ACUTE EFFECTS OF SLOW, CONTROLLED BREATHING EXERCISES ON ARTERIAL PRESSURE AND AUTONOMIC CARDIAC MODULATION IN HYPERTENSIVE PATIENTS

Highlights: 1. Breathing exercises modify the autonomic cardiac modulation and blood pressure. 2. Slow breathing reduced systolic arterial pressure at approximately 4.5 mmHg. 3. These exercises can be a non-pharmacological tool to control hypertension


INTRODUCTION
Systemic arterial hypertension (SAH) is a severe world health problem since it affects around one billion people and it is estimated that the prevalence of this disease will be superior to 1,5 billion patients around the globe until 2025 1 .The autonomic nervous system (ANS) acts on the cardiovascular system through its sympathetic and parasympathetic components, which are responsible for regulating cardiac and circulatory functions, thus modulating the heart rate and the systemic arterial pressure 2 .
Obese patients present ANS alterations, characterized by sympathetic hyperactivity, especially by baroreflex hypersensitivity 3 .The ANS plays a regulative role in cardiac function through sympathovagal modulation, which can be analyzed using the heart rate variability (HRV) 4 .This method can provide quantitative indicators of both sympathetic (LF) and parasympathetic activity (HF), as well as of the autonomic cardiac modulation (LF/HF ratio) 5,6 .However, patients breathing control is not always a monitored variable during data collection, which makes it unknown for the data analysis of the HRV.
Drug therapy is the main tool for handling SAH 7 .However, it is estimated that between 10% to 20% of the patients do not respond to anti-hypertensive drug therapy 8 .With drug management effects reduced, studies that investigate complementary therapies are extremely important.Previous researches have demonstrated the efficiency of non-drug therapies in reducing sympathetic hyperactivity and systemic arterial pressure on these patients [9][10][11] .
Previous studies have demonstrated the positive effects of slow breathing on the heart rate, the autonomic modulation 12,13 , and arterial pressure 14 .However, few studies have investigated the influence of slow breathing on arterial pressure and its autonomic cardiac function in hypertensive patients [14][15][16] .In this context, further investigation on the effects of slow, controlled breathing exercises (SCBE) in the management of cardiovascular disorders is needed, especially in patients experiencing hypertensive crises 17 .The objective of this study was to evaluate the influence of SCBE on arterial pressure and the heart rate variability of hypertensive patients.

Characterization of the study
The present research was conducted in the Evaluation and Function Rehabilitation Laboratories of the Health Sciences Center of the Federal University of Santa Maria (UFSM).
The project was approved by the Institutional Ethics Committee (Protocol: 2.180.257) and respects resolution number 510/2016 of the National Health Council and the Declaration of Helsinki.All the patients were previously informed about the study's protocol and signed the Free and Informed Consent Form.Data was collected in the period comprehended between October 2017 and April 2019.

Subjects
The patients were selected in the primary health attention unities of the city of Santa Maria/RS.Inclusion criteria included volunteers who were previously diagnosed with hypertension (SBP ≥140 mmHg and/or DBP ≥90 mmHg) 18 and were taking anti-hypertensive drugs for at least three months, of both sexes and aged between 20 and 75 years old, alphabetized, and presenting a body mass index lower than 35 kg/m 2 , non-smokers, without musculoskeletal disorders symptoms, without previously diagnosed rheumatologic, cardiovascular, neurological, cancer, immune, and hematologic diseases or psychiatric and cognitive disorders.
The exclusion criteria included the intake of alcoholic or caffeinated beverages in the 12h preceding the tests and the practice of intense physical exercises in the 48h preceding the evaluation.Patients who reported experiencing stressful emotional events in the 72h preceding data collection and those presenting dyspnea or tachypnea in the evaluation day were also excluded.

