Chronic Refractory Angina

Transcutaneous electrical nerve stimulation (TENS)

The gate control pain theory(1) hypothesises that activation of myelinated large diameter sensory nerves (eg proprioceptive fibres) modulates sensory transmission by inhibiting the small diameter unmyelinated fibres carrying pain messages via inhibitory interneurones as sensory nerves enter the dorsal horn of the spinal cord. The application and stimulation of electrically conductive pads to the skin results in activation of the large diameter nerve fibres, which in turn appears to inhibit the onward transmission of painful stimuli to the brain, the inhibition occurring at the level of the spinal cord. Initially, TENS was used to treat localised pain that is of a somatic or neurogenic origin. However, its use in treating labour pain indicates its potential value in visceral pain(2). However, the mechanism of action with respect to treating angina remains unknown. Available data suggest that the effect is in addition to (and possibly separate from) analgesia produced by simple counter-stimulation.
It is often thought that the pain relief obtained by TENS is mainly a placebo effect. A study has demonstrated the analgesic effect of TENS intraoperatively in unconscious subjects, when there is no subjective input(3)
To date, all the work in angina has employed continuous, high-frequency TENS (4,5,6).
The skin should be clean, dry and free from grease or talc to ensure that conduction is not impaired. The electrodes are applied to the chest so that the main area where the patients experience their pain lies between them. They must be more than 1cm apart. The electrodes are connected to the TENS machine which is set to a continuous stimulation at a frequency of 70Hz and the intensity is set to a level immediately below that producing pain.
Patients are advised to use TENS for at least one hour three times a day. They are encouraged to use it during acute angina attacks. Typically this requires a stronger intensity of stimulation than that used for "prophylactic" stimulation. The intensity is increased until pain relief is achieved; by this, patients often describe not a resolution of pain in the first instance, but rather a modulation of their angina, in which they are aware of a sensation, but it is no longer uncomfortable or distressing. This effect is virtually immediate (in contrast to other, non-ischaemic pain states where TENS may not be effective for up to 30 minutes) and is rapidly followed by full resolution of angina. If the angina does continue for longer than 15 minutes, then patients are instructed to seek urgent review at hospital in order to rule out an acute coronary syndrome.

Safety

As a therapy TENS is very safe in most patients. Contact dermatitis is uncommon but if it occurs the type of electrodes, jelly and adhesive tape can all be changed. Permanent cardiac pacemakers are not a contraindication to TENS. However care must be taken as they may need reprogramming(7) and the TENS should not stimulate directly over the pacemaker. It is a sensible to assume that the patient will inadvertently stimulate over the pacemaker and so we routinely take the pracaution of checking the effect of TENS in the safety of the pacemaker clinic in all patients before allowing them home with a device. There is one report of TENS creating an artefact that was misinterpreted by an implantable cardiac defibrillator as ventricular fibrillation, resulting in the patient receiving an inappropriate shock. TENS should thus be used with great caution in such patients. 
It is advisable to avoid placing the electrodes across the throat and patients should be advised not to drive with the TENS switched on as charge could build up unnoticed and produce an unexpected surges of current.

If TENS fails

References

1. Melzack R, Wall PD. Pain mechanisms: a new theory. Science 1965;150:971-9.

2. Carroll D, Tramer M, McQuay H, Nye B, Moore A. Transcutaneous electrical nerve stimulation in labour pain: a systematic review. British Journal of Obstetrics and Gynaecology 1997;104:169-75

3. Bourke DL, Smith BA, Erickson J, Gwartz B, Lessard L. TENS reduces halothane requirements during hand surgery. Anesthesiology 1984; 61:769-72

4. Mannheimer C, Carlsson C-A, Vedin A, Wilhelmsson. Transcutaneous electrical nerve stimulation (TENS) in angina pectoris. Pain 1986;26:291-300.

5. Borjesson M, Eriksson P, Dellborg M, Eliasson T, Mannheimer C. Transcutaneous electrical nerve stimulation in unstable angina pectoris. Coronary Artery Disease 1997;8:843-50.

6. Mannheimer C, Carlsson C-A, Emanuelsson H, Vedin A, Waagstein F, Wilhelmsson C. The effects of transcutaneous electrical nerve stimulation in patients with severe angina pectoris. Circulation 1985;71:308-16.

7. Chen D, Philip M, Philip PA, Monga TN. Cardiac pacemaker inhibition by transcutaneous electrical nerve stimulation. Archives of Physical Medicine and Rehabilitation 1990;71:27-30.

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