Editor’s Note: This is the first installment of a continuing series highlighting researchers sponsored by the Emergency Medicine Foundation (EMF) and illustrating the impact EMF-funded research is having on emergency medicine.

Study Title: Effect of methylene blue on mortality in a porcine model of amlodipine toxicity

Authors: Jenna M. LeRoy, MD; Sean Boley, MD; K.M. Engebretsen, PharmD; Jackie Kelly, MD; Samuel J. Stellpflug, MD, FACEP

Researcher Bios

• Dr. LeRoy is an attending emergency physician and clinical toxicologist at Regions Hospital in St. Paul, Minnesota, and assistant professor of emergency medicine at the University of Minnesota Medical School.
• Dr. Boley is an emergency physician at United Hospital in St. Paul, Minnesota.
• Dr. Engebretsen is a clinical pharmacist and toxicologist at Regions Hospital.
• Dr. Kelly is a quality fellow at HealthPartners Institute for Education and Research and an emergency physician at Regions Hospital.
• Dr. Stellpflug is an attending emergency physician and clinical toxicologist at Regions Hospital and associate professor of emergency medicine at the University of Minnesota Medical School.

Study Background

Cardiovascular medication overdose causes significant morbidity and mortality in the United States. In 2015, the National Poison Data System (NPDS) responded to more than 2.1 million exposures, with 103,339 related to cardiovascular drugs. Cardiovascular drugs were the seventh most frequently involved substance and are now rated as the NPDS top fourth category with the greatest rate of increase in exposure. Despite maximal supportive pharmacologic therapy, including vasopressor administration, high-dose insulin therapy, lipid emulsion therapy, and extracorporeal life support, there are still cases of refractory shock leading to death. In vitro studies on canine arteries exposed to amlodipine have shown that it stimulates release of nitric oxide (NO), leading to peripheral vasodilation. Amlodipine overdose could, therefore, be managed by scavenging NO. Methylene blue (MB) inhibits NO directly but also inhibits NO production by inhibiting guanylyl cyclase and endothelial NO synthase activity. We developed a porcine model of amlodipine toxicity and compared the effects of MB to traditional vasopressor therapy with norepinephrine (NE). Time to death was the primary outcome.

Study Design

The pigs were anesthetized and instrumented with monitoring devices according to previous protocols in our institution, and a pilot study was first completed to establish a lethal model of amlodipine toxicity. Each of the two groups of animals received a toxic dose of amlodipine. A continuous infusion of amlodipine with accelerating doses was given to mimic overdose and continuing gastrointestinal absorption. After 70 minutes of amlodipine infusion, each group was resuscitated with 20 mL/kg of normal saline. Animals in each group were then randomized to receive either MB or NE therapy. Hemodynamic parameters, including mean arterial pressure and cardiac output, were measured every 10 minutes.

Results

The primary outcome was time to death. Survival times were compared using a Kaplan-Meier analysis, and the two groups were compared with the log-rank test. The study was powered at 80 percent to detect a hazard ratio of 0.2 (MB versus NE), assuming a two-sided log-rank test with alpha=0.05. Nine animals per group were required for adequate power.

An interim analysis was conducted after 15 of the initially planned 18 animal protocols were completed (seven MB and eight NE). This revealed that, for the primary outcome, MB was clearly not superior to NE. Furthermore, it would be impossible to achieve a statistically significant effect for the MB hazard ratio with the addition of three pigs, regardless of the outcome. Therefore, the study was terminated early. Overall, one of seven animals (14 percent) in the MB group survived to 300 minutes compared to two of eight animals (25 percent) in the NE group. Median survival time was 100 minutes for the MB group and 177 minutes for the NE group. Survival time did not differ by group (log-rank test P=0.29), but there was a nonsignificant trend toward longer survival in the NE group.

Projected Impact

Our data contribute to a growing body of literature on usage of methylene blue in toxin-induced shock. We hope that this will encourage more research in the field as it is still unclear where this antidote fits in the management of patients.

Dr. LeRoy is an attending emergency physician and clinical toxicologist at Regions Hospital in St. Paul, Minnesota, and assistant professor of emergency medicine at the University of Minnesota Medical School.