Changes in central venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients.

BACKGROUND: In this prospective observational study, we evaluated the effects of fluid bolus (FB) on venous-to-arterial carbon dioxide tension (PvaCO2) in 42 adult critically ill patients with pre-infusion PvaCO2 > 6 mmHg.
RESULTS: FB caused a decrease in PvaCO2, from 8.7 [7.6-10.9] mmHg to 6.9 [5.8-8.6] mmHg (p < 0.01). PvaCO2 decreased independently of pre-infusion cardiac index and PvaCO2 changes during FB were not correlated with changes in central venous oxygen saturation (ScvO2) whatever pre-infusion CI. Pre-infusion levels of PvaCO2 were inversely correlated with decreases in PvaCO2 during FB and a pre-infusion PvaCO2 value < 7.7 mmHg could exclude a decrease in PvaCO2 during FB (AUC: 0.79, 95%CI 0.64-0.93; Sensitivity, 91%; Specificity, 55%; p < 0.01).
CONCLUSIONS: Fluid bolus decreased abnormal PvaCO2 levels independently of pre-infusion CI. Low baseline PvaCO2 values suggest that a positive response to FB is unlikely.

Introduction

The venous-to-arterial carbon dioxide tension difference is an easily-derived metabolic index that can be used to assess the adequacy of tissue perfusion to support the body’s metabolism [1-3]. Applying Fick’s formula for CO2 shows that the difference between the mixed venous and arterial CO2 content equals the ratio between CO2 production (VCO2) and cardiac output. As CO2 content is difficult to assess, it can be replaced with the partial pressure of CO2 in the blood, since there is a linear relationship between these two parameters, at least in a large physiological range [4]. Ideally, venous-to-arterial carbon dioxide tension difference should be derived using pulmonary artery obtained PCO2. Nevertheless, Swan–Ganz catheter is not used often in contemporary intensive care [5]. Central venous venous-to-arterial carbon dioxide tension (PvaCO2) even though is not interchangeable to mixed venous [6] can be used instead as a high PvaCO2 (> 6 mmHg) indicates that tissue perfusion is not sufficiently high to remove the CO2 produced by the tissues [7]. Of note, persistent abnormal PvaCO2 levels can be related to poor outcome in critically ill patients [8, 9]. Accordingly, PvaCO2 might be an interesting target for resuscitation [10].

Unfortunately, the interventions potentially improving PvaCO2 have not yet been adequately evaluated. Observational studies have shown that resuscitation maneuvers improving central venous saturation and arterial pressure might not be related to a decrease in PvaCO2 [6, 11, 12]. Dobutamine can cause a decrease in PvaCO2 due to an increase in CI, although a paradoxical increase might be observed at higher doses [13, 14]. Fluid bolus (FB) might be another therapeutic option in patients with abnormal PvaCO2. Mecher et al. reported that FB decreased high PvaCO2 in patients with septic shock, but the authors enrolled only septic patients with low CI [15]. As elevated PvaCO2 might also represent microcirculatory alterations in the context of preserved CI [16], one may wonder whether FB decreases PvaCO2 independently of the baseline CI.

The aim of this study was to investigate whether FB decreases PvaCO2 and to determine their relationships with CI and oxygenation changes.

Methods

Design and setting

In this prospective observational study, we collected data from patients treated in Brugmann University Hospital’s 33-bed intensive care unit in Brussels between January and June 2015. Approval was obtained from the Ethics Committee (CE2014/122) of CHU-Brugmann.

Inclusion and exclusion criteria

Patients with PvaCO2 > 6 mmHg in whom the attending physician decided for a FB of either colloids or crystalloids within 30–40 min at any time of their stay in the ICU were considered eligible for this study. We included patients using a deferred informed consent as FB was part of standard treatment and we used not invasive methods for monitoring. Informed consent was obtained from all patients or, when that was not feasible, a consent form was gathered from the next-of-kin as soon as possible after FB but before ICU discharge.

Each patient was assessed once. The exclusion criteria were: 1) patients younger than 18 years old; 2) not equipped with jugular or subclavian venous catheter and arterial catheter; 3) measurement of cardiac output with cardiac ultrasound was not possible due to lack of acoustic window; 4) patients receiving extracorporeal membrane oxygenation (ECMO) support; 5) PCO2 higher than 75 mmHg in venous or arterial blood gas analysis; 6) atrial fibrillation; 7) other simultaneous interventions (i.e., introduction or increase in inotrope dosage, mode changes, or the introduction of mechanical ventilation) within 30 min prior to fluid administration.

Data and sample collections

Demographics, the type of fluids used for FB, clinical data concerning treatment (mechanical ventilation, inotropic agents), and laboratory data were collected for each patient. The Acute Physiology and Chronic Health Evaluation (APACHE) II score were used to assess the severity of disease at the time of inclusion in the study.

Using Doppler transthoracic echocardiography (GE Healthcare Vivid S5), we measured the left ventricular outflow tract (LVOT) blood velocity time integral (VTI) just prior to the administration of FB. To calculate stroke volume (SV) and CI, LVOT diameter was measured below the aortic valve at the aortic cusp insertion points in the parasternal long-axis view. Immediately after FB, we repeated the measurements. Both measurements were stored and analyzed off-line. Three consecutive velocity curves were measured, and the average VTI was calculated. We used the same value of LVOT diameter to calculate SV and CI before and after FB. Each patient was assessed once. No interventions were allowed during fluid administration.

Arterial and central venous blood gas analysis were simultaneously obtained just before and after FB. We measured the haemoglobin, arterial, and venous oxygen tensions (PaO2 and PvO2, respectively) and oxygen saturation (SaO2 and ScvO2). Applying the usual formulas, we calculated the arterial (CaO2) and venous (CvO2) oxygen content and oxygen delivery (DO2), and oxygen consumption (VO2). The PvaCO2 and PvaCO2/CavO2 ratios were calculated before and after FB.