Outcomes
The primary outcome was arterial pressure (systolic and diastolic).Secondary outcomes were the autonomic modulation (LF/HF ratio) evaluated in time and frequency domains.The time-domain involves heart rate (HR), the standard deviation of all RR intervals from normal to normal (SDNN), the square root of the mean of the squares of the RR interval successive differences (RMSSD), the percentage of intervals differing more than 50 ms from the previous interval (PNN50%), and the triangular index.In the frequency domain, the variables were total potency (TP), low frequency (LF), and high frequency (HF), in raw data normalized in percentage form.After the data was collected, for three days all the volunteers adapted to the data collection protocol, which involved training SCBE (12 bpm).In the evaluation days, the participants were in 8-hour fasting and had not taken their drugs, which were taken after the data collection was finished.Both data collection were made within 1 to 3 days.First, the spontaneous breathing data were collected (control), followed by SCBE.During the evaluation, the volunteers remained lying down in a supine position for one hour; in the first 30 minutes (min) they rested, and after that, 10 min of spontaneous breathing were collected, as well as 10 min of slow, controlled breathing.The experimental draw is presented in Figure 1.The room temperature was constantly kept between 21 and 24ºC.During spontaneous breathing, volunteers did not suffer any interference, and the breathing was monitored between 12 e 20 breaths per minute using visual inspection and a multi-parameter monitor (Dixtral, 2021 model, Manaus, Brazil).During the SCBE (12 bpm; inspiration/expiration: 2/3) 18,19 , the respiratory frequency was paced by standard verbal stimulus previously recorded in audio.

Arterial pressure evaluation
The evaluation of both systolic arterial pressure (SAP) and diastolic arterial pressure (DAP) was conducted using a multi-parameter monitor (Dixtal, 2021 model, Manaus, Brasil).
The cuff was positioned in the right arm, while the participants were lying down in a supine position in the stretcher.Data were collected before and immediately after spontaneous breathing and the SCBE.

Heart rate variability
Revista Contexto & Saúde -Editora Unijuí -ISSN 2176-7114 -V.24 -N.48 -2024 -e13894 The autonomic function was evaluated by heart rate variability (HRV).The data were collected through a pulse rate monitor (Polar, 810i model, Kempele -Finland).The measurement of heart rate (sample rate -1000Hz) was taken in RR interval time series and obtained with continuous intervals (10min) in the spontaneous breathing and the SCBE.The collected data were transferred to a computer equipped with Polar ProTrainer 5 software.RR intervals of 5 min were selected for the analysis, with a stable heart rate, usually between 3 and 8 min.In each RR 5 min series, interferences were fixed (aberrant errors) using the standard software correction algorithm Polar ProTrainer 5 (that is, moderate filter power and minimum protection zone of 6 beats per min).Furthermore, a visual inspection of the RR time series was conducted and all the remaining artifacts were manually removed 20 .
The data were transferred to the computer and the RR intervals were processed.The analyses of HRV parameter were processed by version 2.1 of Kubios software (Grupo de Análise de Biossinais e Imagens Médicas, Departamento de Física, Universidade de Kuopio, Finlândia, 2012).
The time-domain variables taken into account were the heart rate (HR), the standard deviation of all RR intervals from normal to normal (SDNN), the square root of the mean of the squares of the RR interval successive differences (RMSSD), the percentage of intervals differing more than 50 ms from the previous interval (PNN50%), and the triangular index.In the frequency domain, the variables were total potency (TP), low frequency (LF), high frequency (HF), and the sympathovagal function relationship (LF/HF).
The spectral power analysis was estimated using the fast Fourier transform algorithm.This analysis decomposes the HRV into fundamental oscillatory components, the main ones being: the high frequency (HF) component from 0.15 to 0.4Hz, corresponding to respiratory modulation and the indicator of the vagus nerve's action in the heart; and the low-frequency component (LF) of 0.04 and 0.15 Hz, correlated to the joint action of the vagal and sympathetic components on the heart, with the sympathetic prevailing.The normalized units (n.u) were obtained by dividing the potency of a determined component by the total potency (of which VLF is subtracted) and then multiplying the result by 100 (LF or HF / (Total potency -VLF) x 100).The LF/HF relationship reflects absolute and relative changes among the sympathetic and parasympathetic components of ANS, characterizing sympathovagal influence on the hearth 21 .

