A 48-year-old woman with severe asthma is admitted to the hospital with palpitations. She is initially thought to have atrial fibrillation and is treated with attempts at rate control.
Her physicians avoid β-blockers because she was observed in the past to have worsening bronchospasm with metoprolol. Instead, they treat her with digoxin but have very little success in controlling the arrhythmia.
What does her ECG show?