Regional anesthesia in human and veterinary patients has become invaluable for the treatment of pain in the perioperative period. However, its use in the nonsurgical setting has been less investigated. Regional anesthesia can provide selective and sustained pain control with beneficial systemic effect profiles for critically ill patients. Indications for regional anesthesia in this population are not limited to surgical and postsurgical analgesia but extend to the management of trauma, medical conditions (i.e., acute pancreatitis), and procedural pain at the bedside/cageside.
Pain control in critically ill human and small animal patients still represents a major challenge. A certain degree of pain is expected in nearly all patients in the ICU setting. Furthermore, pain in critically ill patients is often difficult to quantify and is frequently not easily treatable by means of systemic analgesia, while trying to avoid unwanted side effects. In these circumstances, the use of regional anesthesia and its potential impact on patient care and outcome should be highly considered.
Regional analgesia can play an important role in a multimodal approach to pain control in the critically ill patient to achieve optimum patient comfort and to reduce physiologic stress and, although hard to assess in animals, likely psychological stress.
Indications
In the perioperative period, regional anesthesia has been associated with superior analgesia, reduced nausea, vomiting, ileus, and decreased requirement of supplemental analgesics when compared to opioid-based pain control. Moreover, other potential benefits of regional anesthesia in critical care patients are the improved gastrointestinal and hepatic macro- and microcirculation, anti-inflammatory effects, relaxation of bronchial smooth muscle, and antithrombotic effects.
In trauma the main role of regional anesthesia is intraoperative and postoperative pain management. After thoracic trauma, adequate analgesia is effective to reduce pain associated with breathing, subsequently improving the patient’s ventilation, and possibly averting the need for mechanical respiratory support. Use of thoracic paravertebral analgesia and intercostal nerve block has proven effective in humans for this purpose. These techniques are easily applicable to small animal critical care as well.
Nonsurgical patients in the ICU benefit from sustained analgesia and selective sympatholysis provided by regional anesthetic techniques. Indications for neuraxial anesthesia include conditions in which adequate pain control is often difficult to achieve by systemic analgesia, including acute pancreatitis and peritonitis. The sympatholysis conferred by neuraxial techniques is the key mechanism for providing abdominal visceral vasodilation and improving blood flow.
A peculiar use of regional analgesia in ICU is the prevention of procedural pain when performing painful procedures. Examples of this include repeated wound debridement, chest tube placement, and burn treatments. Depending on the anatomical location of a required intervention at the cage side, the choice of a specific peripheral nerve block, plexus block, neuraxial anesthesia, or local infiltration can be considered.
Special Considerations
Altered coagulation is common in critically ill patients, either iatrogenic, primary, or secondary to associated conditions, possibly restricting the use of regional anesthesia.
Prophylactic and therapeutic anticoagulation is prominent in ICU patients because of the presence of numerous risk factors for thromboembolic events, including immobility, indwelling IV catheters, and inflammation.
The American Society of Regional Anesthesia provided guidelines on the suggested management of regional anesthesia in human patients receiving antithrombotic therapy. According to these guidelines, epidural catheter placement should be delayed for 12 hours after the last dose of low-molecular-weight heparin (LMWH), and removal of the epidural catheters should occur 12 hours after the last dose of LMWH. Similar guidelines are not currently available for small animal patients. Therefore, performance of regional anesthesia in this population remains subject to considering the clinical benefits and potential risks on a case-by-case basis.
There is a high prevalence of infections in the ICU, including nosocomial infections and ones inducing sepsis. Recent studies on use of neuraxial analgesia in human septic patients indicate an extremely low risk of central nervous spread of infection, even in patients showing signs of systemic infection, provided that adequate antimicrobial therapy was started. Local infection of the site of the planned insertion of a long-term epidural or nerve block catheter instead is a widely accepted contraindication to their use. According to a recent review on the clinical use of epidural catheterization in human ICU patients, the incidence of superficial skin infection is very low, ranging between at a rate of 0.824/1000 catheter days for femoral nerve catheters and 2.02/1000 catheter days for sciatic catheters. If a catheter infection is identified or suspected, the catheter should be promptly removed
While performing blocks to the brachial plexus, special attention should be given to patients with pulmonary comorbidities, such as asthma or acute lung injury. In this population, hemidiaphragmatic paralysis may lead to worsening of ventilation sufficient to warrant mechanical ventilation. The risk of this side effect should be weighed against the negative consequences of pain on respiratory function.
Doses of epidural drugs should be calculated by using the ideal lean body weight, and injection volumes of solutions containing local anesthetic exceeding 0.25 ml/kg are potentially associated with high blockade of the sympathetic nerve roots. Secondary vasodilation and possible systemic hypotension can result. Systemic hypotension can occur more commonly in animals with pre-existing hypovolemia so ensuring appropriate volume replacement before administering local anesthetics epidurally is indicated.
Neuraxial Anesthesia
Epidural injections and catheterization are increasingly used in small animal patients admitted to the ICU. The technique is particularly suited to the ICU setting, as it provides profound pain relief with minimal sedation or other systemic impairment. Although its use is still limited, Continuous epidural analgesia likely represents the most frequently utilized, yet still limited, regional analgesia technique in critically ill small animal patients.
A study on the use and efficacy of epidural catheters in a small animal ICU demonstrated that a large proportion of patients managed with this technique did not require any supplemental systemic administration of analgesics. Lumbosacral epidural analgesia with local anesthetics provides a dose-dependent anesthesia to the caudal half of the body by blocking the spinal nerve roots and the superficial layer of the spinal cord. In ICU settings, this technique is effective for pain management of hindlimb and pelvic trauma, postoperative pain control following exploratory laparotomy and perineal surgeries, and peritonitis or acute pancreatitis-associated pain. In addition to analgesia, local anesthetics administered epidurally provide regional sympathetic blockade of the paravertebral ganglia innervated by the tract of spinal cord exposed to the local anesthetic solution. This provides selective vasodilation of the vascular beds innervated by those ganglia, increasing organ blood flow. In patients with persistent hemodynamic instability, the use of epidural morphine alone produces onset of analgesia in about 60 minutes and duration of action of 12 to 24 hours.
Peripheral Nerve Blocks
The use of peripheral nerve blocks is gaining increasing popularity in small animal medicine. It is still largely reserved for perioperative orthopedic applications, but provided the patient can be properly positioned for the injection and no absolute contraindications are present, any peripheral nerve block is applicable in a critical care setting as well. Nerve stimulation and ultrasound guidance are incredibly valuable at facilitating the correct placement of needle and catheter, and at maximizing the efficacy and safety of nerve blocks. Possible techniques with particular relevance in the ICU include nerve blocks to the upper or lower extremity, but also a number of blocks to the trunk can also be of use (such as thoracic paravertebral blocks, intercostal blocks, intrapleural blocks, and transverse abdominis plane blocks).
After major trauma or surgical insult to the thoracic wall and pleural cavity, the use of paravertebral, intercostal, and intrapleural blocks should be considered as primary or adjuvant analgesic strategies, aiming to improve comfort and normalize chest wall respiratory mechanics. Transverse abdominis plane block desensitizes the abdominal wall through blockade of the nerve branches within the fascial plane between the transverse abdominis and the internal oblique muscles. This block requires ultrasound guidance and may represent a useful option for postoperative pain control after major abdominal surgery.
Continuous peripheral nerve catheters represent a more advanced strategy of nerve blockade and are still rarely employed in small animal analgesia, arguably due to unavailability or lack of training in performing these procedures. As proved in human ICU settings, they could certainly provide effective and prolonged analgesia for trauma and postsurgical small animal patients admitted to the ICU. Contraindications to the placement of continuous nerve block catheters are similar to the ones of epidural catheterization and include coagulopathy, aggressive anticoagulation, local infections at the puncture site, and lack of physician experience.
Epidural Anesthesia in Acute Pancreatitis and Sepsis
The use of regional anesthesia and specifically epidural catheterization in sepsis is still an object of controversy. The advantages related to long-term effective analgesia and improved visceral blood flow are challenged by the possible complications related to excessive vasodilation and secondary hemodynamic instability, risk of bleeding in coagulopathic patients, and risk of infections. The rationale for the use of continuous epidural anesthesia (EA) in acute pancreatitis is that intestinal and hepatic perfusion is regulated by sympathetic and parasympathetic nerves. Acute stress and pain trigger a massive increase in sympathetic activity resulting in visceral vasoconstriction and consequently reduced blood flow. A pig study on chemically induced severe acute pancreatitis evaluated the therapeutic effect of EA on survival, microcirculation, tissue oxygenation, and histopathologic damage. Epidural anesthesia led to significantly improved survival, enhanced pancreatic microcirculation and tissue oxygenation, and resulted in less histopathologic damage.
In the course of acute pancreatitis, receptor-dependent pulmonary vasoconstriction is reduced, and hypoxic pulmonary vasoconstriction is abolished. In a rat model of acute pancreatitis, continuous EA improved pancreatitis-associated impairment of pulmonary vasoreactivity and gas exchange. Treatment with EA significantly improved both the vasoreactivity to angiotensin II and hypoxia. A number of studies have investigated the use of EA during sepsis, trying to determine the mechanisms, which could, in theory, protect visceral function and reduce morbidity and mortality. However, different animal models have provided conflicting results on whether EA has protective or harmful effects and whether its effect varies depending on the phase of sepsis.
In the course of sepsis, microcirculatory impairment is common and is deemed as one of the most important factors in the pathogenesis of intestinal dysfunction. Sepsis is associated with a redistribution of visceral blood flow and a decrease in gastrointestinal perfusion with secondary impairment of the mucosal barrier, leading to translocation of bacteria and toxins. In some animal models, regional sympathetic blockade by selective epidural anesthesia has been shown to attenuate the impairment of gastrointestinal organ perfusion during endotoxemia and improve intestinal microcirculation during systemic inflammation, without impairing systemic hemodynamics beyond the changes induced by sepsis itself. Pulmonary nitric oxide has been identified to play a critical role in pulmonary dysfunction in sepsis. A recent experimental study in animals showed that EA modulates the NO pathway with positive effects on pulmonary endothelial integrity, but only in hyperdynamic phase of sepsis. A negative effect, however, was identified during the hypodynamic stages.
A canine study investigated the effects of continuous epidural anesthesia (EA) on gastric mucosal microvascular hemoglobin oxygenation (mHbO2). The study demonstrated that, during compromised circulatory conditions, EA causes a reduction in mHbO2 to about 30% of baseline values. Fluid resuscitation was effective at completely restoring this variable, highlighting the importance of using caution when using EA in under-resuscitated septic patients. In a canine model of human sepsis, the effects of sympathetic blockade by EA during bacterial peritonitis on pain relief, hemodynamics, and survival, were investigated. Epidural anesthesia was extremely effective in controlling pain, but the sympathetic blockade induced by EA appeared associated with a decrease in survival times and cardiac function and an increase in creatinine levels. The authors argued that the maintenance of sympathetic tone irrespective of pain relief provided is necessary for maintenance of hemodynamic stability, homeostatic control, and survival. It is clear that the controversy over EA in sepsis is still far from being solved. The main issue with the current literature is that hardly any of the publications are comparable with each other. Additional studies are warranted to assess the effects of EA and sympathetic blockade in septic patients.
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