Introduction Pulsed electromagnetic field (PEMF) therapy is a noninvasive technique, which provides low field electromagnetic stimulation. The therapy of PEMF is achieved by altering biological and physiological systems via low energy and non-ionizing electromagnetic fields. PEMF therapy was originally used clinically to manage osteoarthritis related pain and stiffness and to augment bone healing [1,2]. In addition, recent research has explored the beneficial therapeutic effect of PEMF on microvasculature and circulation . More recently, research interests have expanded their focus to additional mechanisms and syndromes, including a focus on the effects of PEMF on peripheral vascular function and blood flow. It has been suggested that PEMF therapy enhances the binding of free calcium (Ca2+) to calmodulin (CaM) and this phenomenon might alter tissue repair and/or pain [4–6]. Moreover other studies suggested an effect of PEMF on Ca/CaM-dependent nitric oxide (NO) signaling pathways , which is one of the major components for controlling vascular tone and BP. However, the impact of PEMF on NO and blood pressure (BP) regulation is still unclear and there is a lack of studies in humans.
High blood pressure is a foremost clinical feature of metabolic syndrome which is highly associated with cardiovascular disease . In addition, it may have pathophysiological and mechanistic connections with other clinical syndromes including obesity, insulin resistance and hypercholesterolemia [8,9]. Although lifestyle change remains a mainstay for treating hypertension, in the clinical settings, pharmacological treatment is increasingly utilized. However, some hypertensive medications are minimally effective and often elicit metabolic side effects such as weight gain, a reduction in high density lipoprotein (HDL) and an increase in triglycerides level . In addition, the attempts to improve diastolic heart function, an end result of chronic hypertension, via nitrate/nitrite associated pathways have appeared to result in minimal benefit or even a potential negative benefit with chronic utilization [11,12]. Therefore, non-pharmacological treatments such as exercise, dietary changes and/or weight loss remain preferential, however other non-pharmacological strategies to normalize blood pressure especially in higher risk populations is important. Accordingly, the present study investigated the impact of 12 weeks of PEMF therapy on NO bioavailability and BP at rest and during exercise in individuals with metabolic syndrome. It was hypothesized that PEMF therapy would reduce BP at rest and during exercise and increase NO bioavailability.
Methods/Subjects Forty eight subjects with mild to moderate metabolic syndrome participated. For the present study, metabolic syndrome was defined as individuals who had 2 or more of following 5 criteria: hypertension (systolic blood pressure ≥130 mmHg), dyslipidemia 1(triglyceride ≥150 mg/dl), hyperglycemia (fasting glucose ≥100 mg/dl), obesity (body mass index >30 kg/m2) and dyslipidemia 2 (high density lipoprotein <40 mg/dl (male) and <50 mg/dl (female). During participating in the study, subjects were asked to make no changes to their current medication or exercise habits. The study was explained to all subjects and they provided a written informed consent prior to study participation. All aspects of the present study were reviewed and approved by Mayo Clinic Institutional Review Board.
Protocol overview The study was a double blind and between-subject design. The experimental protocol required subjects to report to our cardiopulmonary exercise laboratory on two occasions for pre and post-therapy assessments. Prior to an initial assessment, subjects were randomly assigned to either the PEMF treatment group (PEMF, n = 24) or the control group (SHAM, n = 24) by a random number generator. Of the 48 subjects, four subjects (one from the PEMF group and three from the SHAM group) discontinued the study due to personal issues or time conflicts and thus were not included in the analysis. Thus, 23 subjects from the PEMF group and 21 subjects from the SHAM group successfully completed the study (Figure 1). At the initial assessment, subjects underwent a series of tests to include resting BP measured in triplicate using sphygmomanometer (767 series, Welch Allyn, Germany), a blood draw for NO levels, followed by an incremental exercise test on a cycle ergometer (Corival, Lode, Netherland). The protocol started with a resting phase for 2 min, moving to an incremental phase progressing until the subjects rated their perceived level of exertion (RPE) at ∼14–15 and finished with a constant phase for 3 min. During the incremental phase, workload was increase by ∼20–30 watts every 2 min based on subjects’ age/sex/body weight. Blood pressure was measured manually in triplicate at rest and once during the last minute of exercise. All BP measurements were assessed by the same trained investigator. Heart rate (HR) and oxygen saturation (SpO2) were monitored continuously via forehead pulse oximetry (Radical-7, Masimo, Irvine, CA). Breath by breath respiratory gas exchange was continuously monitored via a metabolic measurement system (Medgraphics, Saint Paul, MN). After completion of pre-assessment, subjects were instructed how to utilize the PEMF device. Thereafter, they were discharged with PEMF device and usage diary and returned the laboratory after 12 weeks for post-therapy assessment. During post-assessment, all measurements made on the initial assessment were repeated and the exercise protocol was kept identical to the initial test for each subject (i.e. the same workloads and duration were utilized).
