The provision of safe and optimal analgesia is a fundamental consideration for veterinary practitioners, ensuring the welfare of patients entrusted to their care. The registered veterinary nurse's (RVN) role is often key with regards to pain scoring patients, identifying pain emergence and liaising with the veterinary surgeon (VS). Jones (2016) highlighted that although there have been large changes in pain scoring and analgesia over recent years, there is still scope for improvement.
For example, Crombie et al (1998) discovered that surgery was potentially responsible for chronic pain in 35% of human patients attending a chronic pain clinic who underwent pelvic limb surgery, suggesting potential for this to occur in veterinary patients. Pelligand and Sanchis Mora (2016) described pain as the fifth vital sign, proposing that successful recognition and management results in improved patient welfare and lower incidences of chronic refractory pain. Similarly, Vadivelu et al (2014) previously evaluated the concept of preventative analgesia for postoperative pain, concluding that pre-emptive and multimodal analgesia are important to reduce sensitisation and subsequent ‘wind up’ phenomena.
Local anaesthetic (LA) blocks can feature greatly in multimodal analgesia; Duke-Novakovski (2016) summarised that they can greatly improve quality of pain control and have anaesthetic sparing effects. RVNs should have a good grasp of multimodal analgesia, the components it can encompass and how to nurse patients that may be receiving a variety of analgesia regimens as increasingly complex surgical procedures are developed.
Introduction
In a recent review, Hunt et al (2015) provided an overview of analgesia prescription in veterinary practice by use of questionnaire; they concluded that there has been a marked increase in perioperative analgesia since 1996. It did however highlight the fact that most respondents are not using LA blocks routinely, with a statistically significant number of the few performing blocks being members of the Association of Veterinary Anaesthetists. This suggests a potential gap in analgesia regimens and indicates an area where pain management could be improved.
Defining pain
Williams and Craig (2016) defined pain as an unpleasant sensory and emotional experience associated with actual or potential tissue damage. Bell (2018) elaborated further to explain pain as an experience requiring the perception of brain activity, noting that this contrasts with nociception, which is the processing of noxious stimuli within the nervous system. This clarifies the importance of multimodal analgesia, where each drug exerts its effects, targeting specific sites along the pathway, culminating in a greater control of pain overall.
Where pain is ill managed, peripheral and central sensitisation can arise, leading to neuroplasticity in response to the prolonged or high-intensity noxious input (Bell, 2018). To understand the action of LA and its role in pain management, the nociceptive pathway must first be considered. As can be seen from the diagram in Figure 1, LA resides in a unique location for activity within the pain pathway, and may be classified as the only true ‘analgesia’. As Lascelles et al (2016) explained, it is one of the most effective ways to prevent nervous transmission and transduction. LA acts by blocking sodium channels, thus blocking nerve action potentials and impulse transmission; this means that nerve impulses cannot reach the dorsal horn of the spinal cord, or the brain to be modulated and perceived (Duke-Novakovski, 2016).
LA can be used in many ways to provide analgesia, but this review will focus on peripheral nerve blocks (PNBs), as interest in them has recently increased within the veterinary field. Human anaesthesia widely makes use of them, as well as regional anaesthesia, to avoid general anaesthesia, improve intraoperative analgesia and increase patient comfort levels postoperatively (Gurney and Leece, 2014). For the veterinary patient, this may mean anaesthetic drug-sparing qualities perioperatively and a reduction in opioid requirements postoperatively.
Human literature also suggests that use of PNB may reduce hospital stay for patients, lead to faster rehabilitation and reduced opioid-induced nausea and vomiting (Gomez-Rios, 2017). Recent veterinary studies looking at the use of PNBs in hind limbs are aimed at attempting different approaches to the nerves and comparing their efficacy with previously described methods. Many papers have also described and studied comparisons of femoral and sciatic nerve blocks with established epidural techniques in order to ascertain whether they can be used instead, with the same efficacy and safety profile.
The aim of the present review is to explore recent literature relating to the use of PNBs, specifically femoral and sciatic nerve blocks, and their effectiveness for patient comfort levels to offer the RVN in practice an understanding of these techniques. Figures 2a, 2b and 3 show locations for sciatic and femoral nerve blocks using a nerve stimulator.
Efficacy of femoral and sciatic nerve blocks
Campoy et al (2012) carried out a prospective, blinded and randomised clinical comparison of canine patients to assess bupivacaine femoral and sciatic nerve block with bupivacaine and morphine epidural for stifle surgery, concluding that the PNBs offer an alternative to epidural with reduced risk of urine retention and reduced postoperative opioid consumption. They also noted that intraoperative mean arterial blood pressure (MAP) was lower in the epidural group, 10 mmHg on average, showing +a p-value of 0.04 where ≤0.05 was considered statistically significant; however, perioperative isoflurane levels were also significantly lower in the epidural group with a p-value of 0.05. The PNB group was still less than minimum alveolar concentration (MAC) with a mean of 1.1%, thus still showing isoflurane-sparing qualities. This could suggest that PNBs pose less of a risk of adding to hypotension under anaesthetic; adding validity to this fact is that all other variables were kept the same, particularly premedication and anaesthesia drug protocol (Cottrell, 2014). The sample size was small with only 20 patients, but the methods were good, with the assessors all being unaware of the treatment the patient had received, which Karanicolas et al (2010) advised increases the validity of results.