Diagnostic definitions

All the diagnostic definitions were set beforehand. The smallest detectable difference (SDD) of PvaCO2 was expected to be ±2.06 mm Hg as it was evaluated in a previous study in critically ill patients [17]. Accordingly, patients were considered as ‘PvaCO2 responders’ if they had a decrease in PvaCO2 > 2 mmHg. ‘Fluid responders’ were defined as patients who had an increase in CI > 15% [18]. Sepsis was defined according to standard criteria [19]. As changes in PvaCO2 may be affected by baseline value and as PvaCO2 is inversely related to cardiac index, we separated patients into ‘low’ and ‘high’ cardiac index using a cut-off value of 2.2 L/min/m2, similarly to a previous study [15]. Of note, the term ‘low CI’ should not be misinterpreted as in some case the low CI may still be adequate [20].

Primary outcome

The primary endpoint was to evaluate whether FB can decrease PvaCO2 by at least 2 mmHg on average.

Secondary outcomes

The secondary endpoint was to investigate changes of PvaCO2 during FB in patients with baseline CI less or more 2.2 L/min/m2 and its relationship with changes of CI and ScvO2. The value of baseline PvaCO2 for the prediction of a decrease in PvaCO2 during FB will be evaluated.

Statistical analysis

We performed statistical analysis using R through the R-studio interface (www.r-project.org, R version 3.3.1). We used a Kolmogorov-Smirnov test to verify the normality of the distribution of the continuous variables. Normally distributed and non-normally distributed data were compared using a Student’s t-test or Wilcoxon signed-rank test, as appropriate. Categorical variables were compared using Fisher’s exact test. Pearson correlation and scatter diagrams were used to assess correlations between values. Univariate regression analysis was performed to evaluate the association between decrease PvaCO2 > 2mmHg and baseline CI, fluid type and mechanical ventilation. Receiver operating characteristics (ROC) analysis was used to derive the prognostic discriminatory performance of baseline PvaCO2 in determining a decrease of PvaCO2 during FB. The sample size was calculated to aim for an AUC of greater than 0.8, which is usually considered as having a good predictive ability. Assuming a fluid responsiveness rate of 30% in mixed population of critically ill patients [21] 40 patients were required to obtain 90% power (alpha 0.05). The Youden index was used to derive the optimal cut-off. Statistical significance was defined as p < 0.05.

Results

We evaluated 80 patients who received FB during the study period. Two patients refused to give informed consent and were excluded from any further analysis. Forty-two patients (73 years (64−83) and APACHE II score on admission 21(15−29)) met our entry criteria (S1 Fig). Twenty-four of the patients (57%) received colloids (Geloplasma®, Fresenius-Kabi AG, Bad Homburg, Germany) and 18 (43%) crystalloids (Plasma-Lyte A, Baxter Healthcare, Deerfield, IL) (S1 Table). The median given volume was 6.3 ml/kg [6.3−7.1] for FB with colloids and 14.9 ml/kg [12.1−19.6] for FB with crystalloids within a median time of 33 min [27− 44]. Central venous pressure increased after FB from 8.5 mmHg [4.0−11.2] to 11.1 mmHg [9.2− 13.0] (p<0.01). No differences were observed in the increases in central venous pressure after FB between the patients who received colloids or crystalloids (33% [30-73] vs 22% [9−44], p = 0.07). Fourteen patients (33%) had an increase in CI >15% after FB. No differences were observed in the changes in CI after FB between the patients who received colloids or crystalloids (13% [0−21]vs 12% [2−25], p = 0.22). Nineteen (45%) of the patients were supported with mechanical ventilation during FB, and 11 (26%) were under sedation. No changes in respiratory rate were observed during FB (21 ± 5 resp/min to 21 ± 6 resp/min, p = 0.92). Sixteen patients had a CI ≤ of 2.2 L/min/m2 before FB, and 26 had a CI of > 2.2 L/min/m2.

The median PvaCO2 before FB was 8.7 [7.6−10.9] and did not differ between intubated and not intubated patients (9.2 [7.7−13.5] mmHg versus 8.4 [7.4−10.2] mmHg; p = 0.23). FB decreased PvaCO2 to 6.9 [5.8−8.6] mmHg (p < 0.01) (Fig 1). Twenty-two patients (52%) had a decrease in PvaCO2 > 2 mmHg (‘PvaCO2 responders’). The hemodynamic and metabolic characteristics of the patients, as well as their changes, are presented in Table 1 and S2 Table. ‘PvaCO2 responders’ had a higher relative and absolute increase in CI compared to ‘PvaCO2 non-responders’.

Fig 1

Evolution of central venous-to-arterial carbon dioxide tension difference (PvaCO2) during fluid bolus.

Table 1

Patients’ baseline hemodynamic and metabolic variables and changes during fluid bolus according to a decrease (or not) in central venous-to-arterial carbon dioxide tension difference (PvaCO2) > 2 mmHg (PvaCO2 non-responders and responders).

Changes are presented as relative (d, %) and absolute values (Δ). Values are presented either as means with standard deviations (±) or as median values and percentiles 25 and 75.

