Coronary Circulation Flashcards
What are the major branches of the coronary arteries?
- Right coronary artery: has PDA branch in 80% of people
- Left coronary artery: bifurcate to LAD with diagonal branches and left circumflex with obtuse marginals
- in 37% of population ramus intermedius trifurcates
What is the ramus intermedius? In what percentage of the population does it exist?
37%
It’s an additional branch in people who have a trifurcation in the left main coronary artery
Do the major epicardial arteries contribute significantly to coronary vascular resistance?
No
Only contribute a small % of resistance, but intramyocardial arterioles contribute most to total coronary vascular resistance
What is capillary density? Is capillary density increased or decreased in the myocardium?
A high capillary-to-cardiomyocyte ration—> shortens diffusion distance and ensures adequate O2 delivery/waste removal
- capillary density is increased in the myocardium
What are the major determinants of myocardial oxygen demand?
HR
Contractility
Systolic wall tension
What are the major determinants of myocardial oxygen supply?
- vascular resistance
- coronary blood flow
- O2 carrying capacity
Is resting O2 consumption of the heart high or low relative to other organs in the body?
High—> 75% of O2 delivered
What is the formula for coronary perfusion pressure?
Coronary perfusion pressure= DBP - LVEDP (or PCWP)
Is O2 highly extracted from blood flowing through the heart?
Yes, the heart extracts O2 to a greater extent than any other organ
- increased O2 demand must be met by increasing coronary blood flow (can only minimally increase O2 extraction)
What factors control coronary blood flow?
- Metabolic control
- autoregulation: 50-150 for MAP
- endothelial control of vascular tone
- extravascular compressive forces
- neural control
Does the majority of coronary blood flow occur during systole or diastole in the left ventricle? Why?
- diastole: extravascular compression of cardiac muscle squeezing during systole decreases blood flow drastically
Which layer of the myocardium is at greatest risk for ischemia?
Subendocardium (inner layer)d/t extravascular compression, especially at low perfusion pressures
- not as affected during diastole
What is coronary flow reserve?
Maximal flow capacity - baseline blood flow
—> difference between baseline blood flow and maximal blood flow
- usually max flow is 4-5 times greater than at rest
- reserve decreases with CAD
Which of the following places a greater O2 cost on the heart? Pressure work vs volume work
Pressure work is more costly since it increases after load and makes the heart work much harder
- pressure work: increased arterial pressures at constant CO (very hard with tachycardia too)
- volume work: increasing CO while maintaining constant pressure
How stenotic do coronary vessels have to be before there is a significant decrease in flow?
Critical stenosis = 60-70% reduction in diameter of the large distributing artery
- coronary arteries ~75% stenosis is significant
What is the final intracellular ion disturbance that leads to impaired myocardial contraction and cell death?
Decreased Na/Ca exchange causes intracellular Ca++ overload, which causes impaired myocardial contraction and CELL DEATH
What are the effects of myocardial ischemia on systolic function?
Loss of inotropy**
- causes downward starling curve—> results in decreased stroke volume and compensatory increase in preload, because of incomplete ventricular emptying
—> increase in EDV and pressure eventually dilates the ventricle
What are the effects of myocardial ischemia on diastolic dysfunction?
LVEDP rises, relaxation is impaired
Myocardial compliance decreases
Ventricular hypertrophy
* impairs the heart’s ability to relax
What is myocardial stunning?
Occurs after brief episode of severe ischemia
- prolonged myocardial dysfunction with gradual return of contractile activity
- completely reversible
- less responsive to inotropy—> can result in cardiogenic shock
What is myocardial hibernation?
Regional cardiac dysfunction d/t chronic ischemia
- impaired resting LV function —> if coronary blood flow is restored the region will regain normal function
What are some of the consequences of myocardial ischemia?
- systolic and diastolic dysfunction
- angina
- CHF or pulmonary edema
- arrhythmias
- MI
- ventricular rupture or VSD
- cardiogenic shock
- death
What are some drugs used for the treatment of ischemia?
- O2
- Ăź- blockers: blunt SNS
- nitrates: decrease SVR—> vasodilation
- anti-platelet/anti-coags.
- analgesics: morphine
- Ca++ channel blockers
What are some of the interventions used in the treatment of ischemia?
- CABG
- percutaneous coronary intervention
- ballon angioplasty
- bare metal or drug eluding stents
How long should you wait before doing elective surgery on a patient who has had a drug eluting stent placed?
- drug eluting stent: waiting 1 year is recommended d/t stopping anticoagulation meds
- bare metal stent: cardiac complications are lowest after 90 days
What are some of the drugs that are used to reduce cardiac ischemic events during surgery?
