Anatomy of the heart

This chapter is relevant to Section G1(i)  of the 2023 CICM Primary Syllabus, which asks the exam candidate to "describe the anatomy of the heart including the chambers, valves, pericardium, and the orientation of the heart".  Prior to the disastrous Question 2 from the second paper of 2023, nothing else in the CICM First Part Exam has ever asked for this, except perhaps a fraction of Question 16 from the second paper of 2019. There, the college wanted us to "compare the structure, function and coronary circulation of the right and left ventricles", where the structure component would have been satisfied by "a simple anatomical description".  From the examiner's inattention, one might have come to the conclusion that the college does not expect their early trainees to become masters of the cardiac anatomy.  Still, it should not have been so surprising that at some random stage in the future they would have asked for this in some detail, considering that this is what happened with the trachea and bronchi. And indeed "outline the anatomy of the cardiac ventricles including the chambers, valves and conduction elements" eventually came up in the second sitting of 2023, with a pass rate of 11%. The examiners, in their comments, provided the following suggested structure:

• position, orientation, relations and characteristics of the chambers including ventricular interdepence
• internal ventricular structures including muscle type, septum,  trabeculae, papillary muscles, infundibulum and moderator band
• valves and valvular rings’ position, structure and attachments
• conduction elements position and divisions
• blood and nerve supply

This structure is therefore canonical and will be used below, as much as possible. Given the nature of the answers expected for that question,  Last's was the natural reference point, as it gives its information in the same structure as the college answer (i.e. that's where they probably copied their marking rubric from). For the pedant, the author was using an old dog-eared ninth edition, where the relevant details can be found on page 265. Also, for the person who has no Lasts of any vintage, an excellent article by Weinhaus & Robers (2005) is somehow available for free. There is of course a lot more detail to cardiac anatomy, but because of the self-imposed limitations of this chapter (keep it simple, keep it text), a lot of the more interesting material had to be exiled to other pages.

Anatomical depiction of the heart

It is at this stage of a textbook chapter that the reader is usually confronted with various cutaway diagrams of the heart, lovingly sketched by artists from the early twentieth century as they sat in prosection labs, choking on formaldehyde fumes. Lovely though they are, the relevance of this practice among publishers is puzzling, particularly where it comes to exam practice. The CICM trainees usually do not have the time or the patience to reproduce those diagrams in the rush of writing their answers. As such, they will usually offer a written description. Moreover, from a purely practical standpoint, it must be acknowledged that most of them will never see or handle their patient's heart in the course of their normal sane behaviour.  Ergo, the most exam-centric thing one can offer here is a textual description of cardiac anatomy, designed for easy regurgitation.

Anatomical description of the heart

If one had to produce a one-sentence description of what the heart is for some sort of introductory statement, one could do worse than Last's, where it is described as "the muscular pump responsible for blood circulation".

Position 

To merely say "it's in the middle of the chest" would probably not be enough to score marks. The specific landmarks have not been asked for in any past papers but they may appear one day in some sort of procedure viva, as the tendency there would be to ask for the anatomical landmarks used for pericardiocentesis (for example).

• Right border: from the lower border of the right 3rd costal cartilage to the lower border of the right 6th costal cartilage; it extends just beyond the right margin of the sternum
• Inferior border: from the right 6th costal cartilage to the apex
• Apex: normally in the left fifth intercostal space about 9 cm from the midline. 
• Left border: from apex to lower border of the left 2nd costal cartilage, about 2 cm from the sternal margin.

Orientation

• Fixed by the greater vessels, but free to move in the pericardium
• The ventricles are narrower and slightly rotated during systole
• During full inspiration, the apex descends more than the base, and the heart become more vertical in position

Basic structural anatomy

• Atria and ventricles are separated by the fibrous skeleton
• Muscular fibres encircle the chambers in spirals
• The conducting system is the only connection between the two muscular systems
• Right atrium:
  • lies between the superior and inferior venae cavae, and forms the right border of the heart
  • Receives the coronary sinus and the venae cava
  • One auricle
• Left atrium
  • Two auricles
  • Entered by four pulmonary veins
• Right ventricle:
  • Irregular concave shape
  • Contains the tricuspid valve, which is attached to the fibrous skeleton
• Left ventricle
  • Conical shape
  • Three times as thick as the right ventricle
• Interdependence
  • Distension of one ventricle (eg. right) affects the distensibility of the other, impacting on its ability to fill

Internal ventricular structures

• The septa:
  • Interatrial septum is a thin wall of atrial muscle forming the posterior  wall of the right atrium above the opening of the coronary sinus.
  • Interventricular septum is  a thick muscular wall (equal in thickness to the rest of the left ventricle) attached to the conjoined atrioventricular rings, and is thinner and more fibrous at this attachment (the membranous portion)
• Trabeculae are irregular muscular columns which project from the inner surface of the ventricles.
• Papillary muscles are extended trabeculae that are attached to the cusps of the tricuspid and mitral valve by chordae tendinae, and their role is to prevent regurgitation by bracing the mitral and tricuspid valves against prolapse during systole.
• The infundibulum is a funnel-like sleeve of right ventricular muscle that  forms the RV outflow tract and forms the fibrous ring that serves as the attachment for ther pulmonary valve. 
• The moderator band, also referred to as the septomarginal trabecula, is a condensation of trabeculae which lies fee from the wall, in the cavity of the RV, attached by its two ends to the interventricular septum and the anterior papillary muscle. It contains part of the right bundle branch of the His system.

