Sinus rhythm electrocardiogram

Sinus Rhythm (SR), also known as Normal Sinus Rhythm (NSR) is the term used to denote normal cardiac activity. A normal beating heart is in sinus rhythm.

In the normal beating of the heart the left and right atria contract followed by the contraction of the left and right ventricles. These atrial and ventricular contractions are separated by a lag time of 1/10th of a second which allows the atria to relax (atrial diastole) while the ventricles contract (ventricular systole). The addition of the normal action of the four heart valves (tricuspid, mitral, aortic, and pulmonary) allows the heart to work as a blood pump and circulate blood within the heart and through the body and lungs.

SR is coordinated by a complex binary conduction system. The first part of the conduction system is composed of heart muscle cells (cardiomyocytes) which are characterized by automaticity. It is important to state here something that is known, but many times ignored: This portion of the conduction system is not composed of nerves... it is formed by specialized cardiac muscle cells! The description of this cardiomyocyte-based system can be read in a separate article here.

The second component of the conduction system is the cardiac autonomic nervous system (CANS), which acts as a moderator (accelerating or slowing) the cardiomyocyte-based conduction system. A separate article will be written soon and linked here. Suffice it to say (for now) that the CANS has two components, an extrinsic component formed by nerves and ganglia outside the heart, and an intrinsic component of nerves and ganglia that forms a complex network within the superficial aspect of the heart muscle and fat surrounding the heart. These are the ganglionated plexuses, and their dysfunction can cause abnormal heart rhythm.

When these two components and their thousands of cardiac muscle cells, ganglia, and nerves are working property, the heart is in sinus rhythm, and beats between 60 to a 100 times per minute.

Personal note: The term Normal Sinus Rhythm (NSR) is redundant. By definition, if the heart is in SR, it is normal. On the other hand, the term Abnormal Sinus Rhythm is also not correct… if the heartbeat is abnormal, the heart is not in SR! (Dr. Miranda)

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Histology image of cardiac muscle*

[UPDATED] Cardiac muscle is one of the three types of muscle found in the human body. It is found exclusively in the heart, where it forms the main component of its middle layer, the myocardium.  

[Myo]=combining form for "muscle"; [-card-]=heart; and [-ium]=layer or membrane. The myocardium is the muscle layer of the heart.

Cardiac muscle has distinct striations and intercalated discs (see accompanying image). The cardiac muscle acts as a functional syncytium. 

The key characteristic of cardiac muscle is automatism or automaticity, its capacity to contract rhythmically in the absence of an external electrical stimulus. The other two types of muscle (smooth and skeletal) lack this characteristic.

The term [cardiomyocyte] can be used to describe each of these cells as the word is composed of [Card(i)o=combining form for "heart"; [myo]=combining form for "muscle"; and the suffix [-cyte]= cell. A cardiomyocyte is a cardiac muscle cell.

One of the components of the conduction system of the heart is made exclusively of cardiomyocytes.

 Image modified from the original on YouTube from the British Heart Foundation.

* Original histology image by S, Girod and A. Becker, courtesy of Wikipedia. 

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Anterior view of the heart*

[UPDATED] These two root terms mean "heart".

The first one, [-card(i)-] arises from the Greek [καρδιά] pronounced kardiá, and can be seen in medical words such as: cardiac, echocardiogram, cineangiocardiogram, cardioplegia, myocardial infarction, etc.

The second one [-cord(i)-] arises from the Latin [cor] or [cordis] and can be seen in words such as: precordial pain, cordial, commotio cordis, etc. 

As a point of interest, the original Greek spelling of [kardium]  was used by Nobel prize winner Dr. Willem Einthoven (1860 - 1927) when he invented the electrocardiograph and the electrocardiogram. The German term is [elektrokardiogramm] and the German abbreviation for the procedure is EKG. Since we use the term in English, [electrocardiogram] we use the abbreviation ECG. Both are correct, although if you are speaking English, ECG should be used.

* Image property of:CAA.Inc.. Artist:Victoria G. Ratcliffe

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Vine tendril. Image courtesy of Jon Sullivan

 

The term [pampiniform] comes from the Latin term [pampinus] meaning "a vine tendril". It refers to a twisted, curved structure as seen in the accompanying image.  The second portion of the word also comes from Latin, [forma] means "shape" or "in the shape of".

