Anatomy of the heart

This chapter is relevant to Section G1(ii)  of the 2017 CICM Primary Syllabus, which asks the exam candidate to " describe the anatomy of the heart, including valves, chambers and the pericardium and coronary circulation".  Unfortunately, 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 surmise that the college does not expect their early trainees to become masters of the cardiac anatomy.

Still, it is not inconceivable that at some random stage in the future they will ask for this in some detail, consider that this is what happened with the trachea and bronchi. 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".

Landmarks

• 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.

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

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

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.