A 46-year-old labourer is freed after six hours trapped beneath a collapsed brick wall. Both thighs are tense, swollen and tender, and he cannot move his toes. At the scene his heart rate was 94/min and regular, and P waves were present. On arrival his rate is 96/min and regular, P waves are no longer visible, the QRS complexes measure 0.16 s and each is followed by a tall, narrow T wave. Serum potassium is 8.2 mmol/L.
Loss of excitability in which one of the following cardiac tissues best explains the disappearance of this man's P waves?
Keep going — a set of ten like this one, one at a time.
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