Effects of heated water-based exercise on blood pressure: a systematic review

Introduction: Systemic arterial hypertension is one of the main cardiovascular risk factors affecting several population. In this context, heated water-based exercise has emerged as a potential alternative to landbased physical exercise to reduce blood pressure (BP) in hypertensive patients. Objective: To systematically synthesize evidence for the lowering effects of heated water-based exercise on BP in a non-specific population. * AYN: Master’s student, e-mail: awassiyn@gmail.com RMA: Doctoral student, e-mail: raphaelmartins.abreu@yahoo.com.br EGC: PhD, e-mail: ciolac@fc.unesp.br


Search strategy and study selection
PubMed (via National Library of Medicine), Scopus (Elsevier) and SciELO (Scientific Electronic Library Online) databases were searched for articles about the effects of HEx on BP.The following MeSH terms or keywords were used for the intervention type (immersion AND immersion exercise AND hydrotherapy AND aquatic exercise) and outcomes (blood pressure OR hypertension).The reference list of original articles within this field of study was also reviewed to identify potential eligible trials.
In addition, the buoyancy effect during HEx reduces loading, facilitating the performance of individuals unable to perform high-impact dynamic exercises [16].However, there is no consensus in literature on which HEx protocol is more appropriate to promote reductions in BP, since the depth of water immersion, exercise intensity and modality, water temperature, and body position during exercise [21][22][23] can influence the cardiovascular responses.Moreover, although there are systematic reviews and meta-analyses about the acute and chronic effects of LEx on BP [9,11,24,25], none of them have distinguished the effects of HEx.Thus, las sesiones.Conclusión: Estos resultados sugieren que el ejercicio realizado en una piscina calentada durante 12 a 24 semanas de entrenamiento puede promover efectos benéficos sobre la reducción de la PA.Por otra parte, la sesión aguda no es suficiente para causar un efecto hipotensor.Sin embargo, no hay homogeneidad en los protocolos utilizados, lo que puede haber llevado la divergencia en los resultados.

Study and subject characteristics
General characteristics of each study included in the present review are displayed in Table 1.Six studies with sample size ranging from 14 to 20 subjects assessed the acute effect of HEx on BP in physically inactive hypertensive [21,26,28] and normotensive [23,26] individuals, recreationally active normotensive [27], and sedentary heart transplant recipients [6], totaling 105 participants.Two studies included only young individuals [23,27], two studies included only middle-aged individuals [26,28] and one study included both middleaged and older individuals [21].All studies assessing acute effect of HEx on BP had randomized cross-over design with one [6,21,23,27,28] or two groups [26], and assessed the effect of HEx versus LEx session [23,27], control session (CON) [28] or both LEx and CON [6,21,26].One study further compared the BP response to HEx between hypertensive and normotensive subjects [26].
Six RCTs with an initial sample size ranging from 26 to 60 subjects assessed the chronic effect of HEx on BP in normotensive [30], pre-hypertensive [32] and hypertensive [29,31]

individuals, in patients with resistant hypertension
Data extraction and analysis Data on study source, sample size, participants characteristics (i.e.age, sex, baseline BP and physical activity levels, comorbidities), method used to measure BP, characteristics of HEx (i.e.type, frequency, intensity and duration of exercise; water temperature; depth of water immersion), LEx (i.e.type, frequency, intensity and duration of exercise) and control interventions (i.e.type, frequency and duration), outcomes and limitations of the studies included were extracted independently by two authors (A.Y. N. and R. M. A.).The two reviewers also independently assessed methodological quality of included studies using the Physiotherapy Evidence Database (PEDro) scale 1 .When there were disagreements between reviewers on data extraction, data analyses and/or quality assessment, a third reviewer (E.G. C.) was consulted to solve the discrepancy.The results of the systematic review are presented descriptively (e.g.means, standard deviations, and minimum and maximum values).

Results
The electronic database search identified 10,461 articles.A total of 54 duplicate articles were removed, and 103,58 articles were excluded after screening by title and abstract.Full-text screening for eligibility of the remaining 49 articles removed 23 non-randomized clinical trials, five articles published before January 2005, six articles who did not present BP outcomes for Finally, the study quality using PEDro-scale is shown in Table 2.The median PEDro score of studies assessing the acute effect of HEx on BP was 6, with a range from 3 to 8. In the studies assessing the chronic effect of HEx on BP, the median PEDro score was 7, with a range from 5 to 9. [17,18], and in postmenopausal women [33], totaling 292 randomized participants.Six studies included only middleaged subjects [17,18,[30][31][32][33] and one study included only older subjects [29].All chronic studies had parallel design and assessed the effect of HEx versus LEx [30], CON [17,18,31,32] or both LEx and CON [29,33].
Page 05 of 16 assessing the chronic effect of HEx on BP reported lowintensity [31,32] and moderate-intensity [17,18,29,30] exercise sessions, with 35 to 60 min of duration; however, one study did not report exercise session duration [30].The exercise sessions were performed three times a week in six studies [17,18,[29][30][31][32], and progress from two to three times a week in one study [33].The water temperature ranged from 27°C to 33.5°C; however, the water temperature during HEx was not reported in one study [33].

