ECG (electro- — not echo- — cardiogram) records electric potential changes between various points on the torso, of which pulse rate is really only a very averag

ECG (electro- — not echo- — cardiogram) records electric potential changes between various points on the torso, of which pulse rate is really only a very averaged metric, obtained via blood flow and pressure changes. No chance you'll detect a QT interval change with this.