Introduction
Increased central (aortic) and systemic (brachial–ankle) arterial stiffness is an independent risk factor for the development of cardiovascular disease [1, 2]. Arterial stiffness increases with age and with various pathological conditions such as obesity, diabetes, smoking, and dyslipidemia, and arterial stiffness has important implications for cardiovascular health [3].
Breathing can directly affect the activity of the autonomic nervous system, including heart rate [4]. Voluntary control of breathing, especially slowing its rate, comes from Eastern traditions and has been used for thousands of years as an important part of meditation and relaxation [4–6]. Heart rate variability (HRV), which is regulated by the activity of the cardiac autonomic nervous system, varies with respiration [7]. For example, controlled breathing at 6 breaths/min increases baroreflex sensitivity [8]. In healthy individuals, slow breathing induces a shift to parasympathetic activity, which is dependent on vagal activity and is primarily associated with energy conservation, rest, and relaxation [9]. Furthermore, even in younger participants, 5 min of slow, deep breathing significantly increased parasympathetic activity and reduced the state of anxiety [10]. Chang et al. [11] reported changes in parasympathetic balance during slow, deep breathing compared to that during breathing at typical rates.
Slow, deep breathing is a useful nonpharmacological intervention to control hypertension [12]. It causes periodic fluctuations in intrathoracic pressure, which affects venous return to the heart, cardiac output, and ultimately blood pressure (BP) [13, 14]. In patients with hypertension, 2 min of slow, deep breathing has been demonstrated to lower systolic blood pressure (SBP) and diastolic blood pressure (DBP) by 8.6 mmHg and 4.9 mmHg, respectively [15]. Changes in BP are strongly associated with arterial stiffness [16]. In addition, slow, deep breathing stimulates stretch receptors in the aortic arch and carotid sinus, thereby activating the baroreflexes and exerting a favorable effect on the cardiovascular system [17]. Thus, a single session of slow, deep breathing may reduce arterial stiffness.
Several breathing control methods are available for this purpose. One of them is the 4-4-8 breathing technique, a breathing pattern developed by a Harvard Medical School physician named Professor Hideyuki Negoro. The 4-4-8 breathing technique involves inhaling for 4 s, holding the breath for 4 s, and exhaling for 8 s. This technique is thought to increase parasympathetic activity and decrease sympathetic activity, thus reducing anxiety and fear [18, 19]. In other words, slow, deep breathing may decrease arterial stiffness via changes in autonomic function and BP. However, the acute effects of the slow, deep, 4-4-8 breathing technique on arterial stiffness is unclear.
Therefore, this study aimed to identify the immediate effects of the 4-4-8 breathing technique on HRV, BP, and arterial stiffness in healthy young men. The hypothesis herein, was that the 4-4-8 breathing technique would decrease arterial stiffness.
Methods
Participants
Because HRV changes differ between men and women owing to hormonal differences [20], only men were recruited in this study to minimize variability. Fifteen healthy men aged 20 years were recruited from the Teikyo University of Science student programs. Prior to the start of the study, all participants provided written informed consent after receiving a complete verbal and written explanation of the purpose and methods of the study. Young men with no medical history who were in good health were included. Individuals with a history of abnormal blood/urine tests, chest radiographs, or electrocardiograms; those with a history of hypertension; and those taking medications that may alter BP were excluded. All participants had no experience with meditation. This study was conducted in accordance with the Declaration of Helsinki and was reviewed and approved by the Ethics Committee of Teikyo University of Science (approval number: 22A009).
This study is a cross-over study. Prior to the testing session, the order in which the two conditions were to be performed was randomly assigned to each participant by an individual not affiliated with the study. None of the participants performed abdominal breathing in the (a) 4-4-8 breathing technique and (b) control (CON) conditions.
To determine the appropriate sample size, a power analysis was performed using IBM SPSS Statistics (ver. 25; IBM Corp., Armonk, NY, USA). The magnitude of the effect of 4-4-8 breathing technique on arterial stiffness was assumed to be 0.5. Using an analysis of variance, it was determined that to detect a difference with 80% power and a two-sided alpha of 5%, nine participants should be included. To account for dropouts, 15 participants were included in the study. This study was conducted in compliance with the Declaration of Helsinki in terms of ethics, human rights, and protection of the participants’ personal information.