Statistical Analysis
The descriptive data are presented as standard mean and standard deviation (SD).The Kolmogorov test was applied to assess the normality of data.The parametric distribution data were compared by the ANOVA of a route for repeated measures, and those of non-parametric distribution by the Friedman test, then both were submitted to post hoc Tukey test.The meaningful results are presented by the mean differences (MD) and their respective confidence intervals of 95% (CI95%).The effect size differences between breaths (spontaneous breathing vs SCBE) were calculated using the d of Cohen and expressed using the following criteria: trivial <0.2, small 0.2-0.49,moderate 0.5-0.79,and large >0.8.All the statistical analyses were made using GraphPad Prism 5.0 software (San Diego, CA, USA).The alpha error rate was of 5% (p<0.05).

RESULTS
This study initially recruited 36 volunteers previously diagnosed with SAH, of which 6 were not present in one of the adaptation to the evaluation protocol days and 1 presented fever in the first evaluation date and was, therefore, excluded (Figure 2).The final sample comprehended 29 patients with an average age of 54 (±14) years old, of which 68.9% were female, 41% were sedentary, 55% were overweight, and 31% were obese.The majority of the patients (59%) were taking Angiotensin II receptor blockers.The patient's characterization and the drugs used in the treatment of hypertension are presented in Table 1.The results for the heart rate variability are presented in In the frequency domain, non-normalized data presented differences in total power between breaths and between the evaluated days (TP ms 2 : p=0.035) and in high frequency (HF ms 2 : p=0.046), which were not confirmed by post hoc Turkey test (p>0.05).After data normalization (n.u.), in the frequency domain, the SCBE reduced sympathetic activity (p<0.001)represented by the LF component by about 18% (1 st evaluation DEM: -20.8;CI95%: -12.5 to -29.1 n.u. and 2 nd evaluation DEM: -18.1;CI95%: -9.9 to -26.5 n.u.) (Figure 3C).On the other hand, SCBE (p<0.001) the parasympathetic activity evaluated by the HF component in 18% (1 st evaluation DEM: 20.7 CI95%: 12.5 to 29.1 n.u. and 2 nd evaluation DEM: 18.1 CI95%: 9.8 to 26.4 n.u.) (Figure 3D).The comparison of the first and the second evaluations of the LF and HF components between spontaneous breathing and SCBE did not present differences.In the LF component, spontaneous breathing in the first evaluation was larger than the slow, controlled breathing exercises in the second evaluation, as well as the SCBE in the first evaluation were smaller than the spontaneous breathing in the second evaluation (DEM: -18.2 CI95%: -9.9 to -26.5 n.u.).These results were repeated in the same proportion for the HF component in the comparison of evaluations (1 st vs 2 nd ).The analyses of the effect size of the means of the two evaluated days demonstrates that SCBE presented a large effect (ES=-1.08) in reducing sympathetic activity (LF) and increasing parasympathetic activity.

Revista
SCBE reduced (p<0.001)autonomic modulation (LF/HF ratio) by about 50% (1 st Evaluation DEM: -1.40 CI95%: -0.64 to -2.2; 2 nd Evaluation DEM: -0.98 CI95%: -0.22 to -1.74) in both days (1 st Evaluation: 45% and 2 nd Evaluation: 56%) (Figure 3E).The comparison of the evaluations (1 st evaluation vs 2 nd evaluation) of LF/HF ratio between spontaneous breaths and SCBE did not present differences.The spontaneous breathing was smaller (DEM: -1.33 CI95%: -0.57 to -2.08 n.u.) in the first evaluation than the SCBE in the second evaluation, as well as the controlled breathing in the first evaluation was smaller than the spontaneous breathing in the second evaluation (DEM: -1.05 CI95%: -0.29 to -1.81 n.u.).These results were repeated in the same proportion for the HF component in the comparison of the evaluations (1 st vs 2 nd ).The analysis of the effect size of the means of the two evaluated days demonstrates that SCBE presented a moderate effect (ES=-0.74) on autonomic modulation.During and after data collection, the patients did not present and did not report any complications.