	PvaCO2 non-responders	PvaCO2 responders	p values
No of patients	20	22
Baseline hemodynamic variables
Mean arterial pressure (mmHg)	71 ± 15	82 ± 14	<0.01
Pulse Pressure (mmHg)	52 ± 16	58 ± 16	0.23
Central Venous Pressure (mmHg)	8 ± 5	8 ± 4	0.28
Velocity Time Integral (cm)	14.7 ± 5	13.6 ± 5	0.91
Stroke Volume (ml)	55 ± 22	51 ± 21	0.54
Heart Rate (beats /min)	87 ± 20	97 ± 19	0.13
Cardiac Index (L/min/m2)	2.7 (1.7−3.3)	2.6 (1.9−3.2)	0.77
Baseline metabolic variables
Oxygen delivery (mL/min/m2)	326 (287−442)	416 (290−472)	0.28
ScvO2 (%)	61 ± 11	65 ± 8	0.17
Oxygen consumption (mL/min/m2)	116 (95−140)	122 (97−159)	0.65
Oxygen extraction (%)	33 (26−46)	33 (29−37)	0.44
PvaCO2(mmHg)	8.2 (6.7−9.7)	10.1 (8.7−12)	<0.01
PvaCO2/ CavO2	1.7 (1.4−2.1)	2.1 (1.7−2.6)	<0.01
Lactate (mmol/L)	1.9 (1.6−3.1)	2.1 (1.6−3.7)	0.47
Hemodynamic variable changes during FB
Δ Mean arterial pressure (mmHg)	2 (-2−9)	4 (-4−10)	0.99
d Mean arterial pressure (%)	3 (-3−13)	5 (-4−12)	0.93
Δ Pulse Pressure (mmHg)	1 (-3−15)	5 (-3−17)	0.97
d Pulse Pressure (%)	2 (-6−33)	10 (-4−24)	0.86
Δ Central Venous Pressure (mmHg)	2 (1−4)	3 (1−4)	0.99
d Central Venous Pressure (%)	16 (12–32)	27 (10–50)	0.78
Δ Velocity Time Integral (cm)	0 (-1−4)	3 (2−4)	0.03
d Velocity Time Integral (%)	5 (-2−19)	21 (14−40)	<0.01
Δ Stroke Volume (ml)	2 (-1−13)	11 (7−17)	0.02
d Stroke Volume (%)	5 (-2−19)	21 (14−40)	<0.01
Δ Heart Rate (beats/min)	-2 (-11−1)	-3 (-6−1)	0.88
d Heart Rate (%)	-2 (-12−2)	-2 (-6−1)	0.81
Δ Cardiac Index (L/min/m2)	0.1 (-0.1−0.4)	0.5 (0.4−0.7)	<0.01
d Cardiac Index (%)	6 (1−13)	19 (11−40)	<0.01
Metabolic variable changes during FB
Δ Oxygen delivery (mL/min/m2)	-25 (-28−13)	46 (-19−97)	0.02
d Oxygen delivery (%)	-7 (-9−5)	10 (-6−32)	0.02
Δ ScvO2 (%)	-1 (-3−3)	1 (-1−4)	0.41
Δ Oxygen extraction (%)	1 (-3−4)	-1 (-3−0)	0.16
Δ Oxygen consumption (mL/min/m2)	1 (-6−15)	4 (-18−32)	0.29
d Oxygen consumption (%)	0 (-9−12)	5 (-7−32)	0.19
Δ PvaCO2(mmHg)	0 (-1−1)	-4 (-5−-3)	<0.01
d PvaCO2 (%)	-4 (-9−10)	-40 (-48−-30)	<0.01
Δ PvaCO2/ CavO2	0.02 (-0.04−0.5)	-0.6 (-0.9−-0.4)	<0.01
d PvaCO2/ CavO2 (%)	2 (-2−33)	-33(-37−-25)	<0.01

SO: central venous oxygen saturation, PCO: venous-to-arterial carbon dioxide tension, CO: arterial-venous oxygen content difference.

There was no association between the decrease of PvaCO2 > 2 mmHg after FB and the pre-infusion levels of CI (i.e. ‘low’ or ‘high CI’). Additionally, the type of fluid used for FB and mechanical ventilation were not found to be associated with the likelihood of decreasing PvaCO2 > 2 mmHg after FB (S3 Table).

A correlation between changes in CI and PvaCO2 was observed only in patients who had a low CI before FB (r = -0.71, p < 0.01). None of the patients who had an increase in CI > 15% (Fig 2) experienced an increase in PvaCO2. Because estimation of the area of LVOT represents the major source of error in calculating cardiac output with transthoracic echocardiography [22] we repeated the analysis using only VTI: similar results were found when PvaCO2 changes were assessed with changes in VTI (S2 Fig).

Fig 2

Relationship between absolute changes in PvaCO2 (Δ PvaCO2) during fluid bolus and absolute changes in cardiac index (Δ CI).

Panel A: Patients with CI ≤ 2.2 L/min/m2; Panel B: Patients with CI > 2.2 L/min/m2. d CI: relative to baseline values changes in CI. The horizontal dotted line corresponds to Δ PvaCO2−2 mmHg. Triangle points represent “Fluid responders” (d CI > 15%) and circle points “Fluid non-responders” (d CI ≤ 15%).

We found no statistically significant correlation between PvaCO2 and ScvO2 changes, independently of the baseline CI. ScvO2 could either increase, decrease, or remained unchanged in ‘PvaCO2 responders’ (S3 Fig).

A value < 7.7 mmHg could exclude a decrease of PvaCO2 during the FB, independently of baseline CI (AUC: 0.79, 95%CI 0.64 ‒ 0.93; Sensitivity, 91%; Specificity, 55%; p < 0.01) (S4 Fig). Baseline PvaCO2 was correlated with the changes in PvaCO2 during FB in patients with low as well as in patients with high CI before FB (low CI before FB: r = -0.55, p = 0.02, high CI before FB: r = -0.72, p < 0.01) (Fig 3).

Fig 3

Relationship between baseline PvaCO2 and changes in PvaCO2 (Δ PvaCO2) during fluid bolus.

Panel A: Patients with CI ≤ 2.2 L/min/m2; Panel B: Patients with CI > 2.2 L/min/m2. The vertical dotted line corresponds to the baseline PvaCO2 8mmHg. The horizontal dotted line corresponds to Δ PvaCO2−2 mmHg.

Discussion

The results of this study can be summarized as follows: 1) FB can decrease an abnormal high PvaCO2 in critically ill patients independently of the before FB CI values, 2) the response of PvaCO2 to FB is highly variable, yet low baseline PvaCO2 (6 to 8 mmHg) can exclude a positive response.

