Anti - Snake Venom - Dose and Mode of Administration
In the management of the snake bite an estimate of the severity of envenomation should be made as soon as possible, before any antivenin is administered, since, for example, in approximately 20% of rattlesnake bites, venom may not be injected.
The preferred route of administration is by intravenous infusion. However, many antivenin polyvalent may be administered intramuscularly. If the intramuscular route is used, the antivenin should be administered into a large muscle mass, preferably into the gluteal area, with care to avoid nerve trunks. It should be kept in mind that maximum blood concentrations may not be attained for 8 or more hours after intramuscular administration.
Reconstituted antivenin polyvalent may be administered intravenously in a 1:1 to 1:10 dilution in 0.9% sodium chloride injection or 5% dextrose injection. Decisions concerning the dilution of antivenin to be used, and the rate of intravenous delivery of the diluted antivenin should take into account the age, weight, and cardiac status of the patient; the severity of the envenomation; and the interval between the bite and the initiation of specific therapy.
The entire initial dose of antivenin should be administered as soon as possible, preferably within 4 hours after the bite. Antivenin is less effective when given 8 hours or more after envenomation and may be of questionable value when given after 12 hours. However, in severe poisonings, it is recommended that antivenin therapy be given even if 24 hours have elapsed since the bite.
The initial 5 to 10 mL of the diluted antivenin should be infused over a 3- to 5-minute period, with careful observation of the patient for evidence of an untoward reaction If no symptoms or signs of an immediate systemic reaction appear, infusion of the diluted antivenin may be continued at the maximum rate considered safe for intravenous fluid administration.
The decision to use additional antivenin should be based on the clinical response to the initial dose and on continuing assessment of the severity of poisoning. If swelling continues to progress, if systemic symptoms or signs of envenomation increase in severity, or if new manifestations appear (for example, fall in hematocrit or hypotension), intravenous administration of an additional 10 to 50 mL (contents of 1 to 5 vials) or more may be necessary.
Pit viper bites on toes or fingers may require as much as 50% more antivenin due to difficulties in achieving adequate antivenin concentrations in the affected area. However, antivenin should never be injected into a finger or toe.
Polyvalent anti-snake venom contains antibodies against cobra, common krait and viper.
To calculate the dose of anti-snake venom needed for a particular patient one has to take into account various aspects. However a rough idea can be made out as below :-
5 vials are given if signs are mild -primarily local manifestations.( Minimal envenomination)
1/3 of the dose should be given subcutaneously (near bite but not in fingers or toes).
The intravenous dose can be repeated every 6 hours till the symptoms disappear. For sea-snake bites, special antivenoms are available.