Effect of Change of Heart Position During Handstand in Water

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

ABSTRACT
　Exposure to microgravity have been reported to affect the central circulation.　Astronauts experience space motion sickness because the fluid shift causes the increase of blood volume in the heart and brain to increase.　This suggests that the change in the position of a visceral organ can be a trigger for stress.　It was reported that the heart position changed to the left during spontaneous breathing （exhalation）, turned left, and changed to the left during deep breathing （exhalation）.　From this, during handstand, we hypothesized that the diaphragm rose to the head direction, and the heart would move toward the head due to gravity.　This study aimed to compare the changes in heart position during a handstand between in water and on land.　Nine healthy young males with no history of heart disease volunteered to participate in this study.　We measured heart position determined using echocardiography, heart rate （excepting hand-standing in water）, blood pressure （standing condition only）.　The conditions in this study are as follows;Standing-on land, Standing-immersion, Handstand on land, Handstand- immersion （hereinafter referred “S-L condition,” “S-W condition,” “H-L condition,” and “H-W condition”）.　Heart position changed significantly by 0.4 ± 1.0 mm to the right under H-W condition compared to the S-W condition （P ＜ 0.05）.　Heart position changed significantly to the right under H-W condition compared to the H-L condition （P ＜ 0.05）.　The apex of the heart entered the left lung side because of the expanding thorax due to water pressure, and relaxing ligaments connected to the diaphragm.　It was reported that the heart position changed to the left during spontaneous exhalation, and heart turned left during deep exhalation.　At handstand, water pressure on the head is greater.　It can be said that the abdominal cavity easily expands but is difficult to return compared to the chest wall because of diaphragm downs.　Therefore, the apex of the heart changed to the right side on the H-W conditions.　The diaphragm rises, and the distribution of blood is increased by the heart, causing the base of the heart to tilt to the left.　As a result, the heart's position changes to the left during inhalation under S-W condition.　The Aorta valve was observed along the short axis during spontaneous breathing （exhalation） under the S-W condition.　This is thought that affected by water pressure to thorax, rising of diaphragm is inhibited in water compared to land, furthermore, diaphragm downs during inhalation.　The main finding of this study is that the heart position changed significantly to the left on handstand in water condition compared to handstand under land condition.

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

　Key words:Ultrasonic diagnostic equipment, Heart position, Handstand, Breathing, Water

I.　Introduction
　Space is a different environment from Earth because of the microgravity, radiation, and closed environment, affecting astronauts during space missions.　Exposure to microgravity have been reported to affect the central circulation^9）.　Astronauts experience space motion sickness because the fluid shift causes increase of blood volume in the heart and brain, and because microgravity affects semicircular canals^5,13）.
　This suggests that the change in the position of a visceral organ can be a trigger for stress.
　It was reported that water immersion at the level of the xiphisternum closely simulates a microgravity environment^5）.　The diaphragm and intercostal muscles are mainly used for respiration.　In water, we do abdominal breathing affected by water pressure.　Therefore, the diaphragm mainly contributes to breathing.　During standing in water, the diaphragm rise was affected by water pressure.　It was reported that the heart position changed to the left during spontaneous exhalation and heart turned left during deep breathing exhalation^13）.　From this, during handstand, we hypothesized that the diaphragm rose to the head direction, and the heart would move toward the head due to gravity.　This study aimed to compare the changes in heart position during a handstand and to gain insight into the mechanism of physiological changes that occur in astronauts under microgravity.

II.　Methods
　Nine healthy young males with no history of heart disease （age:21.8 ±1.0 years, height:170.7 ± 5.1 cm, weight:66.7 ± 7.2 kg, % body fat:17.6 ± 5.6%, mean ± SD） volunteered to participate in this study.　The study parameters were as follows:room temperature at 27.9 ± 0.5℃ and humidity at 85.4 ± 4.0%.　All study protocols were approved by the ethics board at Kawasaki University of Medical Welfare and conformed to the Declaration of Helsinki （20-067）.　Experiments were conducted at the same time on different days for conditions in water and on land.　Each subject was asked to stand and perform a handstand on land and during immersion.　We used the tank（10L Metallicon tank;19.6 Mps, 188φ, K2valve, Japan Aqualung Co., Ltd）for the dives with scuba diving.　It was used to secure breathing during handstand in water.　The conditions in this study are as follows;Standing-on land, Standing-immersion, Handstand on land, Handstand- immersion （hereinafter referred “S-L condition,” “S-W condition,” “H-L condition,” and “H-W condition”）.　Water level was set to clavicle level.
　We measured heart position, heart rate （HR）（excepting hand-standing in water）, blood pressure （standing condition only）.　The change in heart position was measured, with the mitral valve during breath-holding while doing a handstand on land as the starting point and as a control.　We measured heart position using ultrasonic diagnostic equipment （SonoSite M-Turbo;FUJIFILM）.　We measured HR using waterproof electrocardiography （FUKUDA DENSHI Co., Ltd., DS-2202, Japan） with chest bipolar induction.　We measured blood pressure using an aneroid sphygmomanometer （501;KENZMEDICO）.
　Data were analyzed using SPSS ver. 23.0 for MAC and using Paired-samples t-test.　We set p < 0.05 as the threshold for statistical significance.