The clinical implication of the study is that PvaCO2 derived from central venous and arterial blood gas analysis can be used in clinical practice for the evaluation of FB response (S5 Fig). ‘PvaCO2 responders’ had a significantly higher increase in CI, which confirms that the CI augmentation is implicated in the decrease in PvaCO2 during FB. Of note, in theory, FB can cause an increase in PvaCO2 due to acute decrease in hemoglobin concentration [23]. The results of our study showed that an increase in PvaCO2 is rare after FB. Notwithstanding, given that none of the ‘CI responders’ presented an increase in PvaCO2 can be considered as an adverse effect of FB and it can be used as a safety limit for FB in case no CI monitoring is available.

Interestingly, the group of ‘PvaCO2 responders’ was not exactly the same as ‘CI responders’: several ‘CI responders’ did not have a decrease in PvaCO2, whereas PvaCO2 decreases were not always associated with increases in CI >15%. Similar observations were reported by other teams using various measurements related to tissue perfusion [24-26]. Different factors can explain this phenomenon. Increases in CI after FB might not always lead to an improvement in tissue perfusion [27], particularly when CI is not a major contributing factor for microcirculatory abnormalities. Additionally, in patients with high CI changes in PvaCO2 are expected to be limited as the relationship between these two variables is curvilinear [28]. Of note, we detected a statistically significant correlation of CI changes with PvaCO2 only in the group of patients with low baseline CI. Furthermore, ‘CI responders’ are defined based on relative changes in CI. Accordingly, several patients with increases in CI between 0.4–0.5 L/min/m2 were allocated as ‘CI non-responders’. Moreover, evaluation of changes in CI with the method of cardiac echocardiography might not be precise in detecting mild changes [29].

The results of this study add to our knowledge of the optimization of fluid administration in critically ill patients using PvaCO2 values. Recognition of the severity of inadequate tissue perfusion based on the levels of PvaCO2 can guide the physician to decide fluid administration: a low PvaCO2 can be considered as an indication to avoid FB whereas a high level may not always be an indication for FB evaluation of its effects is required. This finding is in line with the results of previous studies, which showed mild microcirculation abnormalities are less likely to be improved after FB [24]. Nevertheless, some patients with high PvaCO2 failed to respond to FB, and therefore, the decision for FB administration should not be based only on the PvaCO2 levels. Furthermore, whether FB is the more appropriate treatment for the treatment of high PvaCO2 levels compared to other interventions aiming to improve tissue perfusion (e.g dobutamine, nitrate) should be further evaluated in future studies.

PvaCO2 changes were not found to be correlated to ScvO2. The meaning of this finding is dual. First, PvaCO2 changes after FB potentially can provide additional information to ScvO2. As in other studies, PvaCO2 may remain altered when ScvO2 is close to normal, so that PvaCO2 can be used in addition to ScvO2 for evaluating the adequacy of resuscitation in critically ill patients [30-32]. PvaCO2 is related to tissue perfusion independently of the presence of tissue hypoxia [3], whereas ScvO2 reflects the balance between oxygen delivery and oxygen consumption [33]. In the majority of patients, improvement in tissue perfusion (‘PvaCO2 responders’) was associated with an increase in ScvO2. However, increases in ScvO2 occurred in some ‘PvaCO2 non-responders’. As multiple patterns were observed, our study underscores the multiple factors implicated in changes in PvaCO2 and ScvO2 after FB. Second, the absence of correlation between PvaCO2 changes and ScvO2 suggests that the Haldane effect has only a minor impact on the changes of PvaCO2 during fluid bolus. Given that arterial saturation and PCO2 did not change in our cohort increases in ScvO2 secondary to a positive fluid response could cause an increase in venous partial pressure of CO2 and consequently an increase in PvaCO2 [34].

The strength of this study is that we assessed the effect of FB on PvaCO2 in a non-selected critically ill population with abnormal high PvaCO2. The high range of the pre-infusion CI permitted the study of PvaCO2 changes after FB in a diversity of hemodynamic conditions, whereas no respiratory variations or other interventions can explain these changes. Nevertheless, this study has several limitations. First, we assessed only acute changes in PvaCO2 so that we cannot ensure that these beneficial effects were maintained. However, evaluation of PvaCO2 over several hours might be challenging as metabolic changes can also occur, especially in non-sedated patients, in addition to other cardiovascular events. Second, metabolic changes independent of FB may have occurred. However, major spontaneous metabolic changes are not expected to occur during the short observational period of the study. Third, only central venous and not mixed venous-to-arterial carbon dioxide tension differences were evaluated. Fourth, we did not investigate thoroughly the effects of other therapeutic interventions (e.g. mechanical ventilation, inotropes) on PvaCO2 as well as its changes during FB.

Conclusions

Abnormal high PvaCO2 can be decreased with FB independently of the levels of the pre-infusion CI. A decrease in PvaCO2 after FB is unlikely in patients with pre-infusion PvaCO2 below 7.7 mmHg. Increases in PvaCO2 can be considered as an indication of negative response to FB. Decreases in PvaCO2can be considered a positive response to FB, even though they might not always be associated with relative increases in CI >15%. Changes in CI can only partially explain decreases in PvaCO2. PvaCO2 and ScvO2 provide complementary information for the effects of FB on tissue perfusion.

Flowchart of patients selection.

(PDF)

Click here for additional data file.

Relationship between changes in PvaCO2 (Δ PvaCO2) during fluid bolus and absolute changes in velocity time integral (Δ VTI).

Panel A: Patients with CI ≤ 2.2 L/min/m2; Panel B: Patients with CI > 2.2 L/min/m2. d VTI: relative to baseline values changes in VTI. Horizontal dotted line corresponds to Δ PvaCO2−2 mmHg.

(PDF)

Click here for additional data file.

Relationship between changes in central venous oxygen saturation (ScvO2) during fluid bolus and absolute changes in PvaCO2 (Δ PvaCO2).

Panel A: Patients with CI ≤ 2.2 L/min/m2; Panel B: Patients with CI > 2.2 L/min/m2. d CI: relative to baseline values changes in CI. Vertical dotted line corresponds to Δ PvaCO2−2 mmHg.

