Hemodynamics

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Hemodynamics or haemodynamics are the dynamics of blood flow. Hemodynamics_sentence_0

The circulatory system is controlled by homeostatic mechanisms, just as hydraulic circuits are controlled by control systems. Hemodynamics_sentence_1

The haemodynamic response continuously monitors and adjusts to conditions in the body and its environment. Hemodynamics_sentence_2

Thus, haemodynamics explains the physical laws that govern the flow of blood in the blood vessels. Hemodynamics_sentence_3

Blood flow ensures the transportation of nutrients, hormones, metabolic waste products, O2 and CO2 throughout the body to maintain cell-level metabolism, the regulation of the pH, osmotic pressure and temperature of the whole body, and the protection from microbial and mechanical harm. Hemodynamics_sentence_4

Blood is a non-Newtonian fluid, best studied using rheology rather than hydrodynamics. Hemodynamics_sentence_5

Blood vessels are not rigid tubes, so classic hydrodynamics and fluids mechanics based on the use of classical viscometers are not capable of explaining hemodynamics. Hemodynamics_sentence_6

The study of the blood flow is called hemodynamics. Hemodynamics_sentence_7

The study of the properties of the blood flow is called hemorheology. Hemodynamics_sentence_8

Blood Hemodynamics_section_0

Main article: Blood Hemodynamics_sentence_9

Blood is a complex liquid. Hemodynamics_sentence_10

Blood is composed of plasma and formed elements. Hemodynamics_sentence_11

The plasma contains 91.5% water, 7% proteins and 1.5% other solutes. Hemodynamics_sentence_12

The formed elements are platelets, white blood cells and red blood cells, the presence of these formed elements and their interaction with plasma molecules are the main reasons why blood differs so much from ideal Newtonian fluids. Hemodynamics_sentence_13

Viscosity of plasma Hemodynamics_section_1

Normal blood plasma behaves like a Newtonian fluid at physiological rates of shear. Hemodynamics_sentence_14

Typical values for the viscosity of normal human plasma at 37 °C is 1.4 mN·s/m. Hemodynamics_sentence_15

The viscosity of normal plasma varies with temperature in the same way as does that of its solvent water; a 5 °C increase of temperature in the physiological range reduces plasma viscosity by about 10%. Hemodynamics_sentence_16

Osmotic pressure of plasma Hemodynamics_section_2

The osmotic pressure of solution is determined by the number of particles present and by the temperature. Hemodynamics_sentence_17

For example, a 1 molar solution of a substance contains 6.022×10 molecules per liter of that substance and at 0 °C it has an osmotic pressure of 2.27 MPa (22.4 atm). Hemodynamics_sentence_18

The osmotic pressure of the plasma affects the mechanics of the circulation in several ways. Hemodynamics_sentence_19

An alteration of the osmotic pressure difference across the membrane of a blood cell causes a shift of water and a change of cell volume. Hemodynamics_sentence_20

The changes in shape and flexibility affect the mechanical properties of whole blood. Hemodynamics_sentence_21

A change in plasma osmotic pressure alters the hematocrit, that is, the volume concentration of red cells in the whole blood by redistributing water between the intravascular and extravascular spaces. Hemodynamics_sentence_22

This in turn affects the mechanics of the whole blood. Hemodynamics_sentence_23

Red blood cells Hemodynamics_section_3

The red blood cell is highly flexible and biconcave in shape. Hemodynamics_sentence_24

Its membrane has a Young's modulus in the region of 106 Pa. Hemodynamics_sentence_25

Deformation in red blood cells is induced by shear stress. Hemodynamics_sentence_26

When a suspension is sheared, the red blood cells deform and spin because of the velocity gradient, with the rate of deformation and spin depending on the shear-rate and the concentration. Hemodynamics_sentence_27

This can influence the mechanics of the circulation and may complicate the measurement of blood viscosity. Hemodynamics_sentence_28

It is true that in a steady state flow of a viscous fluid through a rigid spherical body immersed in the fluid, where we assume the inertia is negligible in such a flow, it is believed that the downward gravitational force of the particle is balanced by the viscous drag force. Hemodynamics_sentence_29

From this force balance the speed of fall can be shown to be given by Stokes' law Hemodynamics_sentence_30

Where a is the particle radius, ρp, ρf are the respectively particle and fluid density μ is the fluid viscosity, g is the gravitational acceleration. Hemodynamics_sentence_31

