Cardiac cycle

A cardiac cycle is the sequence of mechanical and electrical events that occurs with every heartbeat. Now, the heart is shaped like a cone and it contains two upper chambers, called atria; and two lower chambers, called ventricles.

Now, the left atrium receives oxygenated blood from the lungs via the pulmonary veins; while the right atrium receives deoxygenated blood from all of our organs and tissues via the superior and inferior vena cava.

From the atria, the blood flows into the lower chambers of the heart: the left ventricle, which pumps oxygenated blood to all our organs and tissues via the aorta; and the right ventricle, which pumps the deoxygenated blood back to the lungs via the pulmonary arteries.

Alright, so each heartbeat consists of two phases: systole, which is when the heart contracts and pumps blood out of the ventricles; and diastole, which is when the heart relaxes and ventricles fill with blood. Now, the cardiac cycle graph is used to express events during one cardiac cycle.

Along the y-axis are aortic pressure, left atrial pressure, and left ventricular pressure, heart sounds, ventricular volume, right atrial pressure curve, and ECG; while along the x-axis is time.

But, before we continue, here’s something to keep in mind: since there are no valves separating the right atrium from the superior vena cava and the jugular veins, the jugular venous pulse will follow the same pressure changes as the ones that arise in the right atrium.

In other words, an increase in the atrial pressure will result in an increased jugular venous pulse, and vice versa. Therefore, below the right atrial pressure curve let’s write JVP for jugular venous pulse.

And right above the graph, we’ll write the seven phases of the cardiac cycle. The first phase is the atrial contraction, which lasts about 0.1 seconds.

Next, isovolumetric ventricular contraction, rapid ventricular ejection, reduced ventricular ejection, are phases of ventricular systole and together they last about 0.3 seconds.

The last three phases, isovolumetric ventricular relaxation, rapid ventricular filling, and finally, reduced ventricular filling, also known as diastasis, are phases of ventricular diastole and together they last about 0.4 seconds.

Now, the first phase of the cardiac cycle, atrial contraction, starts with the firing of the sinoatrial, or SA node, which sends an electrical signal that propagates outward through the walls of the heart and depolarizes the atria.

On the ECG, this corresponds to the P wave. Now, atrial depolarization is followed by the contraction of the right and left atrium, and as the atria contract, the pressure within the left atrium increases.

Now, as the atria pumps blood into the ventricle, the ventricular volume increases and therefore the ventricular pressure slightly increases.

At the same time, as the right atrium contracts, an increase in the right atrial pressure will be noted as the a wave on the right atrial pressure curve.

In some conditions, such as ventricular hypertrophy, during this phase, the fourth heart sound (S4) can be heard.

This sound is caused by vibration of the stiffened ventricular wall as the blood is pushed from the atria into the ventricles. The next phase of the cardiac cycle is the isovolumetric contraction.

On the ECG, this phase begins with the appearance of the QRS complex, which represents ventricular depolarization. Ventricular depolarization is followed by ventricular contraction.

But, prior to ventricular contraction, when the pressure within the ventricle exceeds the atrial pressure, atrioventricular valves close, producing the first heart sound (S1).

At this point, both the atrioventricular and aortic and pulmonary valves are closed, so the blood volume within the ventricles remain the same - hence the term “isovolumetric”.

But the ventricles contract, so ventricular pressure increases rapidly. However, in the left ventricle, the pressure doesn’t exceed aortic pressure, so the aortic valve is still closed.

At the same time, there’s a slight increase in the left and right atrial pressure because the increasing ventricular pressure makes the atrioventricular valves bulge into the atria.

This increase in the atrial pressure is registered as the C wave, on both left and right atrial pressure curves.

By the end of isovolumetric contraction, the pressure within the ventricles becomes higher than the pressure within the aorta and pulmonary arteries, so the aortic and pulmonary valves open.

This event marks the start of the next phase - rapid ventricular ejection. This phase is called rapid ventricular ejection due to a sudden ejection of a large amount of blood from the ventricles.

Now, as the left ventricle ejects the blood, the pressure from the left ventricle is equally transmitted to the aorta. In other words, both, ventricular and aortic pressures reach their maximum. At the same time, the volume of blood within the left ventricle decreases sharply.

On the ECG, this phase matches the ST segment - the flat section of the ECG between the end of QRS complex and the beginning of the T wave, which represents the period between ventricular depolarization and ventricular repolarization.