^a MEP = motor-evoked potentials, SSEP = somatosensory-evoked potentials.

Source: Reprinted with permission from Schulz-Stubner S et al: Crit Care Med 2005;33:1407.

Clinical Pearls

The most common side effects of epidural blocks are bradycardia and hypotension related to sympathetic block.

Hemodynamic changes can be more pronounced with intermittent bolus dosing, in patients with hypovolemia, or those with reduced venous return secondary to high positive end-expiratory pressure ventilation.

Discontinuation of continuous infusion every morning will allow neurologic assessment when necessary.

There is no compelling evidence of increased risks of epidural bleeding with developing coagulopathy or therapeutic anticoagulation while an epidural catheter is in place. Nevertheless, the benefits of epidural analgesia should be weighed against the risk of this serious complication.

Bolus injections of long-acting local anesthetics such as bupivacaine, ropivacaine, or levobupivacaine or the discontinuation of continuous infusion every morning will allow neurologic assessment when necessary. Monitoring of motor-evoked potentials (MEP) to the lower extremities and somatosensory-evoked potentials (SSEP) of the tibial nerve may serve as indicators when the neurologic examination is doubtful due to the patient’s altered mental status. Although routinely used in the operating room for monitoring spinal cord integrity and for diagnosis and prognosis of spinal cord injury, the use of this technology in the ICU in the context of epidural analgesia has not been adequately assessed.

The most common side effects of epidural blocks are bradycardia and hypotension related to sympathetic block. Hemodynamic changes can be more pronounced with intermittent bolus dosing, in patients with hypovolemia, or those with reduced venous return secondary to high positive end-expiratory pressure (PEEP) ventilation. Based on data from lumbar punctures and meningitis from the beginning of the twentieth century,⁵³ current sepsis and bacteremia are considered contraindications for intrathecal opioid applications and, by analogy, for placement of epidural catheters. However, many ICU patients, especially after trauma and major surgery, present with a clinical picture of SIRS. Fever and increased white blood cell counts alone, that is, in the absence of positive blood cultures, do not provide a reliable diagnosis of bacteremia. The combination of the serum markers C-reactive protein (CRP), procalcitonin, and interleukin-6, on the other hand, have been shown to indicate bacterial sepsis with a high degree of sensitivity and specificity^54–57 and can guide the decision to place an epidural catheter. Regarding the patients coagulation status, the current recommendations of the American Society of Regional Anesthesia (ASRA)^58,59 should be followed. Adequate safety intervals during the administration of anticoagulative drugs are equally important for the placement and removal of epidural catheters.^60,61 Although there is no compelling evidence of increased risks of epidural bleeding with developing coagulopathy or therapeutic anticoagulation while an epidural catheter is in place, the benefits of epidural analgesia should be weighed against this potential, highly detrimental complication.

PERIPHERAL NERVE BLOCKS FOR THE UPPER EXTREMITIES

At the time of this writing, there are no randomized, controlled trials or large prospective trials evaluating the use of peripheral nerve blocks for the upper extremity in critically ill patients. Nevertheless, severe trauma to the shoulder or arm is often part of the multiple injuries due to traffic or workplace accidents, often in combination with blunt chest trauma requiring mechanical ventilation. These injuries can contribute to severe pain, especially during positioning of the patient. If the orthopedic injury is part of complex trauma including brain injury in which the mental status of the patient is altered and opioid-based analgesic regimens might mask the neurologic situation, sufficient analgesia can be achieved for the shoulder or upper limb with either continuous interscalene,^62–64 continuous cervical paravertebral,^64–67 or infraclavicular⁶⁸ approaches to the brachial plexus.

Particular concerns arise concerning the placement of regional blocks in ICU patients with impaired mental status due to neurologic injury or therapeutic sedation. Benu- mof reported a small series of serious complications, including spinal cord injury related to the interscalene approach, which might have been associated with sedation or general anesthesia.⁶⁹ His case descriptions relate to the spinal cord injury in heavily sedated or anesthetized patients and not to injury of the peripheral nerves. Despite this, performance of blocks anatomically close to the centroneuraxis can indeed carry a higher risk of spinal cord needle or injection injury. In sedated critically ill patients a combination of ultrasound and nerve stimulation for the placement of interscalene catheters and a technique with a less medial needle direction^70,71 should help to minimize the risk of complications. Perhaps most importantly, such blocks should be performed only by clinicians with adequate experience. The unavoidable blocking of the phrenic nerve and the loss of hemidiaphragmatic function⁷² should be considered while planning the intervention. Although phrenic nerve blockade has negligible effects in mechanically ventilated patients, it may impair weaning from mechanical ventilation in highrisk patients. Furthermore, the proximity of the insertion site of the interscalene catheter to a tracheostomy tube might increase the risk of infection, and careful, standardized monitoring of the puncture site is therefore needed. Positioning problems might limit the use of the cervical paravertebral approach, which provides good analgesia for shoulder, arm, and hand.

