PCEA Analgesic Agents

There does not appear to be a clearly superior regimen for use in PCEA devices. Local anesthetics are the most commonly used agents. Typically, bupivacaine is used due to its relative resistance to tachyphylaxis. The cardiotoxicity of bupivacaine has led some to use levo-bupivacaine or ropivacaine instead. The usual ranges of concentration for the various local anesthetics are listed in Table 18.7. Often the higher range of concentrations is used for lower extremity orthopedic procedures and the less concentrated solutions used for thoracic surgery; however, there is not any conclusive data to support this practice.

The next most common agents used for PCEA therapy are the opioids. They are usually used in combination with a local anesthetic. There seems to be no benefit of administering lipophilic opioids (i.e., fentanyl or sufentanil) as the sole agent via the epidural route. The site of action of lipophilic opioids when given epidurally is not clear. Studies looking at the efficacy of fentanyl and sufentanil given epidurally versus intravenously are contradictory. Most studies measuring the plasma concentrations of these opioids when given via an epidural or

Table 18.7 Local anesthetic choices for patient-controlled epidural analgesia.

Local anesthetic

Concentration range (%)

Bupivacaine

0.0625-0.2

Levo-bupivacaine

0.0625-0.2

Ropivacaine

0.1-0.2

IV have shown no difference in levels thus suggesting that the major site of action is not at the spinal cord but rather due to systemic absorption.

Hydrophilic opioids (i.e., morphine or hydromorphone), on the other hand, maintain high cerebrospinal fluid levels for an extended period of time and therefore can act at the opioid receptors in the spinal cord. Table 18.8 lists concentrations documented in the literature for the various opioids when administered via the epidural route; the "correct" dose is not known. When used in combination with local anesthetics the lower end of the concentration ranges are suggested. The total 24-h dose of opioid must be kept in mind if high continuous infusion rates or large frequent demand doses are used. It is best to avoid using opioids in epidural infusions for high-risk patients (i.e., elderly, obstructive sleep apnea, morbidly obese) and if systemic opioids are necessary as well for any reason (e.g., pain outside the area the epidural can be expected to cover).

Table 18.8 Opioid choices for patient-controlled epidural analgesia.

Opioid

Concentration range (mcg/ml)

Fentanyl

1-4

Hydromorphone

10-50

Morphine

20-100

Sufentanil

0.5-1

Other adjunctive agents have been studied for addition to local anesthetics and/or opioids for epidural use; however, they are not used extensively, likely because studies are limited and show equivocal results. The major proposed benefits are improved analgesia and the ability to use a lower concentration of local anesthetics and opioids. Table 18.9 lists the adjuvant agents and offers suggested concentrations from the literature; the "correct" doses have not been determined.

Table 1S.9 Other agents for patient-controlled epidural analgesia.

Analgesic

Concentration range (mcg/ml)

Clonidine

1-3

Epinephrine

2-5

Neostigmine

1-7

The major concern with clonidine is an increased incidence of hypotension and sedation. Use of epinephrine has shown improved analgesia with activity though it may lead to an increased incidence of motor block. Epinephrine use also has been shown to reduce plasma concentrations of lipophilic opioids by allowing them to remain in the neuraxis for long enough to actually bind to the opioid receptors in the spinal cord. The concern with neostigmine is that when used for intrathecal administration a dose-dependent incidence of nausea and vomiting occurs as well as sedation; however, epidural use does not seem to have the same incidence of nausea and vomiting and only minimal sedation occurs.

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