ECG Quiz – Cardiology MCQ

Old text books describe electrical alternans in severe left ventricular failure as corresponding to the pulsus alternans (mechanical counterpart) in those cases. Technically when there is pulsus alternans, the left ventricular volume at end diastole alternates. Usually it starts off after a supraventricular ectopic beat which has a low pulse volume due to the shorter diastole, less diastolic filling and end diastolic volume. In the normal heart, the pulsus alternans after the ectopic beat normalises after a few cycles, but it remains so in left ventricular failure. It is known by the Brodie’s effect that QRS amplitude is proportional to the end diastolic volume. So if end diastolic volume alternates, we can have QRS alternans. These are the old explanations which were taught!