From the above equation we can see that the sedimentation velocity of the particle depends on the square of the radius. Hemodynamics_sentence_32

If the particle is released from rest in the fluid, its sedimentation velocity Us increases until it attains the steady value called the terminal velocity (U), as shown above. Hemodynamics_sentence_33

Hemodilution Hemodynamics_section_4

Hemodilution is the dilution of the concentration of red blood cells and plasma constituents by partially substituting the blood with colloids or crystalloids. Hemodynamics_sentence_34

It is a strategy to avoid exposure of patients to the potential hazards of homologous blood transfusions. Hemodynamics_sentence_35

Hemodilution can be normovolemic, which implies the dilution of normal blood constituents by the use of expanders. Hemodynamics_sentence_36

During acute normovolemic hemodilution, (ANH) blood subsequently lost during surgery contains proportionally fewer red blood cells per millimetre, thus minimizing intraoperative loss of the whole blood. Hemodynamics_sentence_37

Therefore, blood lost by the patient during surgery is not actually lost by the patient, for this volume is purified and redirected into the patient. Hemodynamics_sentence_38

On the other hand, hypervolemic hemodilution (HVH) uses acute preoperative volume expansion without any blood removal. Hemodynamics_sentence_39

In choosing a fluid, however, it must be assured that when mixed, the remaining blood behaves in the microcirculation as in the original blood fluid, retaining all its properties of viscosity. Hemodynamics_sentence_40

In presenting what volume of ANH should be applied one study suggests a mathematical model of ANH which calculates the maximum possible RCM savings using ANH, given the patients weight Hi and Hm. Hemodynamics_sentence_41

(See below for a glossary of the terms used.) Hemodynamics_sentence_42

To maintain the normovolemia, the withdrawal of autologous blood must be simultaneously replaced by a suitable hemodilute. Hemodynamics_sentence_43

Ideally, this is achieved by isovolemia exchange transfusion of a plasma substitute with a colloid osmotic pressure (OP). Hemodynamics_sentence_44

A colloid is a fluid containing particles that are large enough to exert an oncotic pressure across the micro-vascular membrane. Hemodynamics_sentence_45

When debating the use of colloid or crystalloid, it is imperative to think about all the components of the starling equation: Hemodynamics_sentence_46

To identify the minimum safe hematocrit desirable for a given patient the following equation is useful: Hemodynamics_sentence_47

where EBV is the estimated blood volume; 70 mL/kg was used in this model and Hi (initial hematocrit) is the patient's initial hematocrit. Hemodynamics_sentence_48

From the equation above it is clear that the volume of blood removed during the ANH to the Hm is the same as the BLs. Hemodynamics_sentence_49

How much blood is to be removed is usually based on the weight, not the volume. Hemodynamics_sentence_50

The number of units that need to be removed to hemodilute to the maximum safe hematocrit (ANH) can be found by Hemodynamics_sentence_51

This is based on the assumption that each unit removed by hemodilution has a volume of 450 mL (the actual volume of a unit will vary somewhat since completion of collection ais dependent on weight and not volume). Hemodynamics_sentence_52

The model assumes that the hemodilute value is equal to the Hm prior to surgery, therefore, the re-transfusion of blood obtained by hemodilution must begin when SBL begins. Hemodynamics_sentence_53

The RCM available for retransfusion after ANH (RCMm) can be calculated from the patient's Hi and the final hematocrit after hemodilution(Hm) Hemodynamics_sentence_54

The maximum SBL that is possible when ANH is used without falling below Hm(BLH) is found by assuming that all the blood removed during ANH is returned to the patient at a rate sufficient to maintain the hematocrit at the minimum safe level Hemodynamics_sentence_55

If ANH is used as long as SBL does not exceed BLH there will not be any need for blood transfusion. Hemodynamics_sentence_56

We can conclude from the foregoing that H should therefore not exceed s. The difference between the BLH and the BLs therefore is the incremental surgical blood loss (BLi) possible when using ANH. Hemodynamics_sentence_57

When expressed in terms of the RCM Hemodynamics_sentence_58

Where RCMi is the red cell mass that would have to be administered using homologous blood to maintain the Hm if ANH is not used and blood loss equals BLH. Hemodynamics_sentence_59

The model used assumes ANH used for a 70 kg patient with an estimated blood volume of 70 ml/kg (4900 ml). Hemodynamics_sentence_60