(PDF)

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ROC curve for baseline values of PvaCO2 for prediction of PvaCO2 decrease during fluid bolus.

(PDF)

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Algorithm of interpretation PvaCO2 in relation to decision and appreciation of fluid bolus.

(PDF)

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Characteristics of the patients received fluid bolus (FB) and included in the study.

(PDF)

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Blood gas analysis derived parameters before and after fluid bolus.

(PDF)

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Univariate logistic regression analysis with positive PvaCO2 decrease > 2mmHg after fluid bolus as the dependent variable.

(PDF)

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5 Jun 2021

PONE-D-21-16244

Changes in venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients

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Reviewer #1: Dear editor, thank you for allowing me to review the manuscript “Changes in venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients” by Pierrakos et al. PavCO2 seems to be an interesting additional tool to evaluate circulatory adequacy and the effect of fluid boluses, and with this study the authors further expand our knowledge of the topic. I have a few remarks though:

MAJOR

2.50 In my view, the mixed venous and arterial CO2 content does not equal the ratio between CO2 production (VCO2) and CI but between CO2 production (VCO2) and CO.

I feel the authors could be more stringent about the terms mixed-venous, central venous and venous, making it clear from the outset that the study deals with central-venous to arterial gradients while the Fick equation applies to mixed venous measurements. Also the fact that mixed venous and central venous gradients are not interchangeable could be mentioned from the start instead of at the end of the manuscript.

The study included both intubated patients and spontaneously breathing patients. Since intubated patients were presumably more heavily sedated, decreasing cerebral oxygen consumption, the difference between CV (reflecting cerebral and upper limb metabolism) and mixed values for O2 and CO2 probably differs for these two groups. Please comment.

5.110 Was the definition of low CI established before data-analysis commenced and what was the source (as mentioned below I am uncertain of whether it is stated by Mecher? Other cut-off values are common but would not have resulted in equally large groups.

5.114 Please rephrase the primary hypothesis more clearly. e.g “FB will decrease PvaCO2 by at least 2 mmHg on average”.

6.133 Please give dates for the study period and describe the targeted number of patients and why in methods.

7.162 I did not understand the additional value of providing data on the correlation between CI and VTI and VTI and PavCO2. Perhaps these could be removed or its importance explained.

7.176 I am not sure the data support statement 3. How else but by changing CO would a FB change PvaCO2? However, as described by Lamia et al (Minerva Anestesiol 2006;72:597-604) with high CO changes in PvaCO2 will be smaller.

10.232 Please expand on this and or rephrase or omit. I agree that changes in CI will probably be equal for the upper and lower body, but I am not sure about the concomitant results on PvaCO2. (see also my comment on 2.50)

10.233 Please rephrase, in its current form it is not a very strong argument.

MINOR

3.63 I could not access the entire article by Mecher but according to the abstract average CI was 2.64.

3.67 I suggest to remove “FB”.

4.78 Please explain the recruitment policy more clearly, also in figure S1. Was consent obtained before or after the attending physician decided to give a FB? Did no one refuse participation?

5.108 I suggest to explain in a little more detail the cut-off value of 2 mmHg (it was the SSD in the study by Mallat).

7.178-180 Please rephrase.

Table 1 and Figures: please be consequent when using “delta” and “d”

Reviewer #2: I thank the editor for giving me the opportunity to review this interesting paper titled “Changes in venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients”.

This is a paper about the effect of fluid bolus on the PVACo2 n critically ill patients with normal or low cardiac output.

I have few suggestions/comments:

1. A critique I have about the results section is the clumping of the crystalloid and colloid fluid bolus together. I will think for a study, you have to standardize the type of the fluid bolus, because 8.4 ml/kg of colloids is clearly different from crystalloids and the effect of this bolus on central venous pressure and cardiac output will be different. I will suggest to separate them out.

2. The paragraph in the results section under secondary outcomes starting in line 159. This result is very confusing when compared to the first result under the same section starting in line 156. This needs to better explained and perhaps figure 2 as well, may be better to use absolute numbers rather Δ changes.

3. Figure 3 needs to be clarified the same as its description in the results section.

4. The assumptions that PVACo2 changes are more pronounced in patients with cardiac index < 2.2 after fluid bolus, how about if this low cardiac index is cardiogenic in nature and that fluid bolus might not be the ideal option, instead, an inotrope might be more appropriate.

5. The patient cohort included only 20 patients with sepsis, what is the disease nature of the remaining 22 critically ill patients, was it of hypovolemic or cardiogenic nature? May be this needs to be clarified in the supplemental section.

6. Perhaps a therapeutic algorithm based on PVACo2 might be helpful to the reader.

**********

6. 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.

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Reviewer #1: No

Reviewer #2: No

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12 Jul 2021

Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. 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 #2: Partly

Answer: We thank the reviewers for the provided comments. We have tried to address their comments below.

________________________________________

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

Reviewer #1: Yes

Reviewer #2: Yes

________________________________________

3. 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 #2: Yes

Answer: All the requested additional data are now presented in main text and in supplementary material. Raw data are available upon publication of the work after reasonable request.

________________________________________

4. 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

________________________________________

5. 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: Dear editor, thank you for allowing me to review the manuscript “Changes in venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients” by Pierrakos et al. PavCO2 seems to be an interesting additional tool to evaluate circulatory adequacy and the effect of fluid boluses, and with this study the authors further expand our knowledge of the topic. I have a few remarks though:

MAJOR

2.50 In my view, the mixed venous and arterial CO2 content does not equal the ratio between CO2 production (VCO2) and CI but between CO2 production (VCO2) and CO.

Answer: We agree with the reviewer and we changed it.

Line 49: “Applying Fick’s formula for CO2 shows that the difference between the mixed venous and arterial CO2 content equals the ratio between CO2 production (VCO2) and cardiac output.”

I feel the authors could be more stringent about the terms mixed-venous, central venous and venous, making it clear from the outset that the study deals with central-venous to arterial gradients while the Fick equation applies to mixed venous measurements. Also the fact that mixed venous and central venous gradients are not interchangeable could be mentioned from the start instead of at the end of the manuscript.