Valvular structures

• Tricuspid valve
  • Position: behind the midline of the lower sternum
  • Structure: three cusps, anterior posterior and septal
  • Attachments: 
    • Fibrous ring that attaches to the cardiac fibrous skeleton
    • The edges and ventricular surfaces of the cusps attach to the chordae tendineae
• Pulmonary valve
  • Position: behind the left border of the sternum, at the third costal cartilage, slightly above the position of the aortic valve
  • Structure: three cusps, anterior, right and left
  • Attachments: fibrous ring at the top of the infundibulum attaches to the crescent-shaped arches of the cardiac skeleton at the root of the pulmonary artery 
• Mitral valve
  • Position: behind the midline of the lower sternum, but higher and to the left of the tricuspid valve
  • Structure: two cusps, anterior and posterior; the anterior cusp sits between the mitral and aortic orifices; thicker and smaller than the tricuspid
  • Attachments:
    • fibrous ring formed by the anterior and posterior fila coronaria arising from the corresponding left and right fibrous trigone of the cardiac fibrous skeleton
    • Chrodae tendinae insert into the edges of the leaflets
• Aortic valve
  • Position: behind the left border of the sternum, at the third costal cartilage, slightly below the position of the pulmonary valve
  • Structure: three cusps, right left and posterior 
  • Attachments: no specific fibrous ring; instead three fibrous triangular arches act as points of attachment for the cusps.

Conductive elements

• SA node: small bundle of cells in the superior right atrium
• Internodal tracts: lines of minimally modified myocytes arranged in parallel along the atrial wall
• AV node: small bundle of cells at the septal part of the right atrial base
• His-Purkinje system: parallel-aligned large heavily modified myocytes, spreading out from the central Bundle of His, along two bundle branches, and into hundreds of terminal ramifications. Insulated by fibrous tissue until the last divisions.

Blood supply

• Right coronary artery: lies in the groove between the right ventricle and the right atrium; runs anteriorly, and then posteriorly to encircle the heart
• Left main: short; divides into tow branches:
  • Left anterior descending (descends anteriorly, as the name suggests)
  • Left circumflex (descends posteriorly in the atrioventricular groove)
• Anastomosis between the arteries exists at the arteriolar level

Venous drainage

• Coronary sinus:
  • Lies in the posterior part of the atrioventricular groove
  • Receive tributaries from the five cardiac veins, which lie next to the coronary arteries:
    • The great cardiac vein (next to the LAD)
    • The middle cardiac vein (next to the PDA)
    • The small cardiac vein (next to the RCA)
    • the posterior vein of the left ventricle
    • the oblique vein of the left atrium (along the posterior surface of the left atrium)
• The anterior cardiac vein is separate from the coronary sinus, and contributes minimally (the sinus gets almost all of the blood)
• Also, there are venae cordis minimae or Thebesian veins, which are small veins in the walls of all four chambers of the heart that open directly into the respective chambers. They have trivial relevance, except to be mentioned by students of physiology when they discuss the causes of shunt. 

Lymphatic drainage

• Lymph channels drain back along the coronary arteries
• They emerge from the fibrous pericardium along with the aorta and pulmonary trunk
• Ultimately, empty into the tracheobronchial lymph nodes and mediastinal lymph trunks

Nerve supply

• The heart is innervated by the cardiac plexus:
  • Multiple contributions from multiple nerves:
    • The superficial part of the cardiac plexus is formed by the union of the inferior cervical cardiac branch of the left vagus and the cardiac branch of the left cervical sympathetic ganglion
    • The deep part of the cardiac plexus receives contributions from:
      • the right vagus
      • the left vagus
      • a branch from each recurrent laryngeal nerve
      • sympathetic fibres from the remaining five cervical sympathetic ganglia
      • from the upper five or six thoracic sympathetic ganglia of both sides
• Supplied territories include:
  • SA node (accelerator or depressor of the heart rate)
  • AV node 
  • Bundle of His
  • Vasomotor to coronary arteries

Now, finally, to answer the structure element of Question 16 from the second paper of 2019, which called for an anatomical comparison between the left and right ventricle. Yes, one must confess thinking about it, there must surely be some relevance to testing this knowledge, as it comes in handy when you embark on the mandatory TTE training which is now a part of the curriculum. It initially seemed logical to put the RV column on the right and LV on the left, but it somehow felt inexplicably wrong.

