What are the indications and dosing protocol for intravenous sodium bicarbonate in a tricyclic antidepressant (TCA) overdose?

Sodium Bicarbonate Treatment Indications for TCA Poisoning

Administer intravenous sodium bicarbonate immediately when QRS duration exceeds 100 milliseconds on ECG, when ventricular dysrhythmias occur, or when terminal rightward axis deviation in lead aVR exceeds 120 degrees. 1, 2

Primary Indications for Sodium Bicarbonate

Electrocardiographic Criteria (Most Important)

• QRS prolongation ≥100 ms is the single most critical indication and better predicts serious complications than serum TCA levels 1, 2, 3
• Terminal rightward axis deviation >120 degrees in lead aVR warrants immediate bicarbonate administration 1
• Ventricular dysrhythmias of any type require bicarbonate as first-line therapy 1, 4
• Wide-complex tachycardia or other conduction abnormalities indicating severe sodium channel blockade 1, 4

Clinical Indications

• Hypotension refractory to initial fluid resuscitation (after 10 mL/kg normal saline bolus) 4
• Cardiac arrest in the setting of known or suspected TCA overdose 5, 4
• Refractory shock unresponsive to standard vasopressor therapy 4

Dosing Protocol

Initial Bolus

• Adults: 1-2 mEq/kg IV bolus (50-100 mL of 8.4% solution) administered slowly over several minutes 5, 1, 2
• Children: 1-2 mEq/kg IV given slowly 1, 3
• Repeat boluses every 5-10 minutes as needed until QRS narrows and hemodynamic stability is achieved 1, 4

Continuous Infusion (if needed)

• After initial bolus, continue with 150 mEq/L solution at 1-3 mL/kg/hour to maintain alkalosis 5, 1
• Some refractory cases may require extraordinarily high cumulative doses (up to 2650 mEq has been reported) 6

Target Parameters

pH Goals

• Target arterial pH: 7.45-7.55 (mild alkalemia is therapeutic) 1, 4
• Do not exceed pH 7.55 to avoid excessive alkalemia 5, 1
• Maintain mild hyperventilation during mechanical ventilation to support this pH range 4

Sodium Monitoring

• Keep serum sodium <150-155 mEq/L to prevent hypernatremia 5, 1, 4
• Monitor serum sodium every 2-4 hours during active therapy 5, 4

Clinical Endpoints

• QRS duration narrowing to <100 ms 1, 2
• Resolution of ventricular dysrhythmias 1, 4
• Hemodynamic stabilization with improved blood pressure 4

Mechanism of Action

Sodium bicarbonate works through two synergistic mechanisms that are both essential for reversing TCA cardiotoxicity 1:

• Sodium loading: Overcomes sodium channel blockade by increasing extracellular sodium gradient 1
• Alkalinization: Reduces the active (ionized) fraction of TCA, decreasing binding to sodium channels and improving cardiac conduction 1, 7

Critical Monitoring Requirements

Continuous Monitoring

• ECG monitoring for QRS duration, dysrhythmias, and conduction abnormalities 4, 2, 3
• Cardiac rhythm strip or 12-lead ECG should be obtained immediately in prehospital setting if available 3

Laboratory Monitoring (Every 2-4 Hours)

• Arterial blood gases to track pH and PaCO₂ 5, 4
• Serum sodium to prevent hypernatremia 5, 4
• Serum potassium as alkalinization causes intracellular shift and hypokalemia 5, 4
• Ionized calcium as large bicarbonate doses can decrease free calcium 5

Essential Airway Management

• Ensure adequate ventilation BEFORE bicarbonate administration because bicarbonate generates CO₂ that must be eliminated 5, 7
• Consider early intubation for respiratory depression or inability to protect airway 4
• During mechanical ventilation, avoid respiratory acidosis and maintain mild hyperventilation (PaCO₂ 30-35 mmHg) to work synergistically with bicarbonate 5, 4
• Failure to eliminate CO₂ can cause paradoxical intracellular acidosis and worsen toxicity 5

Adjunctive and Second-Line Therapies

When Bicarbonate Alone is Insufficient

• Lidocaine (Class Ib antiarrhythmic): 1-1.5 mg/kg IV bolus for refractory wide-complex tachycardia after optimized bicarbonate therapy 4
• Intravenous lipid emulsion (ILE): 1.5 mL/kg bolus of 20% solution followed by 0.25 mL/kg/min infusion for life-threatening toxicity refractory to bicarbonate and lidocaine 4
• VA-ECMO: Consider for cardiac arrest or refractory cardiogenic shock unresponsive to all medical therapies; early consultation improves outcomes 1, 4

Supportive Measures

• IV fluid boluses (10 mL/kg normal saline) for initial hypotension management 4
• Benzodiazepines for TCA-associated seizures 3
• Maintain normothermia as hyperthermia worsens toxicity 4, 7

Absolute Contraindications and Critical Pitfalls

Never Use These Medications

• Flumazenil is absolutely contraindicated even with suspected benzodiazepine co-ingestion—may precipitate seizures or arrhythmias 4, 3
• Physostigmine for anticholinergic symptoms worsens cardiac toxicity 4
• Class Ia antiarrhythmics (quinidine, procainamide), Class Ic (flecainide), and Class III (amiodarone) exacerbate sodium channel blockade 4
• Beta-blockers may precipitate hypotension and cardiac arrest 4

Common Errors to Avoid

• Do not delay bicarbonate while waiting for serum TCA levels—ECG findings (QRS >100 ms) are better predictors of toxicity 2, 3
• Do not allow respiratory acidosis during mechanical ventilation—this potentiates cardiotoxicity 4
• Do not administer bicarbonate without ensuring adequate ventilation to eliminate CO₂ 5, 7
• Do not induce emesis in TCA overdose 3

Prehospital Considerations

• EMS should administer sodium bicarbonate if QRS >100 ms and a protocol exists for its use 3
• Transport immediately with cardiac monitoring; do not delay for activated charcoal administration 3
• Symptomatic patients (weak, drowsy, dizzy, tremulous, palpitations) require emergency department referral 3
• Ingestion >5 mg/kg for most TCAs (>2.5 mg/kg for desipramine, nortriptyline, trimipramine; >1 mg/kg for protriptyline) warrants emergency department evaluation 3

Duration of Therapy and Disposition

• Continue bicarbonate therapy until QRS normalizes, dysrhythmias resolve, and hemodynamic stability is maintained 1, 7
• All patients receiving bicarbonate require ICU-level care with continuous cardiac monitoring 5
• Cardiac monitoring should continue for at least 24 hours after clinical recovery due to delayed toxicity risk 7
• Asymptomatic patients >6 hours post-ingestion are unlikely to develop symptoms and may not require emergency department referral 3