What is cardiac cycle? Draw a normal ECG and label it. What do P-wave and QRS complex represent?

Answer:

Cardiac cycle: the consistent sequence of events (contraction — systole, and relaxation — diastole of atria and ventricles) in one heartbeat is called a cardiac cycle. At 72 beats per minute it takes about 0.8 second (atrial systole 0.1 s, ventricular systole 0.3 s, joint diastole 0.4 s).

[Figure: Normal ECG — a small upward P wave, followed by the QRS complex (small downward Q, tall upward R, downward S), then a dome-shaped upward T wave; P–R interval and S–T segment marked; time on x-axis, voltage on y-axis.]

• P-wave represents depolarization of the atria (spread of impulse from SA node through the atria), leading to atrial contraction.
• QRS complex represents depolarization of the ventricles (spread of impulse through AV node, bundle of His and Purkinje fibres), leading to ventricular contraction; atrial repolarization is masked in it.
• The T-wave represents ventricular repolarization.