The cardiac cycle and the ECG

One complete heartbeat, from one beat to the next, is the cardiac cycle. Follow it from rest:

• Joint diastole: all four chambers are relaxed. The tricuspid and bicuspid valves are open, so blood flows from the veins through the atria into the ventricles. The semilunar valves are closed.
• Atrial systole: the SAN fires, and the atria contract, pushing about 30% more blood into the ventricles.
• Ventricular systole: the impulse passes through the AVN and the AV bundle (bundle of His) into the ventricles, which contract, while the atria relax. The tricuspid and bicuspid valves close, the semilunar valves open, and blood is pushed out into the pulmonary artery and aorta.
• Ventricular diastole: the ventricles relax and their pressure falls. The semilunar valves close, the AV valves open, and filling begins again.

At 72 beats a minute, one cycle lasts about 0.8 seconds.

Each ventricle pumps out about 70 mL of blood per beat: the stroke volume. Cardiac output = stroke volume × heart rate = about 5000 mL (5 litres) a minute. Athletes can reach a much higher output.

The two heart sounds you hear with a stethoscope are valves closing: the first, lub, as the tricuspid and bicuspid valves close; the second, dub, as the semilunar valves close.

An electrocardiogram (ECG) records the electrical activity of the heart. The patient is connected by three leads: one to each wrist and one to the left ankle. Each beat makes a pattern:

• P-wave: the atria are electrically excited (depolarised), leading to atrial contraction.
• QRS complex: the ventricles depolarise; their contraction begins shortly after Q.
• T-wave: the ventricles return to normal (repolarise); the end of the T-wave marks the end of systole.

Counting the QRS complexes in a given time gives the heart rate. Doctors read changes in the ECG to find heart problems.