A range of Hi and Hm was evaluated to understand conditions where hemodilution is necessary to benefit the patient. Hemodynamics_sentence_61

Result Hemodynamics_section_5

The result of the model calculations are presented in a table given in the appendix for a range of Hi from 0.30 to 0.50 with ANH performed to minimum hematocrits from 0.30 to 0.15. Hemodynamics_sentence_62

Given a Hi of 0.40, if the Hm is assumed to be 0.25.then from the equation above the RCM count is still high and ANH is not necessary, if BLs does not exceed 2303 ml, since the hemotocrit will not fall below Hm, although five units of blood must be removed during hemodilution. Hemodynamics_sentence_63

Under these conditions, to achieve the maximum benefit from the technique if ANH is used, no homologous blood will be required to maintain the Hm if blood loss does not exceed 2940 ml. Hemodynamics_sentence_64

In such a case ANH can save a maximum of 1.1 packed red blood cell unit equivalent, and homologous blood transfusion is necessary to maintain Hm, even if ANH is used. Hemodynamics_sentence_65

This model can be used to identify when ANH may be used for a given patient and the degree of ANH necessary to maximize that benefit. Hemodynamics_sentence_66

For example, if Hi is 0.30 or less it is not possible to save a red cell mass equivalent to two units of homologous PRBC even if the patient is hemodiluted to an Hm of 0.15. Hemodynamics_sentence_67

That is because from the RCM equation the patient RCM falls short from the equation giving above. Hemodynamics_sentence_68

If Hi is 0.40 one must remove at least 7.5 units of blood during ANH, resulting in an Hm of 0.20 to save two units equivalence. Hemodynamics_sentence_69

Clearly, the greater the Hi and the greater the number of units removed during hemodilution, the more effective ANH is for preventing homologous blood transfusion. Hemodynamics_sentence_70

The model here is designed to allow doctors to determine where ANH may be beneficial for a patient based on their knowledge of the Hi, the potential for SBL, and an estimate of the Hm. Hemodynamics_sentence_71

Though the model used a 70 kg patient, the result can be applied to any patient. Hemodynamics_sentence_72

To apply these result to any body weight, any of the values BLs, BLH and ANHH or PRBC given in the table need to be multiplied by the factor we will call T Hemodynamics_sentence_73

Basically, the model considered above is designed to predict the maximum RCM that can save ANH. Hemodynamics_sentence_74

In summary, the efficacy of ANH has been described mathematically by means of measurements of surgical blood loss and blood volume flow measurement. Hemodynamics_sentence_75

This form of analysis permits accurate estimation of the potential efficiency of the techniques and shows the application of measurement in the medical field. Hemodynamics_sentence_76

Blood flow Hemodynamics_section_6

Cardiac output Hemodynamics_section_7

The heart is the driver of the circulatory system, pumping blood through rhythmic contraction and relaxation. Hemodynamics_sentence_77

The rate of blood flow out of the heart (often expressed in L/min) is known as the cardiac output (CO). Hemodynamics_sentence_78

Blood being pumped out of the heart first enters the aorta, the largest artery of the body. Hemodynamics_sentence_79

It then proceeds to divide into smaller and smaller arteries, then into arterioles, and eventually capillaries, where oxygen transfer occurs. Hemodynamics_sentence_80

The capillaries connect to venules, and the blood then travels back through the network of veins to the right heart. Hemodynamics_sentence_81

The micro-circulation — the arterioles, capillaries, and venules —constitutes most of the area of the vascular system and is the site of the transfer of O2, glucose, and enzyme substrates into the cells. Hemodynamics_sentence_82

The venous system returns the de-oxygenated blood to the right heart where it is pumped into the lungs to become oxygenated and CO2 and other gaseous wastes exchanged and expelled during breathing. Hemodynamics_sentence_83

Blood then returns to the left side of the heart where it begins the process again. Hemodynamics_sentence_84

In a normal circulatory system, the volume of blood returning to the heart each minute is approximately equal to the volume that is pumped out each minute (the cardiac output). Hemodynamics_sentence_85

Because of this, the velocity of blood flow across each level of the circulatory system is primarily determined by the total cross-sectional area of that level. Hemodynamics_sentence_86

This is mathematically expressed by the following equation: Hemodynamics_sentence_87

Hemodynamics_description_list_0

  • v = Q/AHemodynamics_item_0_0

where Hemodynamics_sentence_88

Hemodynamics_unordered_list_1

  • v = velocity (cm/s)Hemodynamics_item_1_1
  • Q = blood flow (ml/s)Hemodynamics_item_1_2
  • A = cross sectional area (cm)Hemodynamics_item_1_3

