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Before administrating any drug to a patient, the clinician must evaluate the health of the patient to determine whether the patient can tolerate the drug and minimize possible complications resulting from the drug interacting with the patient’s organ systems or with medication the patient is taking. Local anaesthetic actions include depressant effects on the central nervous system and cardiovascular system. Because local anaesthetics undergo bio-transformation in the liver (amides) and blood (esters) and are excreted by the kidneys, the status of these organ systems should be evaluated. A patient’s psychological acceptance of a local anaesthetic needs to be assessed as many patients view the “shot” as the most traumatic aspect of the dental procedure.
While a comprehensive medical history is recommended for all dental patients, the following questions are most pertinent for those patients who are to receive local anaesthesia.
As the confines of this course limit a full discussion of the effects of local anaesthetics on the body and with other drugs the following tables summarize the more common interactions.
| Drugs | Example | Significance Rating | Interaction |
|---|---|---|---|
| Summation interactions of local anaesthetics | Lidocaine plus articaine | Major | Toxicity of local anaesthetics are additive. Total dose of all administered local anaesthetics should not exceed the maximum recommended dose of the drugs. |
| Local anaesthetics with opioid sedation | Local anaesthetic with Demerol | Major | May increase the risk of local anaesthetic overdose. Minimize the dosage of the local anaesthetic. |
| Vasoconstrictor with cocaine | Epinephrine with cocaine Vasopressors should not be administered to patients who have used cocaine within the last 24 hours |
Major | Increases likelihood of cardiac dysrhythmias, tachycardia and hypertension. May lead to MI and cardiac arrest. |
| Vasoconstrictors with general anaesthetics | Epinephrine with halothane | Major | Increases the likelihood of cardiac dysrhythmias. Discuss with cardiologist before administration. |
| Vasoconstrictors with nonselective beta adrenoreceptor antagonists (beta-blocker) | Epinephrine with propranolol | Major | TCAs enhance the cardiovascular actions of administered vasopressors. 5-10X with levonordefrin and norepinephrine. 2X with epinephrine |
| Local anaesthetic induced methoglobinemia (a condition in which the oxygen carrying capacity of the blood is reduced) | Excessive doses of prilocaine | Moderate | Large doses of benzocaine can also induce methoglobinemia |
| Vasoconstrictor with antipsychotic drugs | Epinephrine with chlorpromazine | Moderate | May result in hypotension |
| Vasoconstrictor with thyroid hormone | Epinephrine with thyroxine | Moderate | Increase in effects of vasoconstrictor with excessive thyroid hormones |
| Sulfonamides and esters | Procaine and tetracaine and sulfonamides | Minor | Ester anaesthetics inhibit the bacteriostatic action of sulfonamides. Use amide local anaesthetics. |
| Amide local anaesthetics with metabolic inhibitors (GI disorders) | Cimetidine (Tagamet) and lidocaine. No problem with rantidine (Zantac) and famotidine (Pepcid) | Minor | Inhibits anaesthetic bio-transformation. Increases half-life of anaesthetic. Use minimal dose of amide local anaesthetic. |
| Significance Rating Major – Potentially life threatening or capable of causing permanent damage. Moderate – Could cause deterioration of patient's clinical status; additional treatment or hospitalization might be necessary. Minor – Mild effects that are bothersome or unnoticed; should not significantly affect therapeutic outcome. |
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| Medical Problem | Drugs to Avoid | Type of Contraindication | Alternative Drug |
|---|---|---|---|
| Local anaesthetic allergy, documented | All local anaesthetics in the same class, (e.g., esters) | Absolute | Local anaesthetics in a different chemical class (e.g., amides) |
| Bisulfite allergy | Local anaesthetics containing a vasoconstrictor | Absolute | Local anaesthesia without a vasoconstrictor |
| Atypical plasma cholinesterase | Esters | Relative | Amides |
| Methoglobinemia, idiopathic or congenital | Articaine, prilocaine, topical benzocaine in children younger than 2 years | Relative | Other amides or esters |
| Significant liver dysfunction (ASA III-IV) | Amides | Relative | Amides or esters but judiciously |
| Significant renal dysfunction (ASA III-IV) | Amides or ester | Relative | Amides or esters but judiciously |
| Significant cardiovascular dysfunction (ASA III-IV) | High concentrations of vasoconstrictors (as in racemic epinephrine cords) | Relative | Local anaesthetics with concentrations of 1:200,000 or 1:100,000 or mepivacaine 3% or prilocaine 4% (nerve blocks) |
| Clinical hyperthyroidism (ASA III-IV) | High concentrations of vasoconstrictors (as in racemic epinephrine cords) | Relative | Local anaesthetics with concentrations of 1:200,000 or 1:100,000 or mepivacaine 3% or prilocaine 4% (nerve blocks) |
| Definitions: Absolute contraindication – Implies that under no circumstance should this drug be administered to this patient because the possibility of potentially toxic or lethal interactions is increased. Relative contraindication – Implies that the drug in question may be administered to the patient after carefully weighing the risk of using the drug to its potential benefit, and if an acceptable alternative drug is not available. |
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