A 42-year-old woman is brought to the emergency department by paramedics for evaluation of a day of mental status changes. She has a history of hypothyroidism, major depression, and chronic back pain. Her medications include levothyroxine, amitriptyline, and daily oxycodone. The patient is markedly confused and agitated and talks to herself. Her temperature is 38.9 C (102 F), blood pressure is 92/52 mm Hg, pulse is 110/min, and respirations are 22/min. There is marked flushing, mydriasis, and hyperreflexia. Her electrocardiogram shows sinus tachycardia with a wide QRS interval. She is started on sodium bicarbonate infusion. Which of the following is the primary therapeutic goal of sodium bicarbonate in this patient?

Question

A 42-year-old woman is brought to the emergency department by paramedics for evaluation of a day of mental status changes.  She has a history of hypothyroidism, major depression, and chronic back pain.  Her medications include levothyroxine, amitriptyline, and daily oxycodone.  The patient is markedly confused and agitated and talks to herself.  Her temperature is 38.9 C (102 F), blood pressure is 92/52 mm Hg, pulse is 110/min, and respirations are 22/min.  There is marked flushing, mydriasis, and hyperreflexia.  Her electrocardiogram shows sinus tachycardia with a wide QRS interval.  She is started on sodium bicarbonate infusion.  Which of the following is the primary therapeutic goal of sodium bicarbonate in this patient?

Quizplus · Accepted Answer

Explanation: 11ec2b69_c120_90d3_8f63_15149fdabc04_MD0005_00 This patient's presentation suggests tricyclic antidepressant (TCA) overdose. Patients with TCA overdose can develop central nervous system findings, including mental status changes (eg, drowsiness, delirium, coma), seizures, and respiratory depression. Anticholinergic effects include dry mouth, blurred vision, dilated pupils, flushing, hyperthermia, and urinary retention. Cardiotoxic effects (eg, sinus tachycardia, hypotension, arrhythmias) cause most of the mortality in patients with TCA overdose. TCAs inhibit fast sodium channels in the His-Purkinje tissue and the myocardium to decrease conduction speed, increase phase 0 depolarization, and prolong the refractory period. This can prolong the QRS interval (>100 msec) and cause arrhythmias (eg, ventricular tachycardia, ventricular fibrillation). TCAs can also decrease calcium influx into the myocardium and increase peripheral vasodilation, causing hypotension. Finally, TCA overdose can also cause acidemia, which can mildly increase serum potassium due to cellular exchange of hydrogen and potassium. Sodium bicarbonate is the most effective agent for managing TCA-induced cardiotoxic effects (QRS widening or ventricular arrhythmias). Sodium bicarbonate increases serum pH and extracellular sodium. The increased pH (goal 7.50-7.55) modifies TCAs to their neutral (non-ionized) form, making them less available to bind to the rapid sodium channels. The elevated extracellular sodium increases the electrochemical gradient across cardiac cells and also affects the ability of TCAs to bind to the fast sodium channels. Patients who are refractory to sodium bicarbonate may respond to adjuvant magnesium or lidocaine. (Choice A) Sodium bicarbonate can alkalinize the urine, which is its main action for treating salicylate toxicity. Alkalinizing the urine enhances salicylate excretion by the kidney. However, TCAs have a large volume of distribution and are not effectively eliminated through alkalinization of the urine. (Choice B) Sodium bicarbonate is indicated for correcting severe lactic acidosis with severe acidemia (serum pH <7.1) and serum bicarbonate <6 mEq/L. However, the acidemia in TCA overdose is usually mild and alone does not warrant correction. The goal pH in TCA overdose is actually achieving alkalemia (pH 7.50-7.55) to convert the TCAs to their neutral form.(Choice D) TCAs inhibit central GABA receptors and can cause seizures. However, GABA agonists (eg, benzodiazepines) are preferred to treat seizures in this case. (Choice E) Sodium bicarbonate is an adjunctive therapy for treating significant hyperkalemia or rapidly increasing potassium with electrocardiogram changes (eg, peaked T waves, short QT, increased QRS interval). However, TCA overdose is not associated with severe hyperkalemia. Educational objective: Tricyclic antidepressant overdose can present with central nervous system, cardiac, and anticholinergic findings. Sodium bicarbonate is preferred for treating cardiac arrhythmias. Patients who are refractory to sodium bicarbonate may respond to adjuvant magnesium or lidocaine. References: Tricyclic antidepressant overdose: a review (http://www.ncbi.nlm.nih.gov/pubmed/11435353) Tricyclic antidepressant poisoning treated by magnesium sulfate: a randomized, clinical trial. (http://www.ncbi.nlm.nih.gov/pubmed/22309432)

A 42-Year-Old Woman Is Brought to the Emergency Department by Paramedics