: Yes

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4. Have the authors described where all data underlying the findings will be made available when the study is complete?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception, at the time of publication. 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 #2: Yes

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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 #2: Yes

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6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above and, if applicable, provide comments about issues authors must address before this protocol can be accepted for publication. You may also include additional comments for the author, including concerns about research or publication ethics.

You may also provide optional suggestions and comments to authors that they might find helpful in planning their study.

(Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: General concerns with the protocol:

Based upon the text in lines 66-70, I presumed that the primary variable of interest was tolerance to LBNP. However, Table 3 indicates that this variable is a secondary observation, with differences in the change in cardiac output between trials being the primary observation. This is perplexing given the absence of a physiologically-sound rationale for the mechanisms by which hyperoxia would alter cardiac output during LBNP. Cardiac output changes during LBNP are primarily mediated by reductions in venous return, coupled with baroreflex-driven withdrawal of cardiac vagal tone (affecting heart rate) and accompanying increases in cardiac sympathetic activity (affecting heart rate and contractility). Given this, it is not clear how hyperoxia would alter variables influencing cardiac output during LBNP. A mechanistic rationale should be proposed justifying the primary outcome variable.

I struggle with the assumption that the diameter of the aorta for cardiac output calculations does not change throughout LBNP. There are two possible reasons why that diameter may in fact change. 1) It is highly unlikely that the location of the ultrasound beam (ideally the aortic root, which is not verified and thus unknown) would remain stable throughout LBNP. Possible factors that could influence the location of this beam include subtle angle changes of the probe associated with user “variability”, gradual position changes as the participant is pulled into the LBNP chamber, and/or possible changes in the position of the heart during LBNP. 2) Prior work has shown that low levels of LBNP (e.g., 40 mmHg) change the diameter of the ascending aorta (see PMID 7776239). Given that it is unlikely that the ultrasound beam is consistently focused on the aortic root, there is a high likelihood that LBNP itself is reducing the diameter of the assessed area. Both issues are critical given the substantial error that small differences in aortic diameter have on calculations of stroke volume, and thus cardiac output. As an example, a 10% error in the diameter of the aorta (e.g., 18 mm actual diameter rather than a proposed 20 mm fixed diameter) would result in an ~1 l/min error in cardiac output at a heart rate of 60 bpm. These two potential sources of error for the primary variable are very concerning.

Given the author’s prior work showing that approximately 50-75% of participants can tolerate 4.5 min of 80 mmHg LBNP, for the LBNP tolerance question it is unclear why LBNP does not continue until pre-syncope for all participants. If only 25% of the proposed 15 participants (e.g., ~4 participants) achieve pre-syncope at 80 mmHg, it is unlikely that an effect of hyperoxia on LBNP tolerance will be identified.

Specific recommendations:

Introduction:

Please include a hypothesis statement to inform the reader what the authors are proposing will occur during the hyperoxia trial.

Table 1: The MAP and HR “stop-criteria” are of concern for the LBNP tolerance question. In reviewing the authors’ prior work (and associated figures), I didn’t see any evidence of MAP being 75% below pre-LBNP baseline in the individuals who stopped LBNP prior to 80 mmHg. If someone has a blood pressure of 120/80 (mean = 93 mmHg, depending on the formula used), then a blood pressure of 100/55 during LBNP would equal a 75% reduction from pre-LBNP baseline, thus stopping the LBNP trial. In the hundreds of LBNP trials that I’ve conducted, I can’t think of a single occurrence when someone became pre-syncopal at such a high (relatively speaking) blood pressure. Consider using a more accepted blood pressure threshold for pre-syncope such as a sustained systolic blood pressure below 80 mmHg (see PMID: 25071587). I am also perplexed by the HR statement of “to less than 75% baseline values”. Though HR often decreases during LBNP at pre-syncope, HR values less than 75% below baseline almost never occurs (e.g., resting HR of 60 bpm so a HR of 45 bpm would be a stopping criterion).

Line 134-135: Given the impact of meals on gut blood flow, the authors should ensure similar meals are consumed for both trials.

Line 198-200: I believe there is a mistake in this sentence given that LBNP stages will be 10 mmHg every 3 min, as conveyed in figure 2. Also, please provide a citation supporting the statement that cardiac output decreases linearly at a rate of 0.489 L/min for each 20 mmHg LBNP; do we know that this is a linear response (e.g., the same reduction in cardiac output between 0 and 20 mmHg LBNP as between 60 and 80 mmHg LBNP)? Clearly, adding 20 mmHg LBNP from 0 mmHg would be less of a cardiovascular stress relative to adding 20 mmHg LBNP from 60 mmHg LBNP (e.g., from 60 to 80 mmHg).

Line 201: I am confused regarding the statement of a 15% reduction in oxygen in a protocol where 100% oxygen is administered.

