SM 134 Coronary Blood Flow Flashcards
What determines Oxygen Demand?
Hemodynamic factors such as:
Wall Stress
Heart Rate
Contractility
What is the Double Product?
An index of Oxygen Demand defined as:
HR * Peak Systolic BP
How does oxygen demand relate to coronary flow?
Changes in Myocardial demand require changes in coronary flow
What percent of coronary flow is used at rest in a healthy individual?
6%
What are the components of Resistance to coronary flow?
Total coronary resistance is the sum of:
Condiut Artery Resistance (R1) Microcirculatory Resistance (R2) Compressive Resistance (R3)
What is Conduit Artery Resistance?
Resistance in the Conduit Arteries found on the surface of the heart
Normally small and affected by endothelial and autonomic factors as well as atherosclerosis
Where are Conduit Arteries found?
Conduit Arteries are found on the surface of the heart
How are Conduit Arteries, Microcirculation, and the Muscle of the heart related?
Conduit Arteries on the surface of the heart project into the muscle of the heart using Microcirculation Arteries
What is Microcirculatory Resistance?
Primary mechanism by which flow adjusts to demand and how flow is kept constant when arterial pressure changes
Normally high, relates the caliber of arterial microvessels and open capillaries
What is Poiseuille’s Law Significance?
Says resistance is inversely proportional to radius to the fourth power
How is R2 regulated?
On a local basis, at each given artery branch
What types of factors regulate R2?
R2 is regulated by Metabolic, Endothelial, and Neurohumoral factors
Which Metabolic Factors regulate R2?
Metabolic regulators of R2 include:
Adenosine, PO2
Which Endothelial Factors regulate R2?
Endothelial regulators of R2 include:
EDRF/Nitric Oxide
Endothelial Derived Hyperpolarizing Factor (EDHF)
Prostacyclins
Endothelins
What is EDRF?
Endothelial Derived Relaxing Factor = Nitric Oxide
Made continuously in Endothelial cells, rate varies directly with flow
Vasodilator
How does coronary flow related to EDRF levels?
EDRF/Nitric Oxide levels scale directly with flow
What is EDHF?
Endothelial Derived Hyperpolarizing Factor = EDHF
Flow-induced vasodilation
What are Prostacyclins?
Continuously made by the COX pathway
Vasodilators
Which Neurohumoral agents regulate R2?
Autonomic Nervous System and circulating vasoactive agents, alpha adrenergic vasoconstriction and beta adrenergic vasodilation
What effects do alpha adrenergic agonists have on circulation?
Alpha = vasoconstrict
What effects do beta adrenergic agonists have on circulation?
Beta = vasodilate
What causes Compressive Resistance (R3)?
Compression of coronary blood vessels during Systole
How does Compressive Resistance set the perfusion of the heart?
Compressive Resistance rises so greatly in Systole that perfusion of the heart during Systole is a fraction of what occurs during Diastole
How does Compressive Resistance vary?
Compressive Resistance is greatest in the innermost portion of the ventricular wall and decreases across the wall
How does Compressive Resistance affect Microcirculatory Resistance?
Compressive Resistance is greatest in the innermost portion of the ventricular wall, which forces Microcirculatory Resistance to compensate by decreasing via vasodilation to ensure adequate blood flow
What is Coronary Autoregulation?
Idea that Coronary Microvascular Resistance adjusts to keep flow at the level appropriate to Myocardial demand
What is Reactive Hyperemia?
Increase in coronary flow that follows a brief period of coronary artery occlusion
What is the Coronary Flow Reserve?
Ratio of flow during Maximum coronary vasodilation to flow during resting conditions
To what extent can coronary flow increase during periods of demand?
Coronary flow can increase via the coronary flow reserve to 4-5 times its resting value
What is the “Bruce” Protocol?
BP and HR measured over 4 stages at the end of each 3 minute stage, with gradually increasing speed on a treadmill
Increased speed = More METS
Monitor EKG continually
How is the increase in coronary flow calculated at each stage of the Bruce protocol?
