Coronary Circulation Flashcards
Describe anatomy of right coronary artery
Right coronary artery (RCA) arises from the aortic root just above the AV from coronary ostia.
- RCA supplies blood primarily to RA and RV, but also perfuses SA node and, very commonly, the AV node.
- It terminates as the posterior descending artery (PDA), which supplies the posterior 1/3 of the interventricular septum. LAD perfuses the remainder.
Describe the anatomy of the Left Coronary Artery
Left coronary artery arises from the aortic root just above the AV from coronary ostia.
Begins as the left main stem before dividing into:
- Left anterior descending artery (LAD)
The LAD gives off diagonal and septal branches and perfuses the majority of the LV
- Left circumflex (Cx) artery
The circumflex artery leads to obtuse marginal branches supplying the lateral walls of the left ventricle.
Describe some variations in coronary circulation anatomy
- In ~70% of patients the PDA arises from the RCA (right dominance) but in ~10% it arises from the circumflex (left dominance).
- In 20% of the population, codominance is present with supply from both vessels.
- In most cases, the coronary arteries are located in the epicardium, but in up to 25% they may be intramyocardial
- Differing branching angles and locations of LCx from the left main stem
- Abnormal locations of the coronary ostia which may be more evident in those with bicuspid aortic valves.
- Bicuspid aortic valve is present in ~1% of the population
Describe the venous drainage of the coronary circulation
Anterior surface drainage occurs via the anterior interventricular vein, which lies alongside the LAD coursing between LV and RV.
Forming the great cardiac vein with venous drainage from the circumflex territory, it enters the coronary sinus, located in the posterior atrioventricular groove, and thence to the RA
The RA receives 35% of cardiac venous drainage returns directly through anterior cardiac veins, the remainder draining through the smallest cardiac (Thebesian) veins directly into the LV.
List the determinants of coronary blood flow
- Perfusion pressure and myocardial extravascular compression (e.g. aortic root pressure)
- Myocardial metabolism (O2, CO2, adenosine)
- Neural and humoral control (SNS, PNS, Vasopressin (arginine vasopressin, AVP), VIP, Neuropeptide-Y, Calcitonin gene-related peptide, Arachidonic acid metabolites
- Autoregulation (perfusion, myogenic, metabolic)
What is the effect of Thiopentone on coronary blood flow?
Thiopentone is not used due its:
- Myocardial depressant effect
- Venodilation
- Decrease in central sympathetic outflow
Which results in:
- decreased MAP, CO and increase HR
What effect has Etomidate on coronary blood flow during anaesthesia?
Etomidate has the best haemodynamic profile as it does not depress the myocardium and has no sympathomimetic effects.
But it inhibits adrenal 11-B Hydroxylase production resulting in decreased endogenous cortisol aldosterone formation, even after a single dose.
What effect has Ketamine on coronary blood flow during anasthesia?
Ketamine causes increase in HR and MAP through indirect central and peripheral sympathetic stimulation, so can worsen coronary blood flow in already stenotic arteries.
Preferable in patients who are haemodynamically unstable and in patients with cardiac tamponade physiology
What effect has volatile anaesthetic agents on coronary blood flow?
Volatile anaesthetic agents have been shown to attenuate myocardial ischaemia.
- Non acute manifestations of myocardial ischaemia include hibernating myocardium, stunning and preconditioning.
- Halothane and Isoflurane facilitate the recover of stunned myocardium
- Volatile anaesthetic agents provide a protective response akin to ischaemic preconditioning.
Overall effect of volatile anaesthetics depends on systemic haemodynamics and coronary vasculature. Inhalational anaesthetics cause a mild vasodilatory effect on coronary circulation
What is the perfusion theory of autoregulation?
If there is a change in perfusion pressure it will cause a directionally similar change of pressure in the capillaires, and hence the capillary resistance will increase, resulting in regulation of blood flow.
What is the myogenic theory of autoregulation?
There is an increase in arteriolar tone for enhanced blood flow, as a result of augmented intraluminal pressure.
What is the metabolic theory of autoregulation?
Coronary artery tone is dependent on myocardial oxygen supply and demand.
An increase in flow would wash out the metabolic substrates responsible for feedback control of the coronary artery tone.
Determinants of coronary blood flow.
Describe perfusion pressure
Perfusion pressure and myocardial extravascular compression:
Coronary blood flow depends on the gradient across the coronary circulation known as the critical closing pressure or zero-flow pressure.
The most appropriate measure of driving pressure for coronary flow is the aortic root pressure during diastole - approximated by the aortic diastolic or mean pressure.
Determinants of coronary blood flow:
Discuss Myocardial Metabolism
Various local factors act during rest, exercise and in ischaemia to match myocardial oxygen demand and supply:
Oxygen
CO2
Adenosine
Determinants of coronary blood flow:
Discuss Neural control of coronary blood flow
Coronary blood flow is regulated by SNS and PNS activation.
SNS activation causes increase in HR, contractility and arterial pressure and thus and increase in metabolically mediated coronary blood flow.
Vagal stimulation causes bradycardia, decreased contractility and arterial pressure.