Answer: We agree with the reviewer and we added this to introduction as well as to the title.

Manuscript Title : “Changes in central venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients”

Line 53: “Ideally, venous-to-arterial carbon dioxide tension difference should be derived using pulmonary artery obtained PCO2. Nevertheless, Swan–Ganz catheter is not used often in contemporary intensive care (5). Central venous venous-to-arterial carbon dioxide tension (PvaCO2) even though is not interchangeable to mixed venous (6) can be used instead as a high PvaCO2 (> 6 mmHg) indicates that tissue perfusion is not sufficiently high to remove the CO2 produced by the tissues (7). Of note, persistent abnormal PvaCO2 levels can be related to poor outcome in critically ill patients (8,9)”

The study included both intubated patients and spontaneously breathing patients. Since intubated patients were presumably more heavily sedated, decreasing cerebral oxygen consumption, the difference between CV (reflecting cerebral and upper limb metabolism) and mixed values for O2 and CO2 probably differs for these two groups. Please comment.

Answer: In these study we included a not selected population of critically ill patients in order to avoid any bias. Even though, comparing the PvaCO2 values between invasively not invasively ventilated patients is interesting for the reasons reviewer mentioned, this was not within the scope of this study (We added this to the limitations). However, we compared the baseline values of PvaCO2 between invasively and not invasively ventilated patients we did not find any statistically significant differences. But we think this should be more systematically evaluated.

Line 167: “The median PvaCO2 before FB was 8.7 [ 7.6 −10.9] and did not differ between intubated and not intubated patients (9.2 [7.7−13.5] mmHg versus 8.4 [7.4−10.2] mmHg ; p = 0.23).”

Line 257: “Fourth, we did not investigate thoroughly the effects of other therapeutic interventions (e.g mechanical ventilation, inotropes) on PvaCO2 as well its changes during FB.”

5.110 Was the definition of low CI established before data-analysis commenced and what was the source (as mentioned below I am uncertain of whether it is stated by Mecher? Other cut-off values are common but would not have resulted in equally large groups.

Answer: All the mentioned diagnostic definitions were set before data analysis and were based on Mecher study definitions. Incidentally, SHOCK trial also used 2.2 L/min/m2 cut-off for definition of ‘low’ CI. Nevertheless, we agree with the reviewer that the definition of ‘low’ CI based on cut-off value is not clinical relevant and we used it only for the needs of the study.

Line 114: “All the diagnostic definitions were set beforehand. “

Line 118: “Even though, we acknowledge that the definition of ‘low CI’ is not clinical relevant term (20) for the needs of the study we dichotomized the patients to ‘low’ and ‘high’ cardiac index using as cut-off value 2.2 L/min/m2 similarly to previous study (15)”

5.114 Please rephrase the primary hypothesis more clearly. e.g “FB will decrease PvaCO2 by at least 2 mmHg on average”.

Answer: We rephrased it as suggested.

Line 124: “The primary endpoint was to evaluate whether FB can decrease PvaCO2 by at least 2 mmHg on average.”

6.133 Please give dates for the study period and describe the targeted number of patients and why in methods.

Answer: The study period was added as well as the targeted number .

Line 75 : “In this prospective observational study, we collected data from patients treated in Brugmann University Hospital’s 33-bed intensive care unit in Brussels between January and June 2015.”

Line 142: “Sample size was calculated to aim for an AUC of greater than 0.8, which is usually considered as having a good predictive ability. Assuming a fluid responsiveness rate of 30% in mixed population of critically ill patients (20) 40 patients were required to obtain 90% power (alpha 0.05).”

7.162 I did not understand the additional value of providing data on the correlation between CI and VTI and VTI and PavCO2. Perhaps these could be removed or its importance explained.

Answer: Given that estimation of the area of LVOT represents the major source of error in calculation cardiac output with doppler technique, we performed this additional analysis. We agree with the reviewer that the correlation between CI and VTI do not add much to this study and we eliminated this figure.

Line 183: “Because estimation of the area of LVOT represents the major source of error in calculating cardiac output with transthoracic echocardiography (18) we repeated the analysis using only VTI: similar results were found when PvaCO2 changes were assessed with changes in VTI (S4 Figure)”

7.176 I am not sure the data support statement 3. How else but by changing CO would a FB change PvaCO2? However, as described by Lamia et al (Minerva Anestesiol 2006;72:597-604) with high CO changes in PvaCO2 will be smaller.

Answer: We agree with the reviewer that this statement is confusing and we eliminated it. We added also the curvilinear relationship between CO and PvaCO2 as a factor that can explain the weak correlation between changes in CI and PvaCO2.

Line 215: “Additionally, in patients with high CI changes in PvaCO2 are expected to be limited as the relationship between the relationship between these two variables is curvilinear (26).”

10.232 Please expand on this and or rephrase or omit. I agree that changes in CI will probably be equal for the upper and lower body, but I am not sure about the concomitant results on PvaCO2. (see also my comment on 2.50)

Answer: We agree with the reviewer and we omitted this phrase .

10.233 Please rephrase, in its current form it is not a very strong argument.

Answer: As reviewer suggested in the previous comment we transferred this point to the introduction rephrased.

MINOR

3.63 I could not access the entire article by Mecher but according to the abstract average CI was 2.64.

Answer: In that study CI 2.2L/min/m2 was considered as one of the criteria of hypoperfusion. As we mentioned before this is the reason we used this as cut-off value for defining ‘low’ CI. In the group of patients with high PvaCO2 pre-fluid resuscitation the CI was 2.3 L/min/m2 , compared to 2.6 L/min/m2 the other patients with low PvaCO2 pre-fluid resuscitation. We changed this in the introduction .

Line 66: “Mecher et al. reported that FB decreased high PvaCO2 in patients with septic shock, but the authors enrolled only septic patients with low CI (15).”

