Disentangling Blood Volume and Blood Flow Changes in Hemodynamic Monitoring of Upper and Lower Limbs Reveals Sex Differences in Response to Hypovolemic Stimuli

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This original study provides a detailed assessment of the hemodynamic responses to Lower Body Negative Pressure (LBNP) in males and females, focusing on differences between upper and lower limbs. They used NIRS (Near -infrared spectroscopy) to monitor blood volume and oxygenation changes.

The study used randomized LBNP stimuli with resting intervals to minimize cumulative effects, allowing precise analysis of pressure-dependent responses. LBNP causes a rapid decrease in blood volume in the forearm and an increase in the leg, with slower, progressive changes in the leg over 5 minutes. Both arms and legs exhibit vasoconstriction, indicated by TOI decrease and HHb increase. However, vasoconstriction is stronger in the legs compared to the arms. Males show stronger vasoconstriction than females and no sex differences were observed in blood volume changes.

The manuscript is well written. Table 1 should be corrected. The discussion is extensive and should be shortened. Limitations: they did not account for menstrual cycle phases or adipose tissue thickness, which could influence NIRS measurements.

The findings highlight the utility of NIRS in detecting early hemodynamic changes and provide insights into sex and limb-specific vascular responses to hypovolemic stimuli.
In addition, the study highlights the importance of understanding physiological differences between the sexes to improve targeted medical/non-medical interventions.

Author Response

COMMENT: The manuscript is well written. Table 1 should be corrected. The discussion is extensive and should be shortened. Limitations: they did not account for menstrual cycle phases or adipose tissue thickness, which could influence NIRS measurements.

ANS: We thank the Reviewer for the suggestions. However, we are not sure to have correctly interpreted the requests. We hope that the Reviewer is satisfied with the interventions described below. If this were not the case, we ask them to provide more specific indications.

• Table 1 contains the Pearson’s R values of the repeatability test (correlation of measurements taken from 2 subsequent exposure to LBNP (90 s,30 mmhg). We double-checked that the reported R values are correct. However, we made improvements to the formatting of the table.
• We struggled to find a way to shorten the discussion as all paragraphs are necessary to support the interpretatilon of the data and the logical connection between sentences. However we managed to eliminate about 6 lines without major alterations of the contents.
• The limitation about not accounting for the menstrual cycle phase, was already accounted for at the beginning of paragraph “4.6 Limitations of the study”

A red-lined version of the revised manuscript is provided for the reviewer convenience

Reviewer 2 Report

Comments and Suggestions for Authors

Strengths of the Study:

    Effective methodology: The use of NIRS is appropriate because it is a non-invasive and consistent technique. Allows blood volume and blood flow estimation

    Non-invasive monitoring: Upper and lower extremity blood pressure monitoring is an appropriate noninvasive technique. Similarly, the use of lower body negative pressure to simulate hypovolemic conditions is relevant.

The statistical analysis is adequate and consistent with the data management obtained. The graphs are adequate and summarize the results obtained.

Weaknesses of the Study:

    Study design limitations: The sample size is small and the differences between genders may be important but more variables such as endocrine aspects that contribute to cardiovascular regulation should be considered.

    Lack of comparative analysis: Comparison with existing methods would strengthen the validation of the proposed method by contextualizing its performance, accuracy and ease of use. However, it is not mandatory

 

Major comments: NA

Minor Comments:

The abstract should include at the beginning a sentence explaining the objective of the study (line 14).     .

Please consider including a picture with the volunteer subject and the devices used.

Author Response

Minor Comments:

COOMMENT 1: The abstract should include at the beginning a sentence explaining the objective of the study (line 14).     .

ANS We attempted to improve understanding of the aims by adding a short introductive sentence, hoping that the editor accepts a slight exceeding of the maximum word limit.

COMMENT 2:Please consider including a picture with the volunteer subject and the devices used.

ANS We thank the Reviewer for this suggestion. A scheme illustrating the experimental set-up, including  the subject and the performed measurements has been added (Fig. 1A) and the text and figure caption were modified accordingly.

A red-lined version of the revised manuscript is provided for the reviewer convenience

Reviewer 3 Report

Comments and Suggestions for Authors

It is only suggested for future trials, it would be possible to include age groups to evaluate the response.

Additional comments:

What is the main question addressed by the research? In this work they analyze the temporal evolution of the Near Infrared Spectroscopy signals in response to different negative pressure stimuli in the lower part of the organism to elucidate three questions: if it is possible to determine blood volume displacements and changes in blood flow, the second thing they seek is to demonstrate if arms and legs can have different changes in blood volume and third to show if women present less vasoconstriction than men.
• Do you consider the topic original or relevant to the field? Does it
address a specific gap in the field? Please also explain why this is/ is
not the case. There are some approaches to the topic in previous work; the present study addresses it from a different perspective: that of gender, that is, the differences between men and women.It can be considered an original topic in its field and relevant to it, helping to build knowledge.
• What does it add to the subject area compared with other published
material? The research allowed for the elucidation and quantification of blood volume based on changes in blood flow and the detection of more intense vasoconstriction in the legs than in the arms, and in men than in women. These data provide new knowledge in their field of application.
• What specific improvements should the authors consider regarding the methodology? The methodology proposed seems adequate. In another study, age groups could be created, adding an additional variable and establishing a correlation between the data obtained from the limbs, gender differences, and age.
• Are the conclusions consistent with the evidence and arguments
presented and do they address the main question posed? Please also
explain why this is/is not the case. The findings are consistent with the evidence and show that vasoconstriction is more intense in the legs than in the arms, and is greater in men and less in women, which answers the question posed.
• Are the references appropriate? The references are adequate and related to the topic reviewed, and the number of references is within the recommended range for a research paper, which should be at least 40, although 48 were used in this paper.
• Any additional comments on the tables and figures. The figures presented are considered to be adequate and have good resolution; no changes are necessary.

Author Response

COMMENT 1: It is only suggested for future trials, it would be possible to include age groups to evaluate the response.

ANS We thank the reviewer for the suggestion. Age dependence of hemodynamic responses is indeed a relevant aspect deserving additional investigations.

A red-lined version of the revised manuscript is provided for the reviewer convenience