Pharmacology of Local Anesthetics: Clinical Implications
Sympathetic Reactions

LA formulations may contain epinephrine or levonordefrin. Healthy adults can safely receive up to 0.2 mg of epinephrine or 1.0 mg of levonordefrin per visit. The inadvertent intravascular injection of a LA containing a vasoconstrictor, the use of a LA containing high concentration of vasoconstrictor, the potentiation of the injected vasoconstrictor by endogenous catecholamines, and concomitant therapy with other sympathomimetic agents may contribute to adverse sympathetic effects (Table 5).^19-25,28,29

Vasoconstrictors must be avoided in patients under the influence of cocaine. Other instances requiring prudence with the use of a vasoconstrictor include patients with blood pressure in excess of 180/110, severe cardiovascular disease (i.e., recent MI, unstable angina pectoris, decompensated heart failure, severe valvular disease, supraventricular arrhythmias with uncontrolled ventricular rate, symptomatic ventricular arrhythmias, and high-grade AV block), and patients with uncontrolled hyperthyroidism.^19-25

The medical history should seek to determine the patient’s functional capacity (FC).⁶³ An individual’s capacity to perform a spectrum of common daily tasks has been shown to correlate well with maximum oxygen uptake by treadmill testing. FC is expressed in metabolic equivalents (METs). The oxygen consumption of a 70-kg, 40-year-old man at rest is 3.5 ml per kg per minute, which requires a FC of 1 MET. Cardiac risk is increased in patients unable to meet a 4-MET demand for oxygen.⁶⁴

The hemodynamic effect of 0.045 mg of epinephrine was reported to be less than that produced by ergometric stress testing at 25 watts in young patients and at 15 watts in older subjects.³⁷ The workload of ergometric stress testing at these levels is about 4 METs, which is approximately equivalent to the work load produced by climbing two flight of stairs, walking 4.8 km/hr., doing light yard work (raking leaves, weeding, or pushing a power mover), painting, or doing light carpentry work.

Based on this report, 0.045 mg of epinephrine can be safely administered to patients who can tolerate the activities noted above with minimal or no symptoms, e.g., diaphoresis, fatigue, shortness of breath, and/or chest pain. Epinephrine, 0.045 mg, is equivalent to the amount of epinephrine 1:100,000 found in 4.5 mL of any LA formulation. This study provides evidence that a practical determinant for a safe dose of vasoconstrictor is the patient’s FC, i.e., the ability to tolerate physical/emotional stress.