A Structural Comparison of the Right and Left Ventricle

Domain	Right ventricle	Left ventricle
Shape	Irregular; vaguely triangular	Conical
Valves	Tricuspid and pulmonic	Mitral and aortic
Thickness	Relatively thin: 2-5mm	Three times thicker: 7-11mm
Mass	~ 26g	~90g
Position in the chest	Right and anterior	Left and posterior
Blood supply	RCA and circumflex	LAD and circumflex

The pericardium

This is not a structure which the examiners have thus far concerned themselves, which is fair, because even though it appears in the syllabus, one can virtually guarantee that the exam candidates will not have spent any time reading about it. At a risk of distracting them from more valuable revision topics, the reader is motioned towards Rodriguez & Tan (2017) whose description of this otherwise boring sack is well-supplied with signposts to features of clinical relevance. 

This structure is usually described as "flask-shaped" or "conical", where the apex of the cone is in the same position as the apex of the heart. 

Structure: this is a sandwich of two layers, the outer fibrosa and the inner serosa. Inside, between serosa and epicardium, one finds some serous fluid of nonspecific character, a thin ultrafiltrate of plasma.  The thickness of both layers together is between 1 and 2mm,  

Function:  Through several resources it appears that most authors attribute to the pericardium the role of stabilising the heart inside the chest, presumably because otherwise it would bounce around randomly, batted about by the lungs like a piece of fluff in the possession of a kitten. This is obviously insane, as patients undergoing total pericardiectomy do not experience this. But everybody who discusses the anatomy of this structure seems to quote Holt (1970), and Joseph P. Holt opened his article by stating that the pericardium:

"...holds the heart in a fixed geometric position and isolates the heart from other structures in the thorax, thus preventing adhesions and spread of infection."

It has at least seven other roles according to Holt, among which are:

• Prevent dilatation of the cardiac chambers in response to fluid load
• Prevents hypertrophy of the heart during strenuous exercise
• Maintains negative pericardial pressure during ventricular systole

And in the same paper, Holt quotes Moore & Shumacker (1953), who after reviewing cases of congenital and experimentally absent pericardium concluded that it "serves no vital function and that it can be safely sacrificed".

Position:  without repeating what has already been said about the heart's position in the chest, one could focus on the position of the pericardium in relation to the heart itself, and the other structures. The inner layer is continuous over the entire surface of the heart and extends all the way to the roots of the greater vessels. It also also covers the anterior and lateral sides of the venae cavae and the anterior part of the pulmonary veins.  The outer fibrosa is connected to the adventitia of the greater vessels and to the central tendon of the diaphragm (the pericardiophrenic ligaments). It is also attached to the sternum, over the surface between the manubrium and the sternum. The posterior edges are in contact with the main bronchi.

Blood supply: branches of the internal thoracic artery and the descending thoracic aorta; venous drainage into the brachiocephalic veins.

Lymphatic drainage: there is no single route and the pericardium drains in all directions, including the epicardial lymphatics of the heart, peribronchial nodes and the mediastinal pleura. 

Innervation: 

• Sensory fibres from the phrenic nerve
• Parasympathetic nerve supply from the vagus and left recurrent laryngeal nerves
• Sympathetic innervation from the first dorsal ganglion, stellate ganglion and the aortic, cardiac, and diaphragmatic plexuses.

This innervation pattern can give pericardial pain a diaphragm-like pattern of referral, with shoulder tip pain predominating. A classical description of this comes from an ancient paper by Capps (1927) who reported on his experiences of tapping effusions and then scratching around inside the cavity with a silver wire, to much complaint form his patients. 

Coronary artery anatomy in more detail:

Right coronary artery:

• Arises from the right aortic sinus
• Passes between the right auricle and the infundibulum of the right ventricle 
• Then passes vertically downwards in the atrioventricular groove
• Then runs posteriorly at the inferior border of the heart
• Important branches:
  • Conus artery which passes upwards and medially on the front of the conus part (infundibulum) of the right ventricle.
  • SA nodal artery, which arises close to the origin of the RCA and supplies the SA node in 60% of patients
    • The SA nodal artery may arise directly from the anterior aortic sinus adjacent to the right coronary origin
  • Right marginal artery passes to the left along the right ventricle
  • AV nodal artery, which is the most posterior branch (in 90% of patients, the RCA supplies the AV node)
  • Posterior descending artery (PDA), otherwise referred to as the posterior interventricular, on the diaphragmatic surface of the heart

Left main:

• Arises from the left aortic sinus behind the pulmonary trunk
• Emerges between the left auricle and the infundibulum of the right ventricle
• Divides into the LAD and LCx after a short course

Left anterior descending:

• Runs towards the apex in the interventricular groove
• Under the apex, posteriorly, forms an anastomosis with the PDA
• Important branches:
  • Diagonals (which extend over the anterior left ventricle towards the apex); these are variable in number, and the first one (D1) tends to be the most prominent
  • Septal perforators, which supply the septum

Left circumflex:

• Continues down to the back of the heart in the LA-LV atrioventricular groove
• Anastomosis with the end of the right coronary
• Important branches:
  • SA nodal artery (it contributes to it)
  • Obtuse marginal arteries  - these travel along the LV surface to the left, and are variable in their position and number.