Lecture 17 - Special Circulations I: Coronary and Cerebral Flashcards
Where do epicardial coronary arteries originate from?
From the base of the aorta:
- Right coronary artery from the right aortic semilunar cusp
- Left coronary artery from the left aortic semilunar cusp
What is the richest microcirculation of the CV system?
Coronary circulation
Describe the collateralization of the coronary circulation.
If there is a blockage in one of the coronary vessels, collateral vessels will take over
These are normally closed but can become well-developed overtime as cardiovascular disease progresses
What is myocardial infarction?
Death of tissue due to ischemia
What do epicardial coronary arteries give rise to?
Branches that penetrate both the myocardium and the endocardium
What are the 3 regions of the myocardium?
- Subepicardium (outermost)
- Middle myocardium
- Subendocardium
Describe the pressure generated in the branches of the epicardial arteries in the 3 regions of the myocardium.
Pressures generated within each layer are different which is important when considering the perfusion of the cardiac wall, especially for the LV
How do the pressures in the myocardium of the LV increase during systole? What are the implications of this?
Extravascular/tissue pressures increase from the subepicardium to the subendocardium => means that during systole the pressure in the subendocardium rises above 120 mmHg in order to generate the ventricular pressure => this exceeds arterial pressure and impedes perfusion of this region of the myocardium during a portion of systole => subendocardial region of LV myocardium is particularly susceptible to ischemia and infarction
Other name of epicardial coronary artery?
Extramyocardial coronary artery
What is the BP in the epicardial coronary arteries?
120/80 mmHg
Are there perfusion issues in the RV myocardium during systole?
NOPE because much smaller pressures generated
In which region of the myocardium intramyocardial pressure the highest during systole?
Subendocardium (inner third)
Describe the flow curve of the right coronary artery.
Follows the aortic pressure curve
Describe the flow curve of the left coronary artery.
- During isometric contraction, preceding the rise in aortic pressure, flow falls to 0 as tissue pressure rises to generate the LV systolic pressure
- For the rest of systole flow curve follows the aortic pressure curve
- During isometric relaxation flow greatly rises due to release of intramyocardial pressure and build up of metabolites (adenosine) during preceding systole which cause vasodilation (aka reactive hyperemia)
- For the rest of diastole flow curve follows the aortic pressure curve
During what cardiac cycle phase does the greatest flow occur in the RCA?
Mid-systole because that is when aortic pressure is highest and there is little intervening pressure
During what cardiac cycle phase does the greatest flow occur in the LCA?
Isometric relaxation
What metabolite is primarily responsible for reactive hyperemia in the LV during isometric relaxation and overall metabolic regulation of coronary arteries?
Adenosine
Describe myogenic regulation of coronary arteries.
Can be demonstrated in the coronary circulation but is less important than metabolic regulation or effects of extravascular pressure
Describe sympathetic regulation of coronary arteries.
Sympathetic constriction can be demonstrated, although in normal function increased work of the heart during SNS activation results in metabolic vasodilation of coronary arteries which overrides sympathetic constriction
When do myogenic regulation and sympathetic regulation of coronary arteries become important?
If the heart is diseased with flow limitations then the effects of those regulations will be much more pronounced since the heart is not working as well and not producing as many metabolites
What is the area under the LVP curve during systole a measure of?
LV work and O2 demand
What is the area under the aortic pressure curve during diastole a measure of?
O2 supply to the LV since the highest flow to the myocardium occurs during diastole
What balance is important to maintain in the heart? When is this balance difficult to maintain?
LV O2 demand and LV O2 supply
Difficult to maintain in ill cardiac patients
In what part of the body is the VO2 the lowest at rest? Why?
Heart because it is the most metabolically active organ in the body and because of the difficulty of perfusion its walls
What are 4 factors that could tip the LV O2 supply and demand balance?
- HR
- Systolic arterial P
- Inotropic drugs
- Diastolic arterial pressure
How would an increase in HR tip the LV O2 supply and demand balance?
Negatively
HR rises => time in systole rises/time in diastole decreases => increased O2 demand
How would an increase in systolic arterial pressure tip the LV O2 supply and demand balance?
Negatively
Increased systolic arterial P = increased work done by heart => increased O2 demand
How would inotropic drugs tip the LV O2 supply and demand balance?
Negatively
Increased heart contractility => increased work done by heart => increased O2 demand
How would increasing diastolic pressure tip the LV O2 supply and demand balance?
POSITIVELY!
Increase diastolic P => increase flow to LV during diastole => increased O2 supply
How can we keep diastolic pressure sufficient enough to perfuse the left ventricular wall?
- Ensure that the BV is adequate, but don’t increase it too much or else the preload will increase and the heart will have to work harder
- Coronary vasodilators
How much time before fainting if blood flow to brain is occluded?
Seconds
Why is cerebral circulation so tricky?
- Flow occurs through a fixed volume and too much flow and pressure can cause cerebral edema which interferes with cerebral function
- Interruption of flow for seconds disrupts neuronal function
=> need for CONSTANT global cerebral flow
What structural component of the brain helps maintain constant global flow of blood? Explain.
Circle of Willis made of anastomosis of vessels going up to the brain provides collateralization of flow to respond to interruption or disturbances of cerebral blood flow
Does the Circle of Willis look the same in all humans?
NOPE because when you have collateralization and redundant pathways some of them can be missing without consequences
What is the major mechanism of control of the global cerebral blood flow?
Autoregulation
In what range of pressures does autoregulation of the global CBF work? How is this range affected in chronic HT patients?
50-150 mmHg MAP
Chronic HT patients: higher range
Other than through autoregulation, what other 2 mechanisms work to regulate global CBF?
- Chemical control
2. Functional vasodilation
Describe the chemical control of global CBF. What to note?
- Levels of arterial CO2 regulate vasodilation/constriction to increase/decrease blood flow locally
Note: this mechanism will normally be irrelevant since arterial CO2 is generally kept constant - Blood levels of PaO2 below 50 mmHg => CNS ischemic reflex in response to cerebral ischemia = INTENSE SNS outflow to maintain CBF because there are no sympathetic nerve endings on cerebral vessels
Does the chemical control of global CBF respond to PaO2 above 50 mmHg?
NOPE
What is Cushing’s reflex? In what patients in this often seen? How is HR affected?
High intracranial pressure => poor perfusion of the brain => SNS activation to raise arterial pressure to perfuse the brain (could make the issue a lot worse, but it’s a last ditch effort)
Often seen in head trauma patients
Low HR because the baroreceptor reflex is responding to the very high arterial pressure caused by the Cushing’s reflex
How does hyperventilation affect CBF? What does this explain?
Hyperventilation => decrease in CO2 levels => reduced CBF => fainting
(explains why people who are panicking breathe into a paper bag to make sure their CO2 levels do not drop and the CBF does not fall)
Describe functional vasodilation in the cerebral circulation. What to note?
FUNCTIONAL VASODILATION is active in the cerebral circulation on a local level: there are differences in flow patterns in the brain during different neuronal activities due to varying O2 requirements
BUT global CBF is still constant
AND different mechanism than metabolic regulation in other organs that is much faster
During aerobic exercise, blood flow to the left ventricular myocardium would be expected to increase, mainly due to…
Metabolic vasodilation
How will aortic stenosis impact the O2 supply demand balance of the coronary circulation?
Negatively because decreased supply
