Myocardial perfusion

Pathway of coronary blood flow

Supply of coronary blood flow

• Right coronary artery is dominant in 50% of individual
• Left coronary artery is dominant in 20%
• Equal in 30%

Drainage of coronary blood flow

• Most (90%) coronary venous blood returns to right atrium via coronary sinus.
• Some reach right atrium via anterior coronary veins
• Some reach cardiac chambers directly via:
  * Arteriosinusoidal vessels
  * Arterioluminal vessels
  * Thebesian vessels

 

Left coronary artery vs right coronary artery

Coronary perfusion pressure
= Aortic blood pressure - (intraventricular pressure or right atrial pressure, whichever is higher)

Left ventricle

Because the pressure is much higher in left ventricle in systole

• Perfusion of LV is mainly in diastole
• Reversal of blood flow can happen in systole

Also,

Transmural pressure is greatest in the endocardium

• Impairment of blood flow during systole is more severe in the endocardial region

e.g.

• In systole, LV perfusion pressure
  = 120-120 (transmural pressure)
  = 0 mmHg
• In diastole, LV perfusion pressure
  = 80 - 5 (RA pressure)
  = 75 mmHg

Right ventricle

Difference in perfusion during systole and diastole less marked in the RV
* Intra-ventricular pressure is much lower (SBP=25)
* Perfusion pressure is same as LV

RV wall is thinner too

 

Regulation of coronary perfusion

Metabolic autoregulation overrides the neural factors.

--> Metabolic demands of the heart regulate coronary blood flow

Metabolites involved include:

[BL8:p233-234]

• NO
• ATP-sensitive K+ channels
• Adenosine

NB:

• Direct noradrenergic stimulation actually causes vasoconstriction via alpha-adrenergic receptors

 

Other notes

 

Coronary steal

... when administration of vasodilator can fully dilate normal vessel branches parallel to the narrowed segment and divert blood away from it.