A 28-year-old woman is brought to the emergency department 2 hours after ingesting an unknown quantity of organophosphate pesticide during a suicide attempt. On examination, she is drowsy, has pinpoint pupils, profuse salivation, bronchospasm with audible crackles bilaterally, and muscle fasciculations. Her heart rate is 52 bpm, blood pressure 88/54 mmHg, and respiratory rate 28/min. Which of the following is the most appropriate immediate pharmacological intervention?