Circulation 7 Flashcards
Breakdown of coronary artery dominance in humans
Right-sided dominant: 50%
Left-sided dominant: 20%
Non-dominance: 30%
Ratio of myocardial capillaries to myocytes
1:1 (not usually all active simultaneously)
When are more myocardial capillaries likely to be recruited?
During episodes of ischemia
What is the primary determinant of coronary blood flow?
Aortic pressure
Primary regulation of coronary blood flow is due to wat?
Metabolic activity and changes in arteriolar resistance
Highest myocardial tissue pressures occur when, and what does this cause?
During early systole; left coronary blood flow may actually reverse during systole
When does maximal left coronary blood flow occur?
Early diastole, when tissue pressure falls to approximately 0
Percentages of diastolic vs systolic perfusion of myocardium
60-65% occurs during diastole, 25-30% during systole
Which layer of heart tissue is the ventricular diastolic pressure greatest?
In the endocardium; least near the epicardium
Consequences of endocardium enduring greater pressure
More likely to be compressed; more likely to undergo ischemic event than the epicardium
Regulation of endocardial vessels under normal conditions
Due to larger compression forces, the vessels are more dilated to accommodate
Diseases that can cause subendocardial infarction
Aortic valve stenosis
Aortic valve regurgitation
Congestive heart failure
Consequences of decreased coronary blood pressure
Endocardial blood flow will be restricted more than epicardial blood flow because of the normally greater endocardial tissue (occurs during sever hypotension or partial coronary occlusion)
Sympathetic adrenergic stimulation activates what receptors?
Alpha receptors in the coronaries, inducing weak vasoconstriction
Beta-1 adrenergic receptors - location in the heart and effect
Present on pacemaker cells and myocardium. During sympathetic stimulation, coronary vasodilation will override alpha receptors due to increases in metabolism
Beta-2 adrenergic receptors - location in the heart and effect
Coronary smooth muscle; mediate vasodilation, but are less sensitive to sympathetic stimulation
Vagal stimulation can be blocked by __ suggesting . . .
atropine; an acetylcholine effect is probably mediated by release of nitric oxide from endothelial cells
__ is the major factor in the regulation of coronary blood flow
Metabolism
Graphical relationship of coronary blood flow and myocardial metabolic activity
Linear relationship
Metabolic substrates for the heart
- Fatty acids - 60%
- Carbohydrates - 35-40%
- Others (ketones, lactate, proteins)
How is oxygen supply to the heart limited?
Flow-limited, because most of the oxygen (~80%) is removed from blood during its one passage through the heart
Oxygen consumption is directly related to the __ of the heart.
work
Cardiac work equation
Mean arterial pressure x systolic stroke volume
__ work consumes much more oxygen than __ work
Pressure, volume
Myocardial oxygen supply equation
Myocardial blood flow x arterial oxygen content
Myocardial blood flow is dependent upon . . .
Diastolic perfusion pressure and coronary vascular resistance
Myocardial oxygen demand depends on these three things
Afterload, heart rate, and contractility
Myocardial ischemia results from this
An imbalance in the ratio of oxygen supply to oxygen demand, creating a relative lack of blood flow
This is NEVER a primary cause of ischemia
Excessive oxygen demand
Collateral circulation
Normal human heart has collateral vessels, but they are insufficient to prevent sudden infarction; can respond to gradual obstruction
Process of gradual collateral circulation
Involves a complex sequence of vessel injury, inflammation, and cellular proliferation
Coronary “steal”
Under certain conditions, an increase in blood flow in one region of the heart can cause a decrease in blood flow in another region
No coronary “steal”
Arteriole resistance in the non-obstructed vessel vasodilates to keep the same amount of resistance in both vessels, keeping a good flow through the obstructed vessel as well
When does coronary steal usually become an issue?
During exercise, when the non-obstructed vessel becomes vasodilated and the obstructed vessel remains at a higher resistance
Clinical manifestations of coronary steal
- Exercise-induced ischemia
- Stress-testing with adenosine
- Peripheral arterial disease that causes caludication
How much cardiac output goes to skeletal muscle during rest?
20% (muscle makes up 40% of body mass)
What is the resting flow of skeletal muscle as compared to brain and kidneys?
3 mL/min per 100g, compared to 50 and 400 mL/min per 100g
What does skeletal muscle’s large flow reserve indicate?
Vasculature in resting muscle has a high degree of vascular tone
What happens to the vessels with dynamic exercise?
Alternation between extravascular compression and free flow, but overall, there is a profound reduction in resistance to blood flow due to metabolic vasodilation
Consequences of isometric tensions of ~70% of max or above
Blood flow through the contracting muscle approaches zero, so the tensions cannot be sustained for more than 1-2 min
Innervation of skeletal muscle arteries
Sympathetic cholinergic receptors that induce vasodilation by acting on mucarinic receptors coupled to NO production
Epinephrine with vascular control
Causes vasodilation at low concentrations through beta-2 adrenergic receptors, but at high concentrations, binds to alpha adrenergic receptors and vasoconstricts
