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Hello, today we are going to talk about the electrical anatomy and physiology of the heart.
Okay let’s start by first reviewing the heart, remember that our heart has four chambers two upper and two lower. the two upper chambers are called the Atria, we have a right and left Atria. The two lower chambers are called the ventricles, we have a right and the left ventricle. It is important to understand that the Atria pump blood into the ventricles, the right ventricle pumps blood to the lungs and the left ventricle pumps blood to the body. In order for the Atria and the ventricles to contract there must be an electrical stimulation. Without an electrical stimulation the heart will not pump effectively. When the heart produces an electrical conduction it causes a mechanical action, meaning the Atria and the ventricles contract.
Mechanical action is simply referring to the heart’s ability to contract. In order for the heart to contract it goes through a cycle (Cardiac Cycle). The cycle starts when the atria receive an electrical stimulus causing the atria to contract. As the atria are contracting they are filling up the ventricles with blood, so the ventricles are in a resting state. When the atria are done filling the ventricles up with blood, they are in a resting state while the ventricles contract. If the Atria are contracting the ventricles are resting , if the ventricles are contracting the atria is resting. The term depolarize and repolarize simply mean contract and relax or resting state. You will hear depolarize and repolarize used more often versus contract or relax. So when the atria are depolarizing the are contracting, and the ventricles are repolarizing or resting. When the ventricles are depolarizing they are contracting and the atria are repolarizing. If you can remember that” re”polarizing means to” re”st or” re”-energize you will know that is the period where the atria or ventricles are resting, and depolarizing when contracting. The Mechanical action of Contracting or depolarizing and resting or repolarizing formulate the waveforms on an EKG
The mechanical action would not be possible without an electrical conduction. This is a quick overview of the electrical pathway that starts with the SA node followed by the AV node then down the bundle of His which separates into the right and left bundle branches and lastly the purkinje fibers. These look at these in more detail in the following slides.
The first step in the electrical conduction is the firing or the electrical stimulation of the SA node. The SA node is referred to as the natural pacemaker of the heart because it is what initiates the electrical conduction. The SA node sends an electrical impulse across the right and left Atria, this causes the atria to contract. The SA node sends an electrical impulse of 60 to 100 beats per minute across the atria. One cool thing to remember about the SA node is that it does not require a signal or stimulus to initiate an electrical impulse, it does it all on its own. The heart does not need the brain or any other organ in order to beat, the SA node will automatically send an electrical impulse to generate a contraction.
After the SA node sends its electrical impulse to the atria, the impulse travels to the AV node. The AV node is located between the Atria and ventricles which is why it’s also called atrioventricular node. The AV node slows down the electrical impulse from the SA node in order to make sure the Atria have contracted completely and the blood is expelled into the ventricles. When the Atria have finished contracting the AV node sends the electrical impulse down to the ventricles. If the SA node is not working or firing like it’s supposed to, the AV node can take over and produce a heart rate of 40-60 beats per minute.
After the electrical conduction passes through the AV node it goes through the bundle of His, which then separates into the right and left bundle branches. The right bundle branch sends its electrical impulse to the right ventricle and the left bundle branch sends its impulse to the left ventricle causing them to contract. Just like the AV node, if the SA node and the AV node were to malfunction the right and left bundle branches can pick up and produce a heart rate of 20-40 beats per minute, its the last effort before they die. A ventricular heart beat of 20-40 beats per minute would not perfuse the body well and the patient will die soon without an intervention like a pacemaker.
The last part of the electrical conduction is the purkinje fibers, These little electrical fibers send the electrical impulse to the ventricles and the rest of the myocardium to make sure the entire ventricle contracts. So without the initiation of the SA node firing across Across the internodal pathways in the atria, traveling to the AV node, down the bundle of His to the right and left bundle branches and ending at the purkinje fibers we would not have a mechanical action.
The key points to remember are that and electrical conduction stimulates the Atria and the ventricles to contract causing a mechanical action. The Atria and the ventricles contract or depolarize and relax or repolarize. When the Atria are depolarizing the ventricles are resting so they are repolarizing because they’re filling with blood while the Atria contract, when the Atria are done contracting it is their turn to repolarize while the ventricles depolarize or contract. Both atria and ventricles cannot be contracting at the same time or relaxing at the same time, your patient is dead! In order to have electrical stimulation it must follow a conduction pathway by starting at the SA node, and travel through the AV node bundle of His right and left bundle branches and purkinje fibers. When the atria and ventricles are depolarizing and repolarizing (contracting and relaxing) due to the electrical stimulation, waveforms on an EKG are being produced. These waveforms are discussed in the upcoming EKG lessons.
Check out our other resources and lessons on the electrical activity of the heart or any other topic you may need help with, as always, go out and be your best selves today and happy nursing!