Haemodynamics RF (1).pptx 2.pptx

Haemodynamics Dr Robert Formosa Definition Heamodynamic s is the study of the physical and physiological principles governing the movement of blood through the circulatory system Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in...

Haemodynamics Dr Robert Formosa Definition Heamodynamic s is the study of the physical and physiological principles governing the movement of blood through the circulatory system Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Blood circulation Left ventricle contracts Aorta Elastic arteries Muscular arteries Arterioles Capilleries Venules Large veins Vena Cava Right aorta – Pulmonary circulation Blood Vessel Composition Definitions: Blood pressure is pulsatile Systole=contraction of heart • Systolic blood pressure= maximum pressure in arteries. • At brachial artery systolic pressure normally 120 mmHg (16kPa). Current guidelines are: • >140 mm Hg systolic marginal hypertension. • >160 mm Hg definite intervention threshold. Diastole=relaxation of heart • Diastolic blood pressure= minimum pressure in arteries. • At brachial artery systolic pressure normally 80 mmHg (10.7kPa). • >90 mm Hg diastolic marginal hypertension. • >100 mm Hg definite intervention threshold. Pulse pressure= difference between systole and diastole. • As arteries are elastic pulse pressure normally decreases slightly from aorta to brachial artery. Pulse pressure • Pulse pressure is (systolic - diastolic) pressure. • Mean arterial pressure (MAP) is calculated as diastolic plus 1/3 (one third) pulse pressure. • So for a systolic of 120 mmHg and a diastolic of 90 mmHg, the MAP is 100 mmHg. (90 + 1/3 of 30) • Remember how to calculate MAP! Hypertension (abnormally high blood pressure) • Hypertension: an important risk factor for vascular disease. • Prehypertension is now defined • a person's blood pressure is elevated above normal, not to the level requiring medication. • Prehypertension: a systolic pressure from 120 to 139 mm Hg or a diastolic pressure from 80 to 89 mm Hg. Doesn’t have to be both as damage happens regardless. • Hypertension is a problem partly because the person is often unaware anything is wrong. • Approximately 30% of hypertensive adults are completely unaware of their condition • up to 40% of people with hypertension are not receiving treatment • of those treated, up to 67% do not have their blood pressure controlled to less than 140/90 mm Hg. Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Terms: • Pulse pressure : systolic – diastolic pressure (around 40mmHg) • Stroke Volume: the volume of blood pumped by the left ventricle per beat/contraction (typically around 70ml) • Compliance : ‘elasticity’ Compliance • The pulse pressure is ‘smoothed out’ in the small arteries and arterioles due to their stretchiness or COMPLIANCE. • Compliance is due to elastin fibres in the arterial walls. • COMPLIANCE IS GOOD (in arteries as well as bronchioles)! • This stretchiness reduces the work of the heart in pumping the blood as some of the blood is stored in the large arteries by them stretching and increasing their volume. The Windkessel Effect • The effect of the compliance of the small elastic arteries is often called the “Windkessel effect” • This is because it is similar to the effect of having an air filled ‘buffer’ chamber called a Windkessel in a water pump. • This chamber evens out the flow of water in a sprayer. The Windkessel Effect • The effect of the compliance of the small elastic arteries is often called the “Windkessel effect” • This is because it is similar to the effect of having an air filled ‘buffer’ chamber called a Windkessel in a water pump. • This chamber evens out the flow of water in a sprayer. The Windkessel Effect • The walls of the aorta and elastic arteries • distend when the blood pressure rises during systole • recoil when the blood pressure falls during diastole. • There is a thus net storage of blood during systole which discharges during diastole. • The distensibility of the large elastic arteries is therefore analogous to a capacitor. Compliance • When people get old (or smoke) their arteries loose some of their elastin which is replaced by collagen. • The arteries loose elasticity or “harden”. • This INCREASES the systolic pressure as the aorta cannot stretch to accommodate the stroke volume. Compliance and age Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Poiseuille's law • The flow of liquid (Q) through a tube is controlled by: • Pressure (P) • Radius of tube (r; to the power of 4) • Length of tube (l) • Viscosity (η) • Viscosity, radius and length of tube collectively contribute to the resistance of flow. Resistance to flow • As blood flows, it encounters various factors that resist flow and movement of blood, known as the vascular resistance. • Resistance to flow is caused by frictional forces within the fluid, and depends on the viscosity of the fluid and the dimensions of the tube – both length and