Pampiniform then means "in or with the shape of a vine tendril"

Although mostly associated with the pampiniform plexuses found in both the testicular veins in the spermatic cord and the ovarian veins found within the infundibulopelvic ligament, the term is also used to denote the coiled aspect of the organ of Rosenmuller, also known as the pampiniform body or paraovarium.

The pampiniform body is a non-functional embryological remnant of the development of the female reproductive system. It is composed of a blind longitudinal duct and 10-15 transverse smaller ducts. It is located in the mesosalpinx, an extension of the broad ligament related to the uterine tube (Fallopian tube).

Thanks to David Van Tol for suggesting this article!

Image courtesy of Jon Sullivan, Public domain, via Wikimedia Commons https://jonsullivan.com/

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Brachial plexus

UPDATED: The term [plexus] comes from the Latin term [plectere] meaning " to twine, or to braid". In anatomy, the term [plexus] refers to a group of structures that are intertwined or form a meshwork.  The plural form is [plexuses], although the Latin plural form [plexi] is also correct. Gabrielle Fallopius used the term to denote "a tangle of nerves"

There are many plexuses described in the human body. Most are formed by nerves, but there are many that are lymphatic or vascular. The best known are the plexuses of nerves formed by the ventral rami of the spinal nerves. These are the cervical plexus, the brachial plexus, the lumbar plexus, and the sacral plexus. The image depicts the brachial plexus. For a larger version, click on the image, and for further information on the cervical and brachial plexuses, click here. 

Images and links courtesy of:www.bartleby.com

 

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Left atrial appendage

The left atrial appendage (LAA) is an embryological remnant of the development of the heart. It represents the primitive left atrium (LA) which is then “pushed to the side” by the development of the final (adult) stage of the LA. While the LAA is thin, tubular, tortuous, and presents with convoluted muscular walls, the adult LA has smooth walls and is considered to be a dilation of the terminal portion of the veins that enter the LA, hence the name “sinus venarum”, another term for the atria. The LAA is also known as the "left atrial appendix", or the "left auricle".

The anatomy of the LAA is presented in the video included at the end of the article, but there are some details that are important to discuss in the involvement of the LAA in the creation of thrombi and emboli in the presence of atrial fibrillation (AFib).

LAA shape and size

The LAA has important anatomical variations, with different shapes that anatomists and physicians have tried to consolidate in groups such as: chicken wing, cactus, windsock, cauliflower, etc. The fact is that recording the shape of the LAA is subjective. as the evaluation depends completely on the observer.

Researchers have tried to determine what shape can lead to a higher potential for stroke-producing emboli when AFib is present. A recent study by Dudzińska-Szczerba (2021) and an editorial by Yong Shin (2021) states that the shape itself is not a good predictor, but the distance between the LAA ostium and the first bend of the LAA is indeed a good predictor. The longer the distance there is increased potential for thrombus and emboli formation.

The size of the LAA has been studied in detail and it ranges ranging from 0.3 cm to 2.0 cm in males, and 0.3 to 1.8 cm in females. (Venoit, 1997), as shown in this table.

The LAA ostium

The LAA ostium is the communication between the LA and the LAA, it is generally oval in shape and its size is variable. The ostium is in some cases slit-like, or an elliptical-shaped variant, “smiley”, and even small circular (DeSimone, 2015; Cabrera, 2014). A study by Wang (2010) classified the LAA ostium into five types: oval (68.9%), foot-like (10%), triangular (7.7%), water drop-like (7.7%), and round (5.7%). It is interesting that devices that are used to occlude the LAA ostium are round and that is only 6% of the population reported in the Wang (2010) study. In a study by Su (2006) it was found that 100% of the specimens studied the LAA ostium had an oval shape with the mean diameter of the opening of 17.4 mm with a range between 10-24.1 mm).

In reference to LAA ostium occluders Su (2006) states that "These percutaneous devices / systems,however, have a round shape to fill or cover the LAA ostium. A previous study and our study show that the shape of the LAA ostium is consistently elliptical rather than round. This suggests that to seal the LAA orifice adequately without oversizing, devices may need to be elliptical for a snug fit. A round implant over an oval-shaped orifice may leave crevices on either side of the implant, leading to incomplete sealing of the orifice."

Lobes

The LAA can also present with different dilations called “lobes” these can range from zero to three or four.

