Volume 104, Issue 5 p. 781-782
RESPONSE TO LETTER
Free Access

Interpreting the impact of water drinking on arterial baroreflex function: When physiology speaks for itself

Lauro C. Vianna

Corresponding Author

Lauro C. Vianna

NeuroVASQ – Integrative Physiology Laboratory, Faculty of Physical Education, University of Brasília, Brasília, DF, Brazil

Correspondence

Lauro C. Vianna, NeuroVASQ – Integrative Physiology Laboratory, Faculty of Physical Education University of Brasília, Darcy Ribeiro Campus, Brasília, DF, Brazil.

Email: lcvianna@unb.br

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Igor A. Fernandes

Igor A. Fernandes

NeuroVASQ – Integrative Physiology Laboratory, Faculty of Physical Education, University of Brasília, Brasília, DF, Brazil

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Daniel G. Martinez

Daniel G. Martinez

Research Unit for Cardiovascular and Exercise Physiology, Faculty of Physical Education and Sports, Federal University of Juiz de Fora, Juiz de Fora, MG, Brazil

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André L. Teixeira

André L. Teixeira

NeuroVASQ – Integrative Physiology Laboratory, Faculty of Physical Education, University of Brasília, Brasília, DF, Brazil

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Bruno M. Silva

Bruno M. Silva

Department of Physiology, Federal University of Sao Paulo (UNIFESP), Sao Paulo, SP, Brazil

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Paul J. Fadel

Paul J. Fadel

Department of Kinesiology, University of Texas at Arlington, Arlington, TX, USA

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Antonio C. L. Nóbrega

Antonio C. L. Nóbrega

Department of Physiology and Pharmacology, Fluminense Federal University, Niterói, RJ, Brazil

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First published: 29 April 2019

Linked articles: This is a reply to a Letter to the Editor by Papaioannou et al. To read the Letter to the Editor, visit http://doi.org/10.1113/EP087298.

We are happy that our study (Vianna et al., 2018) has received the attention of others and pleased to have the opportunity to respond and debate the merits of our approach. In their Letter to the Editor, Papaioannou et al. (2019) raise some interesting points regarding the methods of our study and how they relate to the estimation of cardiac baroreflex sensitivity, which we would like to address and provide further clarification.

We are aware and in agreement with the statement by Dr Papaioannou and colleagues that body posture is a critical factor when assessing neural cardiovascular function at rest and during perturbational conditions. In this regard, our group has recently demonstrated that both cardiac and peripheral sympathetic outflow to postexercise ischemia (i.e. muscle metaboreflex activation) can be modulated by mobilization of central blood volume (Katayama et al., 2018; Teixeira et al., 2018). Whether the arterial baroreflex responses to water drinking are dependent on the body posture is unknown. However, this was not part of our study question. As we have clearly stated in the Methods section, subjects were studied in a supine position, which is a standard position for human neural cardiovascular control studies and the majority of studies on water drinking. It is important to clarify further that subjects ingested the 500 ml of water or saline in this position using a flexible straw; therefore, there was no positional change. This approach also minimizes limb movement and muscle activation, which would jeopardize the muscle sympathetic nerve activity recordings.

In our study, mean arterial pressure remained unchanged after ingestion of either bottled water or saline. Of note, this is a common finding in young healthy subjects (May & Jordan, 2011). With regard to mean arterial pressure calculations, Dr Papaioannou and colleagues commented on the importance of describing how mean arterial pressure was derived, because the use of different formulae might limit the extrapolation and interpretation of our findings. With all due respect, we disagree, and we would like to provide the following points for consideration. Firstly, blood pressure was measured on a beat-to-beat basis using a valid and reliable technique (i.e. finger photoplethysmography). In this sense, mean arterial pressure was calculated by a time-weighted integral of the instantaneous pressures derived from the area under the curve of the pressure–time waveform of the entire cardiac cycle. Secondly, our interpretation has emerged from a study where the subjects participated in both experimental (water drinking) and control (saline drinking) trials, and we used the same blood pressure measurement before, during and after each intervention. Therefore, we are confident that our results and interpretations are valid and reliable and would stand regardless of the way in which mean arterial pressure is calculated.

In their letter, Dr Papaioannou and colleagues also pointed out the lack of standardization in the literature with regard to the use of the sequence technique for the estimation of cardiac baroreflex sensitivity. We agree that there are no absolute or agreed upon ideal parameters for analysis (i.e. systolic pressure and R–R interval thresholds). However, in terms of their concern regarding the R–R interval threshold used in our study, we re-analysed our data using a threshold of 4 ms, as suggested in their letter, and the results remained unchanged. Furthermore, the assessment of agreement between 1 and 4 ms threshold provided a high and significant intraclass correlation coefficient (0.938; P < 0.05). Lastly, Papaioannou et al. (2019) advocate the use of frequency domain analyses to address the research question further. It is important to mention that our hypothesis was focused on the sympathetic branch of arterial baroreflex function, for which we had direct muscle sympathetic nerve activity measurements. Although we would welcome the use of frequency analyses to explore cardiac baroreflex sensitivity further, we are confident in our findings using the sequence technique.

In summary, our data provide new insights on the potential contribution of the sympathetic baroreflex in the maintenance of arterial blood pressure after water drinking in young healthy subjects. Furthermore, independently of the mathematical approach, our findings indicate a minimal role of the cardiac baroreflex in the regulation of arterial blood pressure after water ingestion. Nevertheless, we acknowledge there are no perfect analyses and always welcome debate on different ways to evaluate data, as provided by Dr Papaioannou and colleagues.