Each individual responds differently to noxious stimuli, and so it is no surprise that the same surgery will evoke varied pain responses in different patients, despite the seemingly similar pain generator.

When considering the most appropriate regional anesthetic technique for postoperative pain management, anesthesiologists must estimate the degree of postoperative pain the patient will experience in response to the surgical stimulus. It is well recognized that certain demographic and psychosocial characteristics predispose patients to higher levels of postoperative pain. Younger females and those with a tendency toward catastrophizing and neuroticism are more likely to experience greater pain after the same surgery [6,23–26]. Severe and poorly controlled postoperative pain, as well as prolonged duration, are both associated with the development of PPSP [23]. Other important psychological factors associated with both severe acute pain and PPSP that should be considered are anxiety, depression, and chronic stress [24,25,27].

One of the most important predictors of postoperative pain is pre-existing pain [23–26,28]. Even with minor procedures in the ambulatory setting, patients with pre-existing pain syndromes may experience postoperative pain severe enough to warrant hospital admission [29]. A subset of this population which pose a particular challenge are those patients on preoperative analgesics with baseline opioid tolerance [24]. The preoperative identification of patients with these characteristics can allow for the appropriate planning and implementation of multimodal analgesia techniques, including nerve blocks. A regional anesthetic technique, particularly in these patients, can act in conjunction with other components of the multimodal regimen to reduce acute postoperative pain scores, and the transition from acute to PPSP, aligning with postoperative goals. Such planned aggressive management of these patients' pain reduces central sensitization, which can occur during periods of high-intensity pain [23]. Having an open discussion with the patient and surgical team in these cases is essential. It is not uncommon in caring for patients with chronic pain to prioritize analgesia over other goals such as early ambulation and discharge.

Two emerging areas in our understanding of variable pain responsiveness are genetics and epigenetics. We know that those patients who have a heightened response to certain stimuli preoperatively are more likely to experience higher levels of postoperative pain [6,23], which would suggest that those with pre-surgical sensitization (and evidence of hyperalgesia and allodynia) should be identified early. There is currently no convincing evidence that gene mutations are associated with an increased pain response. However, some data suggest that there may be a link to single mutations in certain genes (e.g., catechol-O-methyltransferase, opioid receptor mu 1, and guanosine-5'-triphosphate cyclohydrolase 1) [7]. There is a role for epigenetics and its contribution to the development of PPSP, which infers that a patient's environment can contribute to the expression (or non-expression) of certain genes involved in pain modulation [23]. Future developments in this field will allow anesthesiologists to potentially identify at-risk individuals in advance and plan for targeted analgesic therapy.

A patient's comorbidities also play a role in postoperative pain management needs. The use of regional analgesia generally reduces the requirement for systemic medications, including opioids. This can be beneficial in those with renal impairment who can experience prolonged effects of opioids due to altered opioid pharmacokinetics [30]. Similarly, reduced opioid consumption can result in less respiratory depression and reduced functional capacity. This is especially important for those with respiratory comorbidities, who are at a higher risk of respiratory sequelae in the postoperative period, and for whom targeted effective pain management with regional techniques is beneficial [30,31]. Neuraxial analgesia for major thoracic and abdominal procedures tends to blunt cardio-acceleratory response and sympathetic activation. This can reduce the risk of myocardial ischemia in patients with coronary artery disease by improving the myocardial oxygen supply-demand ratio, as long as hypotension is avoided [30].

However, it is important to recognize the altered metabolism of local anesthetic drugs in patients with end-stage liver and renal disease, who may need their dosing regimen altered. Similarly, some comorbidities may influence the anesthesiologist's decision regarding specific regional anesthetic techniques. For example, choosing an alternative technique instead of an interscalene nerve block for shoulder surgery can result in better preservation of vital capacity, which may be a relevant consideration in certain patients with significant pre-existing respiratory impairment [32]. Another patient-specific consideration is the requirement for the early resumption of anticoagulation postoperatively with novel oral anticoagulants, which precludes the use of an epidural or deep plexus catheter technique [33].

Pain management should be procedure-specific. Knowing the character and duration of pain a patient will encounter in the postoperative period has an important bearing on the ideal regional analgesic technique for them. Different surgeries have different pain trajectories (Fig. 1) [34], and so anesthesiologists must provide each patient with the right intervention at the right time for the right duration.

An open procedure with a large incision can be expected to induce more pain than a minimally invasive procedure. However, patients can experience severe pain even after ambulatory surgery, particularly with orthopedic, urologic, and general surgeries [35]. Certain types of surgery are particularly associated with increased acute postoperative pain and even increased PPSP. These include herniorrhaphy, mastectomy, TKA, limb amputation, thoracotomy, and cesarean sections [7,25]. Patients undergoing these procedures need early identification and aggressive multimodal pain management, including regional nerve blocks. Particular techniques, such as a paravertebral block, may help to prevent PPSP after breast surgery [36]. The surgical technique in itself is also important. Avoiding nerve injury with careful dissection and different surgical approaches, such as avoidance of injury to the intercostobrachial nerve during mastectomy and avoiding the posterolateral approach with a thoracotomy, may decrease chronic pain [6,25]. Therefore, anesthesiologists must understand the technique and approach of the surgical team with whom they are working on a day-to-day basis and plan for procedures collaboratively in advance.

