宇宙航空環境医学 Vol. 60, No. 2, 79-84, 2023

Original

Effect of Standing Posture in Water on Ventilation Efficiency

Yuka Ukida^†,1, Takuma Wada², Sho Onodera³

¹Graduate School of Human-environment Studies, Kyushu University
²Tottori College
³Kawasaki University of Medical Welfare
^†Deceased June 22, 2022

ABSTRACT
　This study aimed to determine the effect of standing posture in water on ventilation efficiency.　Eight healthy young males volunteered to participate in this study.　Each subject was stood on land at various water levels for 5 minutes （i.e., knee, greater trochanter, navel, xiphisternum, and clavicle）.　The temperature of water was maintained at a thermoneutral level throughout the study （34℃-35℃）.　Heart rate （HR）, tidal volume （VT）, minute volume （MV）, oxygen consumption （VO₂）, respiratory rate （RR）, and ventilatory equivalents for oxygen （VE/VO₂） were evaluated.　HR decreased at the water level up to the greater trochanter, navel, xiphisternum, and clavicle （p < 0.05）.　RR decreased at the water level of the xiphisternum and clavicle （p < 0.05）.　VE/VO₂ decreased at the water level of the greater trochanter, navel, xiphisternum, and clavicle （p < 0.05）.Taking these findings together, we suggest that aquatic exercise （immersion combined with exercise） helps to increase VE/VO₂ and improve ventilatory efficiency.

　（Received:24 March, 2022　Accepted:22 July, 2023）

　Key words:Ventilation efficiency, Water, Standing

I.　Introduction
　During immersion, humans are affected by the physical properties of water, such as buoyancy, water pressure, water temperature, and viscosity.　They exhibit physiological responses that are different from those observed on land^11-13）.
　Previous studies investigated pulmonary functions, including lung capacity, as well as residual volume^2,3,7） and blood distribution during water immersion^4）.　The relationship between water level and venous return^12）, stroke volume^12）, blood pressure^12）, and inferior vena cava cross-sectional area^12） has been reported.　It was found that the chest is compressed and the chest circumstance slightly reduced as the water level rises.　The transition from chest to abdominal breathing occurs, affecting ventilatory volume and efficiency.　This study aimed to determine the effect of standing posture in water on ventilation efficiency.

II.　Methods
　Eight healthy young males （age:21.9 ± 1 years;height:171.6 ± 4.8 cm;weight:66.3 ± 7.6 kg;body fat percentage ［InBody Co., Ltd., InBody770, Japan］:17.6% ± 5.2%;mean ± standard deviation） volunteered to participate in this study.　The study was set at a room temperature of 20.2℃ ± 3.7℃ and a humidity level of 80.6% ± 9.9%.　All study protocols were approved by the ethics committee at the Kawasaki University of Medical Welfare （approval code number:20-067） and adhered to the Helsinki Declaration.
　Each subject was stood on land at various water levels for 5 minutes （i.e., knee, greater trochanter, navel, xiphisternum, and clavicle）.　The temperature of water was maintained at a thermoneutral level throughout the study （34℃-35℃）^10）.
　Heart rate （HR）, tidal volume （VT）, minute volume （MV）, oxygen consumption （O₂）, respiratory rate （RR）, and ventilatory equivalents for oxygen （E/O₂） were evaluated.　The study protocol is depicted in Figure 1.
　Waterproof electrocardiography （Fukuda Denshi Co., Ltd., DS-2202, Japan） with chest bipolar induction was used to measure the HR.　A Douglas bag was used to collect expired gases and a certified dry gas meter, DC-5 （Shinagawa Corporation, Japan）, was used to measure the volume.　A mass spectrometer, the ARCO-2000 （ARCO system, Japan）, was used to analyze gas fractions that had been calibrated and confirmed before each test.
　The data were analyzed in the Statistical Package for the Social Sciences software version 23.0 for MAC using one-way repeated-measures analysis of variance.　Subsequently, Dunnett’s post hoc test was performed to identify any interactions.　A p-value of less than 0.05 was considered the threshold for statistical significance.

Figure 1.　Protocol
HR, Heart rate ; RR, Respiratory rate ; VT, Tidal volume

III.　Results
　Figure 2 shows the HR at various water levels.　HR decreased at the water level up to the greater trochanter, navel, xiphisternum, and clavicle （p < 0.05）.
　Figure 3 shows the VT at various water levels.　No change was found in VT at various water levels compared to land.
　Figure 4 shows no change in MV at various water levels compared to land.
　Figure 5 shows no change in O₂ at various water levels compared to land.
　Table 1 shows the RR at various water levels.　It was found that RR decreased at the water level of the xiphisternum and clavicle （p < 0.05）.
　Table 2 shows E/O₂ decreased at the water level of the greater trochanter, navel, xiphisternum, and clavicle （p < 0.05）.

