A Transcutaneous Electrical Nerve Stimulation (TENS) unit is a portable, non-invasive device that delivers a low-voltage electrical current through electrodes placed on the skin for pain management. This electrical stimulation is generally considered safe for use by the public. However, improper use or use by individuals with specific pre-existing conditions carries a serious risk of cardiac interference.
How TENS Units Relieve Pain
The pain-relieving effects of a TENS unit are primarily attributed to two distinct, yet complementary, neurological theories. When the device is used correctly, the electrical current delivered through the skin is superficial and does not penetrate deep into the body’s tissues, targeting peripheral nerves located just beneath the surface.
One mechanism is the Gate Control Theory, which proposes that a “gate” exists in the spinal cord that controls the flow of pain signals to the brain. The electrical stimulation from the TENS unit activates large-diameter nerve fibers, which effectively “close the gate” to the slower pain signals transmitted by small-diameter fibers. Interfering with the transmission of pain messages at the spinal cord level leads to a reduced perception of pain.
The second established mechanism is the Endorphin Release Theory, focusing on the body’s natural pain-relieving chemicals. TENS stimulation is believed to promote the release of endogenous opioids, such as endorphins and enkephalins, into the bloodstream. These natural substances bind to opioid receptors in the brain and spinal cord, producing an analgesic effect similar to pain medication.
Understanding Electrical Interference and Cardiac Risk
Concern regarding TENS units and heart function stems from the fact that the heart is regulated by its own precise electrical system. The heart’s natural pacemaker, the sinoatrial node, generates electrical impulses that coordinate muscle contractions, creating a regular heartbeat. External electrical current presents a theoretical risk if its path crosses the transthoracic area, or the region around the heart.
If the electrical current is permitted to flow across the chest, it can interfere with the heart’s intrinsic electrical rhythm, potentially leading to the induction of abnormal heart rhythms (arrhythmias) or ventricular fibrillation. Ventricular fibrillation is a disorganized and rapid electrical activity in the ventricles, which prevents the heart from pumping blood effectively and can lead to sudden cardiac arrest.
The risk is significantly higher in individuals with unstable heart conditions. In patients with implanted electronic devices, such as pacemakers or implantable cardioverter-defibrillators (ICDs), the risk is device malfunction. TENS signals can be misinterpreted by these devices as abnormal heart rhythms, potentially causing an ICD to deliver an inappropriate shock or a pacemaker to inhibit its function, leading to a life-threatening pause or bradycardia.
Absolute Contraindications and Critical Placement Zones
Safety for TENS use involves strict adherence to a list of absolute contraindications and critical placement zones. Individuals with implanted electronic devices must avoid TENS therapy, as the electrical impulses can cause electromagnetic interference. This prohibition includes pacemakers, ICDs, and other indwelling stimulators or monitoring devices.
TENS is also contraindicated for individuals with severe heart conditions or a history of arrhythmias, as the external current could aggravate an unstable condition. Since the safety of the device for use during pregnancy has not been definitively established, pregnant women should avoid placing electrodes on the abdomen or lower back.
Electrode placement must strictly avoid several critical zones on the body to prevent harmful interference.
Critical Placement Zones
Electrodes must never be placed across the chest, as this creates a transthoracic current path directly across the heart.
Placement on the front or side of the neck is strictly prohibited because this area contains the carotid sinus, and stimulation here can cause a sudden, dangerous drop in blood pressure and heart rate.
Electrodes should not be placed over the eyes, in the mouth, or near the head, as trans-cerebral current flow is a risk.
General Safety Guidelines for TENS Use
Several general safety guidelines should be followed to ensure the effective and safe use of a TENS unit. It is recommended that a person only use a TENS unit that has been prescribed or recommended by a healthcare professional, especially when starting therapy. Consulting a doctor is important if a person has underlying conditions like cancer, epilepsy, or diabetes.
Proper electrode care is necessary to avoid skin irritation and potential burns. The skin must be cleaned and dried before applying the self-adhesive electrodes to ensure good contact and prevent changes in stimulation intensity. If skin irritation or a rash develops, the treatment should be stopped immediately.
The TENS unit should never be used while driving, operating heavy machinery, or while sleeping, as a shift in pad placement could cause a sudden change in current. The device should also not be used in the shower or near water, which could conduct the electrical current unsafely. If increased pain or any unexpected symptoms occur during use, discontinue treatment and consult a medical provider.