The regional anesthetic technique and local anesthetic chosen should match the predicted pain trajectory of the surgery. Certain procedures result in postoperative pain that far outlast the effects of a single injection peripheral nerve block (sPNB). Adjuvants added to long-acting local anesthetics in sPNB may prolong analgesia and may be beneficial in procedures with an intermediate duration of pain [14,20,37]. However, the only reliable technique that provides analgesia for several days is a continuous peripheral nerve block (cPNB). Perineural catheters can reduce pain scores and opioid consumption in comparison to sPNB [38]. For TKA in particular, a review of postoperative pain suggests that pain scores do not fall below four on a numeric rating scale (0: no pain, 10: worst possible pain) until after postoperative day seven [34]. It is clear that for these patients, an sPNB technique may be inadequate. Similarly, postoperative pain scores after mastectomy, hip arthroplasty, and shoulder arthroscopy all suggest that patients undergoing these procedures have high pain scores for at least three postoperative days, and would, therefore, benefit from a cPNB technique [34]. Unfortunately, the hope of liposomal formulations of local anesthetics in sPNB as a substitute for cPNB techniques has not been realized [20,34,39].

With each surgical procedure comes a different set of postoperative recovery goals on a different timeline. The regional anesthesiologist needs to be cognizant of and work in line with these goals. For example, in lower limb joint arthroplasty, the analgesic technique should allow for the patient to participate in physiotherapy within a day to maximize the functional outcomes. In same-day discharge arthroplasty cases, the technique must also minimize muscle weakness and fall risk, particularly at home. For TKA, more distal nerve block techniques, either at or proximal to the anatomical adductor canal, can offer effective analgesia with quadriceps sparing to allow for early physical therapy [13].

Targeted regional analgesia can also reduce unwanted adverse effects after some surgeries. A paravertebral block may offer equivalent analgesic benefit to a thoracic epidural, with lower rates of urinary retention and hypotension in those undergoing thoracic surgery [40–42]. Fascial plane blocks, such as TAP, rectus sheath, or ESP blocks, can provide analgesia for abdominal procedures while avoiding sympathetic block and hypotension encountered with a thoracic epidural and the risk of epidural hematoma in patients with coagulopathy [20].

As we manage patients as part of a multidisciplinary team, the decision regarding which (if any) regional analgesia technique is best must be agreed upon by the surgeons, anesthesiologists, and of course, the patients themselves. There may be resistance among some surgeons regarding the use of regional analgesia techniques in cases when there is the potential for surgical nerve damage or compartment syndrome. While these may be valid concerns, especially with traumatic injuries of the forearm and lower limbs, robust evidence is lacking. No convincing studies have shown that regional analgesia delays the diagnosis of compartment syndrome [43]. Nevertheless, institutional experience can determine a surgical department's willingness to incorporate regional analgesia in these controversial situations and will affect the number of tools for acute pain management available for the anesthesiologist and acute pain medicine specialist.

The successful implementation of regional anesthesia for postoperative pain management requires proper resources and infrastructure [44]. The infrastructure to support these techniques must be established and embraced by the multidisciplinary team, including anesthesiologists, surgeons, nurses, physiotherapists, and occupational therapists. Evidence-based techniques guided by the procedures performed at the institution should be selected to provide safe, consistent care. Anesthesiologists must be proficient in performing these techniques and lead the development of protocols for patient management in the postoperative period. Ongoing education of all team members is essential along with a process for evaluating the quality of care and patient safety.

Techniques such as epidurals and cPNB require a dedicated acute pain service not only to lead these initiatives but to manage patients with these modalities. Staff on the ward need to be confident and capable of managing these techniques and be able to recognize any adverse effects [38,45]. With the growing pressure to shorten hospital stays, many patients may be discharged with cPNB techniques or the effects of a sPNB in place. Therefore, appropriate patient selection and education are critical [34].

Additional resources are required for regional analgesia include staff, capital, equipment, and consumable costs. Institutions require anesthesiologists trained in performing these techniques [22]. In addition, specialized nurses are invaluable in helping to maintain these programs and provide ongoing staff education. Capital costs include ultrasounds, infusion pumps, and potentially a dedicated block room to perform these procedures efficiently. Specialized epidural and block kits with needles, ultrasound machines, catheters, and local anesthetic solutions are also additional expenses. However, these costs may be offset by improved patient outcomes, reduced complications, shorter lengths of stays, and less downstream healthcare utilization post-discharge [46].

Unfortunately, poor access and inequitable distribution of healthcare resources may limit postoperative analgesic options for patients. There needs to be a minimum standard of care established that must be met at all institutions regardless of each patient's socioeconomic status. This should be a priority not only with postoperative analgesia but across the spectrum of healthcare delivery [47]. Local anesthetic, in some form, should be part of every multimodal pain management strategy. In situations where sophisticated regional analgesia is not available, this may mean meticulous layer by layer local anesthetic infiltration by the surgeon during wound closure or adjustments in the regional anesthesia technique if other limitations exist such as using adjuvants to prolong sPNB [14,20] or elastomeric infusion pumps if dedicated programmable pumps are not available [48].

Increasing patient access to a range of regional anesthesia options for various surgeries starts with having a critical mass of anesthesiologists willing and able to perform the procedures safely and effectively. Then, these techniques need to be incorporated into standardized surgical pathways. Even though it is well-established that paravertebral blockade is effective for pain management after breast surgery, some general anesthesiologists may be hesitant to attempt this due to a lack of experience and fear of complications. Education and training play a major role in increasing patient access to robust multimodal analgesia involving regional nerve blocks. In this example, the ESP block may be an attractive alternative to paravertebral block because the deposition of local anesthetic superficial to the paravertebral space may be perceived as easier and safer by the general anesthesiologist [47]. Given the opioid epidemic, educators have recognized the importance of training every anesthesiologist in a basic armamentarium of regional analgesia options. With a consistent curriculum, every anesthesiologist can achieve the competence and confidence to perform a basic set of nerve blocks and learn to incorporate regional analgesic techniques into routine perioperative care [22].