Effect of TENS on Post-operative Sternal Pain (Post Cardiac Surgery): A Narrative Review

Introduction

Sternum pain signifies the pain in the thoracic cavity that contains the sternum and the cartilage connecting it to the ribs. Commonest cause of sternal pain postoperatively is sternal surgery. The incidence of post-sternotomy can even persists for up to three years. Some of the patients report severe pain which affects their level of function over a period of several months post- surgery. Sternotomies are carried out during surgeries like thymectomies, Coronary Artery Bypass Graft (CABG), heart valve replacement, lung transplant and lobectomy/pneumonectomy. Marcassa et al., (2015) stated that inadequate pain management in acute, subacute and rehabilitative post- surgical period may lead to persistent pain in the long term. Hetmann et al., (2017) supported these findings and suggested that preoperative pain is also a factor that influences the postoperative pain and pain is a factor that tends to induce depression and anxiety in patients.¹

Sternum will heal properly for most of the people, except for those who have some complications or comorbidities. The incidence of postoperative sternal complications, such as sternal dehiscence, infection, and sternal instability, is reported to be as low as 1% to 5%. The sternal complications can in turn result in increased length of stay in the hospital, increased costs, thereby leading to high rates of morbidity and mortality.² Multiple interventions in physiotherapy like Transcutaneous electrical nerve stimulation (TENS), sternal precautions, upper limb and trunk exercises, wound support, restrictions on lifting and transfers, restrictions on mobility aid use, breathing exercises, incentive spirometry, thoracic expansion exercises, coughing and huffing, positioning, and modified postural drainage are commonly used, both pre- and postoperatively, some of which are reported to be found useful for the treatment of sternal pain.¹⁰ Out of all these interventions, TENS is a modality that blocks the pain by pain gate mechanism, and is documented to be safe and effective in all kinds of pain.

Sternal pain post sternotomy

Pain in the postoperative period of cardiac surgery contributes to worsening of the respiratory muscle strength and decrease of lung volumes and capacity, thus reducing the number of deep breaths and effectiveness of cough, without which there can be operative complications. The pain stimulus makes physical therapy even slower due to the lack of cooperation.^1,2 The surgical wound pain restricts lung expansion to a certain extent, thus favouring respiratory complications.³

A significant impairment in pulmonary function can occur after median sternotomy. Substantial reduction in lung volumes were reported in patients after median sternotomy performed during cardiac surgery. The mechanism of volume reduction after sternotomy is unclear. But pain is considered as one of the most relevant factors in immediate postoperative days. Subsequently, abnormalities in the chest wall mechanics may also occur (Chetta et al., 2006). Pain, the main manifestation reported by patients who undergo heart surgery has a multiple factors. The factors that influence pain may include the surgical incision, dissection of tissue, sternal retraction, multiple cannulations, chest drainage tubes and the invasive procedure which patient undergoes during the treatment.⁴

A number of lung defence mechanisms may become impaired or overtly ineffective due to intense incisional pain, deep breathing, body mobilization, in particular coughing.⁵ Hence reducing the post-operative pain can be one of the most important factors that can contribute to improvement of pulmonary function and prevent the post-operative complications after median sternotomy.

Transcutaneous Electrical Nerve Stimulation

Transcutaneous electrical nerve stimulation (TENS) is a non-invasive peripheral stimulation technique used to relieve pain. During TENS, pulsed electrical currents are delivered across the intact surface of the skin to activate underlying nerves. The purpose of conventional TENS is to selectively activate large diameter non-noxious afferents (A-beta) to reduce nociceptor cell activity and sensitization at a segmental level in the central nervous system.⁶

TENS is the delivery of pulsed electrical currents across the intact surface of the skin to stimulate peripheral nerves principally for pain relief.⁷ TENS is a method of electrical stimulation which primarily aims to provide a degree of symptomatic pain relief by exciting sensory nerves and thereby stimulating either the pain gate mechanism and/or the opioid system. The different methods of applying TENS relate to these different physiological mechanisms. The effectiveness of TENS varies with the clinical pain being treated, but research would suggest that when used ‘well’ it provides significantly greater pain relief than a placebo intervention.

