Introduction
In this chapter, we’ll begin our journey exploring the heart and its rhythm by going over the cardiac cycle, and the events linked to it. In further chapters, we’ll elaborate on the available technologies that enable us to measure the heart rhythm and detect possible heart rhythm disorders.
Key messages
✔️ The sequence of mechanical and electrical events that is repeated with every heartbeat is called the cardiac cycle.
✔️ The P-wave on the ECG represents atrial contraction and the start of ventricular filling, while the QRS-complex visualizes the contraction of the ventricles and ejection of blood into the aorta.
✔️ The pulsatile ejection of blood into the circulatory system causes arterial blood pressure waves, resulting in the expansion and relaxation of blood vessels.
✔️ Under normal circumstances, the sinoatrial node determines the duration of the electrical conduction while cardiac myocytes determine the relative duration of contraction and relaxation.
The heart function in a nutshell
The heart is a muscular pump that drives the human circulatory system. With each heartbeat, the ventricles contract which generates an arterial blood pressure wave. As a result, the blood flows through the vascular system. The cardiac cycle consists of a sequence of mechanical (pumping) and electrical events that are repeated with every heartbeat.
Electrical events linked to the cardiac cycle
The cardiac conduction system is a network of nodes, cells, and signals that controls your heartbeat in an orderly fashion. During an average person’s lifespan, the cardiac system is responsible for approximately 2.5 billion heartbeats. The cardiac action potential originates in a group of cells called the sinoatrial (SA) node, or the pacemaker, located in the right atrium. Normally, these cells depolarise spontaneously and fire off action potentials at a regular, intrinsic rate that is usually between 60 and 100 beats per minute. The electrical impulses are conducted from cell to cell throughout the atria.
The P-wave on an electrocardiogram (ECG) reflects depolarization of the right and left atrial muscle cells called the cardiac myocytes. About one-tenth of a second later, after this depolarization, the electrical impulses arrive at the atrioventricular (AV) node. The impulses will then travel from the AV node to the His-Purkinje fiber system, which is a network of specialized conducting cells that carries the signal to the ventricular myocytes. The QRS complex represents the depolarization of the ventricles. Finally, the cells of the His-Purkinje fiber system will repolarize, which resembles the T-wave on an ECG. The consecutive, orchestrated steps within the electrical conduction system result in one cardiac cycle.
The cardiac conduction system sends thousands of signals each day to keep your heart beating. These electrical impulses also cause mechanical events, which will be discussed in the next section.
Mechanical events linked to the cardiac cycle
The heart resembles a pump with a reciprocating piston that alternates between a filling phase (diastole), and an emptying phase (systole). The alternations between both phases occur in an orchestrated manner, caused by the signals from the electrical conduction system. During the P-wave, the atria contract, which enhances ventricular filling. After that, the QRS complex triggers the contraction of the ventricles.
The well-organized interaction between the myocytes within the ventricular wall results in the ejection of oxygenated blood from the ventricles into the aorta causing arterial blood pressure waves. These pressure waves cause the pulsatile propagation of blood in the circulatory system. In the peripheral arterial system, the pressure waves result in the expansion and relaxation of blood vessels and the delivery of oxygen and nutrients to the tissues and vital organs. At the end of the cardiac cycle, ventricular contraction is followed by ventricular relaxation.
The effect of the autonomic nervous system
Under normal circumstances, the body’s own pacemaker, or SA node, determines the duration of the electrical conduction. The cardiac myocytes determine the relative duration of contraction and relaxation. This pattern remains relatively steady as long as the heart rate remains unchanged. The heart rate and intensity of contraction are modified by the sympathetic and parasympathetic divisions of the autonomic nervous system.
The sympathetic system, which is a network of nerves that helps your body to activate the “fight-or-flight” response, acts as an accelerator, speeding up the heart rate and increasing the contractility of the heart. Whenever more oxygen is necessary, for example during exercise or if blood pressure drops, the input of the sympathetic system increases. This causes the heart rate and intensity of the heart contractions to increase.
The parasympathetic input, a network of nerves that relaxes your body after periods of stress or danger, acts like a brake, slowing down the heart. The parasympathetic system causes relaxation after periods of stress or danger. It causes your heart rate to slow down when you relax.
In some individuals, the electrical conduction system doesn't work correctly which may cause issues related to the heart rate and/or the heart rhythm. Abnormal signaling results in the fact that the heart beats too fast (tachycardia), too slow (bradycardia), or irregularly. One of the most commonly known cardiac arrhythmias is atrial fibrillation, an irregular rhythm that causes impaired blood flow which results in an increased risk to develop stroke.
⏩ Learn more about the ECG signal in the next chapter!
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