With similar findings, a research paper by Portela et al (2013) described a prospective, anatomical research and clinical study performed to examine a novel approach to a femoral nerve block. Rather than an inguinal approach, they approached more proximally from the lateral lumbar muscles, cranial to the iliac crest with a needle direction of 30–45°, thus entering the caudal portion of the psoas compartment. Their results showed that this, in combination with a sciatic nerve block, is suitable for providing perioperative analgesia in patients undergoing pelvic limb surgery, highlighting that 86.8% of dogs had no cardiovascular response, even when maintained in a light plane of anaesthesia using lessthan-standard MAC values. Some variables were limited; for example, all patients had only acepromazine premedication with no other analgesia component. The cohort was however limited at only 15 patients; therefore, extrapolation from this is made cautiously. Furthermore, the approach to the femoral nerve was novel and so perhaps the results cannot be generalised. In addition, Vettorato et al (2013) claimed from a retrospective study that 78.5% of blocks for pelvic limb surgery examined were deemed successful, and again MAC was reduced, giving support to the aforementioned studies. In the patient records assessed, only 27.1% of patients required methadone in the postoperative period. It is interesting to note that 44.8% of patients received perioperative fentanyl indicating a non-successful block, based on parameters set by the anaesthetist, but that only 27.1% went on to have post-operative methadone. This could suggest that basing success on perioperative analgesia alone would be a mistake and that, as highlighted in this study, the patient should be thoroughly pain-scored postoperatively using a validated pain-scoring tool. The potential for bias cannot be ruled out in this paper, however, as the kennel nurses carrying out the pain scoring were not blinded to the patient's block status; there is always therefore a chance that this could have influenced their scoring as found by Karanicolas et al (2010).
Romano et al (2016) approached proving effectiveness in a different manner and claimed that PNBs prevented the glycaemic and cortisol responses to surgery, promoted better recoveries and reduced pain scores postoperatively. The study was robust, with 45 patients split into three groups of either fentanyl, spinal analgesia or PNB, and a separate 15 patients as a control group undergoing non-invasive procedures. As both cortisol and glucose can be markers of a stress response potentially triggered by surgery (Muir et al, 2011), this study highlights the different tools that can be employed with less risk of bias than pain scoring methods. The inclusion of a control group and the spinal analgesia group seemed to validate the findings of efficacy of the PNB, as all three of these groups had similar blood glucose and serum cortisol levels. The control group is significant as they had no invasive procedure carried out; they were simply under anaesthetic for diagnostic procedures, therefore offering an important comparison as described by Lund et al (1994).
A study by Bartel et al (2016) compared PNBs with neuraxial analgesia finding no significant difference between the two techniques with regards to need for rescue analgesia administration, which again corroborates the findings above by Romano et al (2016), but shows slight differences to Campoy et al (2012) who found postoperative opioid consumption higher for the neuraxial group. Bartel et al (2016) also concluded however that neither technique is 100% effective, which again shows the need for multimodal approaches and careful post-operative pain scoring as highlighted by Vettorato et al (2013).
In addition to Campoy et al (2012) and Portela et al (2013), Romano et al (2016) discussed that they found an apparent isoflurane-sparing effect from the regional anaesthesia combinations compared with fentanyl infusion, highlighting once again the diverse benefits to the patients. Reduced isoflurane levels can help to lower the risk of hypotension under general anaesthesia and, as with any drug, can reduce dose-dependent side effects including the potential for accidental overdose (Pang, 2017).
Wick et al (2017) reviewed postoperative analgesia using non-opioid techniques in response to the American opioid shortage, describing the enhanced recovery after surgery (ERAS) pathway. They claimed that multimodal pain management is an essential component in aiding recovery, reducing hospital stays, and reducing opioid consumption with its related side effects. Their overview concurred with the previously outlined studies, asserting that PNBs, when effective, are associated with less pain for the patient compared with systemic opioid use, and an overall reduced opioid consumption.
Conclusion
The use of PNBs can be seen to positively correlate to reduced opioid consumption in the perioperative and postoperative period. This would in turn reduce opioid-induced side effects such as nausea, inappetance, dysphoria and potentially the duration of hospitalisation (Wick et al, 2017). Their use has also been shown in numerous studies (Campoy et al, 2012; Portela et al, 2013; Romano et al, 2016) to reduce requirements for anaesthetic inhalational drugs, proving their value as part of a multimodal regimen where all drugs can be used at lower levels thus reducing side effects.
In patients that are higher anaesthetic risks, the inclusion of PNBs, if deemed acceptable by the attending VS, may mean that even lower doses of other drugs can be used. Romano et al (2016) showed that PNB use is associated with stable perioperative biomarker levels in dogs, compared with intraoperative opioid use where levels were three times as high. It is important that the RVN has sound knowledge of the modality of analgesia being used, and a thorough understanding of pain scoring is vital to ensure patient comfort levels are maintained both perioperatively and postoperatively.
KEY POINTS
- It is important that the registered veterinary nurse (RVN) has sound knowledge of pain scoring to ensure patient comfort levels are maintained both perioperatively and postoperatively.
- RVNs should have a good grasp of multimodal analgesia and the components it can encompass to enable them to nurse and monitor the patients under their care.
- Pre-emptive or preventative and multimodal analgesia can help to reduce sensitisation in patients, thus potentially lowering the risk of chronic pain.
- There is a potential to improve analgesia regimens in practice for the patient's wellbeing and reduce overall opioid consumption thus minimising risk of unwanted side effects.