TENS is the application of electrical current through electrodes placed on the skin for pain control. It can be applied with varying frequencies, from low (< 10 Hz) to high (> 50 Hz). Intensity may also be varied from sensory to motor intensities. Sensory intensity is when the patient feels a strong but comfortable sensation without motor contraction. High intensity usually involves a motor contraction but is not painful. In general, higherfrequency stimulation is delivered at sensory intensity, and low-frequency stimulation is delivered at motor intensity. Prior literature from our laboratory shows that, regardless of intensity, different frequencies activate central mechanisms to produce analgesia. Specifically, we show that low-frequency TENS activates μ-opioid receptors in the spinal cord and the brainstem, whereas high-frequency TENS activates δ-opioid receptors in the spinal cord and the brainstem.⁸

Need of the review

Patients who experience pain after any cardiac surgery may also experience prolonged immobilization, insufficient respiratory functions, and the inability to cough as a result of the surgical procedure, specifically post median sternotomy. Pain is  considered as one of the most important factor influencing in post-operative pulmonary dysfunction and in developing a restrictive ventilatory pattern.²² Therefore, duration of mechanical ventilation and length of hospital stay of these patients may increase significantly.²³ TENS has been proven to be useful as an adjunct or a single intervention in reducing any kind of post-surgical thoracic pain.²⁴ Hence this narrative review was conducted to gather the studies on the effect of TENS in post- operative thoracic pain, to further strengthen the evidence and document its effectiveness on the same.

Aim and purpose of the review

1. To investigate the efficacy of TENS on postoperative sternal pain.

2. To determine the parameters of TENS used in postoperative sternal pain for pain reduction and vital capacity.

3. To investigate the efficacy of TENS on short-term transcutaneous electrical nerve stimulation after cardiac surgery, its effect on pain, pulmonary function, electrical muscle activity, length of hospital stay, post-operative atelectasis.

Methods

Database search was conducted for relevant peerreviewed articles, using the search terms “TENS”, “Physiotherapy”, and “Sternal pain”. Databases searched were CINAHL Plus with full text, Cochrane Library (Wiley), Healthcare Journals, Medline/PubMed, Google, Google Scholar, Science Direct (Elsevier), Scopus etc. Other potentially relevant literature was identified through bibliography searches of included articles and expert recommendation. Studies published between 2005 -2020 were included. Titles and abstracts of citations were then screened for relevance to the inclusion. “MeSH” terms were used for the search. RCTs (Randomised Control Trials) were included in this search. Out of 70 studies that were screened, six fulfilled the inclusion criteria and were included in the review.

Inclusion criteria

Studies conducted between 2005-2020

Published and unpublished studies

Studies specifically focussing on cardiac surgery

Randomised controlled trials

Studies using TENS in conjunction with an intervention or as the main intervention

