Coronary Circulation

Describe the anatomy of the ~~heart, the pericardium and~~ coronary circulation

Vascular Anatomy

The left main coronary artery:

Arises from the posterior aortic sinus superior to the left coronary cusp of the aortic valve
Eddy currents produced in the sinuses of Valsalva (outpouchings of the aortic wall) prevent the valves occluding the os of the LM and RCA during systole, so they remain patent throughout the cardiac cycle.
The left main is 5-10mm long, and bifurcates to form the LAD and LCx
The intermediate artery is an anatomical variant that arises from the bifurcation, creating a trifurcation

The LAD:

Courses along the anterior interventricular groove to the apex of the heart
Here, it anastomoses with the posterior descending artery from the RCA.
Supplies the anterolateral myocardium and anterior 2/3^rds of the interventricular septum
Branches of the LAD include:
- Diagonal vessels
  Branches are named successively from proximal to distal (i.e. LADD₁, LADD₂, etc), and wrap around the anterior wall of the ventricle.
  - Proximal LAD is defined as the area of LAD between the origin and D₁, mid LAD is between D₁ and D₂
- Septal perforators
  Penetrate into the myocardium.

The LCx:

Courses along the left atrioventricular groove between the LA and LV in the epicardial fat pad
Supplies the inferolateral wall of the LV
Gives off three obtuse marginal branches (OM₁, OM₂, etc) which follow the left margin of the heart
Runs in close approximation with the coronary sinus for much of its course

The RCA:

Arises from the anterior aortic sinus, superior to the right coronary cusp of the aortic valve
Courses vertically downwards in the right atrioventricular groove
Supplies the RA and RV
RCA gives off:
- Acute marginal arteries
  Almost never grafted.
- Posterior branch to the LV (PLV)

The posterior descending artery:

Arises from either the LCx or RCA
These vessels travel in opposite directions around the atrioventricular groove.
Descends in the posterior interventricular groove before coursing along the base to anastomose with the LAD at the apex of the heart
Is also known as the posterior interventricular artery

Coronary dominance refers to which vessel gives rise to the PDA:

Additionally:

85% of venous drainage occurs via the coronary sinus
- Drains the LV
- Formed from the cardiac veins:
  - The great cardiac vein runs with the LAD
  - The middle cardiac vein follows the PDA
  - The small cardiac vein runs with the RCA
  - The oblique vein follows the posterior part of the LA
Most of the remainder is via anterior cardiac veins which drain directly into the RA
A small proportion of blood from the heart is drained via the thebesian veins directly into four the cardiac chambers
Most into the right atrium, and least into the left ventricle. The portion of blood draining into the left side of the circulation contributes to physiological shunt.

Coronary Blood Flow:

Normal is ~250ml.min^-1 (~5% of resting CO)
May increase 4x during strenuous exercise
Myocardial work may increase up to 9x, though as myocardial oxygen extraction is unchanged efficiency is actually improved during exercise.

CBF is dependent on:

Coronary vascular resistance
Coronary perfusion pressure
The difference between aortic root pressure and the greater of RAP or intracavity pressure: i.e.
- Note that the pressure gradient is usually Aorta-Cavity rather than Aorta-RA
  This is because the pressure in the ventricle acts as a Starling resistor - coronary flow is independent of RAP whilst $RAP < P_{Cavity}$
Heart rate
LV CBF is affected in systole due to the changes in perfusion pressure, and compression of intramuscular vessels (causing an increase in CVR).
- RV CBF is less affected, as the force of contraction is significantly smaller and a pressure gradient is maintained
- Tachycardia reduces diastolic time and subsequently LV CBF

Left Ventricular coronary flow, demonstrating diastolic-dependent perfusion:

Right Ventricular coronary flow, demonstrating flow throughout the cardiac cycle:

CBF is autoregulated:

Myogenic autoregulation
This is common to many organ systems, and occurs within the coronaries.
- Increasing transmural pressure increases the leakiness of smooth muscle membranes, depolarising them
- Resistance increases proportionally to pressure, such that flow remains constant
Metabolic autoregulation
Anaerobic metabolism results in production of vasoactive mediates such as lactate and adenosine, which stimulate vasodilation and therefore increase flow (and oxygen delivery).
- This is the predominant means for autoregulation in the heart
- Typical myocardial oxygen extraction is 70% and raising this further is difficult
  Therefore, increasing oxygen supply requires an increase in blood flow.

Autonomic mechanisms also control some aspects of coronary blood flow:

Direct effects include:
- Parasympathetic and sympathetic innervation of coronary vessels, with release of ACh or NA and A decreasing or increasing coronary blood flow
Indirect effects
- Are more important than direct effects
- Are related to autoregulation occurring with changing levels of myocardial work in response to parasympathetic or sympathetic stimuli

Hall, JE, and Guyton AC. Guyton and Hall Textbook of Medical Physiology. 11th Edition. Philadelphia, PA: Saunders Elsevier. 2011.
CICM July/September 2007
McMinn, RMH. Last's Anatomy: Regional and Applied. 9th Ed. Elsevier. 2003.
Coronary Artery Graph based on Coronary Arterial Circulation - es. 2/3/2013. (Image). By Addicted04 (Own work) CC BY 3.0, via Wikimedia Commons.

Last updated 2021-08-23