Study design
Participants were asked to come to the laboratory early in the morning after an 8-h of overnight fasting. Next, the participants were asked to assume the supine position in a quiet room and underwent resting HRV, BP, and arterial stiffness testing. The researcher then explained and demonstrated the 4-4-8 breathing technique using a simplified instruction manual so that the participants could fully understand it. The participants performed the 4-4-8 breathing technique in a quiet room, lying in the relaxed supine position with their eyes closed. HRV, BP, and arterial stiffness were tested in the supine position at 30 min, 60 min, and 24 h after the 4-4-8 breathing technique was completed.
Body composition
Body height was measured in 0.1-cm increments using a height meter (Sanwa Corporation, Tokyo, Japan), and body weight and fat percentage were measured noninvasively (impedance method) using a body composition analyzer (InnerScan Dual Black RD-E04BK, Tanita Corporation, Tokyo, Japan). Body mass index (BMI) was calculated by dividing weight (kg) by height squared (m2) (kg/m2).
Arterial stiffness
Pulse wave velocity (PWV) is an established measure of arterial stiffness. To evaluate arterial stiffness, PWV was calculated from the pulse wave propagation time between two arterial points as follows: PWV = arterial length/pulse wave propagation time. As an index of systemic arterial stiffness, brachial–ankle PWV (baPWV) was evaluated using a blood pressure pulse wave testing device (BP-203RPEII; Fukuda Colin Co., Tokyo, Japan) by attaching oscillometric sensors to the left and right upper arms and ankles [21]. Carotid–femoral PWV (cfPWV) was measured by recording arterial pressure waveforms (SphygmoCor XCEL TM-2805V; A&D Corp., Tokyo, Japan) using tonometric and oscillometric sensors affixed to the left common carotid and left femoral arteries, respectively. The linear distance between the sensors was measured using a tape measure and divided by the automatically calculated time difference in the increase in arterial pressure between the sensors, which was used as an index of aortic stiffness [22]. The intra- and inter-rater coefficients of variation for the PWV measurements were 3% and 4%, respectively.
Cardiovascular indices
Arterial BP waveforms were recorded using oscillometric sensors attached to both upper arms with a BP pulse wave testing device (PWV/ABI; Fukuda Colin, Inc., Tokyo, Japan) [23] to evaluate the BP in the brachial arteries. The same device with electrocardiograph sensors attached to both wrists [23] was used to evaluate the heart rate. A tonometric sensor attached to the left carotid artery was used to record arterial BP waveforms using a BP pulse wave testing device (SphygmoCor XCEL TM-2805V; A&D Corp., Tokyo, Japan) to evaluate the SBP of the aortic artery, augmentation index (AIx), ejection pulse pressure and reflex pulse pressure. The intra- and inter-rater coefficients of variation for BP readings were 2% and 3%, respectively.
HRV analysis
The high-frequency (HF) and low-frequency (LF) HRV components reflect parasympathetic and sympathetic activation is generally assessed as the overall sympathetic vagal balance and degree of autonomic excitation [24]. The RR intervals recorded from the wearable heart rate sensor WHS-1/RRD-1 (Union Tool Corporation, Tokyo, Japan) were downloaded and analyzed using the HRV analysis software RRI Analyzer 2 (Union Tool Corporation, Tokyo, Japan). The software converted the RR intervals into the frequency domain indices LF (ms2) and HF (ms2). Each index was calculated at 2-min intervals based on the recommendations of the standard. It is generally accepted that 1 min is needed to evaluate the HF component of HRV, whereas approximately 2 min are needed to address the LF component [25]. Automatic artifact correction was performed on all recordings before analysis. Data were subjected to HRV spectral analysis; the LF and HF bands were defined as 0.04–0.15 Hz and 0.15–0.4 Hz, respectively [26].
4-4-8 breathing technique
The participants were guided by a video of a droplet on a computer screen moving up and down. As the droplet rose, the participants inhaled, and as the droplet fell, they exhaled. Initially, the participants had to (1) exhale completely through the mouth, (2) breathe in through the nose while placing their hands on the belly while it expanded (4 s), (3) hold their breath (4 s), and (4) breathe out through the nose to depress the belly (8 s). The exercise lasted for 5 min (19 cycles).
