Hypertension has high prevalence and poor control rates and is considered one of the major modifiable risk factors for cardiovascular system diseases, and one of the most important public health problems. The imbalance of the autonomic cardiovascular control, which can be considered as a major etiologic factor in the development of essential hypertension is characterized by increased sympathetic activity and reduced parasympathetic activity possible.

Cardiovascular disease is the leading cause of death and disability in the world and autonomic imbalance is associated with several pathological conditions, and may be a final common pathway for the increased morbidity and mortality of cardiovascular diseases. In this sense, the heart rate variability (HRV) is a quantitative marker of sympathetic and parasympathetic activity, which can be used to assess disease and mortality as a noninvasive technique.

TENS is a noninvasive therapeutic modality, easy to handle, it has no side effects or interactions with medications being used for the relief of pain by sensory stimulation through peripheral nerves and mainly for control and treatment of acute and chronic pain . Research on the effect of TENS on the sympathetic and parasympathetic nervous system activity remain controversial, especially regarding the parameters to use.

The aim of this study is to investigate the effects of TENS on the sympathetic and parasympathetic nervous system in patients with hypertension by heart rate variability and blood pressure variability.

This is a randomized clinical trial, double blind, where hypertensive patients were randomized to three groups: high-frequency TENS (100 Hz) n = 20, low frequency TENS (4 Hz) n = 20 and n = 20 placebo. Evaluations will be made in a single session. The results will be assessed by a blinded investigator and randomization will be done electronically.

It is expected to evaluate how often TENS exerts more influence in the modulation of sympathetic and parasympathetic system.

Data collection was performed at the Laboratory of Clinical Investigation (LIC) Institute of Cardiology. Initially, patients rested for 30 minutes, and during this period has been completed the evaluation form with the identification data and questions about the patient's clinical status, such as checking blood pressure, weight and height.

Patients were randomized to group of high frequency (100 Hz / 200μs), low frequency (4 Hz / 200μs) or placebo. The placebo group received an active current for 30 seconds, and then gradually decreased for 15 seconds to not pass any current. This approach aims at masking the investigator and subject (RAKEL et al., 2010). Eligible subjects were randomly assigned in blocks, by electronic randomization (www.randomization.com).

The adhesive electrodes were used, size 9x5 cm, placed in bilateral paravertebral region of the T1 segment of the thoracic spine at L2 lumbar spine. Before the application of TENS (IBRAMED®), the skin was cleaned in place of the current application with alcohol to avoid any barrier conduction of electrical current.

The sessions took place at the same time of the day, throughout the protocol, the participants were comfortably accommodated in an acclimatized room (23ºC) in a supine position, head elevation of 30º with knees resting on a wedge. All the participants were forbidden to perform exhaustive exercises and intake caffeine at least two hours before the intervention and instructed to have a meal before the assessment.

TENS was applied for thirty minutes, with the frequency defined according to randomization. The intensity of the current was delivered at sensory-level intensity, adjusted every 5 minutes by the sensory threshold, during the 30 minutes as tolerated by each subject, but without motor contraction or pain reported by the subject.

The analysis of autonomic control was performed by means of a sensor placed on the middle finger, and connected to the Finapres (Ohmeda 2300, Colorado, USA) by recording beat to beat. Then, the conversion signal was performed using the powerlab (LAB chart). The analysis will be performed using the software Kubios.

Inclusion Criteria:

Previous Hypertension diagnosis, systolic blood pressure (SBP) > 140 mm Hg and/or diastolic blood pressure (DBP) >90 mm Hg and clinical stability with no change in medications for at least 2 months preceding the study.

Exclusion Criteria:

Presence of any comorbidity such as diabetes; congestive heart failure; unstable angina; myocardial infarction; acute inflammation; peripheral vascular disease; associated neurological disease; acute respiratory illness; infectious disease or fever; cardiac pacemaker; unstable ventricular arrhythmia; active smokers; obesity; musculoskeletal disease in the lower limbs; changes in the electrocardiogram; smoker; treatment with beta blockers, steroids, hormones or chemotherapy or change in drug therapy were excluded from the study.