DISCUSSION
The present study demonstrates that 10 minutes of SCBE reduces SAP and sympathetic activity, modifying autonomic modulation in hypertensive patients.In SAH, it is observable the imbalance of this modulation agent, which presents an excessively excited sympathetic component and can lead to the emergence of arrhythmia and sudden death 2,5 .In studies that evaluated normotensive patients, SCBE represented an important increase in the parasympathetic component and autonomic modulation 12,13,22 .These results with healthy volunteers reinforce the positive benefits of respiratory control over autonomic modulation, which is clinically relevant to the sympathetic hyperactivity of those patients 3 .
In the present study, the SCBE (12 bpm) reduced the sympathetic activity and increased the parasympathetic activity, which changed the autonomic cardiac modulation in hypertensive patients.These results are due to the respiratory sinus arrhythmia mechanism, which increases parasympathetic activity through SCBE, inducing the sympathetic activity reduction in the cardiac muscle 23,24 .In such a context, the findings are justified by the feedback mechanism 23 , given the modulating effect of ANS on the cardiorespiratory function, especially due to the physiological event of respiratory sinus arrhythmia 24 , which results in the influence of breathing on cardiac function.During slow, controlled breathing, the expiratory phase excites, and the inspiratory phase inhibits the solitary tract, thus generating stimulus, especially in the X pair of cranial nerves (Vagus), which promotes autonomic cardiac modulation control 22 .This mechanism occurs mostly in the expiratory phase and, in the present study, that phase was longer than the inspiratory phase (inspiration/expiration: 2/3).This influenced the results for the exercise program on the autonomic modulation and on the SBP reduction found by the present study in a healthy way.
Previous studies with hypertensive patients reinforce the results of the present research [25][26][27] .Stress control in patients with SAH using slow breathing presents itself as a scientific perspective for the non-pharmacological treatment of this cardiovascular disease 25,28 .
SCBE programs (30 min sessions, 2 times a week for a month) reduced the respiratory frequency (14 bpm to 10 bpm), the SAP (-12 mmHg), the DAP (-7 mmHg), and increased the tidal volume, which is inversely associated with the patients' arterial pressure reduction 27 .A recent study demonstrated that a SCBE program (10 bpm, during 15 min, 3 times a week, for 8 weeks) with the help of device (Resperate®) reduced SAP (-8,3 mmHg) and DAP (-4.9 mmHg) in patients with SAH 26 .Li et al, demonstrated that 5 min SCBE (8 bpm), when compared with normal standardized breathing (16 bpm), reduces the heart rate (~5 bpm) and the arterial pressure (SAP: -6 mmHg and DAP: -5 mmHg), as well as modifies the cardiac autonomic modulation of the patients 14 .These studies confirm the present study results, demonstrating that SCBE reduce sympathetic activity, restore the sympathovagal balance, and decrease arterial pressure, as well as can serve as a non-pharmacological tool in the management of SAH 14- 16,26,27 .Among this study's limitations, we highlight the lack of measurement of baroreflex sensitivity and the lack of evaluation of the plasma catecholamines levels (norepinephrine, acetylcholine, and epinephrine) 29 .Another worth considering factor is the lack of microneurography, which is the invasive and direct evaluation of the autonomic nervous system activity 30 .However, the techniques employed in this study are widely used in researches, as well as recommended by the TASK Force guidelines 6 .

SCBE ( 12
breaths per minute; Inspiration / Expiration: 2/3) modify the autonomic cardiac modulation and reduce hypertensive patients' arterial pressure.These alterations are demonstrated by the decrease of the sympathetic activity, the increase of the parasympathetic activity, and the reduction of the SBP.The present research findings show the importance of considering the role of breathing control, even if in physiological values, on the autonomic cardiac function during scientific studies data collection.Clinically, SCBE can be a nonpharmacological tool in the reduction of the arterial pressure of hypertensive patients.

Table 1 .
Baseline characteristics of patients.

Table 2 .
Heart rate variability data analyzed in the time and frequency domain of hypertensive patients.