Studies including patients above 18 years of age

Exclusion criteria

Studies conducted before 2005 or after 2020

Studies relating to other surgeries apart from cardiac surgery

Studies using any other electrotherapeutic modality apart from TENS

Studies including patients below18 years of age

Mechanism of TENS in pain reduction

TENS is a method of electrotherapeutic method in which stimulation of nerve fibres occur. Most commonly used technique of TENS is the conventional TENS. Conventional TENS are low – intensity pulsed currents administered at high frequencies. During conventional TENS, low-intensity pulsed currents are administered at the pain site with high frequencies ranging between 10-200 pps. Patient experiences tingling sensation or paraesthesia. Activation of large diameter nerve fibres by conventional TENS closes pain gate at spinal level, thus causes reduction of pain. One more method of therapeutic TENS application is acupuncture like TENS with high intensity and low frequency less than 10pps. This TENS will be applied over the muscles, acupuncture points and trigger points. Acupuncture like TENS will activate small diameter afferents which closes the pain gate via extra segmental mechanisms. High intensity high frequency TENS has a counter-irritant effect.¹⁵ Clinical application of TENS will be with varying frequencies, intensities, and pulse durations depending upon the therapeutic needs. Frequency is classified as high frequency (>50 Hz), low frequency (<10 Hz), or burst (bursts of high frequency stimulation applied at a much lower frequency) TENS. Intensity is decided based on patient’s response to the treatment at sensory level/ motor level.¹⁶

High frequency TENS provides analgesia by activating endogenous inhibitory mechanisms in the central nervous system involving opioid GABA, and muscarinic receptors. Low frequency TENS acts on descending inhibitory pathways involving the PAGRVM pathway activating opioid, GABA, serotonin and muscarinic receptors to reduce dorsal horn neuron activity and thereby pain.¹⁷ Their review suggests TENS as an effective method in reducing pain postoperatively, osteoarthritis, painful diabetic neuropathy and some acute pain conditions if appropriate dosage is provided.

TENS and post-operative pain

In addition, some studies have also documented a lower incidence of postoperative complications like paralytic ileum, atelectasis reduction in pulmonary function post TENS application.²¹ The use of TENS has been documented to control postoperative pain after multiple procedures like cardiac operations, cholecystectomy, caesarean delivery, and thoracotomy. In a study conducted by Lima et al., (2011), TENS was administered to on pump CABG patients post sternotomy, and this study demonstrated significant effectiveness of TENS in pain reduction and the increase in respiratory muscle strength at first-day after CABG surgery.²⁰ A study conducted by Fabrizio et al., suggested that TENS is not effective in cases of very painful operation. Opioid and non-opioid analgesics were documented to be better compared to TENS. In contrast, TENS can be used as an adjunct to other medications when pain is moderate and can be the only pain therapy when pain is mild.²¹ TENS acts as an analgesic, is non-invasive, has no side effects and documented to be effective in pain management.² TENS reduced atelectasis, improved peak expiratory flow rate, six-minute walk test distance and Visual Analogue Scale following CABG surgery in a study.¹² TENS has been shown to be effective in acute and chronic pain in both medical and surgical conditions in multiple studies conducted since 1970.⁹ Some studies have stated that the role of TENS can be controversial in post-operative pain,^2,9,10 but have demonstrated its effectiveness in incisional pain and cough associated pain.¹⁰

Outcome measures

Many of the studies in this review have taken postoperative pain as the primary outcome measure. Some other outcome measures considered were shortterm transcutaneous electrical nerve stimulation after cardiac surgery, its effect on pain, pulmonary function and electrical muscle activity,  thus determining its application as an adjunct to physiotherapy, or as a sole treatment. One investigation studied the effect on extubation time, rescue analgesic medication, length of intensive care unit stay, or length of hospital stay.³ Postoperative atelectasis following surgery was one of the outcomes considered in one of the studies.⁶

Postoperative pain that was taken as an outcome measure was divided into early postoperative pain,³ and late postoperative pain. Early postoperative pain was also classified as recorded at 4h, 5h, 6h, 7h, 8h postoperatively³ or as the pain on first day of surgery.¹⁵

Results

Out of all the studies, six have shown the effect of TENS on sternotomy / cardiac surgery. Following were the findings of the studies:

Discussion

Six randomized controlled trials fulfilling the inclusion criteria were selected. Out of the six studies, five studies have demonstrated the effectiveness of TENS in reducing post- operative pain in cardiac surgery. One study has demonstrated that parasternal block is more effective than TENS. All the five studies have shown the clinical efficacy of TENS post sternotomy.

