Understanding the Different Phases of the Cardiac Cycle

The cardiac cycle is most commonly described in terms of a graph showing the increase or decrease in the “heartbeat rate” (bpm). For those of you who don’t know what the graph is talking about, I’ll give you a quick definition before moving on. The cardiac cycle is essentially the function of the heart in the recovery process from a cardiac arrest to the beginning of another heartbeat after cessation of coronary artery activity. It consists of two phases: one during which the heart pumps rapidly to pump blood to the different parts of the body, and one during which it relaxes and fills with blood called diastole after a time of rapid contraction and emptying of the heart’s chambers. The second phase of the cardiac cycle happens in between these two events. This is the cardiac cycle definition I’m going to give you in this article.

Different Phases of the Cardiac Cycle

In order to understand the nature and clinical significance of this Phase of the Cardiac Cycle, we must first have an understanding of how it actually works. As mentioned above, the second phase occurs between the two chambers filling with oxygenated blood during the cardiac cycle. The amount of oxygenation depends on the state of our cardiac cycle. In other words, the longer the duration or higher the degree of ventricular relaxation, the greater the amount of oxygen delivered to the heart. For simplicity’s sake, I’ll refer to this as the cardiac cycle “respiration”, and it applies equally well to shortness of breath and cardiac arrest.

It’s important to understand the difference between ventricular events and heart sounds. A heart sound is an actual, audible sound, produced by the heart’s muscular contractions. Vibrations or rhythms are produced by ventricular events, such as when the heartbeats. However, the term “heart sound” can apply to any rhythmic, periodic waveform (i.e., beat) and not just to the rhythms produced by the heart. For instance, the term “rhythmic” can apply to a heartbeat, to the sound of a bell (which is technically a “vibration” and not a “beat”), to the beating of a heart inside the body, and even to a tick.

Now, the second cardiac cycle definition I’d like to give you is one that has to do with the length of time from the beginning of respiration until the end of the heartbeat. It’s called diastole. In cardiac cycle terms, this is the period of time from beginning the cardiac cycle to end the cardiac cycle. In layman’s terms, it’s the period of time from the start of breathing to the end of expiration. This is an easy concept to understand. We start with diastole (the beginning of respiration) to the end of expiration (the end of the cardiac cycle).

The next definition is for those who may be more interested in how long the heartbeat lasts during respiration. This is simply the duration (in beats) of the heartbeat. There are two different periods of respiration; tidal respiration (breathing at a rate equal to body weight divided by 4) and respite or expiration. The diastole is actually the period from the start of tidal respiration to the end of expiration.

Finally, we come to the end of the cardiac cycle. This is the point of maximum pressure or saturation of the blood. This is again a function of the atrium; the larger the atrium, the greater the amount of blood that can fill the atrium and therefore, maximum pressure. At this point, the ventricle has ceased pumping blood, and the atrium is full; therefore, there is no further need to fill the atrium.

Now that we have the basics, we can better understand the different phases of the cardiac cycle and how these heart functions. When we first begin to circulate blood through the body, the ventricular systole is immediately apparent. It is a contraction that occurs in the walls of the atrium binding the atrial diastole together. As we move into the main part of the cardiac cycle, the atrial diastole relaxes, the ventricular systole is again evident, but this time the ventricular tachycardia begins. With a slight delay, the ventricular fibrillation also occurs.

Throughout the entire cardiac cycle, the ventricles and the atria maintain their unique purpose of filling with gas and releasing this gas to supply oxygenated blood throughout the body. They do this through distinct chambers, also known as chambers of demand, that is located in the heart. Although it appears that the heart does not move around much during these different stages, it is important to note that all of these chambers are extremely complex and require a lot of information to be processed efficiently and thus form the various essential components of the heart.