4. Myocardial Blood Flow and Oxygen Consumption Flashcards
What arteries supply perfusion to the heart?
Left and Right Coronary Arteries
What is the equation for coronary blood blow?
Coronary blood Flow = Perfusion Pressure / Resistance
What is perfusion pressure?
How is coronary blood flow related to perfusion pressure?
- The difference between top and bottom end pressures of coronary circulation
- Top end - diastolic arterial blood pressure = 80mmHg
- Bottom end - LV EDP = 8mmHg
- Perfusion Pressure = Diastolic Arterial BP - LV EDP
- Coronary blood flow is directly proportional to Perfusion Pressure
What is resistance of vessels determined by?
How is coronary blood flow related to resistance?
- Length of vessel (longer vessel = increased resistance)
- Viscosity of blood (more viscous = increased resistance)
- Size of arterial lumen (as r4, small change in radius gives large change in resistance)
- Coronary blood flow is indirectly proportional to resistance
What happens to the coronary arteries when the heart contracts?
What does this mean for coronary perfusion?
- Resistance of coronary arteries increases as they are squeezed until they are obstructed
- Coronary perfusion occurs in diastole
How does the oxygen demand of the heart change with exercise?
What implication does this have for coronary blood flow?
- Oxygen demand of the heart dramatically increases with exercise
- Require a way to increase coronary blood flow to increase oxygen supply during exercise
What is different about the heart’s blood supply compared to other organs?
How does the heart ensure it receives enough oxygen?
- It does not receive an excess blood supply i.e. only receives what it requires
- It extracts more oxygen from the blood it receives
What is the equation for Oxygen Delivery?
Oxygen Delivery = Arterial Oxygen Concentration x Coronary Blood Flow
What determines arterial oxygen concentration?
- Relatively little dissolved in the plasma
- Mainly determined by Oxygen bound to Haemoglobin (1 molecule can bind 4 Oxygen molecules)
What is a condition that can decrease Arterial Oxygen Concentration?
Anaemia due to decreased haemoglobin
What is the primary determinant of Oxygen Delivery?
As arterial oxygen concentration is normally constant, coronary blood flow is the primary determinant of Oxygen Delivery
What 4 effects regulate coronary blood flow?
- Mechanical effects (pressures in heart)
- Local effects (vasoconstrictor/dilator metabolites)
- Neural effects (ANS)
- Humoral effects (adrenaline)
Describe an aortic pressure trace.
- When the heart contracts and aortic valve opens, generate a peak pressure - Systolic BP (120mmHg)
- When heart begins to relax blood wants to flow back into heart but closure of aortic valve supports blood pressure - Diastolic BP (80mmHg)
Describe a left ventricular pressure trace.
- When the ventricle contracts, generate a peak pressure - Systolic pressure (120mmHg)
- When ventricle begins to relax there is no aortic valve to support pressure therefore pressure drops - LV EDP (8mmHg)
Describe a left ventricle to aorta pressure trace.
- Providing aortic valve opens normally, systolic pressures are the same
- Diastolic pressures are different as closure of aortic valve supports pressure in aorta
- Difference between Diastolic BP and LV EDP is the Coronary Perfusion Pressure
Where is the window for coronary flow?
What 3 things can change the window for coronary flow?
- Between diastolic BP and LV EDP
- Tachycardia disproportionately reduces diastole and therefore time for coronary perfusion
- Increased LV EDP (hypertension/heart failure) reduces perfusion pressure
- Decreased diastolic BP reduces perfusion pressure
Define autoregulation.
The ability of an organ to maintain a constant blood flow despite changes in perfusion pressure
How does autoregulation maintain coronary blood flow?
- If perfusion pressure decreases (blood loss) the heart responds by decreasing resistance of coronary arteries to maintain constant blood flow
- i.e. coronary blood flow is maintained despite continued reduction in perfusion pressure
What mediates the fall in resistance in autoregulation?
Local vasodilator metabolites
What is the evidence that local metabolites mediate regulation of coronary blood flow?
Hypoxia induces marked vasodilation in situ but not in isolated coronary artery, therefore must be local metabolites causing vasodilation
How does the heart increase coronary blood flow in hypoxia?
Produces metabolites (Adenosine, K+ ions, H+ ions, CO2, lactic acid) that stimulate vasodilation to decrease resistance and increase coronary blood flow
What receptors mediate coronary blood flow in larger and smaller coronary vessels?
- Larger have α-adrenoceptors that cause vasoconstriction
- Smaller vessels have β2-adrrnoceptors that cause vasodilation
What hormones can the heart release?
- Atria - Atrial Natriuretic Peptide (ANP)
- Ventricles - Brain Natriuretic Peptide (BNP)
What stimulates release of ANP and BNP?
- Stretching of myocardial fibres
- Increased atrial/ventricular pressures
- Volume overload
What are the main effects of ANP and BNP oon the kidney and vasculature?
- Increases renal excretion of Na+ (natriuresis) and water (diuresis)
- Decreases VSMC contraction (except in efferent arteriole to maintain GFR)
- Increases vascular permeability
What do ANP and BNP block the release/actions of?
- Aldosterone (normally increases Na+ and water retention
- Angiotensin II (normally causes vasoconstriction)
- Endothelin (normally causes vasoconstriction)
- ADH (normally increases water retention)
What are cardiac natriuretic peptides metabolised by?
Nepralysin (neutral endopeptidase)