To determine the impact of PEMF on plasma NO, a blood sample was collected at rest prior to submaximal exercise during the pre and 12 week post- therapy assessments. Plasma NOx (NO2+NO3) level was measured via the Griess reagents method (Nitrate/Nitrite colorimetric assay kit, Cayman Chemical Co. Ann Arbor, MI). Blood was collected into ethylenediaminetetraacetic acid containing tubes and immediately centrifuged at 3000 rpm, 4 °C for 15 min. Afterward, plasma was transferred into a 10 kDa molecular weight cut-off ultrafiltration centrifuge cryovial (Sartorius Vivaspin 500, Cole Palmer, Vernon Hill, IL) and centrifuged at 15,000 × g, 4 °C for 5 min. Filtered plasma was frozen at −80 °C and all analysis performed in one batch. For quantification of NOx, 40 µL of the plasma was diluted with 240 µL assay buffer and mixed with 10 µL nitrate reductase and 10 µL enzyme cofactor. After the plasma had been incubated at room temperature for 3 hours to convert nitrate to nitrite, total nitrite was measured at 540 nm absorbance by using the Griess reagents reaction.
Pulsed electromagnetic field therapy For the present study, a portable PEMF device (Bioboosti, Biomobie, Shanghai, China) was utilized. For the PEMF group, the bioboosti included adjustable magnetic field strength range (X-axis: 0.22 ± 0.05 mT, Y-axis: 0.20 ± 0.05 mT and Z-axis: 0.06 ± 0.02 mT) and working frequency (30 ± 3 Hz). This magnetic strength range and frequency were maintained during 12 weeks of the study period. For the SHAM group, the sham PEMF devices were modified to deliver no micromagnetic field when turned on. The subjects were instructed to use their device three times per day: providing micromagnetic emitting on both hands during a morning session, both hands again during an afternoon session and both feet during a night session. Each session took 16 min and thus subjects were exposed to therapy for 48 min per day. To obtain adherence of PEMF, the participants recorded usage on a daily log collected at the completion of the study. Subjects were off from the therapy on the day for post-assessment to avoid acute impact of PEMF.
Statistical analysis To determine the effect of PEMF therapy on BP, plasma NO and HR, the initial analysis was performed via a repeated measure analysis of variance (ANOVA). For BP, systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) were analyzed separately. In addition, this analysis was conducted based on (1) difference in BP between the PEMF group and the SHAM group (all subjects), (2) a relationship between pre BP and the Δ in BP from pre to post in the PEMF group and the SHAM group and (3) difference in parameters between PEMF group and the SHAM group in the subset of participants who had uncontrolled hypertension at the initial visit (hypertension defined as resting SBP > 140 mmHg and/or resting MAP > 100 mmHg). For this analysis, 11 subjects from the PEMF group and 9 subjects from the SHAM group were identified. In addition, a paired sample t-test and an independent t-test were conducted for specific difference between pre- and post-therapy and the PEMF and the SHAM groups. Two-tailed statistical significance was determined using an alpha level set at 0.05 and SPSS (version 22) was utilized for all analysis.
Results The anthropometric characteristics of the forty-four subjects with metabolic syndrome who completed the study are described in
. Adherence rate was 92.1 ± 8.2% for the PEMF group and 93.5 ± 8.0% for the SHAM group. Following PEMF therapy, although the PEMF group demonstrated a fall in SBP, DBP and MAP when comparing to the SHAM group, these differences were not significant (p = .16, p = .20 and p = .14, respectively). During exercise, the PEMF group demonstrated a reduction in peak SBP at the same workload after 12 week therapy (p = .04) while the SHAM group did not (p = .57). However, no significant differences in peak DBP and peak MAP were observed between groups (p = .50 and p = .18, respectively). In addition, there was no significant difference in resting HR and peak HR between groups (p = .24 and p = .34, respectively) and resting SpO2 and peak SpO2 were not different between groups (p = .66 and p = .66 respectively). However, there was a significant difference in plasma NO levels following 12 week-PEMF therapy between groups (p = .04). The PEMF group showed a significant improvement in plasma NO levels after therapy (p = .04), but the SHAM group did not (p = .37). illustrates the alterations in parameters at rest and during exercise following therapy.