Increase in coronary flow is calculated by taking the ratio of Double Product (SBP * HR) at the end of the stage to the Double Product at the beginning, to infer what factor of vasodilation had to occur to support the activity
What findings on a stress test indicate that the coronary reserve has been exhausted?
Development of Chest Pain and/or ST segment depression on an EKG indicate coronary reserve has been exhausted
Compare and contrast Imaging to Exercise testing?
Imaging can reveal regional limitations in flow reserve, but exercise testing can provide quantitative values for flow reserve
Use both together
When would cardiac radionuclide imaging be performed?
In patients who cannot be physically active enough for an exercise stress test
How does cardiac radionuclide imaging work?
Adenosine is infused via IV so that a cardiac radionuclide imaging procedure can be performed with maximum Microvascular dilation
Identifies differences in regional flow, but like all imaging studies, cannot provide quantitative values of flow
What are endothelium independent components of coronary flow reserve?
Increased metabolic demand of exercise and vasodilatory effects of adenosine dilate microvasculature without Endothelial cell contributions
What are endothelium dependent components of coronary flow reserve?
Increases in production of Nitric Oxide and other endothelial cell vasodilators affect microvasculature resistance
Which component of coronary flow reserve is affected by disease?
Endothelium dependent component of microvasculature resistance is reduced by diabetes, hypercholesterolemia, and smoking
Reduces maximum flow response to exercise or adenosine
What are the causes of Angina Pectoris and Myocardial Ischemia?
Imbalance between O2 demand and O2 supply:
Inability to increase coronary flow sufficiently when O2 demand rises = exertional angina
Primary reduction in O2 supply from thrombus formation or vasoconstriction of a stenosed epicardial artery
What is the effect of a coronary artery stenosis?
Abnormal increase in resistance
Why does the pressure drop across a stenosed artery decrease alinearly?
Due to separation losses
How can coronary flow be maintaned at rest with stenosis?
Increased conduit resistance offered by stenosis is compensated for by microvascular vasodilation, at the cost of coronary reserve flow
How does coronary flow reserve change with increasing stenosis?
As the severity of stenosis increases, microvascular coronary flow reserve is exhausted producing exertional angina
Why is the subendocardium most susceptible to ischemic injury?
Microvascular coronary flow reserves are exhausted first here, so ischemia damages the subendocardium first
How can coronary collateral flow develop?
Develops frequently as stenosis becomes critical and offers modest degree of perfusion to areas in jeopardy
What are the goals of treating obstructions to coronary blood flow?
Limit increases in Myocardial O2 demand and augment flow to ischemic areas
How can increases in Myocardial O2 demand be limited?
Minimize increases in systolic blood pressure, heart rate
How can flow to ischemic areas be augmented?
Counter increases in stenosis severity, endothelial dysfunction, and abnormal vasoconstriction
Increase collateral flow or use PTCA/CABG
Which component of coronary resistance is most readily manipulated?
R2 = Microvascular resistance; can contract or dilate vessels easily
In deeper layers of the heart, such as the subendocardium, how do the components of coronary resistance change?
In the subendocardium and other deep layers, compressive resistance is high while microvasculature resistance is low to compensate; conduit resistance stays constant
How do abnormalities alter the slope of the Coronary Flow vs Coronary Pressure graph?
Abnormalities such as hypertrophy and tachycardia decrease the slope, reflecting a lower coronary reserve
What are the major factors that affect the extent of pressure decrease across a stenosis in an artery?
Degree of stenosis, flow rate and length of stenosis all increase the pressure decrease due to a stenosis
What is the %stenosis cutoff for serious loss of flow?
70% stenosis leads to serious loss of flow that can cause ischemic damage during periods of activity
90% stenosis borders on insufficient for resting function
By what factor does coronary flow need to expand to during exercise?
Coronary flow rises to approximately 3x the resting value