diameter. Vasoconstriction or Vasodilation • In other words if you keep pressure constant but double the radius you increase flow by 24 (16) times the previous level. • In a rigid tube this equation applies to the flow at all times. p r 4 dP Flow= 8h L • In a tube with compliance the average flow is controlled by Poiseuille’s law. • Small increases in radius increase flow as Δr4. • Similarly small decreases in diameter reduce flow as Δr4. Atheroma • An ATHEROMA is a fatty deposit on the inside of an artery. • Due to Poiseuille’s Law even a small atheroma can dramatically decrease blood flow • This can render tissue hypoxic. • An advanced atheroma (right) would reduce blood flow to perhaps 5% of normal Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Cardiac Output • The total blood flow out of the heart (liters/min) is the cardiac output. Cardiac output is a key parameter in heart function. Measurement of C.O. to is essential to diagnose many heart problems. How can we measure cardiac output? Fluid flow through a pipe = Pressure/Resistance • So by analogy: C.O. = Blood pressure/Total peripheral resistance • We can easily measure BP but we cannot measure peripheral resistance- unfortunately we cannot measure C.O. this way! Cardiac Output C.O. = heart rate x stroke volume Doppler Ultrasound • -used to measure stroke volume and velocity. • The blood velocity through the first part of the aorta (left ventricular outflow tract) causes a Doppler shift in the frequency of the returning ultrasound waves – this can be used to measure velocity. • The cross-section area (diameter) of the tract can be measured by ultrasound, and thus flow (cardiac output) can then be obtained. • Doppler ultrasound is non-invasive, accurate and inexpensive; it has high levels of reliability and reproducibility. https:// www.youtube.com/watch? v=zs1TXChwEis&ab_chann el=ClariusMobileHealth Factors affecting CO Oxygen requirements of vital organs AT REST: • How is this distributed ? • Highest requirement at rest is for Liver and digestive system, kidneys and skeletal muscles. Vascular requirements of vital organs • What causes variations to this: 1. Digestive state: • When fasting, very little goes to the gut (probably less than 0.5 L/min) • After a meal, at least 1.5 L/min goes to the gut to allow for digestion, leaving only 1.2-1.5 L/min for the muscles and skin. 2. Temperature: • Blood flow through skin is extremely variable: essentially it can increase (via vasodilation) or decrease (via vasoconstriction) to allow other organs to have what they want. 3. Exercise • What mechanisms increase cardiac output during exercise? • heart rate increases (up to about 2.5 times resting) • stroke volume increases (up about 1.5 times, from about 70 ml to 100ml). • So a normally fit man could increase his resting output about (2.5 x 1.5), I.e. 3.75 times • 3.75 x 5 = (approx) 19 l/min = max • Increase to cope with increased oxygen demand An example of cardiovascular changes during exercise Details of changes- stroke volume • Note that her stroke volume reaches a plateau (100 ml) during moderate work and is unchanged even when she exercises at high intensity. • Ventricular filling is maintained despite an ever-shortening filling time; how is this done? • The stroke volume is maintained due to the contraction of the atria which help transfer blood into ventricles during diastole. • During exercise there is also increased ventricular contractility causing decreased end systolic (residual) volume. Details of changes- oxygen uptake • Note finally that her oxygen uptake has increased from 0.25 L/min to 2.5 L/min i.e. ten fold. This is also normal. • The oxygen uptake (VO2) actually increases more than cardiac output during exercise as relatively more oxygen is taken up by the lungs. • During exercise respiration rate and depth increases, increasing pulmonary PO2. • Pulmonary arterioles relax due to this increased PO2. • There is a small increase in pulmonary arterial pressure improving perfusion of the lungs, particularly the apices. • The lungs become more efficient at taking up oxygen as almost all blood becomes fully oxygenated (VO2 Max) Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Capillary structure • Capillaries have a small diameter (5 to 10 micrometres (μm)) • They have no smooth muscle in their walls. • Muscle would impede the exchange of fluids and gases • Thus they can withstand fluid pressures of >20 mm Hg using only the small tension generated by the basement membrane. Capillary structure Capillaries Blood flow • Capillaries may be 5 µm or less, smaller than the diameter (7 µm) of the red cells, and so the red cells have to bend or deform to get through. • One would expect this to mean the effective viscosity of the blood in the capillaries is high- in fact the viscosity is anomalously low. • The reason for this low viscosity is not known, but it means that the work the heart has to do to push the blood through the capillaries is