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Chronic pain and green anaesthesia – niche topics in pain management

Dr. Anne Chan
Specialist in Anaesthesiology
Hong Kong
Dr. Pak-Chung Yu
Specialist in Anaesthesiology
Hong Kong
10 Nov 2020

Low awareness of chronic postsurgical pain (CPSP) is behind inappropriate use of acute-pain medication and an apparent lack of symptom amelioration following some corrective surgeries. Widespread use of volatile-gas anaesthesia has a non-negligible and increasing impact on global emissions. In an interview with MIMS Doctor, Dr Anne Chan and Dr Pak-Chung Yu, specialists in anaesthesiology in private practice in Hong Kong, discuss two niche topics with far-reaching implications in modern-day pain management.

 

Chronic postsurgical pain

CPSP typically begins as acute postoperative pain that is difficult to control, but soon becomes persistent and develops neuropathic features unresponsive to opioids. [Lancet 2019;393:1537-1546]

“Chronic pain is defined as any moderate pain lasting >3 months and not relieved by simple analgesics, covering a very wide spectrum of manifestations,” explained Chan. “The underlying mechanisms are different from those of acute pain and the two should be treated differently, as acute-pain medication will not be effective for chronic pain. However, lack of awareness often means that patients continue to take acute-pain medication long after their pain becomes chronic, increasing the dose in the absence of pain relief.”

“Acute pain is mainly generated through the peripheral pathway, where the surgical wound represents the noxious stimulus in the immediate postoperative period. Subsequent insults, such as nerve injury, inflammation or infection, wind up the peripheral mechanism, leading to sensitization. This sensitized state may lead to amplification of the central mechanism, whereby the pain transforms from acute to chronic,” elaborated Chan. [F1000Prime Rep 2015;7:56; Pain Reports 2017;2:e588]

Total knee and hip replacements, back surgery, mastectomy, limb amputation, inguinal hernia repair and thoracotomy are associated with a high chance of CPSP. [Lancet 2019;393:1537-1546] For instance, the prevalence of CPSP of any severity is as high as 44 percent for knee arthroplasty, with 15 percent of patients reporting severe-to-extreme persistent pain. [Pain 2011;152:566-572]

“Factors predisposing to CPSP include pre-existing chronic pain, where the central mechanism is already wound up, female gender, young age, and high BMI. Underlying psychiatric conditions, such as depression and anxiety, and a catastrophising personality also have a role due to CNS interplay,” noted Chan. [Lancet 2019;393:1537-1546]

“Patients often continue to experience pain after joint replacement, even after the anatomical issues have been fixed, because the pre-existing chronic pain that necessitated the operation in the first place was not treated with appropriate medication,” explained Chan. “Since these patients receive surgery after having established chronic pain, in spite of taking opioids, and continue to experience pain after the operation, a change to antidepressants or anticonvulsants would be appropriate.”

Preventing chronic pain: Multimodal analgesia

“Timely use of appropriate medication may help prevent transformation of acute pain into chronic pain through sparing nerve injury and avoiding the wind-up phenomenon,” said Chan. “Pain management is evolving to bridge the gap between the immediate postoperative pain and what is recognized and treated as chronic pain. Some countries, such as Canada and Finland, are beginning to address this by referring patients to a multidisciplinary transitional pain clinic and offering CPSP management <3 months after the operation.” (Figure 1) [Lancet 2019;393:1537-1546]

HK-AVI-094_01

Intraoperative multimodal analgesia (IOMA) is a form of early pain treatment that prevents chronic pain and reduces the dose of opioids required. “The original incentive for introducing IOMA was the imperative for early postoperative discharge, as patients were less willing to mobilize while being administered intravenous [IV] opioids postoperatively,” noted Chan. “Use of opioids as part of a balanced protocol is generally safe, but reliance on opioids alone can become problematic, as medication for chronic pain may be given indefinitely.”

A multimodal analgesic regimen implies the use of different classes of medication and different routes of administration. [Med J Aust 2016;204:315-317] “For example, for patients undergoing knee surgery, anticonvulsants, which aim to prevent nerve injury, and NSAIDs are used preoperatively. Intraoperatively, the patients receive a nerve block and an intra-articular injection of a local anaesthetic and an opioid. Postoperatively, anticonvulsants are given alongside a combination of strong and mild oral opioids. This achieves much better outcomes than traditional IV opioid patient-controlled anaesthesia, with fewer side effects. Patients become ambulatory the following day and are discharged early,” shared Chan.

In addition to pharmacotherapy, patients should be offered behavioural therapy and introduced to psychological techniques to cope with CPSP. “Since most medications are unable to eliminate pain completely, but rather reduce it by half or by one-third, with some baseline pain remaining in the long term, the approach should be one of holistic management as opposed to pure treatment,” remarked Chan. “Patients with CPSP also receive an active exercise programme, which helps release endorphins, thereby reducing the perception of pain.”

