Posted by For this STEAM project, I decided to cover the topic “Explain how the heart translates an electrical signal to a contraction” (#39). By doing so, I created a booklet designed to interpret ECG’s (Electrocardiogram) for various rhythms that the heart can transmit. In each rhythm, and by understanding basic ECG interpretation, one can figure out what exactly is wrong with the heart and what part of the electrical signaling pathway is being affected.
For background, the electrical pathway starts at the pacemaker of the heart, known as the sinoatrial node. Firing at a rate of 60-100 BPM, it travels down the internodal pathways to the AV Node (Atrioventricular Node) as well as Bachman’s Branch to the left atrium. The AV node processes the signal, slowing it down to a rate of 40-60 BPM. The signal then transmits down the Bundle of His, to the right and left bundle branches. Lastly, it travels to the Purkinje Fibers stimulating the ventricles for contraction. Then the process starts all again from the top.
Starting with our Sinus Rhythm, this would be showing what our heart is doing at rest. The P-wave is seen first, and represents atrial depolarization. Next is our QRS Complex, which signifies ventricular depolarization. And last is the T-wave, showing ventricular repolarization. There are important intervals to take note of: first is the PR interval, showing the movement of electricity from the atria to the ventricles, and second is the width of the QRS complex, which should normally be under 0.12 seconds otherwise it is considered “wide”. In addition, the rate of the rhythm is between 60-100 beats per minute and it comes at regular time intervals (R-R interval). The P-wave is also present and the ST segment follows the isoelectric line. An ST segment that is above the isoelectric line could indicate a myocardial infarction, otherwise known as a STEMI (ST Elevation Myocardial Infarction).
Ventricular Fibrillation is our next rhythm; it is life threatening and is characterized by rapid and irregular ventricular contraction. What is happening here is the sinoatrial node is not the main pacemaker of the heart, instead, the ventricular pacemakers of the myocardium are firing randomly and at their own pace. Early on, this rhythm can often present with chest pain, palpitations, and shortness of breath. It is commonly caused from previous myocardial infarction or Coronary Artery Disease. This is one of two rhythms that need immediate defibrillation.
Supraventricular Tachycardia occurs when there is a recurret loop and the electrical signal travels back through the AV node causing ventricular contractions to occur at a rapid but regular rate. This will reduce preload, causing the heart to want to beat faster than it already was to compensate for the reduced cardiac output. It is life threatening and could lead to serious arrhythmias and death. People with SVT often present with shortness of breath, lightheadedness, syncope (fainting), and palpitations.
Ventricular Tachycardia can present with a pulse or without one. Those with a pulse may have symptoms of chest pain and SOB, while those without a pulse are in cardiac arrest. We see the ECG showing wide QRS complexes due to forceful ventricular contractions. As well, the ECG can show monomorphic (one shape) or polymorphic (multiple shapes) QRS complexes. Based on the peer-reviewed journal titled Regulation of Cardiac Conduction and Arrhythmias by Ankyrin/Spectrin-Based Macromolecular Complexes, this rhythm “can arise from disruption to normal states of conduction but are also the consequence of genetic or acquired remodeling of the cardiac action potential.”
Atrial fibrillation and atrial flutter are also further arrhythmias that can arise from genetic predisposition or disruption. These atrial inconsistencies on an ECG are often seen as “F waves”. Multiple ectopic pacemakers in the heart are firing at the same time trying to dominate over the other. The atrial myocardium will continue to depolarize by the “wavelets” (not true waves because of how small the signal is) and rapid contractions will occur. A-flutter can develop into a high ventricular rate while V-fib can increase risks of strokes or embolisms, yet both are maintainable in life if dealt with properly.
The following rhythms are AV Nodal Blocks, all varying in degrees.
A First Degree Heart Block occurs when there is a prolonged PR-interval (greater than .20 seconds). It is often due to vagal stimulation, medications, or disease. Everything else on the ECG is presented as normal and people are often asymptomatic. A Third Degree Heart Block occurs when there is no connection between the atriums and ventricles. The atriums are firing at random times, and so are the ventricles. This is known as a complete heart block. Based on the journal Development and Function of the Cardiac Conduction System in Health and Disease, “conduction disease in the VCS [Ventricular Conduction System] can manifest as a complete heart block.” This can lead to further ventricular arrhythmias and asystole (death).
A Second Degree Heart Block – Mobitz Type I, is characterized by a progressive prolongation of the PR interval. Eventually, the QRS will be dropped. This is often caused by a diseased AV node having a long refractory period (a brief period of unresponsiveness from myocardial nerves). Refractory is usually normal, but this presents with a period that is too long causing a potential QRS drop.
Lastly, a Second Degree Heart Block – Mobitz Type II, is similar to Type I, yet there is no prolonged PR interval, there is only a drop of the QRS complex (ventricular contraction). This is often caused by failure of the His-Purkinje conduction system, and will eventually lead to a complete heart block.
Sources:
Nassal, D., Yu, J., Min, D., Lane, C., Shaheen, R., Gratz, D., & Hund, T. J. (2021). Regulation of Cardiac Conduction and Arrhythmias by Ankyrin/Spectrin-Based Macromolecular Complexes. Journal of cardiovascular development and disease, 8(5), 48. https://doi.org/10.3390/jcdd8050048
Park, D. S., & Fishman, G. I. (2017). Development and Function of the Cardiac Conduction System in Health and Disease. Journal of cardiovascular development and disease, 4(2), 7. https://doi.org/10.3390/jcdd4020007
Schwartz, P. J., Ackerman, M. J., Antzelevitch, C., Bezzina, C. R., Borggrefe, M., Cuneo, B. F., & Wilde, A. A. M. (2020). Inherited cardiac arrhythmias. Nature reviews. Disease primers, 6(1), 58. https://doi.org/10.1038/s41572-020-0188-7
Konner’s project covered the objective that asked you to explain how the heart translates an electrical signal to a contraction. I thought Konner’s booklet translated well to the objective he covered. In his explanation and his booklet, he highlighted many disorders or situations that could happen if your heart’s rhythm is thrown off track. Some of these he listed off included First degree heart block which is where there is a prolonged PR-interval meaning that it’s greater than 20 seconds which can be from a few things. There is also supraventricular tachycardia which occurs when there is an ongoing loop and electrical signals being sent through the AV node which causes contractions to occur at a rapid rate. I really liked his artwork I feel like this is something that could be used in the medical field by other health professionals and could be a guide to them. It gives a breakdown of all the conditions covered in his writing but there was also a physical drawing and a small description of each condition. I learned a lot because there was a lot of good information in his writing and his heart which was easy to understand.