SM 134a - Coronary Blood Flow Flashcards
What is the “double product?”
How is it calculated?
The double product estimates the oxygen needs of the heart.
Double product = Systolic blood pressure * Heart Rate
What is R1, as it relates to coronary blood flow?
What governs it?
R1 is the resistance in conduit arteries
It is ususally small, and governed by endothelial and autonomic factors
What is R2, as it relates to coronary blood flow?
What governs it?
Microcirculatory (aka autoregulatory) resistance in vessels usually <200 uM. R2 is the primary mechanism for maintaining the balance of oxygen supply and demand in the myocardium
- Metabolic factors predominate at rest
- Endothelial and neurohormonal factors are activated during exercise
What is R3, as it relates to coronary blood flow?
What governs it?
R3 is compressive resistance
It is governed by the compression of coronary blood vessles during systole
What is the primary governer of resistance to blood flow in the endocardium?
R3: compressive resistance
There are more R3 mechanisms in the endocardium than in the epicardium
What is the primary governer of resistance to blood flow in the epicardium?
R2: Microcirculatory resistance
There are more R2 mechanisms in the epicardium than in the endocardium
Under basal conditions, is R2 in the myocardium high or low?
Why is this important?
At rest, R2 is high.
This means there is a lot of “stretch” reserved for vasodilation if the heart needs it (i.e. during exercise)
What metabolic factors govern R2?
When do they predominate?
Metabolic factors adenosine and PO2 govern R2 during rest
What endothelial factors actively impact R2 during exercise?
-
Nitric Oxide (aka Endothelial-derived relaxing factor, EDRF)
- Greatest impact in vessels <100 um
- Increased flow = increased synthesis
-
Endothelial derived hyperpolarizing factor (EDHF)
- Increased flow = increased synthesis
-
Prostacyclins
- Synthesized continuously (via the cyclooxygenase pathway
What neurohormonal factors actively govern R2 during exercise?
- Autonomic nervous system
- Alpha-adrenergic constriction to non-essential capilary beds
- Beta-adrenergic dilation to essential capillary beds (ex: the heart)
- Vasoactive agents
Which resistance mechanism is the primary mechanism for maintaining oxygen supply and demand in the heart?
R2: Resistance in microcirculatory (<200 uM) vessels
What is endothelial-derived hyperpolarizing factor (EDHF)?
When is it synthesized?
A vasodilator synthesized by endothelial cells during periods of increased flow (ex: during exercise)
It decreases R2, allowing for increased blood flow to the endocardium
What is Endothelial-Derived Relaxing Factor (EDRF)?
When is it synthesized?
EDRF is nitric oxide. It is synthesized by endothelial cells (especially coronary vessels between 100-200 uM) during periods of increased flow (ex: during exercise)
It decreases R2, allowing for increased blood flow to the endocardium
Where is R2 the greatest?
Epicardium, during rest
Where is R3 the greatest?
Endocardium