3.67 I suggest to remove “FB”.

Answer: Thank you, we removed “FB”.

4.78 Please explain the recruitment policy more clearly, also in figure S1. Was consent obtained before or after the attending physician decided to give a FB? Did no one refuse participation?

Answer: We included patients using a deferred informed consent as fluid bolus was a standard treatment and not invasive methods for monitoring were used. Two patients refused to be included in the study. We included this information in the methods as well as in the figure S1.

Line 82: “We included patients using a deferred informed consent as FB was part of standard treatment and we used not invasive methods for monitoring. Informed consent was obtained from all patients or, when that was not feasible, a consent form was gathered from the next-of-kin as soon as possible after FB but before ICU discharge.”

Line 149: “Two patients refused to give informed consent and were excluded from any further analysis.”

5.108 I suggest to explain in a little more detail the cut-off value of 2 mmHg (it was the SSD in the study by Mallat).

Answer: We added this information in the methods.

Line114: “The smallest detectable difference (SDD) of PvaCO2 was expected to be ±2.06 mm Hg as it was evaluated in previous study in critically ill patients (17). Accordingly, patients were considered as ‘PvaCO2 responders’ if they had a decrease in PvaCO2 > 2 mmHg.”

7.178-180 Please rephrase.

Answer: We thank the reviewer for this point and we rephrased it.

Line 200: “The clinical implication of the study is that PvaCO2 derived from central venous and arterial blood gas analysis can be used in clinical practice for the evaluation of FB response (S8 Figure). ‘PvaCO2 responders’ had a significantly higher increase in CI, which confirms that the increase in CI is implicated in the decrease in PvaCO2 during FB.”

Table 1 and Figures: please be consequent when using “delta” and “d”

Answer: Changes are presented as relative (d, %) and absolute values (Δ). We made the appropriate changes and we explained this in the figure capture .

Reviewer #2: I thank the editor for giving me the opportunity to review this interesting paper titled “Changes in venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients”.

This is a paper about the effect of fluid bolus on the PVACo2 n critically ill patients with normal or low cardiac output.

I have few suggestions/comments:

1. A critique I have about the results section is the clumping of the crystalloid and colloid fluid bolus together. I will think for a study, you have to standardize the type of the fluid bolus, because 8.4 ml/kg of colloids is clearly different from crystalloids and the effect of this bolus on central venous pressure and cardiac output will be different. I will suggest to separate them out.

Answer: We thank the reviewer for the interest to our paper as well as to very constructive criticism to our results. This study was an observational study and the decision for the type of fluids was rested to the attending physician. The design of this study was not made for evaluating the differences between the effects of colloids and crystalloids on PvaCO2. Nevertheless, we further investigated the reviewer’s hypothesis: First we clarified in the results that the amount of fluids that was given was higher for crystalloids compared to crystalloids (14.9 ml/kg [12.1−19.6] vs 6.3 ml/kg [6.3− 7.1]). Second we evaluated the differences in the in the changes of CVP or CI between the patients who treated with crystalloids or colloids and we did not find any differences. Third, we performed univariate analysis that do not show any association between the type of fluids and the PvaCO2 response after FB. Even though we acknowledge it as a limitation of our study we think that this limitation has only minor effect on our results.

Line 154: “The median given volume was 6.3 ml/kg [6.3− 7.1] for FB with colloids and 14.9 ml/kg [12.1−19.6] for FB with crystalloids within a median time 33 min [27− 44].”

Line 157: “No differences were observed in the increases in central venous pressure after FB between the patients who received colloids or crystalloids (33% [30 –73] vs 22 % [9−44], p = 0.07).”

Line 159: “No differences were observed in the changes in CI after FB between the patients who received colloids or crystalloids ( 13% [0−21]vs 12% [2−25], p=0.22).”

Line 175: “Additionally, the type of fluid used for FB were not found to be associated with the likelihood for decreasing PvaCO2 > 2 mmHg after FB (S4 Table).”

Line 257: “ Fourth, we did not investigate thoroughly the association of other therapeutic interventions (e.g mechanical ventilation, inotropes, type of fluids) with PvaCO2 changes during FB.”

2. The paragraph in the results section under secondary outcomes starting in line 159. This result is very confusing when compared to the first result under the same section starting in line 156. This needs to better explained and perhaps figure 2 as well, may be better to use absolute numbers rather Δ changes.

Answer: We agree with the reviewer. We eliminated this paragraph and this result. Instead we added univariate analysis to demonstrate the absence of any association between the presence of ‘low’ or “high” pre-infusion CI and the possibility of decrease of PvaCO2 > 2mmHg. Furthermore , in figure 2 we present changes in absolute numbers (not relative,%).

Line 174: “There was no association between the decrease of PvaCO2 > 2 mmHg after FB and the pre-infusion levels of CI (i.e. ‘low’ or ‘high CI’).”

3. Figure 3 needs to be clarified the same as its description in the results section.

Answer: Slightly rephrased the description in result section the Figure 3

Line 196: Baseline PvaCO2 was correlated with the changes in PvaCO2 during FB in patients with low as well as in patients with high CI before FB (low CI before FB: r = -0.55, p = 0.02, high CI before FB: r = -0.72, p < 0.01) (Figure 3)

4. The assumptions that PVACo2 changes are more pronounced in patients with cardiac index < 2.2 after fluid bolus, how about if this low cardiac index is cardiogenic in nature and that fluid bolus might not be the ideal option, instead, an inotrope might be more appropriate.

Answer: We agree with the reviewer that baseline PvaCO2 should not be the only parameter to decide giving FB. We mention it in the line 232: “Nevertheless, some patients with high PvaCO2 failed to respond to FB, and therefore, the decision for FB administration should not be based only on the PvaCO2 levels.”. Based on our results we do not suggest the PvaCO2 are more pronounced in patients with cardiac index <2.2 L/min/m2. Our main finding is that changes in PvaCO2 are independent to the baseline CI. Nevertheless, we agree with the reviewer that whether FB is most optimal treatment option for high PvaCO2 should be further evaluated.