Statistical analysis
A total of 15 participants were included in the full analysis set. Clinical response rates and 95% confidence intervals were calculated, and statistical analyses were performed in a blinded manner. Data for the outcome variables were tested for normality and log normality using the Shapiro–Wilk test. Repeated-measures two-way analysis of variance was used to evaluate the between-trial change in each measure in each intervention using a post hoc test (Bonferroni method). All statistical analyses were performed using IBM SPSS Statistics (ver. 25; IBM Corp., Armonk, NY, USA), with a statistical significance level of 5%. All data are presented as mean ± standard deviation.
Results
Physical characteristics
Twenty subjects were recruited, screened, 5 excluded, and 15 included. All the enrolled participants (n = 15) completed the study sessions without any adverse events. The participants’ height, weight, body fat percentage, BMI, and resting BP are summarized in Table 1.
|
Value |
|
|
Age [years] |
20 ± 0 |
|
Height [cm] |
170 ± 5 |
|
Weight [kg] |
56 ± 3 |
|
Body fat [%] |
16 ± 2 |
|
BMI [kg/m2] |
19 ± 2 |
|
Brachial SBP [mmHg] |
106 ± 6 |
Arterial stiffness
The cfPWV values are shown in Figure 1A. The cfPWV values for both trials did not significantly change at 30 min, 60 min, or 24 h compared to those at baseline, and there were no between-trial differences. baPWV values are shown in Figure 1B. In the 4-4-8 breathing technique trial, the baPWV values significantly decreased at 30 min (p < 0.05), but no significant change was observed at 60 min, and 24 h. In the CON trial, baPWV values at 30 min, 60 min, and 24 h were not significantly different from those at baseline. The baPWV at 30 min was significantly lower in the 4-4-8 breathing technique trial than in the CON trial (p < 0.05).
± standard deviation; PWV — pulse wave velocity; CON — control conditions; *p < 0.05 vs. baseline;
†p < 0.05 vs. CON.
Cardiovascular indices
Blood pressure values are shown in Table 2. In the 4-4-8 breathing technique trial, brachial SBP, PP and AIx decreased significantly at 30 min (p < 0.05) compared to baseline, but there was no significant change in the value at 60 min and 24 h. In the CON trial, brachial SBP, PP and AIx did not change significantly at 30 min, 60 min, and 24 h compared to that at baseline. Brachial SBP, PP and AIx at 30 min was significantly lower in the 4-4-8 breathing technique trial than in the CON trial (p < 0.05). In both trials, brachial DBP, ankle SBP, DBP, PP, aortic SBP, aortic ejection pulse pressure, and aortic reflex pulse pressure did not change significantly from baseline at 30 min, 60 min, or 24 h, and were not significantly different in both trials.
|
Baseline |
After 30 min |
After 60 min |
After 24 h |
|
|
Brachial SBP [mmHg] |
||||
|
4-4-8 breathing |
105 ± 10 |
99 ± 4*† |
106 ± 12 |
105 ± 7 |
|
CON |
106 ± 6 |
104 ± 6 |
105 ± 6 |
106 ± 10 |
|
Brachial DBP [mmHg] |
||||
|
4-4-8 breathing |
67 ± 6 |
67 ± 5 |
67 ± 5 |
67 ± 4 |
|
CON |
68 ± 5 |
68 ± 5 |
68 ± 6 |
66 ± 6 |
|
Brachial PP [mmHg] |
||||
|
4-4-8 breathing |
38 ± 5 |
32 ± 4*† |
39 ± 4 |
37 ± 6 |
|
CON |
37 ± 6 |
36 ± 8 |
37 ± 4 |
39 ± 8 |
|
Ankle SBP [mmHg] |
||||
|
4-4-8 breathing |
117 ± 10 |
117 ± 6 |
117 ± 8 |
117 ± 6 |
|
CON |
118 ± 6 |
118 ± 6 |
118 ± 6 |
117 ± 6 |
|
Ankle DBP [mmHg] |
||||
|
4-4-8 breathing |
61 ± 6 |
60 ± 8 |
63 ± 8 |
62 ± 6 |
|
CON |
61 ± 6 |
63 ± 4 |
62 ± 6 |
59 ± 6 |
|
Ankle PP [mmHg] |
||||
|
4-4-8 breathing |
55 ± 8 |
56 ± 8 |
54 ± 8 |
54 ± 4 |
|
CON |
56 ± 10 |
55 ± 8 |
55 ± 6 |
57 ± 8 |
|
Aortic SBP [mmHg] |
||||
|
