Question

Excessively high heart rate (> 180) can reduce cardiac output because

• A. Blood is moving too fast through the lungs to pick up enough oxygen
• B. It tires out the heart muscles and so they pump slower
• C. It reduces the time for ventricular filling which reduces stroke volume
• D. The PR-interval increases which leads to longer ventricular diastole and shorter ventricular systole

Answer 1

Answer: (C)

Heart rate affects both stroke volume and minute volume by altering the length of diastole and thereby the degree of filling and force of contraction. Minute volume and B.P may rise even if the stroke volume falls. This happens with a moderate rise in the heart rate. But if the heart rate is too high, the stroke volume becomes so low that the minute output falls far below the normal.

During muscular exercise, the output may be 30-40 litres i.e., 6-10 times the normal minute volume( stroke volume 170-200 ml; heart rate 150-180 per minute).

So, the correct answer is 'It reduces the time for ventricular filling which reduces stroke volume'.