Question

Short / Long type answer type questions.
Give a detailed account of the events of a cardiac cycle and correlate these with an electrocardiogram.

Answer 1

• The cardiac cycle is the sequence of events in one heartbeat.
  

It can be divided into three stages:

1. Diastole: The chambers of the heart (atria and ventricles) are relaxed and filling up with blood from the veins. The atrioventricular valves (bicuspid on the left and tricuspid on the right) are open and the semilunar valves (pulmonary and aortic) are closed. This stage lasts for about 0.4 seconds.

2. Atrial systole: the atria contract and top up the ventricles. This takes about 0.1 seconds.

3. Ventricular systole: the ventricles contract and pump blood out of the heart into the arteries. During this stage, the atrioventricular valves are closed and the semilunar valves are open. This takes about 0.3 seconds.

• A typical human electrocardiogram has five waves – P, Q, R, S, and T.
  

1. The P-wave indicates atrial depolarization. During this wave, the impulse of contraction is generated by the SA node. The PQ-wave represents atrial contraction.

2. The QR-wave is preceded by ventricular contraction. It represents the spread of the impulse of contraction from the AV node to the wall of the ventricle. It leads to ventricular depolarization.

3. The RS-wave represents a ventricular contraction of about 0.3 sec.

4. The ST-wave represents ventricular relaxation of about 0.4 sec. During this phase, the ventricles relax and return to their normal state.

5. The T-wave represents ventricular relaxation.