DROMOTROPISM: CARDIAC ELECTROPHYSIOLOGY, CLINICAL CONSIDERATIONS - ANATOMY AND PHYSIOLOGY - 2024

The term dromotropism refers to the ability of the heart to conduct the electrical impulse. It is synonymous with conductivity in cardiac physiology and can be observed objectively on the electrocardiogram.

The myocardium is a muscle that contracts periodically at a rate of approximately 80 contractions per minute. These movements are due to an electrical stimulus propelled through its fibers by the electrical contraction system of the heart. The periodic contractions of the heart in a certain time is what is known as a rhythm or heart rate.

From Ske from French Wikipedia - Enregistrement sur 10s de la derivation V2. Mesure réalisée sur et par ske et tracé à l'aide du logiciel Médistory de http://www.prokov.com., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid = 1625975

In order for the heart to rhythmically contract and maintain this rhythm with the perfect movement of all its chambers, a series of complex physiological events occurs through the electrical network that controls it.

The set of elements that achieve the conductivity of the electrical impulse is called the electrical conduction system.

Any pathology that alters this system will have direct repercussions on the rhythm or heart rate, a condition that will affect the supply of blood and oxygen to the organs.

There are diseases that decrease the electrical impulse and others that increase it causing a decrease or increase in the heart rate respectively. For either of the two situations there are medications that normalize them.

Medications that increase the electrical impulse are called positive dromotropics and those that decrease it are known as negative dromotropics.

Cardiac electrophysiology

Cardiac electrophysiology is the science that deals with investigating the proper electrical functioning of the heart muscle and diagnosing and treating pathologies related to this process. It is a clinical branch of cardiology.

By The US Food and Drug Administration - FDA's Biophysics Lab - Studying Medical Devices and the Heart, Public Domain, A small percentage of the muscle fibers of the heart are specialized elements that have the ability to generate electrical potentials, which are essential in its proper functioning.

The rhythmic contraction of the heart muscle occurs by a coordinated sequence of electrical impulses from one of the specialized portions, which is responsible for maintaining that rhythm.

This area is called the sino-atrial node and is known as a physiological pacemaker since it acts by directing the action potentials that trigger the electrical impulse to generate the heartbeat.

Electrical conduction system of the heart

It is known by the name of electrical conduction system, to all the cardiac elements that are responsible for propagating the electrical impulse generated by the sino-atrial node.

The conduction capacity of these elements is what is known as dromotropism and is one of the four fundamental properties of the heart along with contraction, excitability and automatism.

The action potential that generates the electrical impulse begins in the sino-atrial node. From there it travels, through specialized cells of the atrium, to a second station: the atrio-ventricular (AV) node. This is located between a septum between the atrium and the ventricle.

From Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below) Bartleby.com: Gray's Anatomy, Plate 501, Public Domain, https://commons.wikimedia.org/w/index. php? curid = 567268

The electrical impulse propagates from the atria to the ventricles, through a set of preferential fibers or channels with conduction capacity called His bundle.

Once the electrical impulse reaches the ventricles, ventricular contraction and heart beat occur, completing the cardiac cycle.

When all this process is accomplished, the specialized cells of the sino-atrial node are ready to unleash a new action potential that triggers another electrical impulse.

Sino-auricular node (SA)

The sino-atrial node is a set of myocytes, specialized muscle cells, that have the ability to generate electrical impulses.

It is located in the right atrium, which is one of the four heart chambers. It is oval in shape and measures about 3.5 mm, being the largest physiological pacemaker. It is supplied by the homonymous artery, which is a direct branch of the right coronary artery.

By Stephenson RS, Boyett MR, Hart G, Nikolaidou T, Cai X, Corno AF, et al - Contrast Enhanced Micro-Computed Tomography Resolves the 3-Dimensional Morphology of the Cardiac Conduction System in Mammalian Hearts. PLoS ONE, CC BY 4.0, The main function of this cardiac component is to initiate the action potential that results in an electrical impulse. This impulse travels through the electrical conduction system of the heart, causing the myocardium or heart muscle to contract.

The mechanism of action of the sino-atrial node was discovered in 1907 by physiologists and scientists Martin Flack and Arthur Keith, after exhaustive investigation of more than a year in mammalian hearts.

The SA node is called the physiological pacemaker of the heart because, thanks to its correct functioning, the heart rate is maintained. This process happens automatically. The cells of the node initiate the action potential and as the electrical impulse travels through the conduction system, the cell assembly prepares to initiate the next potential.

Damage to this physiological system would result in a rhythm problem in the patient, which could lead to serious complications including death. The SA node can be affected by the lack of blood supply to its cells caused by angina pectoris, or by necrosis caused by a heart attack.

Clinical considerations

Any pathology that affects the electrical conduction system, or the cardiac physiological pacemaker, has an impact on the heart rhythm and the oxygenation of the patient's tissues.

In addition, an altered heart rhythm can generate small blood clots within the heart, which can travel through the circulatory system and block small blood vessels causing the pathology known as thrombosis.

By Madhero88 - Own work, CC BY-SA 3.0, For this reason, it is important to identify these problems in order to treat them in a timely manner and avoid complications.

When conditions affecting conductivity result in a decrease in heart rate, medications should be administered to normalize this situation. That is, the patient is given a substance that increases the frequency and brings it to normal.

By Sinusbradylead2.JPG: James Heilman, MDderivative work: Mysid (using Perl and Inkscape) - This file was derived from: Sinusbradylead2.JPG:, CC BY-SA 3.0, https://commons.wikimedia.org/w/index. php? curid = 22055720

Medications that have a positive dromotropic effect are those that increase conductivity. Adrenaline, or epinephrine, is one of the most widely used medications for this purpose.

There are other pathologies that alter the conduction system, increasing the heart rate generating tachycardia in the patient and, frequently, arrhythmias.

In these cases, drugs are used that have a negative dromotropic effect, thus reducing the conductivity so that the impulse travels more slowly.

One of the most widely used negative dromotropic drugs is verapamil, which acts specifically on the SA and AV nodes, decreasing impulse conduction and protecting the myocardium from an inadequate heart rhythm.

References

1. Kashou, AH; Basit, H; Chhabra L. (2019). Physiology, Sinoatrial Node (SA Node). StatPearls Treasure Island. Taken from: nlm.nih.gov
2. Silverman, M. E; Hollman, A. (2007). Discovery of the sinus node by Keith and Flack: on the centennial of their 1907 publication. Heart (British Cardiac Society). Taken from: nlm.nih.gov
3. Francis, J. (2016). Practical cardiac electrophysiology. Indian Pacing and Electrophysiology Journal. Taken from: nlm.nih.gov
4. Jabbour, F; Kanmanthareddy, A. (2019). Sinus Node Dysfunction. StatPearls Treasure Island. Taken from: nlm.nih.gov
5. Park, D. S; Fishman, GI (2011). The cardiac conduction system. Circulation. Taken from: nlm.nih.gov
6. Sampaio, K. N; Mauad, H; Michael Spyer, K; Ford, TW (2014). Chronotropic and dromotropic responses to localized glutamate microinjections in the rat nucleus ambiguus. Brain research. Taken from: nlm.nih.gov