III.　Results
　Table 1 shows the change in heart position at various conditions.　It was reported that the heart position changed to the left during spontaneous exhalation and heart turned left during deep breathing exhalation^14）.
　During breath-holding, the heart position changed significantly to the left under S-W condition, and to the right under H-L condition（P < 0.05）.　Heart position changed significantly to the right under H-W condition （P < 0.05）.　Heart position changed significantly to the right under H-W condition （P < 0.05）.　During spontaneous respiration （inhalation）, the heart position changed to the right under S-L condition.　The results of changes in the heart position were the same for the S-L condition and the S-W condition.　The aortic valve was clearly observed along the short axis under the H-W condition.　During spontaneous respiration （exhalation）, the heart position changed to the left under S-L condition.　The heart position changed to the left under S-W condition.　The mitral valve was clearly observed along the long axis during the H-L and the H-W condition.　During deep breathing （inhalation）, heart position changed to the right under S-L condition.　 The heart position changed significantly to the left under S-W condition （P < 0.05）.　The heart position changed significantly to the left under H-L condition （P < 0.05）.　 The heart position changed to the right under H-W condition.　The heart position changed to the left under H-W condition.　During deep breathing （exhalation）, the heart position changed to the left under the S-L condition.　The mitral valve was clearly observed along the long axis under the S-W, H-L, and H-W conditions.
　Figure 2 shows heart rate at various conditions.　HR is decreased during a handstand from a standing^8）.　Because venous return is increased during a handstand affected by gravity^1）.
　During breath-holding, the HR was significantly decreased under the S-W and H-L conditions （p < 0.05）.　During spontaneous respiration, HR was significantly decreased under H-L condition （p< 0.05）.　During deep breathing, HR was significantly decreased under H-L condition （p < 0.05）.
　Figure 3 shows changes in blood pressure.　we considered that systolic blood pressure was decreased.　When floating in microgravity, the human body causes a fluid shift of blood.
　The systolic blood pressure （SBP） significantly decreased from 108.7 ± 5.7 mmHg to 103.8 ± 6.5 mmHg （p<0.05） in water.　Also, the diastolic blood pressure （DBP） decreased from 63.1 ± 3.6 mmHg to 58.4 ± 9.2 mmHg in water.

Table 1　Amount of changes in heart position at various conditions（mm） S-L , Standing-o n land;S -W , Standing-i mmersion;H -L , Handstand on land;H -W;H andstand- immersion
Breathing	Amount of changes in heart position （mm）
Breathing	S-L	S-W	H-L	H-W
Breath-h olding	control	0.8 ± 0.5*	－0.1 ± 1.0	－0.4 ± 1.0^†‡
Spontaneous respiration （inhalation）	－0.4 ± 1.1	－0.4 ± 1.1	－0.1 ± 1.6	Aortic valve （short axis）
Spontaneous respiration （exhalation）	0.2 ± 1.3	0.3 ± 1.1	Mitral valve （long axis）	Mitral valve （long axis）
Deep breathing （inhalation）	－0 .9 ± 1.7	0.9 ± 1.3*	－0.6 ± 1.3	0.5 ± 1.7
Deep breathing （exhalation）	－0.4 ± 1.6	Mitral valve （long axis）	Mitral valve （long axis）	Mitral valve （long axis）
（mean ± SD） *:p <0.05 （vs. S-L） ^†:p <0.05 （vs. H-L） ^‡:p <0.05 （vs. S-W）


Figure 2　Heart rate at various conditions （bpm） S-L, Standing-on land ; S-W, Standing-immersion ; H-L, Handstand on land	Figure 3.　Blood pressure at various conditions （mmHg） SBP, Systolic blood pressure ; DBP, diastolic blood pressure ; L, On land ; W, immersion

