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Heart rate, Endurance training, Blood flow restriction, Blood pressure, What&,rsquo s Known Endurance exercise improves heart rate variability (HRV) and blood pressure (BP), but the effects of combining low-intensity endurance training with blood flow restriction (BFR) in mild hypertension are unclear. What&,rsquo s New Low-intensity endurance exercise combined with BFR reduces BP and HRV similar to exercise and increases HRV more effectively, without causing adverse effects. IntroductionHeart rate variability (HRV) refers to variations in time intervals between successive heartbeats. These variations reflect the balance between the sympathetic and parasympathetic divisions of the autonomic nervous system (ANS). 1, Aging is associated with an increase in sympathetic nervous system (SNS) activity and a decrease in parasympathetic nervous system (PNS) activity, which leads to reduced HRV. 2, HRV reduction plays a role in the progression of hypertension and raises the long-term danger of cardiovascular diseases and mortality. 3, , 4, Exercise training is a non-pharmacological intervention to improve cardiovascular function and HRV. 5, , 6, It enhances cardiovascular health by increasing capillary density, mitochondrial function, and oxygen delivery. 6, , 7, Blood flow restriction (BFR) involves partial arterial and complete venous occlusion. Enhances the benefits of resistance exercise at low intensities and is particularly beneficial for elderly populations unable to perform high-intensity exercise. 8, , 9, A study reported that 12 weeks of low-intensity resistance training with BFR reduced blood pressure in elderly adults but had no effect on HRV. 10, Additionally, 6 weeks of walking with BFR improved time-domain HRV parameters and reduced systolic blood pressure in middle-aged men. 11, , 12, Considering that endurance exercise is more effective in improving cardiovascular function, combining this type of exercise with BFR may yield beneficial or adverse effects due to increased nervous system stimulation and additional metabolic stress. Due to limited information and insufficient knowledge of the benefits and disadvantages of this exercise model on the cardiovascular system, the present study investigated the effects of 10 weeks of endurance training on a cycle ergometer with blood flow restriction on HRV, ECG parameters, mean arterial pressure (MAP), resting heart rate, and heart rate recovery time (HRRT) in individuals aged 50 to 65 with grade 1 hypertension. Materials and MethodsThis randomized double-blind clinical trial was registered with the Iranian Registry of Clinical Trials (IRCT20230528058311N1) and received approval from the Ethics Committee of Kerman University of Medical Sciences (IR.KMU.AH.REC.1402.029). It was conducted in spring 2024 at Kerman University of Medical Sciences. All participants were fully aware of the study events, and written informed consent was obtained from each participant. Participants, Sample Size, and Study Design Participants in this study were aged between 50-65 years and had a body mass index (BMI) of less than 30. The participants were assessed by a cardiologist in the Javad-al-Aeme Hospital in Kerman (Iran). Persons were assessed by 48-hour ambulatory blood pressure monitoring. Based on the American Heart Association&,rsquo s guidelines, a systolic pressure of 130-139 mmHg and/or a diastolic pressure of 80-89 mmHg was measured as grade 1 hypertension. 13, Exclusion criteria include regular exercise for the previous 6 months, consuming any medicines that lesser blood pressure, positive history of joint or bone disorders, cancer, cardiovascular, liver, kidney, or pulmonary diseases, diabetes, and being overweight.The sample size was calculated using G*Power software (version 3.1.9.2, Heinrich-Heine-Universit&,auml t D&,uuml sseldorf, Germany) based on the number of groups and a one-way ANOVA design (&,alpha =0.05, power=0.80, effect size=0.5), resulting in a total sample size of 43 participants. Participants were randomly assigned to three groups, Ex+BFR (n=15) performed cycle ergometer sessions with blood flow restriction, Ex (n=15) performed identical training without BFR, and Control (Con) (n=13) maintained usual activity. In the Ex+BFR group, a cuff was applied to the thigh to restrict blood flow. Complete arterial occlusion pressure (AOP) for the femoral artery was estimated using the formula, 14, Lower body arterial occlusion pressure (mmHg)=(5.893&,times thigh circumference)+(0.912 systolic pressure)(0.734 diastolic blood pressure)-220.046The cuff pressure was set at 30% of AOP and maintained throughout the exercise program. The exercise duration was increased by five to 10 min each week. The control group continued their normal lifestyle without participating in any exercise activities. Measurement of Heart Rate (HR) and Calculation of MAP and VO2 Peak Participants were instructed to avoid alcohol, caffeine, intense physical activity, and stressful situations for at least 