Muscular Wall Structure

Cow LAA internal structure

The LAA has internal ridges that form a muscular meshwork. The term used to describe these is “trabeculated”. It makes sense that in the case of atrial fibrillation, the slow to non-existent flow of blood within the deep recesses of the trabeculated muscular wall of the LAA will cause blood to pool and coagulate, forming thrombi. The presence of these LAA trabeculations have been found to be associated with stroke risk by Dudzińska-Szczerba (2021). The accompanying image shows the trabeculations in a cow's LAA. They are not as deep or as convoluted as those found in a human heart.

Crenellations

This is a rarely used term. It is a pattern along the top of a fortified wall, as in a castle, forming multiple, regular, rectangular spaces. These crenellations are found in the edge of the LAA compounding the irregularity of the wall and increasing the chance for thrombus formation and stroke-inducing thrombi. Crenellations are shown by yellow triangles in the first image in this article.

Function of the LAA

As stated, the LAA is an embryological remnant, but it does have a function in the adult. It generates a peptide involved in the control of salt in the circulatory system. This is the atrial natriuretic peptide (ANP), a hormone that is secreted by both  the right and left atria and their appendages in response to circulatory volume and pressure changes. ANP helps the elimination of excess sodium through the kidneys (natriuresis), control of urine elimination (diuresis), and antifibrotic and antihypertrophic effects within the heart (Sandeur, 2023)

While removing both the right and left atrial appendages could cause ANP deficiency, surgical removal or exclusion of only the LAA does not cause an ANP problem (8).

Involvement of the LAA in AFib

The LAA is an electrically active structure. The cardiomyocytes that form its walls have automatic activity and it has been described as an area that can trigger AFib. The accompanying video shows an LAA that has been separated from the heart (in this case using a surgical stapler) and it can be seen how the LAA continues fibrillating on its own. Video courtesy of Dr. Randall K. Wolf

This is the why LAA exclusion is a must in the case of AFib and potentially in any cardiovascular procedure where the pericardial sac is opened (this is a subject for discussion). The problem is that devices that only occlude the LAA ostium do not disconnect the LAA wall from the LA wall, leaving this potential AFib-producing connection intact.

Personal note: In May 5th, 202 Dr. Randall K. Wolf invited me to a live webcast where we reviewed the anatomy of the left atrial appendage, the problems the LAA can cause in atrial fibrillation leading to stroke, and the reasons for its exclusion in AFib surgery. This video is next. You can watch other videos on the topic here.  Dr. Miranda.

Sources:
1. “Anatomy of the Normal Left Atrial Appendage: A Quantitative Study of Age-Related Changes in 500 Autopsy Hearts: Implications for Echocardiographic Examination” Veinot, JP; et al. 1997 Circulation; 96:3112–3115
2. “A Review of the Relevant Embryology, Pathohistology, and Anatomy of the Left Atrial Appendage for the Invasive Cardiac Electrophysiologist” De Simone, CV, et al. J AFib 2015; 8:2 81-87
3. “Left atrial appendage: anatomy and imaging landmarks pertinent to percutaneous transcatheter occlusion” Cabrera,JA; Saremi, F; Sanchez-Quintana, D. 2014 Heart 2014 100:1636-1650
4. Left Atrial Appendage Studied by Computed Tomography to Help Planning for Appendage Closure Device Placement” Wang Y. et al. J Cardiovasc Electrophyisiol 2010 21:9 973-982
5. Is the Left Atrial Appendage (LAA) anatomical shape really meaninglessmeasure for stroke risk assessment?
6. “Assessment of the left atrial appendage morphology in patients after ischemic stroke” Dudzińska-Szczerba, K. et al. Int J Cardiol 2021 330:65-72
7. “Atrial Natriuretic Peptide” Sandeur, CC; Jialal, I. Stat Pearls 2023. StatPearls https://www.ncbi.nlm.nih.gov/books/NBK562257/
8. Personal communication, Dr. R. Wolf 2023
9. "Slide Atlas of Human Anatomy" Gosling, J.A.; Whitmore, I; Harris, P.F.; Humpherson, J.R., Et al; ISBN: 0723426570 Hong Kong: Times Mirror, 1996
10. "Atrial and brain natriuretic peptides: Hormones secreted from the heart" Nakagawa Y, Nishikimi T, Kuwahara K.  Peptides. 2019 Jan;111:18-25.
11. "Occluding the left atrial appendage: anatomical considerations" Su, P; McCarthy, KP; Ho, SY. 2008 Heart 94:1166–1170

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