4-4-8 breathing |
95 ± 10 |
96 ± 10 |
95 ± 7 |
95 ± 10 |
|
CON |
96 ± 10 |
95 ± 8 |
96 ± 6 |
96 ± 8 |
|
Ejection pulse pressure [mmHg] |
||||
|
4-4-8 breathing |
22 ± 2 |
23 ± 4 |
23 ± 4 |
23 ± 4 |
|
CON |
23 ± 6 |
23 ± 6 |
24 ± 4 |
23 ± 4 |
|
Reflex pulse pressure [mmHg] |
||||
|
4-4-8 breathing |
11 ± 4 |
10 ± 4 |
11 ± 5 |
12 ± 4 |
|
CON |
11 ± 4 |
11 ± 5 |
11 ± 4 |
11 ± 5 |
|
AIx [%] |
||||
|
4-4-8 breathing |
14 ± 5 |
10 ± 6*† |
12 ± 5 |
14 ± 5 |
|
CON |
14 ± 5 |
14 ± 6 |
14 ± 5 |
14 ± 7 |
Sympathetic response (LF)
The LF values are shown in Figure 2A. In the 4-4-8 breathing technique trial, the LF value was significantly lower at 30 min (p < 0.05) than at baseline, but there was no significant change in the value at 60 min and 24 h. In the CON trial, In the CON trial, LF values at 30 min, 60 min, and 24 h were not significantly different from those at baseline. The LF value at 30 min was significantly lower in the 4-4-8 breathing control trials than in the CON trial (p < 0.05).
Parasympathetic response (HF)
The HF values are shown in Figure 2B. In the 4-4-8 breathing technique trial, the HF value was significantly higher at 30 min (p < 0.05) than at baseline, but there was no significant change in the value at 60 min and 24 h. In the CON trial, HF values at 30 min, 60 min, and 24 h were not significantly different from those at baseline. The HF value at 30 min was significantly higher in the 4-4-8 breathing control trials than in the CON trial (p < 0.05).
Discussion
According to available research, this is the first study to investigate the acute effects of the 4-4-8 breathing technique on PWV in young participants. This study included young men to avoid the influence of sex on HRV [20]. In this study, the HF value increased and the LF, baPWV, and SBP values decreased after performing the 4-4-8 respiratory technique. However, values after 24 h were not significantly different between the two trials.
The typical number of breaths per minute is 12–20 [27], but the 4-4-8 breathing technique is similar to slow, deep breathing as it takes 16 s per breath, which is approximately 4 less breaths per minute. According to Vierra et al. [28], slow breathing helps improve HRV. Magnon et al. [10] reported that slow, deep breathing in healthy young participants significantly increased the HF value and restored vagal tone, similar to the findings of the present study. Thus, long, slow, and deep breathing with prolonged exhalation may increase parasympathetic activity and decrease sympathetic activity.
This study also found that the 4-4-8 breathing technique reduced BP. The results of this study are consistent with those of previous studies. Many studies have shown that deep breathing reduces BP by 4–54 mmHg in individuals of various ages and with various BP levels [12]. In this study, the 4-4-8 breathing technique reduced values by an average of 5.8 mmHg after 30 min. In previous studies, slow, deep breathing decreased SBP in patients with hypertension and increased the HF value [29, 30]. Lin et al. [31] instructed healthy college students to breathe at 6.0 breaths/min and 5.5 breaths/min and found that the HRV was higher at 5.5 breaths/min. However, no positive effect on HRV has been identified when the ratio of inhalation to exhalation is 1 to 1 [32]. In this study, the 4-4-8 breathing technique resulted in longer exhalation times. Thus, practicing slow, deep breathing with a relatively long exhalation time may decrease BP owing to oxygenation and activation of the parasympathetic nervous system. This issue needs to be investigated in detail in future studies.