Turbulence Hemodynamics_section_8

Blood flow is also affected by the smoothness of the vessels, resulting in either turbulent (chaotic) or laminar (smooth) flow. Hemodynamics_sentence_89

Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Hemodynamics_sentence_90

The Reynolds number (denoted NR or Re) is a relationship that helps determine the behavior of a fluid in a tube, in this case blood in the vessel. Hemodynamics_sentence_91

The equation for this dimensionless relationship is written as: Hemodynamics_sentence_92

The Reynolds number is directly proportional to the velocity and diameter of the tube. Hemodynamics_sentence_93

Note that NR is directly proportional to the mean velocity as well as the diameter. Hemodynamics_sentence_94

A Reynolds number of less than 2300 is laminar fluid flow, which is characterized by constant flow motion, whereas a value of over 4000, is represented as turbulent flow. Hemodynamics_sentence_95

Due to its smaller radius and lowest velocity compared to other vessels, the Reynolds number at the capillaries is very low, resulting in laminar instead of turbulent flow. Hemodynamics_sentence_96

Velocity Hemodynamics_section_9

Often expressed in cm/s. Hemodynamics_sentence_97

This value is inversely related to the total cross-sectional area of the blood vessel and also differs per cross-section, because in normal condition the blood flow has laminar characteristics. Hemodynamics_sentence_98

For this reason, the blood flow velocity is the fastest in the middle of the vessel and slowest at the vessel wall. Hemodynamics_sentence_99

In most cases, the mean velocity is used. Hemodynamics_sentence_100

There are many ways to measure blood flow velocity, like videocapillary microscoping with frame-to-frame analysis, or laser Doppler anemometry. Hemodynamics_sentence_101

Blood velocities in arteries are higher during systole than during diastole. Hemodynamics_sentence_102

One parameter to quantify this difference is the pulsatility index (PI), which is equal to the difference between the peak systolic velocity and the minimum diastolic velocity divided by the mean velocity during the cardiac cycle. Hemodynamics_sentence_103

This value decreases with distance from the heart. Hemodynamics_sentence_104

Hemodynamics_table_general_0

Relation between blood flow velocity and total cross-section area in humanHemodynamics_table_caption_0
Type of blood vesselsHemodynamics_header_cell_0_0_0 Total cross-section areaHemodynamics_header_cell_0_0_1 Blood velocity in cm/sHemodynamics_header_cell_0_0_2
AortaHemodynamics_cell_0_1_0 3–5 cmHemodynamics_cell_0_1_1 40 cm/sHemodynamics_cell_0_1_2
CapillariesHemodynamics_cell_0_2_0 4500–6000 cmHemodynamics_cell_0_2_1 0.03 cm/sHemodynamics_cell_0_2_2
Vena cavae inferior and superiorHemodynamics_cell_0_3_0 14 cmHemodynamics_cell_0_3_1 15 cm/sHemodynamics_cell_0_3_2

Blood vessels Hemodynamics_section_10

Vascular resistance Hemodynamics_section_11

Main article: Vascular resistance Hemodynamics_sentence_105

Resistance is also related to vessel radius, vessel length, and blood viscosity. Hemodynamics_sentence_106

In a first approach based on fluids, as indicated by the Hagen–Poiseuille equation. Hemodynamics_sentence_107

The equation is as follows: Hemodynamics_sentence_108

In a second approach, more realistic of the vascular resistance and coming from experimental observations on blood flows, according to Thurston, there is a plasma release-cell layering at the walls surrounding a plugged flow. Hemodynamics_sentence_109

It is a fluid layer in which at a distance δ, viscosity η is a function of δ written as η(δ), and these surrounding layers do not meet at the vessel centre in real blood flow. Hemodynamics_sentence_110

Instead, there is the plugged flow which is hyperviscous because holding high concentration of RBCs. Hemodynamics_sentence_111

Thurston assembled this layer to the flow resistance to describe blood flow by means of a viscosity η(δ) and thickness δ from the wall layer. Hemodynamics_sentence_112