Line 229: “Furthermore, whether FB is the more appropriate treatment compared to other interventions aiming to improve tissue perfusion (e.g dobutamine, nitrate) for the treatment high PvaCO2 levels should be further evaluated in future studies.”

5. The patient cohort included only 20 patients with sepsis, what is the disease nature of the remaining 22 critically ill patients, was it of hypovolemic or cardiogenic nature? May be this needs to be clarified in the supplemental section.

Answer: In seven patient the disease was cardiogenic nature. The others was clinical suspicion of hypovolemia. We added this information in S2 Table.

6. Perhaps a therapeutic algorithm based on PVACo2 might be helpful to the reader.

Answer: We added an algorithm in the supplement material.

________________________________________

6. 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

Submitted filename: PLOs_reviewers_120721_v2.docx

Click here for additional data file.

27 Jul 2021

PONE-D-21-16244R1

Changes in central venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients

PLOS ONE

Dear Dr. Pierrakos,

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.

==============================

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Vincenzo Lionetti, M.D., PhD

Academic Editor

PLOS ONE

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

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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 #2: 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 #2: Yes

**********

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

Reviewer #1: Yes

Reviewer #2: (No Response)

**********

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 #2: No

**********

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

**********

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: Dear authors,

Thank you for your well-performed revision. I highly appreciated your efforts and feel that the manuscript has improved significantly. I only have a few minor comments left, most of them related to editing.

2.118 I agree with the content of the new sentence however the English could be improved.

11.257 The new line in the manuscript differs from the line given in the answers to the reviewers: I prefer the latter “Fourth, we did not investigate thoroughly the effects of other therapeutic interventions (e.g. mechanical ventilation, inotropes) on PvaCO2 as well AS its changes during FB.”

This makes it clear that some conditions impacting on the correlation between central and mixed venous values might not only influence the effect of FB but the whole concept of PvaCO2.

Some content appears to be missing (Figure 3 and S3, S5 and S7)

S8 Figure: I liked this tentative algorithm. However some ambiguity is introduced when using “<” and/or “>” in cases of negative changes (in this case in PvaCO2). Is a change of “-2” “<” or “>” compared to a change of “-5”? In the algorithm I would use “> -2 mmHg” but opinions probably differ on this. Perhaps words could be used instead.

Reviewer #2: I think the authors addressed all the questions raised during the review process adequately.

I think the results section of the abstract needs to be rephrased to reflect the clear findings presented in the first paragraph of the discussion section. The abstract as it is now is still a little confusing.

Also, I am not sure if the authors included S3 table, S5 figure, S7. I am not able to see them in the attached manuscript.

**********

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 #2: 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.]

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20 Aug 2021

Journal Requirements:

Please review your reference list to ensure that it is complete and correct. If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice.

Answer: We reviewed our reference list and we can confirm that it is complete and correct. None of the included reference was noticed as “retracted”.

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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 #2: 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 #2: Yes

________________________________________

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

Reviewer #1: Yes

Reviewer #2: (No Response)

________________________________________

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 #2: No

________________________________________

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

________________________________________

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: Dear authors,

Thank you for your well-performed revision. I highly appreciated your efforts and feel that the manuscript has improved significantly. I only have a few minor comments left, most of them related to editing.

2.118 I agree with the content of the new sentence however the English could be improved.

Answer: We improved the language of this sentence and we hope that now it is more appropriate.

Line 112: As changes in PvaCO2 may be affected by baseline value and as PvaCO2 is inversely related to cardiac index, we separated patients into ‘low’ and ‘high’ cardiac index using a cut-off value of 2.2 L/min/m2, similarly to a previous study (15). Of note, the term ‘low CI’ should not be misinterpreted as in some case the low CI may still be adequate (20).’

11.257 The new line in the manuscript differs from the line given in the answers to the reviewers: I prefer the latter “Fourth, we did not investigate thoroughly the effects of other therapeutic interventions (e.g. mechanical ventilation, inotropes) on PvaCO2 as well AS its changes during FB.”

This makes it clear that some conditions impacting on the correlation between central and mixed venous values might not only influence the effect of FB but the whole concept of PvaCO2.

Answer: We agree with the reviewer and we changed this phrase to the text.

Some content appears to be missing (Figure 3 and S3, S5 and S7)

Answer: Our apologies for this inconvenience. We think that these contents are visible now.

S8 Figure: I liked this tentative algorithm. However some ambiguity is introduced when using “<” and/or “>” in cases of negative changes (in this case in PvaCO2). Is a change of “-2” “<” or “>” compared to a change of “-5”? In the algorithm I would use “> -2 mmHg” but opinions probably differ on this. Perhaps words could be used instead.

Answer: We thank the reviewer for the appreciation of the suggested algorithm and we agree that the mathematical expressions can cause misunderstandings. We replaced them with words.

Reviewer #2: I think the authors addressed all the questions raised during the review process adequately.

I think the results section of the abstract needs to be rephrased to reflect the clear findings presented in the first paragraph of the discussion section. The abstract as it is now is still a little confusing.

Answer: We rephrased the results section of the abstract. We hope now it is easier to be read.

Also, I am not sure if the authors included S3 table, S5 figure, S7. I am not able to see them in the attached manuscript.

Answer: Our apologies for this inconvenience. We think that these contents are visible now.

________________________________________

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Reviewer #1: No

Reviewer #2: No

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Submitted filename: Reviewers_comments_18821_f.docx

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31 Aug 2021

Changes in central venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients

PONE-D-21-16244R2

Dear Dr. Pierrakos,

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Reviewer #1: All comments have been addressed

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Reviewer #1: (No Response)

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Reviewer #1: (No Response)

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Reviewer #1: No

3 Sep 2021

PONE-D-21-16244R2

Changes in central venous-to-arterial carbon dioxide tension induced by fluid bolus in critically ill patients

Dear Dr. Pierrakos:

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.

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