Prolongation Of Action By Vasoconstrictors

The effect of a local anaesthetic is terminated by its removal from the site of application. Anything that delays its absorption into the circulation will prolong its local action and can reduce its systemic toxicity where large doses are used. Most local anaesthetics, with the exception of cocaine, cause vascular dilation. The addition of a vasoconstrictor such as adrenaline (epinephrine) reduces local blood flow, slows the rate of absorption of the local anaesthetic, and prolongs its effect; the duration of action of lidocaine is doubled from one to two hours. Normally, the final concentration of adrenaline (epinephrine) should be 1 in 200 000, although dentists use up to 1 in 80 000.

A vasoconstrictor should not be used for nerve block of an extremity (finger, toe, nose, penis). For obvious anatomical reasons, the whole blood supply may be cut off by intense vasoconstriction so that the organ may be damaged or even lost. Enough adrenaline (epinephrine) can be absorbed to affect the heart and circulation and reduce the plasma potassium. This can be dangerous in cardiovascular disease, and with co-administered tricyclic antidepressants and potassium-losing diuretics. An alternative vasoconstrictor is felypressin (synthetic vasopressin), which, in the concentrations used, does not affect the heart rate or blood pressure and may be preferable in patients with cardiovascular disease.

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