The blood resistance law appears as R adapted to blood flow profile : Hemodynamics_sentence_113

where Hemodynamics_sentence_114

Hemodynamics_unordered_list_2

  • R = resistance to blood flowHemodynamics_item_2_4
  • c = constant coefficient of flowHemodynamics_item_2_5
  • L = length of the vesselHemodynamics_item_2_6
  • η(δ) = viscosity of blood in the wall plasma release-cell layeringHemodynamics_item_2_7
  • r = radius of the blood vesselHemodynamics_item_2_8
  • δ = distance in the plasma release-cell layerHemodynamics_item_2_9

Blood resistance varies depending on blood viscosity and its plugged flow (or sheath flow since they are complementary across the vessel section) size as well, and on the size of the vessels. Hemodynamics_sentence_115

Assuming steady, laminar flow in the vessel, the blood vessels behavior is similar to that of a pipe. Hemodynamics_sentence_116

For instance if p1 and p2 are pressures are at the ends of the tube, the pressure drop/gradient is: Hemodynamics_sentence_117

The larger arteries, including all large enough to see without magnification, are conduits with low vascular resistance (assuming no advanced atherosclerotic changes) with high flow rates that generate only small drops in pressure. Hemodynamics_sentence_118

The smaller arteries and arterioles have higher resistance, and confer the main blood pressure drop across major arteries to capillaries in the circulatory system. Hemodynamics_sentence_119

In the arterioles blood pressure is lower than in the major arteries. Hemodynamics_sentence_120

This is due to bifurcations, which cause a drop in pressure. Hemodynamics_sentence_121

The more bifurcations, the higher the total cross-sectional area, therefore the pressure across the surface drops. Hemodynamics_sentence_122

This is why the arterioles have the highest pressure-drop. Hemodynamics_sentence_123

The pressure drop of the arterioles is the product of flow rate and resistance: ∆P=Q xresistance. Hemodynamics_sentence_124

The high resistance observed in the arterioles, which factor largely in the ∆P is a result of a smaller radius of about 30 µm. Hemodynamics_sentence_125

The smaller the radius of a tube, the larger the resistance to fluid flow. Hemodynamics_sentence_126

Immediately following the arterioles are the capillaries. Hemodynamics_sentence_127

Following the logic observed in the arterioles, we expect the blood pressure to be lower in the capillaries compared to the arterioles. Hemodynamics_sentence_128

Since pressure is a function of force per unit area, (P = F/A), the larger the surface area, the lesser the pressure when an external force acts on it. Hemodynamics_sentence_129

Though the radii of the capillaries are very small, the network of capillaries has the largest surface area in the vascular network. Hemodynamics_sentence_130

They are known to have the largest surface area (485 mm^2) in the human vascular network. Hemodynamics_sentence_131

The larger the total cross-sectional area, the lower the mean velocity as well as the pressure. Hemodynamics_sentence_132

Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Hemodynamics_sentence_133

Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing arterial pressure. Hemodynamics_sentence_134

If the blood viscosity increases (gets thicker), the result is an increase in arterial pressure. Hemodynamics_sentence_135

Certain medical conditions can change the viscosity of the blood. Hemodynamics_sentence_136

For instance, anemia (low red blood cell concentration), reduces viscosity, whereas increased red blood cell concentration increases viscosity. Hemodynamics_sentence_137

It had been thought that aspirin and related "blood thinner" drugs decreased the viscosity of blood, but instead studies found that they act by reducing the tendency of the blood to clot. Hemodynamics_sentence_138

Wall tension Hemodynamics_section_12

Regardless of site, blood pressure is related to the wall tension of the vessel according to the Young–Laplace equation (assuming that the thickness of the vessel wall is very small as compared to the diameter of the lumen): Hemodynamics_sentence_139

where Hemodynamics_sentence_140

For the thin-walled assumption to be valid the vessel must have a wall thickness of no more than about one-tenth (often cited as one twentieth) of its radius. Hemodynamics_sentence_141

The cylinder stress, in turn, is the average force exerted circumferentially (perpendicular both to the axis and to the radius of the object) in the cylinder wall, and can be described as: Hemodynamics_sentence_142

where: Hemodynamics_sentence_143

Hemodynamics_unordered_list_3

  • F is the force exerted circumferentially on an area of the cylinder wall that has the following two lengths as sides:Hemodynamics_item_3_10
  • t is the radial thickness of the cylinderHemodynamics_item_3_11
  • l is the axial length of the cylinderHemodynamics_item_3_12

Stress Hemodynamics_section_13

When force is applied to a material it starts to deform or move. Hemodynamics_sentence_144