Effects of HEx on BP and secondary outcomes
The main results and conclusions of acute and chronic water-based exercise on BP are described in Table 3.Five studies showed significant acute BP reduction (systolic and/or diastolic) after HEx [6,21,[26][27][28].On the other hand, one study did not observe

Characteristics of HEx
The studies assessing the acute effect of HEx on BP reported low-intensity [27], moderate-intensity [6,26,28], and high-intensity aerobic exercise sessions [21,23], with 30 min [6,21,27] or 45 min [26,28] of duration; however, one study did not report exercise duration [23].The water temperature ranged from 28.5°C to 32°C.The studies acute hypotensive effects after HEx [23].Considering the chronic effects, six studies showed significant BP reduction after HEx training when compared to control group and/or land-based exercise group [18,29,30,[32][33][34], and only one study showed no effect of HEx training in BP [31].Indeed, one study observed similar BP reduction in both HEx and LEx group, which were greater than in CON group [29].Therefore, the results of this review demonstrated some divergent effects in chronic or acute approach post-HEx in BP.Luza et al. [26] In the hypertensive group, the exercise protocol on the ground caused an average reduction of 16.5 ± 3.7 mmHg (P = 0.01) in SBP at 90 minutes post-exercise.In the normotensive group, the protocol of rest in the water caused an average reduction of 14 bpm (P < 0.01) in HR.
The volume of diuresis was increased when compared with the protocols accomplished on the ground (P < 0.01).
The results suggest that when a physical exercise lasts 45 minutes, at submaximal intensity, it causes reduction of SBP in hypertensive individuals.
Water based exercise induces greater increase in cerebral blood flow velocity than land-based exercise of matched intensity.The water immersion may enhance the recurrent episodic increases in cerebral blood flow and shear stress that occur during exercise and, subsequently, amplify cerebrovascular health benefits associated with exercise training.
Cunha et al. [28] Overall (n = 18), DBP did not change after the HEx and CON, and SBP decreased at 10 and 20 min post exercise compared to the CON.Among overweight women, SBP decreased at 10 and 20 minutes post exercise.In contrast, among obese women, SBP decreased only at 10 minutes post exercise.SBP variation was 2.68 mmHg in overweight and 2.4 mmHg in obese women.

NA
The HEx session leads to a reduction in SBP, but not in DBP, during 10 and 20 minutes post exercise recovery.

Castro et al. [6]
No significant differences between interventions were found in 24h and nighttime BP.However, daytime DBP was significantly lower after HEx than CON (-4 ± 1.6 mmHg, P = 0.03), and daytime DBP tended to be lower after LEx than CON (-2.3 ± 1.1 mmHg, P = 0.052).Hourly analysis showed that SBP and DBP values were lower after HEx (average reductions of 6.6 to 12.3 mmHg, P < 0.01) and LEx (average reductions of 5 to 8.3 mmHg, P < 0.05) than after CON in several hours.No significant differences between HEx and LEx were found in any ambulatory BP data.

NA
HEx and LEx promoted similar reductions in ambulatory BP of heart transplant recipients.This post exercise hypotension occurred even though the heart transplant recipient patients had lower control ambulatory BP levels (average 24h of 122/81 mmHg), which does not occur in non-heart transplant recipient populations.These results suggest that both exercises may be tools to counteract hypertension in this high-risk population.

Studies assessing the acute response to heated waterbased exercise
Garzon et al. [23] BP and systemic vascular resistance were not different in exercise on HEx and LEx in ergocycle.The stroke volume and cardiac output were significantly higher during exercise on immersible ergocycle (P < 0.05, g = 0.59 and 0.20 respectively).The stroke volume, ejection fraction and contractility index were higher (P < 0.05, g = 0.64, 0.71 and 0.19 respectively).
During exercise and recovery in immersion, arteriovenous differences were reduced in healthy young participants, while stroke volume and cardiac output were increased for the same external power output.During the recovery, central hemodynamic responses remained higher in immersible ergocycle.BP and systemic vascular resistance were not different between the two conditions.

NA
The HEx preferentially ameliorates blood pressure reactivity to exercise stress, which would then be predicted to prevent or delay the onset of chronic diseases such as essential hypertension.

Silva et al. [32]
The SBP and DBP significantly reduced only in HEx (5.89 mmHg for SBP and 5.15mmHg for DBP).

NA
The results show that regular HEx causes significant decreases in systolic and diastolic pressures in prehypertensive men.