Take-home messages  

“In Hong Kong, there is low awareness of CPSP as a phenomenon separate from acute postoperative pain and of the availability of specific treatments among patients and physicians alike,” stated Chan. “Postoperative pain should reduce once the wound heals. Pain that is changing in nature [eg, development of paresthesia], is worsening or is expanding to an area larger than the surgical wound should be a signal for doctor review. Anyone with pain lasting >3 months that does not improve with acute medication should consult a pain specialist.”

Green anaesthesia

“At public hospitals in Hong Kong, approximately 200,000 operations are performed each year. Just over half of these are major or ultra-major operations, which require general anaesthesia, which can be achieved with volatile anaesthetic gases or IV agents,” said Yu. [Hong Kong Hospital Authority statistical report 2018–2019]  

All currently used volatile anaesthetics, namely, isoflurane, sevoflurane and desflurane, are halogenated compounds destructive to the ozone layer and, as such, have an impact on global warming. [Anesth Analg 2011;112:213-217]

Volatile anaesthetics are evaporated into a stream of medical gases (oxygen, N2O, and medical breathing air) administered to the patient using an anaesthetic machine designed as a semi-closed breathing system. In the absence of mandatory anaesthetic gas capture systems, to protect the medical personnel in the operating theatre, the anaesthetics used are vented outside and allowed escape into the atmosphere. [Geophys Res Lett 2015;42:1606-1611]

“With the scavenging closed-circuit system in low-flow anaesthesia, the volatile agents can be trapped and reused within the closed loop, which nevertheless requires some replenishment with fresh gas at a rate of 1 L/min, due to patient consumption and leakage,” said Yu. “In minimum-flow anaesthesia in a modern anaesthesia machine with very tight connections, the gas flow of oxygen can be reduced to 0.3–0.5 L/min, thereby extending the life cycle to a greater extent.” [Can J Anaesth 2012;59:785-797; Anesth Analg 2010;110:101-109]

Consumption data from a decade ago indicate that anaesthetic use of NO2 alone contributed 3 percent of the total emissions in the US, while halogenated anaesthetics accounted for a further 1 percent. [Anesth Analg 2011;112:213-217] “Although there are no Hong Kong-specific data, it is reasonable to assume that the situation is similar here,” said Yu. Furthermore, studies suggest that the influence of halogenated anaesthetics on global warming will be of increasing relative importance, given the decreasing level of chlorofluorocarbons globally. [Br J Anaesth 1999;82:66-73]

Observed emissions of long-lived medicinal greenhouse gases suggest a global combined release of 3.1 ± 0.6 million tonnes of CO2 equivalent in 2014, of which approximately 80 percent stemmed from desflurane. (Figure 2) [Geophys Res Lett 2015;42:1606-1611] “The CO2 equivalent of using desflurane or sevoflurane for 1 hour is the same as 230 or 30 miles travelled in a modern car, respectively,” noted Yu. [Lancet Planet Health 2017;1:e216-e217]

HK-AVI-094_02

Volatile-gas anaesthesia vs TIVA

“Total IV anaesthesia [TIVA] is an alternative to volatile-gas anaesthesia,” said Yu. “However, it has a number of limitations, making complete replacement of volatile-gas anaesthesia by TIVA impossible.”

“Volatile anaesthetic agents have a long history, with a safety profile including pregnant women, neonates and unstable trauma patients,” noted Yu. “Models which inform the level of the drug needed to maintain TIVA are derived from healthy volunteers. No consensus programme exists for TIVA in neonates. The use of TIVA poses a questionable profile in maternal safety, foetal safety and the issue of teratogenicity. Some IV agents can cross the placenta, leading to respiratory depression in neonates.” [Int J Obstet Anesth 2010;19:71-76; Anesthesiology 1998;88:1467-1474; Can J Anaesth 1996;43:653-659; Indian J Anaesth 2016;60:234-241]

“In addition, TIVA is more time-consuming to set up, as it usually requires concurrent use of at least two infusion pumps. Furthermore, the depth of anaesthesia needs to be monitored very closely, as inappropriate use of TIVA carries a higher risk of awareness than volatile-gas anaesthesia, where end-tidal anaesthetic agent level acquired from the sample of exhaled gas has a well-established profile that corresponds to the depth of anaesthesia,” explained Yu. [Int J Surg 2017;41:44-49]

“Finally, while TIVA avoids the use of greenhouse gases, it is associated with a carbon print of its own, from single-use syringes, IV cannulas, extension tubing, and electricity for the infusion pump. It is therefore not clear whether TIVA has much less of an impact on global warming,” remarked Yu.

Take-home messages

“While concern for global warming has increased considerably in the medical profession in the past 15 years, it remains insufficient. In Hong Kong, cost-effectiveness still represents a greater concern than climate change,” shared Yu. “Macroscopically, the Hong Kong government, the health authority and educators should strive to raise awareness among next-generation medics, to minimize the use of volatile agents, plastic and undegradable materials. Microscopically, waste can be reduced by segregating nonhazardous and biomedical items and by using fewer disposables. However, patients’ safety should always come first – if the risk of infection can be eliminated, recycling should be done wherever possible.”

 

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