As the force needed to deform a material (e.g. to make a fluid flow) increases with the size of the surface of the material A., the magnitude of this force F is proportional to the area A of the portion of the surface. Hemodynamics_sentence_145

Therefore, the quantity (F/A) that is the force per unit area is called the stress. Hemodynamics_sentence_146

The shear stress at the wall that is associated with blood flow through an artery depends on the artery size and geometry and can range between 0.5 and 4 Pa. Hemodynamics_sentence_147

Under normal conditions, to avoid atherogenesis, thrombosis, smooth muscle proliferation and endothelial apoptosis, shear stress maintains its magnitude and direction within an acceptable range. Hemodynamics_sentence_148

In some cases occurring due to blood hammer, shear stress reaches larger values. Hemodynamics_sentence_149

While the direction of the stress may also change by the reverse flow, depending on the hemodynamic conditions. Hemodynamics_sentence_150

Therefore, this situation can lead to atherosclerosis disease. Hemodynamics_sentence_151

Capacitance Hemodynamics_section_14

Main article: Capacitance of blood vessels Hemodynamics_sentence_152

Veins are described as the "capacitance vessels" of the body because over 70% of the blood volume resides in the venous system. Hemodynamics_sentence_153

Veins are more compliant than arteries and expand to accommodate changing volume. Hemodynamics_sentence_154

Blood pressure Hemodynamics_section_15

The blood pressure in the circulation is principally due to the pumping action of the heart. Hemodynamics_sentence_155

The pumping action of the heart generates pulsatile blood flow, which is conducted into the arteries, across the micro-circulation and eventually, back via the venous system to the heart. Hemodynamics_sentence_156

During each heartbeat, systemic arterial blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. Hemodynamics_sentence_157

In physiology, these are often simplified into one value, the mean arterial pressure (MAP), which is calculated as follows: Hemodynamics_sentence_158

Hemodynamics_description_list_4

  • MAP ≈ ​⁄3(BPdia) + ​⁄3(BPsys)Hemodynamics_item_4_13

where: Hemodynamics_sentence_159

Hemodynamics_unordered_list_5

  • MAP = Mean Arterial PressureHemodynamics_item_5_14
  • BPdia = Diastolic blood pressureHemodynamics_item_5_15
  • BPsys = Systolic blood pressureHemodynamics_item_5_16

Differences in mean blood pressure are responsible for blood flow from one location to another in the circulation. Hemodynamics_sentence_160

The rate of mean blood flow depends on both blood pressure and the resistance to flow presented by the blood vessels. Hemodynamics_sentence_161

Mean blood pressure decreases as the circulating blood moves away from the heart through arteries and capillaries due to viscous losses of energy. Hemodynamics_sentence_162

Mean blood pressure drops over the whole circulation, although most of the fall occurs along the small arteries and arterioles. Hemodynamics_sentence_163

Gravity affects blood pressure via hydrostatic forces (e.g., during standing), and valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure in veins. Hemodynamics_sentence_164

The relationship between pressure, flow, and resistance is expressed in the following equation: Hemodynamics_sentence_165

Hemodynamics_description_list_6

  • Flow = Pressure/ResistanceHemodynamics_item_6_17

When applied to the circulatory system, we get: Hemodynamics_sentence_166

Hemodynamics_description_list_7

  • CO = (MAP – RAP)/TPRHemodynamics_item_7_18

where Hemodynamics_sentence_167

Hemodynamics_unordered_list_8

  • CO = cardiac output (in L/min)Hemodynamics_item_8_19
  • MAP = mean arterial pressure (in mmHg), the average pressure of blood as it leaves the heartHemodynamics_item_8_20
  • RAP = right atrial pressure (in mmHg), the average pressure of blood as it returns to the heartHemodynamics_item_8_21
  • TPR = total peripheral resistance (in mmHg * min/L)Hemodynamics_item_8_22

A simplified form of this equation assumes right atrial pressure is approximately 0: Hemodynamics_sentence_168

Hemodynamics_description_list_9

  • CO ≈ MAP/TPRHemodynamics_item_9_23

The ideal blood pressure in the brachial artery, where standard blood pressure cuffs measure pressure, is <120/80 mmHg. Hemodynamics_sentence_169

Other major arteries have similar levels of blood pressure recordings indicating very low disparities among major arteries. Hemodynamics_sentence_170

