The cardiac cycle refers to the sequence of events that occur from the start of one heartbeat to the start of the next, and this lasts approximately 0.8 seconds. Each cardiac cycle is started by the spontaneous production of an action potential in the sinus node. This node is located near the opening of the superior vena cava in the superior lateral wall of the right atrium, and the action potential travels swiftly from here via both atria and then through the A-V bundle into the ventricles.
There is a delay of more than 0.1 second during the passage of the cardiac impulse from the atria into the ventricles due to this unique layout of the conducting system from the atria to the ventricles. This delay permits the atria to contract before the ventricles, allowing blood to flow into the ventricles before the forceful ventricular contraction begins. As a result, the atria serve as starter pumps for the ventricles, which in turn offer the primary source of energy for transporting blood through the body’s circulatory system.
Diastole and Systole
The cardiac cycle consists of diastole, a time of rest during which the heart fills with blood, followed by systole, a period of contraction. The reciprocal of the heart rate is the overall duration of the cardiac cycle, including systole and diastole. For example, if your heart rate is 72 beats per minute, the cardiac cycle lasts 1/72 minute every beat—roughly 0.0139 minutes or 0.833 seconds.
Increasing Heart Rate Decreases Duration of Cardiac Cycle
The duration of each cardiac cycle, including the contraction and relaxation phases, reduces as heart rate rises. The length of the action potential and the time of contraction (systole) both shorten, but not by as much as the relaxation phase (diastole). Systole accounts for around 0.4 of the cardiac cycle at a normal heart rate of 72 beats per minute. Systole accounts for around 0.65 of the cardiac cycle at three times the normal heart rate. This implies that a fast-beating heart does not remain relaxed long enough to allow the cardiac chambers to fill completely before the following contraction.
Relationship of the Electrocardiogram to the Cardiac Cycle
An electrocardiogram is a record of the electrical signals in your heart. It is represented by the P, Q, R, S, and T waves. They are electrical voltages generated by the heart and recorded from the body’s surface by an electrocardiograph.
The P wave is caused by spread of depolarization through the atria and is followed by atrial contraction, which causes a slight rise in the atrial pressure curve immediately after the electrocardiographic P wave.
About 0.16 second after the onset of the P wave, the QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the ventricles and causes the ventricular pressure to begin rising. Therefore, the QRS complex starts slightly before ventricular systole.
Finally, the ventricular T wave denotes the point at which the ventricular muscle fibers begin to relax during repolarization. Therefore, the T wave occurs just before ventricular contraction ends.