Silva et al. [31]
Comparisons of the BP levels observed before and after the intervention revealed no statistically significant differences.
NA Despite evidence demonstrating the benefits of swimming for arterial pressure, it is necessary to emphasize the need for further studies to determine the optimal parameters for the prescription of physical activity for hypertensive individuals.Thus, the activities or forms of exercise can be selected to meet the personal characteristics of each individual in order to enhance the prevention, treatment, and control of hypertension.

Colado et al. [33]
The DBP significantly reduced (P ≤ 0.01) in both exercise groups (6.8 mmHg in HEx group and 4.8 mmHg in the elastic band group).

NA
Training with aquatic resistance exercises is a viable alternative to traditional resistance with elastic bands, and may provide more benefits to individuals who would be more sensitive to heavier loads or to impact, which may occur when training on LEx with certain devices and exercises.
There was no significant effect of age, baseline body mass index and stage of hypertension on the exercise-induced changes in BP.
A 10-week course of HEx markedly reduced the systolic and mean arterial BP of patients with essential hypertension and is especially recommended for the obese and the elderly who have orthopedic problems or bronchospasm.
a greater activation of major muscle groups due to the resistance imposed by the water, thus triggering hypotension.In this context, heart failure patients who underwent an exercise program associating LEx and HEx (water temperature of 31ºC) showed substantial reduction in diastolic BP (11 mmHg) after 24 weeks of training, which did not occur in patients who underwent LEx only for the same period [25].This result supports the superior effects of HEx due to the mechanisms previously mentioned.Therefore, the present systematic review identified a variety of training protocols that make it difficult to identify the best dose response of training to promote changes in BP effectively.However, HEx can be an optional exercise tool for reducing BP, isolated or in association with other exercise interventions.HEx also allows the practice of exercise for those individuals who are unable to perform LEx due to some physical limitation.

Study limitations
The present review was limited by the heterogeneity and degree of the pathological conditions among the studied populations, associated with the inclusion of both gender in the same samples, and the variety of training protocols, which made it impossible to carry out a meta-analysis.

Conclusion
Although negative findings exist, the present review suggests that HEx training performed for 12 to 24 weeks may reduce BP in normotensive, prehypertensive, hypertensive, postmenopausal women populations, whereas a HEx session may be efficient to acutely reduce BP in normotensive, hypertensive and heart transplant individuals.However, there is no homogeneity in the protocols used, which may have led to the heterogeneity in magnitude and duration of HEx-induced BP reductions.

Acknowledgement
This work is part of Awassi Y. Ngomane Master of Sciences dissertation under the guidance of Dr. Emmanuel G. Ciolac.Awassi Y. Ngomane was supported
There are several physiological mechanisms triggered by the aquatic environment which benefits the neurohumoral control of BP [22,35,36].The physical properties of water, such as the hydrostatic pressure, are responsible for facilitating venous return, which stimulates baroreceptors to trigger the increase in cardiac filling volume and stroke volume, reflexively reducing heart rate and BP [6,[35][36][37].In addition, heated water ranging from 30 to 32°C triggers a reduction in peripheral vascular resistance due to dilatation of arterioles [6,18,38].
Moreover, HEx reduces circulating levels of adrenergic neurotransmitters epinephrine and norepinephrine, renin and endothelin-1, as well as increases circulating levels of nitric oxide, which may results in reduced peripheral vascular resistance [38].Regarding neurohumoral regulation, the improved renal system undergoes inhibition of renin-angiotensinaldosterone system, increases factors that excrete sodium (atrial natriuretic peptide), and favors diuresis, which reduce blood volume [9,18,38].Other hypothesis is that there are improvements in sympathomodulatory and arterial baroreceptors after HEx, which reduce BP.
In contrast to the beneficial results of HEx, two studies did not identify differences in BP values in patients with hypertension [31] and healthy normotensive individuals [23].These effects can be attributed to methodological differences in relation to training protocols, when compared with the other studies that found reduction of BP in the same populations [18,21,[26][27][28][29][30]32].One possible explanation for the lack of hypotensive effect in the above mentioned studies is that the exercise protocol did not follow the recommendations for dynamic exercises, such as an exercise intensity between 50% and 80% of peak oxygen consumption [2].
Muscle mass is another important factor that may contribute to the post-exercise hypotension after HEx.The larger muscle mass involved in HEx may result in larger production of vasodilatory agents, such as adenosine, potassium, lactate, nitric oxide and prostaglandin [24].Although some land-based exercises (e.g.walking, running, cycling) require the participation of large muscle groups, it appears that HEx requires by Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP # 2015/05259-2) during this project.

Figure 1 -
Figure 1 -Flowchart of the process of screening and selection of articles for inclusion in the review.

Table 2 -
PEDro score of the included studies

Table 3 -
Main results on blood pressure variables, other hemodynamic parameters, secondary measurements and the conclusions of