Line 219: Wouldn’t LBNP tolerance be assessed between trials via a paired T-test?

Reviewer #2: Manuscript Number: PONE-D-21-37763

Full Title: Effects of supplemental oxygen on systemic and cerebral hemodynamics in experimental hypovolemia: Protocol for a randomized, double blinded crossover study

Thank you for the privilege in reviewing this protocol for the above study. The reviewer works as an academic anaesthetist in a university teaching hospital, with clinical and research interests in an anaesthesia for complex cardiac surgery and liver transplantation. This reviewer has no conflicts of interests.

In summary, this research is unique, commendable, and addresses a clinically important and relevant question. Moreover, this research could provide critical pilot data for the design of larger clinical trial for trauma patients. The safety aspects for the healthy volunteers have been carefully considered and the primary and secondary end points are valid. The study design is thorough. This is an excellent study and the authors are to be congratulated.

Minor points

1. Please comment on the preop fluids allowances and ensure this is built into the protocol and is consistent for all experiments. The authors state that patients may have a lights meal before, but volaemic status is arguably more important to control for, as this could confound the results. Please consider.

2. As a secondary outcome, please consider adding in the measurements of venous blood gases (baseline, mid experiment, end). Translating the changes in biochemical outcomes e.g., base deficit, lactate, hemoglobin, hematocrit, may further strengthen the scientific integrity of this excellent study. A venous blood gas imposes minimal additional risks and there would be little reason to think that these participants would not consent for this small additional step.

3. As an exploratory outcome, please consider whether funding allows for the assessment of markers of endothelial/glycocalyx function (e.g., heparin sulphate, syndecan 1 etc.). This will be an exploratory outcome but may inform the design of a larger clinical trial, especially if a signal is seen between the groups. If the samples are frozen and even analyzed at a later date, this would provide further valuable information about strategies to protect the endothelium during major trauma.

4. Will all experiments be undertaken at the same time of the day? If not, could the hydration status from overnight fasting impact on any of the outcomes, especially if the cross over is such that first experiment for a given participant is conducted in the morning and the second experiment for the same participant in the afternoon. Please comment.

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Reviewer #1: No

Reviewer #2: Yes: Laurence Weinberg

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Effects of supplemental oxygen on systemic and cerebral hemodynamics in experimental hypovolemia: Protocol for a randomized, double blinded crossover study

PONE-D-21-37763R1

Dear Dr. Lindvåg Lie,

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.

An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org.

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,

Quan Jiang, Ph,D.

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. Does the manuscript provide a valid rationale for the proposed study, with clearly identified and justified research questions?

The research question outlined is expected to address a valid academic problem or topic and contribute to the base of knowledge in the field.

Reviewer #1: Partly

Reviewer #2: Yes

Reviewer #3: Yes

**********

2. Is the protocol technically sound and planned in a manner that will lead to a meaningful outcome and allow testing the stated hypotheses?

The manuscript should describe the methods in sufficient detail to prevent undisclosed flexibility in the experimental procedure or analysis pipeline, including sufficient outcome-neutral conditions (e.g. necessary controls, absence of floor or ceiling effects) to test the proposed hypotheses and a statistical power analysis where applicable. As there may be aspects of the methodology and analysis which can only be refined once the work is undertaken, authors should outline potential assumptions and explicitly describe what aspects of the proposed analyses, if any, are exploratory.

Reviewer #1: Partly

Reviewer #2: Yes

Reviewer #3: Yes

**********

3. Is the methodology feasible and described in sufficient detail to allow the work to be replicable?

Descriptions of methods and materials in the protocol should be reported in sufficient detail for another researcher to reproduce all experiments and analyses. The protocol should describe the appropriate controls, sample size calculations, and replication needed to ensure that the data are robust and reproducible.

Reviewer #1: Yes

Reviewer #2: Yes

Reviewer #3: Yes

**********

4. Have the authors described where all data underlying the findings will be made available when the study is complete?

The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception, at the time of publication. 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 #2: 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 #2: Yes

Reviewer #3: Yes

**********

6. Review Comments to the Author

Please use the space provided to explain your answers to the questions above and, if applicable, provide comments about issues authors must address before this protocol can be accepted for publication. You may also include additional comments for the author, including concerns about research or publication ethics.

You may also provide optional suggestions and comments to authors that they might find helpful in planning their study.

(Please upload your review as an attachment if it exceeds 20,000 characters)

Reviewer #1: Despite the authors' replies, several methodological concerns remain that I believe will adversely affect both the publishability and associated conclusions of this work. That said, I recognize that this is not the ideal forum to debate those concerns.

Reviewer #2: I am satisfied that the detailed responses to the Reviewers have been adequately addressed. The responses to both reviewers have been suitably responded to.

Reviewer #3: I have no statistical comments: randomization, statistical analysis plan, sample size are all clearly delineated.

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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 #2: No

Reviewer #3: No