In this article we will discuss about the features and measurement of coronary blood flow in humans.
Features of Coronary Blood Flow:
The following summarizes important features of coronary blood flow:
i. Flow is tightly coupled to oxygen demand. This is necessary because the heart has very high basal oxygen consumption (8-10 ml O2/min/100 g) and the highest A-VO2 difference of a major organ (10-13 ml/100 ml). In non-diseased coronary vessels, whenever cardiac activity and oxygen consumption increases, there is an increase in coronary blood flow (active hyperemia) that is nearly proportionate to the increase in oxygen consumption.
ii. Good auto-regulation between 60 and 200 mm Hg perfusion pressure helps to maintain normal coronary blood flow whenever coronary perfusion pressure changes due to changes in aortic pressure.
iii. Adenosine is an important mediator of active hyperemia and auto-regulation. It serves as a metabolic coupler between oxygen consumption and coronary blood flow. Nitric oxide is also an important regulator of coronary blood flow.
iv. Activation of sympathetic nerves innervating the coronary vasculature causes only transient vasoconstriction mediated by α1-adrenoceptors. This brief (and small) vasoconstrictor response is followed by vasodilation caused by enhanced production of vasodilator metabolites (active hyperemia) due to increased mechanical and metabolic activity of the heart resulting from β1-adrenoceptor activation of the myocardium.
Therefore, sympathetic activation to the heart results in coronary vasodilation and increased coronary flow due to increased metabolic activity (increased heart rate, contractility) despite direct vasoconstrictor effects of sympathetic activation on the coronaries. This is termed “functional sympatholysis.”
v. Parasympathetic stimulation of the heart, (i.e. vagal nerve activation) elicits modest coronary vasodilation (due to the direct effects of released acetylcholine on the coronaries). However, if parasympathetic activation of the heart results in a significant decrease in myocardial oxygen demand due to a reduction in heart rate, then intrinsic metabolic mechanisms will increase coronary vascular resistance by constricting the vessels.
vi. Progressive ischemic coronary artery disease results in the growth of new vessels (termed angiogenesis) and collateralization within the myocardium. Collateralization increases myocardial blood supply by increasing the number of parallel vessels, thereby reducing vascular resistance within the myocardium.
During contraction of the ventricular myocardium (systole), the subendocardial coronary vessels (the vessels that enter the myocardium) are compressed due to the high intraventricular pressures. However, the epicardial coronary vessels (the vessels that run along the outer surface of the heart) remain patent. Because of this, blood flow in the subendocardium stops. As a result most myocardial perfusion occurs during heart relaxation (diastole) when the subendocardial coronary vessels are patent and under low pressure.
In the presence of coronary artery disease, coronary blood flow may be reduced. This will increase oxygen extraction from the coronary blood and decrease the venous oxygen content. This leads to tissue hypoxia and angina which is intense chest pain. Severe ischemia can cause the heart muscle to die from hypoxia, such as during a myocardial infarction.
If the lack of blood flow is due to a fixed stenotic lesion in the coronary artery (because of atherosclerosis), blood flow can be improved within that vessel by:
1. Placing a stent within the vessel to expand the lumen.
2. Using an intracoronary angioplasty balloon to stretch the vessel open.
3. Bypassing the diseased vessel with a vascular graft. If the insufficient blood flow is caused by a blood clot (thrombosis), a thrombolytic drug that dissolves clots may be administered. Anti-platelet drugs and aspirin are commonly used to prevent the recurrence of clots.
If the reduced flow is due to coronary vasospasm, then coronary vasodilators can be given (e.g. nitrodilators, calcium channel blockers) to reverse and prevent vasospasm.
Measurements of Coronary Blood Flow:
1. Kety Method:
This method is based on Fick’s principle.
It states that the amount of a substance taken up by an organ (or by the whole body) per unit of time is equal to the arterial level of the substance minus the venous level (A – V difference) times the blood flow.
Amount of substance taken/min. = (A – V difference) × blood flow/min
Blood flow/min = Amt. of substance taken/min/(A – V) difference of the substance
The subject inhales a mixture of air and an inert gas till the gas has distributed itself in tissues in accordance to partition coefficient. Arterial blood sample can be collected from any peripheral artery. Venous blood sample can be collected from any coronary sinus. Coronary blood flow is calculated as the ratio of the amount of the inert gas passing through the coronary arteries in unit time to the time—integrated average (A-V) gas concentration difference. Inert gases suitable are N2O radio-nucleotide, hydrogen and helium.
2. Radionuclide Utilization Technique:
Radioactive tracers that can be detected with scintillation cameras over the chest have been used to study regional blood flow in the heart and to detect areas of ischemia and myocardial infarction.
A suitable radionuclide such as radioactive thallium (201Th) is injected. Gamma cameras are placed in front of the (heart) chest for monitoring 201Th uptake by the heart. The uptake proportional to blood flow. So areas of ischemia can be detected by their low uptake.
Conversely, radiopharmaceuticals, e.g. technetium 99m stannous pyrophosphate (99mTc-PYP) are selectively taken up by infarct tissue and make infarcts stand at as Hot Spots on scintiscans of the chest.
3. Coronary Angiography:
This can be combined measurement of 133Xe washout to provide detailed analysis of coronary blood flow. Radiopaque contrast medium is first injected into the coronary arteries, X-rays being used to outline their distribution. The angiographic camera is thus replaced with a multiple-crystal scintillation camera and 133Xe wash out is measured.
4. Direct Measurement:
Flow can be measured directly by placing an electromagnetic flow meter around a coronary artery. This method can be used in man only during open heart surgery.