Can TENS Units Really Cause a Heart Attack?

Transcutaneous electrical nerve stimulation (TENS) units are commonly used for pain relief, delivering low-voltage electrical currents through the skin. While generally considered safe, concerns have been raised about whether these devices could interfere with heart function or even trigger a heart attack.

Given that the heart relies on electrical signals to maintain its rhythm, it’s important to understand how external electrical currents might affect it.

Mechanism of TENS Signal

TENS units deliver low-voltage electrical impulses through electrodes on the skin, targeting nerves to modulate pain perception. These impulses typically range from 1–200 Hz, with intensities varying from 1 to 100 milliamperes (mA), depending on device settings and individual tolerance. The primary mechanism stimulates afferent nerve fibers, particularly Aβ fibers, which transmit non-painful sensory information. By activating these fibers, TENS inhibits nociceptive signals from Aδ and C fibers, which are associated with pain perception. This aligns with the gate control theory of pain, suggesting that non-painful stimuli can suppress pain signals at the spinal cord before reaching the brain.

TENS also influences endogenous pain modulation. High-frequency TENS (50–100 Hz) primarily activates inhibitory interneurons in the spinal cord, reducing pain transmission through presynaptic inhibition. Low-frequency TENS (1–10 Hz) has been linked to the release of endogenous opioids, such as enkephalins and endorphins, which bind to opioid receptors to produce analgesic effects. These dual mechanisms provide both immediate and prolonged pain relief, depending on the frequency and duration of stimulation.

TENS impulses are designed to remain localized, minimizing systemic effects. However, electrical conductivity varies based on tissue composition, hydration, and electrode placement, influencing current spread. While most current remains superficial, some studies suggest deeper structures, including autonomic nerve fibers, may be affected under certain conditions. This has led to investigations into whether TENS could influence autonomic functions, such as heart rate variability, particularly when applied near the cervical or thoracic spine.

Cardiac Conduction and External Electrical Currents

The heart’s rhythmic contractions are governed by an electrical conduction system that ensures coordinated blood pumping. This system originates in the sinoatrial (SA) node, the heart’s natural pacemaker, generating impulses that travel through the atria, prompting contraction. The signal then reaches the atrioventricular (AV) node, where it experiences a brief delay before propagating through the bundle of His and Purkinje fibers, triggering ventricular contraction. This sequence is maintained by ion channel activity within cardiac myocytes, particularly the movement of sodium, potassium, and calcium ions. Any disruption can result in arrhythmias, which may range from benign to life-threatening.

External electrical currents, such as those from a TENS unit, can interact with cardiac conduction when applied near the heart. While TENS devices primarily affect peripheral nerves, unintended stimulation of autonomic nerve fibers could influence heart rate or rhythm. The autonomic nervous system, particularly the vagus nerve and sympathetic pathways, plays a key role in cardiac function. Electrical stimulation near the cervical or thoracic spine has been shown in some studies to alter heart rate variability, suggesting even low-intensity currents can exert physiological effects beyond the targeted area. Although TENS is not designed to penetrate deeply enough to directly affect myocardial tissue, variations in physiology, electrode positioning, and current intensity could theoretically create conditions for electrical interference.

Experimental research has explored how external electrical currents influence cardiac electrophysiology. Studies in animal models have shown that high-intensity impulses can alter cardiac excitability, particularly near the vagus nerve or stellate ganglia. In humans, clinical investigations have reported cases where unintended electrical stimulation—such as from transcutaneous electrical stimulation used for other therapies—has triggered transient bradycardia or tachycardia. However, these effects are generally associated with settings far exceeding those used in standard TENS therapy. Regulatory guidelines, such as those from the FDA, caution against electrode placement near the anterior chest wall or over the carotid sinus due to potential autonomic interactions that could disrupt heart function.

Body Regions for TENS Electrode Placement

Electrode placement is critical in optimizing TENS therapy while minimizing unintended effects. Most commonly, electrodes are positioned over or near the pain site, such as the lower back for chronic lumbar discomfort, the knee for osteoarthritis, or the shoulder for musculoskeletal injuries. Placement strategies often follow dermatome maps to ensure stimulation of the appropriate nerve pathways, enhancing pain relief.

Certain placements are discouraged due to the potential for unintended neuromodulation. Areas such as the anterior neck, particularly over the carotid sinus, are avoided because stimulation in this region can activate baroreceptors, leading to sudden drops in blood pressure or reflex bradycardia. Similarly, positioning electrodes over the thoracic cavity or near the sternum raises concerns about unintended stimulation of cardiac autonomic fibers. While TENS units are not designed to interfere with myocardial conduction, applying electrodes too close to the heart may theoretically influence autonomic regulation, especially in individuals with cardiovascular sensitivities.

Electrode spacing affects current distribution. Placing electrodes too close together creates a concentrated stimulation zone, which may cause discomfort or ineffective nerve activation. Conversely, positioning them too far apart can reduce current efficiency, limiting therapeutic benefits. Most guidelines recommend maintaining at least a few centimeters of separation while ensuring they remain within the same dermatome or myotome. Electrode polarity also influences sensation and effectiveness, with some users reporting greater analgesic effects when cathodal stimulation is placed proximally in certain configurations.

Observations in Pre-Existing Cardiac Conditions

Individuals with pre-existing cardiac conditions may have heightened sensitivity to external electrical stimuli, raising questions about TENS safety in this population. Conditions such as atrial fibrillation, ventricular arrhythmias, and heart failure involve altered electrical conduction, which could theoretically make the heart more susceptible to external influences. While TENS units are designed to stimulate peripheral nerves rather than cardiac tissue, variations in physiology mean some patients may experience unexpected responses, particularly if electrodes are placed near autonomic control centers regulating heart rate and vascular tone.

Clinical observations suggest that TENS therapy generally does not induce adverse cardiac events in individuals with stable heart disease. A study published in The Journal of Pain examined TENS effects on patients with coronary artery disease and found no significant changes in electrocardiographic parameters or heart rate variability when electrodes were placed on the limbs or lower back. However, case reports have documented transient bradycardia in patients with compromised autonomic function, particularly when stimulation occurs near the cervical spine. This suggests that while the risk of direct myocardial interference is low, certain electrode placements could trigger autonomic reflexes problematic for those with dysautonomia or other autonomic dysfunctions.

Interactions With Implantable Cardiac Devices

Individuals with implantable cardiac devices, such as pacemakers and implantable cardioverter defibrillators (ICDs), require careful consideration when using TENS units. These devices rely on precise electrical signaling to regulate heart rhythm, and external electrical currents could interfere with their function. Manufacturers of implantable devices warn about electromagnetic interference (EMI) from various sources, including medical and consumer electronics, to ensure reliability. While TENS impulses are not typically strong enough to disrupt implanted devices under normal circumstances, electrode placement near the chest could increase the likelihood of unintended interactions.

Research on TENS effects on pacemakers and ICDs has produced mixed findings. Some studies show no significant interference when electrodes are placed on the lower extremities or back, while others document instances of inappropriate sensing or inhibition of pacing when stimulation occurs near the thoracic region. The interference mechanism likely involves the misinterpretation of TENS pulses as intrinsic cardiac activity, leading the device to suppress necessary pacing or deliver inappropriate shocks. To mitigate these risks, guidelines from organizations such as the American Heart Association recommend positioning electrodes several inches away from the pacemaker or ICD site and using the lowest effective current intensity. Individuals with implanted cardiac devices should consult their healthcare provider before using TENS therapy to ensure safe application tailored to their condition.