Synergistic Understanding of Cardiac Conduction, ECG, & Anatomy of the Heart

The heart is pretty important – fact. Just kidding, that is actually an opinion, but I would imagine that opinion is rather universal. The heart plays several roles in our survival, but the most important is to keep beating to pump blood throughout our body; it does so through some extremely interesting mechanisms. In this article, we will examine the electrical system of the heart, match it to its EKG/ECG reading, and equate those readings to the anatomical steps that occur for the heart to fulfill just one beat.

Cardiac Conduction System
The heart, believe it or not, has its own electrical conduction system [1]. So, this electrical stimulus originates from the heart itself as opposed to coming from the brain like most of the other functions of the body are regulated [1]. This electrical signaling is made possible due to groupings of different types of cardiomyocytes (heart muscle cells) serving a variety of functions depending on where the signal is located [2].

Electrocardiogram (ECG/EKG)
While I will not be going into the applied details of this mechanism, the reading it provides is key to bridge the gap between electrical signaling and visualizing the anatomical movements of the heart. An ECG is simply a reading system of each sub-step necessary for the heart to fulfill each beat. It informs the observer of rate and quality (strength and normalcy) of the heart beat [3].



 

Anatomy of the Heart
This is not a dedicated “anatomy of the heart” article, so I will only discuss four major players and their constituent parts briefly.

Blood passes in this order, one way or another (Beginning to end, descending):


1. Right Atrium
2. Tricuspid Valve
3. Right Ventricle
4. Pulmonary Valve
5. Left Atrium
6. Bicuspid Valve
7. Left Ventricle
8. Aortic Valve (not viewable)


 

1

2

3

4

5

6

7

8

How does this occur?
Ok, so now we have a rudimentary understanding of what each piece of this puzzle is, yet here comes the challenging portion of this article; this is the part where we put it all together, in detail.

Let us begin by familiarizing ourselves with the starting point of this entire process.


                                                                              P-wave: Atrial Depolarization


 

Take a quick peek up to the “Anatomy of the Heart” section and notice that the starting chamber of the heart is the right atrium – this is where we will begin. Imagine that blood has moved into, and filled, the right atrium. It is at this point that the cardiac conduction system reacts to begin the pulse of the heart. Specifically, near the top of the right atrium are a grouping of nerve cells that make up the sinoatrial node (SA node), and these neurons, sensing the fullness of the atrium, release an electrical pulse to start a chain reaction of events [5]. This pulse depolarizes the right atrium, which, if we were to look at an ECG, would be represented by the P-Wave, or the initial “bump” if reading from left to right.

At this point, the SA node has just depolarized and is sending a signal across the heart muscle cells to contract the right atrium and the left atrium. It is able to contract the right atrium, because the SA node is located on the right atrium, however it also affects the muscle cells of the left atrium by sending the impulse across the bachmann’s bundles (also known as the “interarterial bundles”)[6]. This impulse, although barely noticeable, impacts the right atrium just slightly before the left atrium. As the muscle cells are activated, the tricuspid valve located between the right atrium and right ventricle is opened and 70% of the blood previously located in the right atrium falls into the right ventricle while the following 30% is then squeezed in by a small contraction of the muscles making up the right atrium (atrial diastole) [7]. Meanwhile, a similar phenomenon occurs in the left atrium and the left ventricle via the bicuspid valve [5].


The yellow arrows show where the impulse is sent from the SA node. As this happens, the atriums contract, allowing blood (red arrow) to move into the ventricles.

Sinoatrial node (SA node)

Impulse moving across the Bachmann Bundles to reach the left atrium.

Why does this occur at both the right and left atrium?
This action must occur almost simultaneously, because as blood is moved on one side of the heart in preparation for the next step, blood must also be moved from the other side in preparation for the next step. In a healthy heart beat, this is not an issue. In pathological cases, it can be an issue.

What is the sinoatrial node’s speed of conduction?
If the source is correct, roughly .05 meters/sec [9].

 

Intra P-Q: Signal Slowing

This is not an “official” step of the process, but important, because it is the point immediately post P-wave and explains the flat line to the Q-wave. At this point, the signal has contracted the muscles of the atriums, but as blood does not move as quickly as the electrical signaling, the signal is slowed upon reaching the arterioventricular node (AV node) so as, again, to allow the blood to fully fill both ventricles [5]. This is represented by the flat line between the P-wave and the Q-wave.

Arterioventricular node (AV node)
Note: This diagram does not distinguish between oxygenated and deoxygenated blood.
Q-R-S waves: Ventricular Depolarization

What is the arterioventricular node’s speed of conduction?