- volatile anesthetics—> anesthetic preconditioning
- Ăź-blockers: have pt continue taking
- statins: stabilize plaque
- anti-inflammatory
- alpha - 2 agonists
- Ca++ channel blockers
Is isoflurane an appropriate agent to use during cardiac surgery? Why or why not?
Yes
Minimal cardiac effects
What are collateral blood vessels in the heart? How are they formed?
In response to chronic slow developing occlusion—> new vessels for between branches of occluded and non-occluded arteries
- originate from pre-existing arterioles that undergo changes of endothelium and smooth muscle
- vascular endothelial growth factor
- monocytes chemoattractive protein
What is anesthetic preconditioning in the heart?
Anesthetics have effect that mimics ischemic preconditioning
- K+ atp channels play an important role
What parts of the heart does the LCA supply?
Anterior and left lateral portion of the LV
What part of the heart does the right coronary artery supply?
Most of RV and posterior part of the heart in 80-90% of people
Where does 75% of total coronary blood flow from the heart muscle return to?
The coronary sinus
How is the remaining coronary blood returned to the heart?
Small anterior cardiac veins that flow directly into the RA
- a very small amount of RV blood blows back to heart through tiny thespian veins that empty into all chambers of the heart
What is normal coronary flow and how does it change during exercise?
Normal coronary flow is ~225 mL/min (4-5% of CO
- increases 3-4 fold during exercise
Epicardial coronary arteries supply most of the muscle of the heart.
T/F
True
In all vascular bed (including heart tissue), where are the primary sites of vascular resistance?
Small arteries and arterioles
—> primary site for regulation of blood flow
Does the heart utilize anaerobic metabolism?
The heart has very little capacity for anaerobic metabolism
- normally it consumes lactate
- myocardial lactate production is a sign of severe ischemia
What percent of O2 delivered to the heart does it consume at rest?
75%
- has no significant reserve because of this
How many mL/min of blood does cardiac muscle consume?
9.7mL/100G/min blood
Blood flow through coronary system is regulated mostly by what?
Local arterial vasodilation in response to needs of cardiac muscle
- as need increases, flow increases
- as need decreases, flow decreases
Coronary circulation is exquisitely sensitive to __________________________________.
Myocardial O2 tension
Increasing O2 demand results in higher or lower tissue O2 tension?
Lower
How does low tissue O2 tension cause vasodilation and increased blood flow?
From the release of:
- adenosine
- nitric oxide
- prostaglandins
- K+ atp channels
Damage to endothelial cells leads to:
- decreased NO and prostacyclin production (stops vasodilation)
- increased endothelium production—> vasoconstriction
- Will lead to vasoconstriction, vasospasm, thrombosis **
What are the different effects of neural control?
- direct effects: SNS stimulation, epi and norepinephrine—> increaseHR, inotropy and cardiac metabolism
- indirect: dilates coronary vessels increasing flow(since metabolism increases)
- contrast: vagal stimulation (from acetylcholine release) slows HR, slight decrease in inotropy
- decreases O2 consumption- indirectly constricts coronary arteries
What are the constrictor receptors?
What are the dilator receptors?
Constrictor: alpha receptors
Dilator: beta receptors
What is one of the worst things you can do to a pt with severe CAD?
Cause tachycardia—> they have no reserve
What does O2 consumption mean?
Volume of O2 consume/min/100g of tissue
What is meant by wall tension?
Tension generated by myocytes that results in given intravascular pressure at a particular ventricular radius
- late of la place explains wall tension. Pr/n
What are 2 settings that lead to myocardial supply and demand mismatch?
- profoundly low perfusion pressures
- irreversible stenosis
What are factors that increase myocardial O2 consumption?
- # 1 is tachycardia
- increased inotropy
- afterload
- preload (to a lesser extent than the others)
What is unstable angina caused by?
Thrombosis
What is meant by stenosis?
Abnormal narrowing of an artery or decreased cross sectional area of heart valve when opened
According to Pousielles law, decreasing the radius by 1/2 will increase resistance by _______.
16 fold
What do Ăź-blockers do?
Block sympathetic Ăź-adrenergic receptors, preventing increase in HR and cardiac metabolism
- decreases cardiac need for extra O2 during “stressful” conditions
What do Ca++ channel blockers do?
Vasodilation: increases coronary blow
Decrease HR
What happens if a CABG is done before severe damage occurs? After?
If a CABG done before severe damage occurs—> may provide pt with normal survival expectation
- if done after severe damage occurs the CABG is likely to be of little value
What to drug eluting stents do?
Slowly release drug and may help prevent excess growth of endothelial scar tissue that causes restenosis
What is myocardial preconditioning?
Phenomenon where an intervention or trigger before a prolonged ischemic insult to myocardium results in decrease in the infarcted area
