Chronic postsurgical pain (CPSP) is defined as the persistence of pain at surgical site or referred area, at least 3 months following the surgical procedure. CPSP after median sternotomy is not uncommon after cardiac surgery. The reported incidence ranges from 28% to 56% up to 2 years postoperatively. A number of factors can contribute to the variation in incidence, including the variations in presentation of chronic post-sternotomy pain and under-reporting from patients. Given the nature of cardiac surgery and considerable anxiety of patients at perioperative period, patients would tend to live with the chronic pain and rationalize it as a normal consequence of the surgery. Patients therefore may not report any symptoms, resulting in a delay in diagnosis of chronic post-sternotomy pain.

The mechanisms for development of CPSP after sternotomy are not fully known. The up-regulation, activation and neural sensitization during the acute pain phase are important factors to the development of chronic pain. Causes of pain from sternotomy include neuropathy from anterior intercostal nerves entrapment or injury during surgical dissection and internal mammary artery harvesting, musculoskeletal injury from surgical incision and dissection, sternal fracture or incomplete bone healing, and development of sternal wound infections. Poorly controlled acute pain in the perioperative period can trigger central sensitization, a stepwise modification of spinal pain pathways involving protein synthesis and synaptic strength, which can lead to hyperalgesia and chronic pain. Therefore, good perioperative acute pain control not only provides postoperative pain relief, but also prevents occurrence of chronic pain.

In most centres, opioids such as fentanyl and morphine are used mainly for the management of acute pain after cardiac surgery. However, perioperative opioids are known to be associated with dose-dependent side effects such as nausea and vomiting, pruritis, respiratory depression, postoperative chronic opioid use and increased risk of chronic pain. Multimodal approach such as the use of NSAIDs may not be optimal after cardiac surgery due to the risk of bleeding and renal failure. Regional analgesia has opioid-sparing effect and seems to be a good choice for perioperative anaglesia. Regional techniques such as thoracic epidural and paravertebral blocks have been described but they are not ideal due to the potential complications from systemic heparinization in cardiac surgery. Erector spinae plane (ESP) block was reported to decrease perioperative morphine consumption and early postoperative pain score. However, the transverse process tip is not always easy to visualize on ultrasound, especially in obese patients, resulting in variation in ESP block efficacy.

Transversus thoracis muscle plane (TTP) block covers the anterior branches of intercostal nerves T2 to T6. It has been described in cardiac surgical patients that provides analgesia for sternotomy. TTP block is a superficial block and therefore is relatively safe when compared with thoracic epidural and paravertebral nerve block during heparinization. The investigators hypothesized that pre-emptive TTP block can provide effective perioperative analgesia and prevent the occurrence of CPSP in cardiac surgical patients requiring median sternotomy. A prospective, double-blinded randomized controlled trial was conducted to assess the analgesic efficacy of TTP block in open cardiac surgery.