reduced. • If the cells are too large or malformed (eg as in sickle cell disease) they clog up the capillary and oxygen delivery is compromised. • They may also rupture and cause haemolysis. <5 µm The issue with stagnant blood • When blood does NOT move through a vessel clots may form in the stagnant blood. • Atrial fibrillation • Most common type of arrhythmia • Common cause of clots in atria that travel through the body and can cause aneurysms • Similarly if blood is stagnant in leg veins there is a risk of deep vein thrombosis (DVT). Blood viscosity • The work of the heart depends on the viscosity (thickness) of blood as well as the diameter of the arterioles • The viscosity depends mainly on the haematocrit (proportion of red cells in blood by volume, normally ~45%). • If the haematocrit is too high, the viscosity is too high and the heart has to work much harder to pump the blood around the body. • On the other hand if the haematocrit is too low, not enough oxygen is transported. • The viscosity also depends on the mechanical properties (mainly the deformability) of the red cells (erythrocytes) The determinants of blood viscosity • The primary determinants of blood viscosity are haematocrit, red blood cell deformability, red blood cell aggregation, and plasma viscosity. • Haematocrit has the strongest impact on whole blood viscosity. • One unit increase in haematocrit can cause up to a 4% increase in blood viscosity. • This relationship becomes increasingly sensitive as haematocrit increases. Clinical conditions increasing viscosity • Polycythaemia – when a person has too many red blood cells their blood becomes more viscous • Sickle cell disease – malformed red blood cells that are inflexible can make blood more viscous • Macrocytosis – if blood cells are big they can make the blood more viscous Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Discuss capillary structure and blood viscosity • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Laplace’s Law • Arterioles have much thinner walls than arteries. This enables them to contract or relax efficiently. • How do they manage to sustain the arterial pressures with these thin walls? • The answer is Laplace’s Law Laplace’s Law • Laplace’s law states that the pressure that an elastic vessel can withstand depends on the tension produced in the walls by their elasticity divided by the radius • In a CYLINDER (eg a blood vessel) the pressure withstood is (diameter)toofT the vessel. proportional (tension)/ r P(withstood) =k( T / R) • Tension = Pressure X radius • The smaller the radius of a vessel, the greater the pressure that a given wall strength can withstand. Thus, smaller vessels with smaller radius can withstand greater pressure with thinner walls. Aneurysm • If an artery wall weakens or gets a tear, its radius increases and so the balancing pressure (from Laplace’s law) that the elastic tissue generates is less. • The wall balloons out and this further reduces the effectiveness of the wall to withstand the pressure. • Eventually an aneurysm may occur. The aorta is a common site for an aneurism Hypertension and aneurysm • This happens especially when the person has high blood pressure anyway. • So hypertension and artery disease are major risks factor for aneurysm formation. • There is often a local inflammation reaction in the atheroma which causes destruction of elastin fibres in the artery wall, weakening it • This making it more prone to stretch when there is a blood pressure spike. Intended learning objectives • Define the terms pulse pressure and mean blood pressure, and state values for these in the normal healthy young adult. • Explain the concept of arterial compliance, and describe the relationship between pulse pressure, stroke volume and compliance. • Comment on the importance of Poiseuille's law relating vessel radius and resistance to flow, and the relevance of this to changes in pressure in the circulation. • Explain the relationships between cardiac output, peripheral resistance and blood pressure. • Comment on the importance of Laplace's law relating vessel radius and pressure, and how this relates to aneurysm formation. Summary of haemodynamics LO1: • Pulse Pressure (PP)= Systolic pressure – Diastolic pressure • MAP = Diastolic Pressure + 1/3 of PP (80 + [1/3 *40] = 93.33mmHg) LO2: • Compliance is the elasticity of the blood vessels, calculated C = V/P • Compliance decreases with age • Compliance is responsible for the Windkessel effect that assures that blood keeps flowing between heartbeats. LO3: • Poisuille’s Law: blood flow is directly proportional to the pressure and radius4 and inversely proportional to the length of the vessel and the viscosity LO4: • Cardiac Output = BP/PR OR HR * SV (70bpm * 70ml = 5L/min) • Different organs have different vascular requirements depending on different physiological states (particularly exercise) LO5: • Blood has unexpected low viscosity which is composed mainly by haematocrit. LO6: