Cite as: Grzybowski, J.  Nerve blocks under general anesthesia: safety versus benefits. ASRA Pain Medicine News 2024;49. https://doi.org/10.52211/asra110124.009.

Introduction

The practice of performing pediatric regional anesthesia procedures while the patient is under general anesthesia (GA) is commonplace and has gained increasing acceptance since the early 2010s. During that decade, two large multicenter studies found similar rates of postoperative neurologic symptoms and complications between children receiving regional blocks under general anesthesia and those receiving blocks under mild sedation or awake.^1-3 Despite advances in ultrasound technology and the ability to use peripheral nerve stimulation or injection pressure monitoring to verify needle tip location, there is no current scientific evidence that these precautions reduce rates of neurologic injury. Thus, the practice of performing regional anesthesia in various adult patient cohorts remains controversial. 

The most recent ASRA Pain Medicine practice recommendations published in 2015 suggest that “neuraxial regional anesthesia or interventional pain medicine procedures should be performed rarely in adult patients whose sensorium is compromised by general anesthesia or deep sedation.” At the time, members of the ASRA Pain Medicine Practice Advisory Panel described wakefulness as a monitor of patient well-being and considered it a component of vigilant patient care. The panel also reaffirmed their previously issued advice not to routinely perform regional anesthetic procedures in anesthetized or deeply sedated adult patients.⁴ While the guiding ethical principle of nonmaleficence, or “do no harm,” would likely support the 2015 practice recommendations, other technical advances and increasing trends of fascial plane blocks cannot be ignored. This article aims to provide further context for the historical avoidance of nerve blocks under GA and review evidence of the relative safety of nerve blocks under GA by examining the literature on this controversial topic.

A Cautionary History

A landmark case report from 1998 describes the difficult placement of a lumber epidural for total knee replacement performed under GA. After four unsuccessful attempts, an epidural catheter was inserted above the upper end of a prior laminectomy scar. The patient awoke with new onset of permanent paraplegia and sensory levels terminating at T5 bilaterally. An MRI revealed a region of increased T2-weighted signal in the anterior aspect of the spinal cord between T4 and T5, consistent with infarction.⁵ In 2000, Benumof published a case series of four inter scalene blocks (ISB) resulting in intracord injections and permanent loss of cervical spinal cord function. All blocks were performed under GA. Three of the blocks used nerve stimulation localization, while one was performed using a landmark-based technique. Patient coughing, straining, and neck movement were noted during performance of one of the blocks with a block needle in situ. The author notes several clinical lessons; among them are GA can be considered as a relative contraindication for ISB, ISB needle length should be limited, and the physician should attempt to limit the likelihood of unexpected patient movement.⁶

In addition to the case reports detailed above, other safety standards and dogmatic assumptions can help account for the avoidance of performing regional anesthesia in adults under GA. Anesthesiologists who avoid blocks under GA may reason that an awake or minimally sedated patient ought to be able to report developing symptoms of local anesthetic systemic toxicity (LAST) before a toxic dose might be injected. Likewise, the same patient might be able to recognize and report pain or other atypical symptoms from an errant needle before neurological injury occurs. However, both assertions are relatively unproven in their ability to offer any real protection against LAST or neurologic injury. Proponents of performing blocks under GA or deep sedation argue that their practice increases safety by decreasing the chance of unexpected patient movement, resulting in aberrant needle placement. Additionally, patient acceptance generally increases with this practice, increasing the number of patients who will benefit from regional anesthesia. The following sections will take a closer look at some of the safety concerns and warning mechanisms pertinent to the practice of regional anesthesia.

Local Anesthetic Systemic Toxicity (LAST) Identification

Early central nervous system symptoms indicating a rising local anesthetic plasma concentration include tongue or circumoral numbness followed by “lightheadedness” and visual or auditory disturbances. Light sedation, typically offered for comfort in conjunction with preoperative nerve blocks, unfortunately, can decrease the patient’s ability to detect intravascular injection of local anesthetics. An un-pre-medicated subject can detect an intravenous bolus of lidocaine (1.5 mg/kg), 2-chloroprocaine (90 mg), or bupivacaine (25 mg) with 100% sensitivity. This decreases to between 60%-80% with even small doses of sedatives or opioid analgesics (eg, 1.5-2.8 mg midazolam and 60-96 mcg fentanyl).^7,8 Moreover, sedative-hypnotics, such as benzodiazepines, propofol, and barbiturates, significantly raise the threshold for local anesthetic-induced seizures and may increase the safety margin for local anesthetic CNS toxicity.⁹ Similarly, benzodiazepine administration can be considered to decrease clinical symptoms of LAST, making subjective reporting of symptoms less likely. Bernards et al. recommended that the potential ability of GA to obscure early signs of LAST is not a valid reason to forego performing peripheral nerve or epidural blocks in anesthetized or heavily sedated patients, given their reduced capability to identify prodromal CNS symptoms.⁹

An intravascular injection is more reliably detected with an intravenous test dose containing epinephrine, typically 5 mcg/mL (1:200,000), than by unsedated patient reporting.⁹ Studies performed on the identification of inadvertent intravascular catheterization while attempting epidural catheter placement concluded that the minimum effective epinephrine dose to detect intravascular injection was 15 mcg based on HR and SBP criteria. However, 5 mcg intravascular epinephrine resulted in decreased T wave amplitudes in all anesthetized adults.¹⁰

Ultrasound guidance has numerous purported mechanisms that may reduce the risk of LAST.¹¹ Real-time guidance of needle trajectory is used to help avoid vascular trauma. If an inadvertent vascular puncture occurs, ultrasound imaging may detect entry of local anesthetic into a blood vessel or the lack of injectate spread around the neural target, alerting the physician to cease injection. The findings of a study by Barrington et al demonstrated that ultrasound-guided peripheral nerve blocks are associated with a reduced dosage of ropivacaine.¹¹ The ultrasound-guided technique typically involves assessing and reassessing the local anesthetic injectate spread with incremental dosing. This reduces the maximum local anesthetic blood level following the nerve block and potentially the risk of LAST.¹²

Neurologic Injury

Concern about the risk of nerve injury is an often cited reason for not performing peripheral nerve blocks in anesthetized or heavily sedated patients. The prevailing assumption is that a patient with unaltered sensorium can recognize impending nerve injury before it occurs, thereby preventing it. One issue with this is whether needle paresthesia is a sensitive and specific indicator of potential nerve damage. Additionally, has the damage already been done by the time a paresthesia is reported, or can injury still be prevented by withdrawing the needle and redirecting injection elsewhere? Both needle trauma and injection trauma (via saline or local anesthetic injection) are concerns to the anesthesiologist, while causation of either mechanism is difficult to prove, given the multifactorial nature of peripheral nerve injuries.

A prospective series of 21,278 peripheral nerve blocks found four cases of radiculopathy, all of which were accompanied by paresthesia during puncture.¹³ This translates to an incidence of 1.9/10,000 blocks. The degree of sedation used was not specified. The study indicated that all injections were halted when patients complained of pain/paresthesia.¹³ The fact that patients developed injury despite prompt termination of the injection could be interpreted as evidence that the patient’s warning is insufficient to prevent injury and, thus, clinically useless.⁹ Also, failure to report the incidence of pain/paresthesia not associated with nerve injury makes it impossible to calculate the specificity of these symptoms with regard to injury. Sufficient data to establish a protective effect of wakefulness is not currently available. While awake patients may not be able to sound a protective warning, an anesthetized patient will never be able to do so. ⁹ This remains a guiding logic for conservative practices.

Injection Pressure

Needle-based trauma from injection in or close to peripheral nerves can result in neuropathy. Conservative wisdom regarding peripheral nerve blocks would dictate aborting injection if significant subjective resistance is noted upon attempted injection. Before 2014, no study had examined the association between needle-nerve contact and opening injection pressure. A study of 16 patients undergoing shoulder surgery with interscalene block suggested that high³ (15 psi) opening injection pressure may indicate needle-nerve contact, and avoiding injection at this site might improve patient safety.¹⁴ Injection of solution 1 mm from a brachial plexus root resulted in low (8.2 psi) opening injection pressure, whereas injection with the needle apposed to the root, resulted in high (20 psi) opening injection pressure.¹⁴

Another study focused on opening injection pressures and needle-nerve contact occurring during femoral nerve blockade. The opening injection pressure was 15 psi or greater for 90% and 100% of cases when the needle indented the femoral nerve and fascia iliaca, respectively. The opening injection pressure was less than 15 psi for all 20 patients when the needle was withdrawn 1 mm from the nerve and at the subfascial position.¹⁵

These results offer some guidance in the safe practice of regional anesthesia. While in-line pressure monitoring may improve safety by avoiding injections against high opening injection pressures, this is a limited adjunct for safety as similarly high pressures were noted with both fascia iliaca indentation and needle-nerve contact. More precise localization is key to block success and should still be confirmed via nerve stimulation and/or ultrasound guidance. In the setting of an anesthetized patient, however, increased opening injection pressure might serve as an important warning to the anesthesiologist in a population who either may not have the appropriate sensorium to report pain/paresthesia or the capacity to remain still for the block.

Nerve Localization—Ultrasound versus Nerve Stimulation

A review of the different nerve localization techniques is warranted when considering nerve blocks under GA or deep sedation. There is no human data to support the superiority of one nerve localization technique with regard to another in terms of reducing the likelihood of peripheral nerve injury.⁴ Peripheral nerve stimulation resulting in an evoked motor response at a current of <0.5 mA(0.1 ms) indicates intimate needle-nerve relationship, needle-nerve contact, or intraneural needle placement. Likewise, the absence of a motor response at a current of up to 1.8 mA does not exclude needle-nerve contact or intraneural needle placement.⁴

Ultrasound localization has shown the ability to detect intraneural injection. Current technology, however, does not have adequate resolution to discern between interfascicular and intrafascicular injection.⁴ Importantly, all operators in all patients do not consistently obtain adequate images of the needle-nerve interface.⁴ The use of ultrasonography in conjunction with peripheral nerve stimulation is supported by a statistically significant finding of lower seizure rates related to systemic local anesthetic in the combined group versus that only utilizing landmark identification with peripheral nerve stimulation.¹⁶ The same study, however, found there to be no significant difference in neurologic injury between the two groups.¹⁶

The safest practice in peripheral nerve localization would be to utilize as many of the following modalities simultaneously as possible: ultrasonography, peripheral nerve stimulation, injection pressure monitoring, and patient-reported pain/paresthesia. These practical applications depend on numerous patient-, anesthesiologist-, and facility-related factors. The availability of quality ultrasound machines at a facility may vary due to the purchase costs or ability to maintain and store such machines. Injection pressure monitoring devices also incur additional costs to a facility and may not be readily adopted by all anesthesiologists. Patient anxiety and needle phobias may limit the acceptance of nerve block techniques being done with unaltered patient sensorium. Due to many patients considering injections under conscious sedation to be unacceptable, Masaracchia et al implemented a protocol for the performance of PECS blocks under general anesthesia. Quality assurance follow-up found that multiple patients who received a block under conscious sedation would not repeat the block if additional surgery were needed. After performing a series of 40 PECS blocks under GA, no quality-compromising issues were identified immediately during PACU care or the post-discharge follow-up.¹⁷ Other non-modifiable patient characteristics limit either the safety, feasibility, or both with regard to performing peripheral nerve blocks in an awake patient. Some of these will be discussed in the next section.

As with most areas of medical decision-making, a thorough risk/benefit analysis specific to each patient scenario is the best guide for the appropriateness of conditions for placing nerve blocks.

Special Populations

Children

Routine performance of peripheral nerve blocks in children is a widely accepted practice. Except for emancipated minors, this population of patients cannot truly consent to procedures and make their own medical decisions. As such, it stands to reason that, in coordination with parental consent, the benefits of improved analgesia expected from successful regional anesthesia or analgesia procedures often outweigh the risk of harm and relatively supersede the ethical principle of autonomy. Without an adult-level ability to follow instructions and remain relatively motionless, ensuring a compliant patient (under general anesthesia) is likely the safest strategy for successful nerve block placement. Additionally, in the youngest of patients, their wakefulness may not provide any layer of protection if they are not developmentally able to reliably identify and describe paresthesias to the anesthesiologist. 

Two large, multicenter prospective studies conclude that the overall rate of complications for regional anesthesia procedures in children is very low.^2,3 In the Pediatric Regional Anesthesia Network (PRAN) study, a total of 14,917 regional blocks yielded no deaths or complications with sequelae lasting >3 months (95% CI 0-2:10,000).³ The French-Language Society of Paediatric Anaesthesiologists analyzed 29,870 GAs associated with regional blocks and 1,262 purely regional blocks. Complications (41 in 40 patients) were rare and usually minor. They did not result in any sequelae. The study revealed an overall complication rate of 0.12%; CI 95% [0.09-0.17], significantly six times higher for central than peripheral blocks.²

Intellectual Disability and Advanced Dementia

Intellectual disabilities often present in patients with Down’s Syndrome, Autism Spectrum Disorder, Fragile X Syndrome, etc., can pose a significant barrier to safe regional anesthesia procedures. The inability to follow instructions and the increased risk of sudden movement in this population, while not anesthetized, significantly increase the risk of inadvertent vascular puncture during a block. For patients who would likely derive benefits from a regional block and the minimization of respiratory depressing opioids, it may not be unreasonable to perform the nerve block under GA. Likewise, patients with advanced dementia may be both incapable of articulating if they feel paresthesia and cooperating during critical portions of the procedure.

Polytrauma

Individuals subjected to polytraumatic injuries comprise a group that would likely benefit significantly from one or more regional anesthesia interventions. However, many of these patients initially present intubated/sedated upon hospital arrival due to extensive thoracic wall injuries or traumatic brain injuries. To minimize ICU days and time requiring mechanical ventilation, regional blocks may be best suited to be performed while the patient is still heavily sedated for comfort, altered mental status, or inadequate ventilation. Traditionally, epidural analgesia has been the regional anesthetic technique with the best evidence of efficacy in rib fractures. Meta-analysis has shown that thoracic epidurals provide superior analgesia to paravertebral blocks, intercostal blocks, and IV analgesia.¹⁸ Given the potential for hemodynamic perturbations and the prevalence of anticoagulant use in the general population, the erector spinae plane and the serratus anterior plane blocks have gained popularity in the last decade. These blocks have improved safety profiles and are appropriate targets for continuous nerve catheter techniques. While largely unproven in efficacy, a greater case could be made for doing these blocks under deep sedation, given the decreased likelihood of doing significant harm relative to thoracic epidural placement.

Conclusion

The decision to perform nerve blocks in deeply sedated or anesthetized patients is not one to be made lightly. Evidence to date supports the routine performance of nerve blocks under GA for the pediatric population due to the lack of persisting complications. Other special patient populations outlined above can be reasonable cohorts to consider blocks under GA as they either cannot communicate needle-nerve contact eliciting paresthesia, are unable to reliably remain still for a procedure, or have another indication to be heavily sedated. Even highly anxious or needle-phobic patients may be more amenable to receiving helpful regional blocks if they were to be performed under GA, thus potentially leading to higher patient satisfaction.

However, one should consider patient alertness as a possible safety mechanism to complement ultrasonography, opening injection pressure monitoring, and peripheral nerve stimulation. It would behoove any anesthesiologist to perform nerve blocks in awake or mildly sedated patients most of the time. Loss of spinal cord function in patients receiving blocks under GA is well documented, and a patient under GA cannot warn of possible impending neurologic injury (paresthesia). The earliest subjective CNS manifestations of LAST are undetectable in anesthetized patients. As with most areas of medical decision-making, a thorough risk/benefit analysis specific to each patient scenario is the best guide for the appropriateness of conditions for placing nerve blocks.

References

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