# Development and evaluation of a novel prehospital antidote service providing methylthioninium chloride (methylene blue) for sodium nitrite poisoning

## TL;DR

A new prehospital antidote service using methylene blue was developed to treat sodium nitrite poisoning, showing potential but needing more evidence for broader use.

## Contribution

A novel prehospital methylthioninium chloride service was developed and evaluated for sodium nitrite poisoning.

## Key findings

- Three patients received prehospital methylthioninium chloride, with MetHb levels decreasing and all surviving.
- The service had minimal consumable costs and identified areas for improvement.
- Only nine cases were observed, suggesting specialist team restriction is proportionate.

## Abstract

Sodium nitrite has become established as a method of self-harm and suicide. Toxicity occurs primarily through the formation of methaemoglobin (MetHb). In response to a coroner request, West Midlands Ambulance Service developed a prehospital methylthioninium chloride (methylene blue) capability within the specialist Hazardous Area Response Team (HART) to treat methaemoglobinaemia. A service evaluation was planned to understand the impact.

A retrospective observational series of patients, during a planned evaluation period from 1 July 2020 to 1 July 2024. All patients receiving a HART response for suspected sodium nitrite poisoning were included. A Patient Group Direction (PGD) for the treatment of methaemoglobinaemia by paramedics was produced, allowing treatment with intravenous methylthioninium chloride 1–2 mg/kg in specific circumstances. MetHb levels were assessed using handheld pulse CO-Oximeters.

Nine patients were attended for suspected sodium nitrite toxicity and three were administered prehospital methylthioninium chloride under PGD indications. In addition, one patient had no evidence of toxicity, four had conditions unequivocally associated with death and one was in cardiac arrest. The patient in cardiac arrest received sodium nitrite in hospital from the HART team under verbal direction from medical staff but did not survive. Serial CO-Oximeter readings for two of the three patients who received methylthioninium chloride indicated a decrease in MetHb levels and all three survived. Consumable costs associated with the new capability were minimal. Potential improvements to the service were identified, including changes to PGD indications.

The limited number of cases seen in the evaluation period suggests that restricting the prehospital provision of methylthioninium chloride to specialist teams is proportionate, and it appears feasible for specialist paramedics to deliver prehospital methylthioninium chloride under PGD indications. However, prehospital use of methylthioninium chloride for the treatment of methaemoglobinaemia currently lacks sufficient evidence to support widespread adoption without further evaluation.