In the innominate artery, the average reading is 110/70 mmHg, the right subclavian artery averages 120/80 and the abdominal aorta is 110/70 mmHg. Hemodynamics_sentence_171

The relatively uniform pressure in the arteries indicate that these blood vessels act as a pressure reservoir for fluids that are transported within them. Hemodynamics_sentence_172

Pressure drops gradually as blood flows from the major arteries, through the arterioles, the capillaries until blood is pushed up back into the heart via the venules, the veins through the vena cava with the help of the muscles. Hemodynamics_sentence_173

At any given pressure drop, the flow rate is determined by the resistance to the blood flow. Hemodynamics_sentence_174

In the arteries, with the absence of diseases, there is very little or no resistance to blood. Hemodynamics_sentence_175

The vessel diameter is the most principal determinant to control resistance. Hemodynamics_sentence_176

Compared to other smaller vessels in the body, the artery has a much bigger diameter (4  mm), therefore the resistance is low. Hemodynamics_sentence_177

The arm–leg (blood pressure) gradient is the difference between the blood pressure measured in the arms and that measured in the legs. Hemodynamics_sentence_178

It is normally less than 10 mm Hg, but may be increased in e.g. coarctation of the aorta. Hemodynamics_sentence_179

Clinical significance Hemodynamics_section_16

Pressure monitoring Hemodynamics_section_17

Hemodynamic monitoring is the observation of hemodynamic parameters over time, such as blood pressure and heart rate. Hemodynamics_sentence_180

Blood pressure can be monitored either invasively through an inserted blood pressure transducer assembly (providing continuous monitoring), or noninvasively by repeatedly measuring the blood pressure with an inflatable blood pressure cuff. Hemodynamics_sentence_181

Remote, indirect monitoring of blood flow by laser Doppler Hemodynamics_section_18

Noninvasive hemodynamic monitoring of eye fundus vessels can be performed by Laser Doppler holography, with near infrared light. Hemodynamics_sentence_182

The eye offers a unique opportunity for the non-invasive exploration of cardiovascular diseases. Hemodynamics_sentence_183

Laser Doppler imaging by digital holography can measure blood flow in the retina and choroid, whose Doppler responses exhibit a pulse-shaped profile with time This technique enables non invasive functional microangiography by high-contrast measurement of Doppler responses from endoluminal blood flow profiles in vessels in the posterior segment of the eye. Hemodynamics_sentence_184

Differences in blood pressure drive the flow of blood throughout the circulation. Hemodynamics_sentence_185

The rate of mean blood flow depends on both blood pressure and the hemodynamic resistance to flow presented by the blood vessels. Hemodynamics_sentence_186

Glossary Hemodynamics_section_19

Hemodynamics_description_list_10

  • ANH: Acute Normovolemic HemodilutionHemodynamics_item_10_24
  • ANHu: Number of Units During ANHHemodynamics_item_10_25
  • BLH: Maximum Blood Loss Possible When ANH Is Used Before Homologous Blood Transfusion Is NeededHemodynamics_item_10_26
  • BLI: Incremental Blood Loss Possible with ANH.(BLH – BLs)Hemodynamics_item_10_27
  • BLs: Maximum blood loss without ANH before homologous blood transfusion is requiredHemodynamics_item_10_28
  • EBV: Estimated Blood Volume(70 mL/kg)Hemodynamics_item_10_29
  • Hct: Haematocrit Always Expressed Here As A FractionHemodynamics_item_10_30
  • Hi: Initial HaematocritHemodynamics_item_10_31
  • Hm: Minimum Safe HaematocritHemodynamics_item_10_32
  • PRBC: Packed Red Blood Cell Equivalent Saved by ANHHemodynamics_item_10_33
  • RCM: Red cell mass.Hemodynamics_item_10_34
  • RCMH: Cell Mass Available For Transfusion after ANHHemodynamics_item_10_35
  • RCMI: Red Cell Mass Saved by ANHHemodynamics_item_10_36
  • SBL: Surgical Blood LossHemodynamics_item_10_37

Etymology and pronunciation Hemodynamics_section_20

The word hemodynamics (/ˌhiːmədaɪˈnæmɪks, -moʊ-/) uses combining forms of (which comes from the ancient Greek haima, meaning blood) and dynamics, thus "the dynamics of blood". Hemodynamics_sentence_187

The vowel of the hemo- syllable is variously written according to the ae/e variation. Hemodynamics_sentence_188

See also Hemodynamics_section_21

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Hemodynamics.