The theory behind the technique of TENS is the gatecontrol theory of pain by Melzack and Wall. The activity of myelinated A fibres can block the pain caused by unmyelinated C fibres. When these fibres are stimulated, they prevent painful peripheral stimuli from gaining access to higher cortical centres. The release of endorphins and activation of inhibitory reflex areas in the brain stem are the alternative mechanisms suggested for pain control through TENS.⁹

There are studies supporting the evidence that TENS is effective in reducing post- operative cardiothoracic pain. Endorgan et al., concluded that TENS helps to reduce the need for opioid intake during the postoperative period; also, it has been proved to be effective in post thoracotomy pain. Similarly, Benedetti and colleagues reported that TENS is useful in controlling mild or moderately acute post thoracotomy pain caused by muscle-sparing thoracotomy, median sternotomy, and video-assisted thoracoscopic surgery.⁸ Gerson et al., have mentioned in their study that although the exact mechanism(s) for TENS-related hypoalgesia is currently unknown, they could include peripheral blocking of nociceptive input, activation of descending inhibitory feedback circuits, ‘gating’ of noxious stimuli at spinal segmental level, and or activation of either opioid and non-opioid systems at a cortical level. Even in patients with more severe pain, TENS showed to be useful at least when applied for a short period of time (4 h).¹⁰ Sukhyanti et al., stated that studies with both peri-incisional placement of electrodes and acupoint stimulation reported TENS to be effective in reducing pain and additional analgesics requirement.¹¹

Bjordal et al., published a meta-analysis with studies using TENS as an analgesic resource in the postoperative period (cardiac surgery) between 1966 and 2001. In that meta-analysis, they stated that TENS helped decreasing the use of analgesic medication during the first three postoperative days, which helped decrease the side effects of the medication like depression of the respiratory centre. They also demonstrated a significant difference in pain assessment in patients undergoing placebo versus therapeutic TENS.¹¹ Kiran et al., stated that a combination of TENS along with conventional physiotherapy is much more efficient than conventional physiotherapy alone in reducing pain and atelectasis in patients undergoing median sternotomy.¹²

A study reported by Vishwas et al., stated that there was also evidence that TENS can directly decrease the conduction and amplitude of painful stimuli through the A- fibers. This study has stated that TENS can be used for pain relief during removal of chest drains after cardiac surgery, in conjunction with analgesics. TENS is simple to use.¹³

Thus, TENS has been hypothesized to reduce pain by many theories that included effects on sensory nerves, interference with sensory discriminative pathways, stimulation of release of natural chemicals that affect the way pain is perceived and transmitted, for example, enkephalins and endorphins or through increased blood flow in treated areas such as the skin or heart. Recent data also suggested that pain relief from low- and highfrequency TENS is mediated by the release of mu- or delta-opioids in the CNS and reductions in substance P.¹⁴ All these studies provided a strong evidence that TENS can be used in controlling post-operative pain.

Applications and implications

Sternal pain can be a disabling factor in postoperative patients and can significantly deteriorate their quality of life. Using TENS as an adjunct treatment for pain reduction in a hospital set up can help the patients recover faster and also help in contributing to reducing the psychological trauma and apprehension postsurgery which is associated with this pain. If TENS is incorporated in an IPD or OPD setup on a daily basis, then the reduction of pain can contribute to a faster functional recovery in the patients.

Strength of the review

Studies included were all recent studies.

limitation of the review

More research can be conducted on effect of TENS on post-operative sternal pain to strengthen the evidence in these kind of reviews.

Conclusion

The application of TENS in sternotomy has seen positive effects as far as improving forced vital capacity, reducing postoperative pain, improving functional capacity, proving as an adjunct to physiotherapy is concerned. Majority of the studies stated TENS as a positively effective intervention in sternotomy cases and very few studies stated that it was not very effective. This review emphasises that TENS can be a safe, effective and feasible adjunct or a main treatment modality for use in clinical setup, thus providing comfort to the patients of sternotomy in the hospital admission period as well as post discharge.