Clinical Pearls

In patients with altered mental status in whom opioid- based analgesic regimens might make neurologic evaluation difficult, excellent analgesia can be achieved for the shoulder or upper limb with continuous interscalene, cervical paravertebral, or infraclavicular approaches to the brachial plexus.

Performance of blocks anatomically close to the cen- troneuraxis can carry a higher risk of spinal cord needle or injection injury. In heavily sedated critically ill patients such blocks should be performed only by clinicians with adequate experience.

An interscalene brachial plexus block results in the loss of hemidiaphragmatic function. Although phrenic nerve blockade has negligible effects in mechanically ventilated patients, it may impair weaning from mechanical ventilation in high-risk patients.

The continuous infraclavicular^{64,68,73–75} and axillary^64,76–78 approaches provide good analgesia for most of the arm, elbow, and hand. Bolus injection of local anesthetic through the catheter should especially be considered in patients who need surgical anesthesia, for example, for painful dressing changes or debridements for burns or large soft tissue wounds in the affected area. The small risk of pneumothorax must be considered against the slightly higher success rate and easier catheter maintenance of the infraclavicular versus the axillary approach, but can be ignored if a chest tube is already in place. A more lateral infraclavicular approach^79–81 might help to reduce the pneumothorax risk further.

PERIPHERAL NERVE BLOCKS FOR THE LOWER EXTREMITIES

Femoral nerve catheters are helpful in the management of acute pain from femoral neck fractures, in the period between the injury to shortly after surgical stabilization of the fracture.^82,83 Skilled use of ultrasound⁸⁴ might limit the unavoidable pain with nerve stimulation in this situation, which otherwise can be treated with small doses of intravenous remifentanil (0.3-0.5 mcg/kg) or ketamine (0.2-0.4 mg/kg). A fascia iliaca compartment block^85,86 might be a technical alternative.

Continuous femoral catheters in combination with a sciatic block provide excellent pain relief for the whole leg and even surgical anesthesia for procedures like external fixation.⁸⁷ Whether an anterior⁸⁸ or posterior approach (midgluteal,⁸⁹ subgluteal⁹⁰ classical Labat approach with one or two injections⁹¹) to the sciatic nerve is chosen depends largely on the skills of the operator and the ability to adequately position the patient for the procedure. If a combination of catheter techniques is used, as is often needed for the lower extremity, the total daily dose of local anesthetic should be adjusted based on catheter location, admixtures like epinephrine, drug interactions, and disease states as summarized in a recent review by Rosenberg and coworkers.⁹²

Bolus injection of long-lasting local anesthetics in combination with clonidine⁹³ or buprenorphine⁹⁴ may help to reduce the overall amount of local anesthetics needed and minimize the effects of local anesthetic toxicity, although research results about these adjuvants are equivocal at present.^95,96

OTHER REGIONAL ANALGESIC TECHNIQUES

Celiac plexus blocks may provide excellent analgesia for pancreatitis and cancer-related upper abdominal pain, but technical difficulties in the critically ill (CT guidance, fluoroscopy, or transgastric ultrasound) and the need for repeated injections limit its value for acutely critically ill patients.

Intrapleural catheters for pain control after chest trauma are of limited value secondary to concurrent drainage from chest tubes. The risk of pneumothorax limits their benefits for the management of pain after conventional cholecystectomy compared with the epidural or paravertebral technique in ventilated patients.

Thoracic paravertebral catheters can be a valuable alternative to epidural catheters for the management of unilateral pain restricted to a few dermatomes (eg, rib fractures⁹⁷ or zoster neuralgia).

Table 67-2 provides a summary of the most often utilized continuous peripheral catheters.

Single-injection nerve blocks, for example, intercostal blocks for the placement of chest tubes, scalp blocks⁹⁸ for the placement of halo fixation, and sufficient local infiltration anesthesia for typical ICU procedures like placement of arterial and central venous catheters, lumbar punctures, or ventriculostomies are often forgotten, although they are easy and safe to perform. If EMLA cream is used for topical anesthesia it needs to be applied 30-45 min before the procedure to achieve an optimal effect.

Intrathecal morphine^99–102 injections as a single shot and via spinal catheters (microcatheters are currently not approved in the United States but are available in Europe) can be an alternative to epidural catheters, especially if only shortterm use after surgery is anticipated.

SYSTEMIC EFFECTS & COMPLICATIONS OF LOCAL ANESTHETICS IN THE CRITICALLY ILL PATIENT

Local anesthetics have been shown to have several positive systemic effects (including analgesic, bronchodila- tory, neuroprotective, anti-inflammatory, antiarrhythmic, and antithrombotic properties)¹⁰³ when given or absorbed in adequate quantities (the exact dose-response relationships are widely unknown). They also have negative effects, such as, neurotoxicity (dose-dependent), myotoxicity,^104,105 inhibition of wound healing, cardiotoxicity (dose-dependent), and central nervous excitation or depression (dose-dependent).¹⁰⁶

Table 67–2.

Continuous Peripheral Nerve Blocks in the Critically Ill