Most vascular functions are regulated by the contractile state of smooth muscle cells in the arterial wall [33]. Heart rate has an important influence on PWV and is closely related to HRV [34]. Changes in HRV are associated with changes in PWV [35]. In young adults, baPWV correlates with sympathetic nervous system activation [36, 37]. In the present study, the 4-4-8 breathing technique increased the HF value and decreased the LF and baPWV values. In other words, long, slow, and deep breaths with longer exhalations may decrease the PWV by regulating autonomic function. Another study examining the effects of the autonomic nervous system activity on arterial stiffness reported that the autonomic nervous system did not play a direct role in the regulation of cfPWV in healthy participants [38]. Consistent with these results, cfPWV did not change after the 4-4-8 breathing technique trial in this study. This may involve the organic effect of central arteries having more elastic tissue and less smooth muscle than peripheral arteries to adapt to the high pressures caused by ventricular ejection [39].
In humans, altering the contractile state of the brachial artery smooth muscle with the sympathetic neurotransmitters norepinephrine and nitroglycerin increases and decreases brachial artery stiffness, respectively [40]. Exposure to acute intermittent hypoxia and increased PWV increase norepinephrine levels [41, 42]. In other words, slow, deep breathing may suppress the PWV by increasing norepinephrine inhibition. However, norepinephrine was not measured in this study and is a subject for future research; important predictors of PWV are age and SBP [43]. As noted above, a recent study demonstrated that SBP decreased with slow, deep breathing compared to spontaneous breathing [30]. In this study, SBP and PWV decreased after the 4-4-8 breathing technique trial. Therefore, SBP may be involved in the decrease in baPWV with the 4-4-8 breathing technique. Further details need to be verified in future studies. Decreased respiratory sinus arrhythmias during deep breathing are associated with increased coronary atherosclerosis [44]. In this study, 4-4-8 breathing technique increased HF value and decreased baPWV in healthy young men, suggesting that the phenomenon of respiratory sinus arrhythmias occurred normally. Further detailed studies are needed in the future.
Arterial stiffness depends on arterial structural characteristics (calcification, intimal tunica media thickening, increased collagen, and decreased elastin) [45]. Increased sympathetic activity damages elastin fibers and decreases arterial compliance [46]. Improvements in HRV were observed after 4 weeks of respiratory training, and respiratory training lowered BP [47]. In other words, although this study had an acute effect, long-term breathing exercises might have resulted in organic changes and a lower baseline baPWV. Therefore, future studies should investigate the relationships among long-term 4-4-8 respiratory control, autonomic function, and PWV. Future long-term studies should also investigate whether breathing exercises contribute to better sleep quality.
Limitations of the study
This study has several limitations. First, because the participants were healthy young adults, caution should be exercised when generalizing the results to older adults or those with impaired sleep quality, such as those with metabolic syndrome. Second, norepinephrine or electroencephalogram was not assessed, which might have had important effects on arterial stiffness. The small sample size warrants caution when interpreting the results and limits the generalizability of the present findings.
Conclusions
In this study, the 4-4-8 breathing technique transiently reduced the BP and baPWV. According to available research, this is the first study to evaluate the effects of the 4-4-8 breathing technique on arterial stiffness. The results of this study are significant because they highlight the positive effects of slow, deep breathing on vascular function.
Data availability statement: Data described in the manuscript, code book, and analytic code will be made available upon request pending [e.g., application and approval, payment, other]. Before the study began, all participants received a complete explanation of the study’s purpose and methods before providing their written informed consent.
Ethics statement: This study was conducted in accordance with the Declaration of Helsinki and approved by the Ethics Committee of Teikyo University of Science (approval No.: 22A009).
Author contributions: R.K. and H.N. designed the research; R.K. and H.N. conducted the research; R.K. and H.N. analyzed data; and R.K. wrote the paper. R.K. had the primary responsibility for the final content. All authors read and approved the final manuscript.
Funding: None.
Acknowledgments: None.
Conflict of interest: None declared.
Supplementary material: None.
