Antivenom Dosing Algorithm

Antivenom Dosing Calculator
Envenomation Severity
Recommendation: Minimal Envenomation
Minimal Envenomation
No antivenom needed. Local wound care, observe 8-12 hours. Monitor for progression.
Doses antivenom (CroFab only, not Anavip) for pit viper snakebites.

Why Use

Approximately 9,000 snakebites are treated yearly in the US, with 5 of those patients dying annually. The case-fatality rate is reported at 1 death per 736 patients. The algorithm specifies the manifestations of crotaline envenomation that necessitate aggressive management.

When to Use

Patients with known or suspected crotaline envenomation (rattlesnake, copperhead, cottonmouth). Not valid for snakebites to the head or neck or with cardiovascular collapse: call Poison Control (1-800-222-1222).

Formula

Algorithm, as follows: Are there signs of envenomation? Signs include: Swelling, tenderness, redness, ecchymosis, or blebs at bite site. Elevated protime, decreased fibrinogen or platelets. Systemic signs (hypotension, bleeding beyond puncture site, refractory vomiting, diarrhea, angioedema, neurotoxicity). If YES, proceed to 2. If NO, this is an apparent dry bite or no bite. Observe patient ≥8 hours, repeat labs prior to discharge, and do not administer antivenom. Check for indications for antivenom: Swelling that is more than minimal and is progressing. Elevated protime, decreased fibrinogen or platelets. Any systemic signs. If YES, proceed to 3. If NO, this is an apparent minor envenomation. Observe patient 12-24 hours, repeat labs at 4-6 hrs and prior to discharge, and do not administer antivenom. Administer antivenom: Establish IV access, give IV fluids. Mix 4-6 vials of crotaline Fab antivenom in 250 mL NS and infuse over 1 hour. Increase dose to 6-8 vials for patients in shock or with serious active bleeding. Reexamine after 1 hour, and proceed to 4. Check for control of envenomation (after initial antivenom): Swelling and tenderness not progressing. Protime, fibrinogen, platelets normal or clearly improving. Clinically stable. Neurotoxicity resolved or clearly improving. If YES, monitor patient and perform serial examinations and maintenance therapy (2 vials of antivenom every 6 hours x 3 doses (given 6, 12, and 18 hours after initial control). Maintenance therapy may not be needed if close observation by physician expert is available. If NO, then repeat antivenom until control is achieved.

Pearls / Pitfalls

Based on the Unified Treatment Algorithm ( Lavonas 2011 ), which was developed with the goal of quick identification and management of patients who may benefit from Crotalidae Polyvalent Immune Fab. There is significant variability among envenomation patients, and this algorithm does not represent a standard of care. Report all cases of suspected/confirmed envenomation to poison control (1-800-222-1222).

Management

Maintenance therapy: 2 vials of antivenom every 6 hours x 3 doses (given 6, 12, and 18 hours after initial control). May not be needed if close observation by physician expert is available. Follow-up planning: Patient should return for worsening swelling not relieved by elevation, or abnormal bleeding (e.g. melena, gum bleeding, easy bruising). If fever, rash, or muscle/joint pains occur (i.e., suggesting serum sickness), patient should return. Patient should be given bleeding precautions (no contact sports, elective surgery, or dental work for 2 weeks). Follow-up: Patients who did not require antivenom: as needed. Copperhead victims: as needed. Cottonmouth and rattlesnake victims: repeat labs (CBC, PT, fibrinogen) twice (2-3 days and 5-7 days after discharge), then as needed.

Critical Actions

Report to poison control (1-800-222-1222) all cases of suspected/confirmed envenomation. Avoid the following: Cutting or suctioning wound. Ice. NSAIDs: avoid in rattlesnake/cottonmouth victims ( Pham 2018 ). Prophylactic antibiotics. Prophylactic fasciotomy. Routine use of blood products. Electrical shock therapy. Steroids, unless allergic phenomena observed. Tourniquets.

Advice

Leading edge of swelling and tenderness should be marked every 15-30 minutes. Elevate and immobilize extremity, treat pain aggressively with IV opioids, and update tetanus status as needed.

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