Drug Interactions between bupivacaine / fentanyl and dyphylline / ephedrine / guaifenesin / phenobarbital

GENERALLY AVOID: Barbiturates may potentiate the central nervous system (CNS) depressant effects of opioids. Concomitant use may result in profound sedation, respiratory depression, coma, and death. On the other hand, some barbiturates can also induce the hepatic metabolism of opioids that are metabolized by CYP450 3A4 such as butorphanol, fentanyl, hydrocodone, methadone and oxycodone, resulting in enhanced clearance. Reduced analgesic efficacy or withdrawal symptoms may occur in patients maintained on their opioid regimen following the addition of a barbiturate. Conversely, discontinuation of the barbiturate may increase plasma concentrations of the opioid and potentiate the risk of overdose and fatal respiratory depression.

MANAGEMENT: The use of opioids in conjunction with other CNS depressants such as barbiturates should generally be avoided unless alternative treatment options are inadequate. If coadministration is necessary, the dosage and duration of each drug should be limited to the minimum required to achieve desired clinical effect, and patients should be closely monitored for signs and symptoms of CNS and respiratory depression. Particular caution is advisable when a barbiturate is added to or withdrawn from therapy in patients receiving opioids that are CYP450 3A4 substrates, as there may be an increased risk of withdrawal symptoms (e.g., restlessness, insomnia, sweating, lacrimation, or rhinorrhea) following initiation of the barbiturate and overdose following discontinuation. A dosage adjustment for one or both drugs may be required.

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MONITOR: Coadministration of local anesthetics with other oxidizing agents that can also induce methemoglobinemia such as antimalarials (e.g., chloroquine, quinine), nitrates and nitrites, sulfonamides, aminosalicylic acid, dimethyl sulfoxide (DMSO), metoclopramide, nitrofurantoin, phenazopyridine, phenobarbital, and phenytoin may increase the risk. Additional risk factors include very young age (e.g., infants less than 6 months), cardiac or pulmonary disease, genetic predisposition, and glucose-6-phosphate dehydrogenase (G6PD) deficiency. Data surrounding the incidence of methemoglobinemia are agent-specific and, in many instances, have primarily been reported in case reports and/or in overdose situations.

MANAGEMENT: Monitoring for signs and symptoms of methemoglobinemia is recommended if local anesthetics must be used with other methemoglobin-inducing agents. Signs and symptoms of methemoglobinemia may occur immediately or hours after drug exposure. Patients or their caregivers should be advised to seek medical attention if they notice signs and symptoms of methemoglobinemia (e.g., cyanotic skin discoloration, abnormal blood coloration, nausea, headache, dizziness, lightheadedness, lethargy, fatigue, dyspnea, tachypnea, tachycardia, palpitation, anxiety, and confusion). In severe cases, patients may progress to central nervous system depression, stupor, seizures, acidosis, cardiac arrhythmias, syncope, shock, coma, and death. Methemoglobinemia should be considered if central cyanosis is unresponsive to oxygen. Calculated oxygen saturation and pulse oximetry are generally not accurate in the setting of methemoglobinemia. The diagnosis can be confirmed by an elevated methemoglobin level of at least 10% using co-oximetry. Methemoglobin concentrations greater than 10% of total hemoglobin will typically cause cyanosis, and levels over 70% are frequently fatal. However, symptom severity is not always related to methemoglobin levels. Experts suggest that treatment of methemoglobinemia varies from supplemental oxygen and symptom support to the administration of methylene blue, depending on severity of symptoms and/or the presence of G6PD deficiency. Institutional guidelines and/or individual product labeling should be consulted for further guidance.

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Ephedrine-methylxanthine combinations are used for the treatment of asthma but the efficacy of the combination has been questioned. This combination may lead to increased xanthine side effects. The mechanism is unknown, but may be related to synergistic pharmacologic effects. Patients using this combination should be closely monitored for side effects such as nausea, vomiting, tachycardia, nervousness, or insomnia. If side effects are noted, the dosage of the xanthine may need to be decreased.

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