IV.　Discussion
　The main finding of this study is that the heart position changed significantly to the left on handstand in water condition compared to handstand under land condition.　The hypothesis was verified.　The heart position changed to the left under the S-W condition.　This result was the same as in the previous study^15）.　The ribs deflected to the anterior superior^8） because the thorax was affected by water pressure.　Then, increasing gastric pressure^10） as the abdominal organs moved upward^8） resulted in diaphragm rise^8）.　Anatomically, the upper sternopericardial and sternopericardial ligaments are connected to the pericardium covering the heart through the sternum.　The diaphragm is connected to the pericardium through the pericardiacophrenic ligaments.　The left lung is slightly smaller than the right lung^2） because the apex of the heart is located on the left lung side.　Therefore, the apex of the heart entered the left lung side because of the expanding thorax due to water pressure, and relaxing ligaments connected to the diaphragm.
　During a handstand, we speculated that the diaphragm shifts cranially with the effect of gravity^12）, with the abdominal viscera moving toward the head^7）.　Therefore, the functional residual capacity decreases, increasing negative pressure in the chest cavity.　The deeper the water is, the greater the water pressure.　At a standing posture, water pressure on the lower legs is greater.　However, at handstand, water pressure on the head is greater.　That is, during a handstand while immersed in water, the water pressure exerts a greater effect on the chest cavity compared to the abdominal cavity.
　It can be said that the abdominal cavity easily expands but is difficult to return compared to the chest wall because of diaphragm downs.　Therefore, the apex of the heart changed to the right side on the H-W conditions.
　The diaphragm, which is the respiration muscle, moves downward during inhalation and rises during exhalation^3）.　The inferior vena cava hiatus of the diaphragm opens during inhalation, allowing blood flow to the heart^4）.　The diaphragm rises because water pressure affects the thoracic on S-W condition^8）.
　From this, the diaphragm rises, and the distribution of blood is increased in the heart, causing the base of the heart to tilt to the left.　As a result, the heart's position changes to the left during inhalation under S-W condition.
　The mitral valve was observed along the long axis at handstand posture.　The diaphragm moves in the cranial direction^8）, and blood volume to the heart increases owing to venous return increases^1）.　Therefore, the apex of the heart comes near front of the lung.　the left lung.　The Aortic valve was observed along the short axis during spontaneous breathing （exhalation） under the S-W condition.　This is thought that affected by water pressure to thorax, rising of diaphragm is inhibited in water compared to land, furthermore, diaphragm downs during inhalation.
　Water pressure causes a venous return increase, and HR is decreased due to increased stroke volume in water^11）.　We considered that water pressure caused HR decrease under S-W condition.
　HR is decreased during a handstand from a standing^9）.　Because venous return is increased during a handstand affected by gravity^1）, similar results were obtained in this study.　However, the left ventricular ejection fraction and systolic average rate of the left ventricular emptying （dV/dt） are decreased during a handstand compared to when standing^3）.　On this basis, we expect the cardiac output to decrease while HR increases.　It is likely that the handstand position is perceived as stressful, causing the vagus nerve reflex to stimulate the parasympathetic nervous system in the cardiovascular center, eventually decreasing HR as a result.
　As the water level rises, venous return increases and peripheral vascular resistance decreases^10）.　Therefore, we considered that systolic blood pressure was decreased.　When floating in microgravity, the human body causes a fluid shift of blood.　We considered that this fluid shift in microgravity affects the heart.　
　This study was conducted in water, a simulated microgravity environment.　Therefore, there is a difference in pressure and gravity effects between microgravity and underwater.　However, the results of this study suggest that changes in heart position are affected by gravity.　Thus, we considered that the heart is in a different position than on the ground simply because it is under microgravity.
　We showed the limitations of this study;the impossibility of measuring blood pressure, ;uses a cylinder to acquire breathing, so it does not necessarily match the conditions under natural breathing, ;the influence of water temperature cannot be excluded, it is not possible to create conditions that separate water pressure and air pressure, ;inability to create conditions that separate buoyancy and gravity.

V.　Conclusion
　The heart demonstrated the following positional changes:
　1.　During breath-holding, the heart position changed significantly to the left on standing under water condition compared to standing under land condition.
　2.　During breath-holding, the heart position changed significantly to the right on handstand in water condition compared to standing under water condition.
　3.　During breath-holding, the heart position changed significantly to the left on handstand in water condition compared to handstand under land condition.
　4.　During deep breathing （inhalation）, the heart position changed significantly to the left on standing in water condition compared to standing under land condition.

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.

Corresponding author:Sho Onodera
　　　　　　　　　　　　288 Matsushima, Kurashiki, Okayama 701-0193, Japan
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