Figure 2.　HR at various water levels	Figure 3.　VT at various water levels
Figure 4.　MV at various water levels	Figure 5.　VO2 at various water levels

Table 2.　E/O2 at various water levels E/O2, Ventilatory equivalents for oxygen
Ventilatory equivalents for oxygen
On land	Knee level	Greater trochanter level	Navel level	Xiphisternum level	Clavicle level
36.4±7.1	33.6±5.4	32.3±2.8*	31.7±3.9*	32.6±5.3	31.1±4.6*
（mean±SD） *p<0.05 （vs. On land）

IV.　Discussion
　The remarkable finding of this study was the improvement of E/O₂ above the water level of greater trochanter.　It was reported that the inspiratory and expiratory reserve volumes decreased when the water level reached the shoulder, and decreased lung functions, especially lung capacity, were seen when the water level reached the clavicle^2,3,7）.　The chest wall is affected by water pressure and acquires more negative pressure, and the inferior vena cava hiatus expands.　With the rise in water level, the blood distribution in the thoracic cavity expands^2,3,15）, the central venous pressure rises^15）, and pulmonary congestion occurs.　This results in increase of pulmonary vascular resistance, closing the peripheral airways in the lungs and increasing the closing volume^3）.　However, abdominal breathing was maintained in water, and O₂ level did not vary.
　It was found that E/O₂ dropped and ventilatory efficiency improved above the water level of greater trochanter.　Abdominal breathing in is affected by water pressure.　The diaphragm is mostly used in abdominal breathing.　Abdominal breathing （diaphragmatic breathing） improves E/O₂ as compared to spontaneous breathing^5）.　This is because abdominal breathing increases VT^5）.
　Abdominal breathing in water decreases the inspiratory and expiratory reserve volumes and lung functions, especially lung capacity.　Therefore, VT was reduced in water as compared with that on land.　However, if we breathe abdominally in water, E/O₂ is decreased and VT is increased.　Abdominal breathing decreases E/O₂ both in water as well as on land.　As a result, abdominal breathing does not change VT on land and in water.
　We breathe abdominally in water because of the water pressure.　Buoyancy causes the parasympathetic nervous system to dominate and relaxation is maintained^11）.　Therefore, we assumed that RR was increased because parasympathetic nerves were dominating.　The central venous pressure rises with the rise in water level^15）.　This change is decrease in FRC（functional residual capacity;sum of expiratory reserve volume and residual volume） and vital capacity （VC）.　RR is increased as VC is reduced.　In this study, RR decreased in water immersion, of which result was different in the previous study^1）.　The research was a study on women.　VC in women decreases 10%-12% compared with men of same height age^1）.　VT was seen to be decreased in water immersion up to the neck^16）.　While MV did not change because of increase in RR^16）.　Both VT and MV did not change in this study.　It was considered that the rate of decline in lung function in water immersion was lower in men than in women.　Because, in women, the radial rib cage dimension is smaller^1） and the diaphragm length is shorter^1） when compared with same height men.
　Water pressure increases venous return^12）, which increases stroke volume and decreases HR^12）.　As water level increases while standing, venous return increases and the amount depends on the water level at standing immersion^12）.　It was reported that HR decreased as water level increased^14）.　HR increased as water level was reduced.　However, it was assumed that not only water pressure but also automatic nervous system affects changes in HR.　Because, changes in HR in water immersion differ between increasing and decreasing water levels.
　This study says standing immersion alone improves ventilatory efficiency.　Continuous exercise lowers E/O₂^6,9）, and exercise that uses abdominal breathing also lowers E/O₂^18）.　With these findings, we suggest that aquatic exercise （immersion combined with exercise） helps to improve ventilatory efficiency.　When a human is immersed, parasympathetic nerves dominate and HR and RR fall^8,12） while stroke volume increases because of increased venous return^12）.
　As the water level rises above the xiphisternum, lung function decreases^2,3,7） and the heart position changes^17）.　Water pressure affects the chest wall during thoracic immersion, which is expected to maintain and improve respiratory muscles.
　Based on the above, training in an underwater environment is expected to increase E/O₂ and improve ventilation efficiency.

V.　Conclusion
　Above the water level of greater trochanter, E/O₂ dropped, and ventilatory efficiency improved.

VI.　Conflict of Interest
　The authors declare that there is no conflict of interest regarding the publication of this article.

Acknowledgments
　We thank Editor-in-chief Dr. Masanori Fujita （Professor of National Defense Medical College） and the editorial board for their special consideration for the publication of this paper.
　We would like to thank Associate Professors Yasuko Ishimoto （Kawasaki University of Medical Welfare）, Assistant Professor Yasuo Ishida （Okayama University of Science） and Director Giho So （Nippon Barance Posturist Federation） for their help in revising the paper.

REFERENCES

Corresponding author:Sho Onodera
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