1 day before measurements. They were allowed to rest for 10 min to stabilize their heart rate. A Polar smartwatch (Polar, Finland) was utilized to automatically measure the resting heart rate. Arterial BP was measured using a digital monitoring device (Omron M2 Comfort, Japan). Measurement of arterial BP was calculated from the left hand for two periods with an intermission of 10 min, and the mean of these two values was recorded. MAP was calculated using the formula, MAP=DBP+1/3 (SBP-DBP) where SBP is systolic BP, and DBP is diastolic BP.Maximal oxygen consumption (VO2 Peak) was assessed by the Astrand test to calculate aerobic capacity. This test includes participants cycling for 6 min on a cycle ergometer (Monark, Ergomedic 839 E, Sweden) connected to a gas analyzer (Cortex, Metalyzer 3B, Germany), maintaining a cycling rate of 50&,plusmn 5 rebellions per min and a HR between 120 and 140 bpm. 15, The test measured oxygen saturation (SpO2) using a pulse oximeter (Beurer, Germany), HR, oxygen uptake (VO2), and respiratory exchange ratio (RER). Mean HR and output wattage were used to estimate the VO2 peak, with age adjustment. The test was considered valid if the participants maintained HRs between 120 and 140 bpm throughout the 6-min protocol. Training Protocol The 10-week exercise program consisted of three weekly sessions on a bicycle ergometer. The initial exercise duration was set at 15 min at 50-60% VO2 peak intensity, which corresponded to the target heart rate zone, and gradually increased by 5-10 min per week, reaching 55 minutes by week 10. Exercise intensity was adjusted every 2 weeks based on heart rate and rating of perceived exertion (RPE scale). For the Ex+BFR group, cuff pressure was adjusted based on RPE level during the sessions and temporarily released/reapplied as needed to ensure safety and comfort while maintaining exercise efficacy. A 10-min warm-up and cool-down period was included in each session. Measurement of HRRT Before the first exercise session, resting HR was measured as described above. Then, immediately after the exercise ended, the time it took for the heart rate to return to resting levels was recorded and considered as the baseline HRRT for each participant. This procedure was repeated in the last session of the protocol, and the final HRRT was recorded. ECG Recording and HRV Assessment Before and after the exercise program, participants were instructed to abstain from caffeinated beverages for 24 hours and strenuous exercise for 48 hours. Lead II electrocardiograms (ECGs) were recorded in the supine position after 10 min of rest at 25 &,deg C using a PowerLab system (ADInstruments, Australia). ECG parameters, including PR interval, QRS duration, JT interval, and heart rate-corrected QT (QTc), as well as HRV parameters such as time domains (RMSSD, SDSD, pRR50), frequency domains (LF, 0.04&,ndash 0.15 Hz HF, 0.15&,ndash 0.4 Hz LF/HF ratio), and nonlinear parameters (SD1, SD2, SD1/SD2) from Poincar&,eacute plots were calculated using dedicated software. To calculate the percentage changes in HRV, ECG, and MAP variables, the following formula was used, Percentage Change=(Final Value-Initial ValueInitial Value)&,times 100 Statistical Analysis Statistical analyses were conducted using Prism software (version 9, GraphPad Software, USA). Data were presented as mean&,plusmn SEM. The normality of the data was evaluated by the Shapiro-Wilk test. One-way ANOVA compared pre- and post-intervention values among groups. Significant group differences were examined using Tukey&,rsquo s post hoc test. Intragroup changes were analyzed with a paired t test. P value less than 0.05 was considered the level of significance.ResultsOf the 90 screened patients, 45 were randomized (15/group). Attrition occurred only in controls (n=2 withdrew final n=13). Both Ex and Ex+BFR groups maintained full retention (n=15 each) (figure 1,). Figure 1. The diagram depicts the CONSORT flow diagram of the investigation.Table 1, presents demographic characteristics, medication, and clinical history of each group. No differences across groups were detected at baseline.ParameterEx group (n=15)Ex+BFR group (n=15)Control group (n=13)P valueAge (years, mean&,plusmn SEM)54.2&,plusmn 5.859.1&,plusmn 6.357.4&,plusmn 5.50.124Sex, n (%)Male3 (20)3 (20)3 (23.07)0.975Female12 (80)12 (80)10 (76.92)0.975Weight (Kg, mean&,plusmn SEM)72.3&,plusmn 9.178.6&,plusmn 8.775.2&,plusmn 9.40.231BMI (Kg/m2, mean&,plusmn SEM)25.8&,plusmn 2.528.3&,plusmn 2.727.1&,plusmn 2.60.092SBP (mmHg, mean&,plusmn SEM)132.58&,plusmn 0.66134.23&,plusmn 0.54133.06&,plusmn 0.730.20DBP (mmHg, mean&,plusmn SEM)81&,plusmn 1.9281.63&,plusmn 1.8783.08&,plusmn 1.340.68Ethanol intake (%)000-Current smoker, (%)000-Family history of HTN, n (%)5 (38.5%)7 (46.7%)6 (40.0%)0.714BFR, Blood Flow Restriction SEM, Standard Error of Mean HTN, Hypertension BMI, Body mass index SBP, Systolic blood pressure DBP, Diastolic blood pressure Con, Control group Ex, Exercise training group Ex+BFR, Exercise training+Blood flow restriction group |