Again, if the source is trustworthy, roughly .05 meters/sec [9]. This may seem quick, but in relation to other conduction speeds, it is quite slow.


At this point, the signal is released from the AV node, following down the Bundle of His and splitting into two branches lining each of the ventricles in the heart’s septum (middle section separating right and left sections of heart)[5]. On the ECG, this is observed as the Q-wave dip. As the signal flows down the septum, the tricuspid (between right atrium and right ventricle) and the bicuspid (between left atrium and left ventricle) valves are closed once more [5]. This is done in preparation for the next step so as to stop backflow into either atrium.

Then, as the signal has passed through the left and right bundle branches, it enters the Purkinje fibers that connect straight to the myocytes (muscle cells) located in the lower, meaty areas of the heart’s ventricles [5]. There, depolarization of the cells across both entire ventricles leads to muscle contraction and blood being pushed out of the ventricles [5]. I should mention that the left ventricle contracts just a bit sooner than the right ventricle [5]. This initial contraction of the left ventricle is marked on an ECG by the R-wave, and the remaining contraction of the right ventricle is outlined by the S-wave.

These contractions lead to the pushing of blood from the right ventricle through the pulmonary artery via the opening of the pulmonary valve, as well as the pushing of blood from the left ventricle into the aorta through the aortic valve. Both of these are considered ventricular systole, because pressure is being exerted.


Bicuspid valve closed again to stop backflow.
Tricuspid valve closed again to stop backflow.
Signal continuing via the Bundle of His and splits into two branches.
Signal enters the purkinje fibers to contract ventricles.

Why is the R-wave so large?
As the area around the ventricles is rather thick and filled with muscle cells for an even more powerful contraction (generally against gravity), the signal is then stronger as more cells are activated and this is seen in a tall ECG R-wave [8].

What is the Bundle of His and Purkinje fibers’ speed of conduction?

While the Bundle of His is quick, clocking in at about 1 meter/sec, the purkinje fibers are known to be the fastest in this entire process running somewhere between 1-4 meters/sec [9].

T-wave: Relaxation

This is the final step and consists of your heart muscles relaxing due to ventricular repolarization (resetting)[8]. At this point, blood has left both ventricles and the cycle resets by filling both atriums once again for the next pulse (beginning at the P-wave). This depolarization is depicted by the T-wave on the electrocardiogram.

Both atriums refill with blood.
Ventricles relax back out to resting position.

Now that we have completed a full cycle of blood movement, it should be clear how electrical conduction and anatomy play an intimate role together in performing just a single heartbeat. Here is a final review illustration that shows most of the steps of cardiac conduction.

Writer: Nicolas Verhoeven

                                                                                                          Citations


[1] Understanding the Heart's Electrical System and EKG Results - NHLBI, NIH. (n.d.). Retrieved from http://www.nhlbi.nih.gov/health/health-topics/topics/hb/understanding

[2] Christoffels, V. M. (2009). Basic Science for the Clinical Electrophysiologist. American Heart Association, 195-207. Retrieved from http://circep.ahajournals.org/content/2/2/195.full

[3] Electrocardiogram (ECG or EKG). (n.d.). Retrieved from http://www.heart.org/HEARTORG/Conditions/HeartAttack/SymptomsDiagnosisofHeartAttack/Electrocardiogram-ECG-or-EKG_UCM_309050_Article.jsp#.ViFowvlVhBc

[4] The Sinoatrial Node: The Body's Natural Pacemaker. (n.d.). Retrieved from hyperphysics.phy-astr.gsu.edu/hbase/biology/sanode.html

[5] Your Heart's Electrical System - NHLBI, NIH. (n.d.). Retrieved from https://www.nhlbi.nih.gov/health/health-topics/topics/hhw/electrical

[6] J.H. van Campenhout, M. (2013). Advances in Arrhythmia and Electrophysiology.American Heart Association, 1041-1046. Retrieved from http://circep.ahajournals.org/content/6/5/1041.short

[7] The P Wave - Sinus Rhythm - Normal Function of the Heart - Cardiology Teaching Package - Practice Learning - Division of Nursing - The University of Nottingham. (n.d.). Retrieved from http://www.nottingham.ac.uk/nursing/practice/resources/cardiology/function/p_wave.php

[8] Conquering the ECG - Cardiology Explained - NCBI Bookshelf. (n.d.). Retrieved from http://www.ncbi.nlm.nih.gov/books/NBK2214/

[9] Conduction speed. (n.d.). Retrieved from http://www.ld99.com/reference/old/text/2878909-395.html

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