WINDSOR UNIVERSITYSCHOOL OF MEDICINE

Hemodynamics

Dr. Samuel Taiwo Alawode B.Sc, MSc, MD.

Hemodymamics

• William Harwey (1578-1657) Discovery of blood circulation and heart function (published 1628)

• This theory was fully accepted after discovery of pulmonary capillaries (Marcello Malpighi - 1661).

Diastolic work the filling blood (made on the heart)

Systolic work of the heart

Intraventricularpressure

Ventricular volume

afterload Stroke volume

Systolicresidualvolume

Enddiastolicvolume

preload

Diastolic

fillin

g

Izotonic maximaIz

ovol

umic

max

ima

Learning Objectives

• Describe the organization of the circulatory system and its function

• Explain how the systemic and pulmonary circulations are linked physically and physiologically

• Understand the relationship between flow, velocity, and cross-sectional area

Learning Objectives

• Understand the relationship between pressure, flow, and resistance in the vasculature .

• Define resistance and conductance. Understand the effects of adding resistance in series vs. in parallel on total resistance and flow.

Functions of circulation

• Supply the tissues with nutrients• Removal of waste product of tissue

metabolism• Control blood flow to the skin and limbs to

regulate heat loss• Aids in body’s defence mechanisms by

delivering antibodies ,platelets and leucocytes to the affected area of the body.

Hemodynamics• Hemodynamics is an important part of cardiovascular

physiology dealing with the forces the pump (the heart) has to develop to circulate blood through the cardiovascular system.

• Adequate blood circulation (blood flow) is a necessary condition for adequate supply of oxygen to all tissues, which, in return, is synonymous with cardiovascular health, survival of surgical patients, longevity and quality of life.

• A significant majority of all cardiovascular diseases and disorders is related to systemic hemodynamic dysfunction. Hypertension and congestive heart failure are two best known systemic hemodynamic disorders.

Hemodynamics

• A healthy cardiovascular system maintains adequate supply of oxygen to all tissues under all metabolic conditions by a dynamic variation of levels of four modulators.

• Three of them are the systemic hemodynamic modulators, • One is the perfusion flow modulator.

The body changes levels of these four modulators for every heart beat in response to a varying oxygen demand of all tissues.

Hemodynamic modulators• The three hemodynamic modulators are the causes of

changes of levels of blood pressure (MAP = Mean Arterial Pressure) and blood flow (SI = Stroke Index). These modulators are:

1. intravascular volume 2. inotropy 3. vasoactivity

• Subsequently, the fourth modulator which relate to perfusion flow modulation is :

• Chronotropy

Normal Hemodynamic state

Only a patient who has all four modulators attheir respective normal levels, i.e., who isNormovolemic, Normoinotropic, Normovasoactive and Normochronotropic

can be in a normohemodynamic and normoperfusion

Change in Hemodynamic state

Note that: 1.In a healthy patient, between the rest (the

lowest oxygen demand) and a strenuous exercise, such as running a marathon (the highest oxygen demand), the SI increases by 66%, CI by 500%, while MAP changes a little or not at all.

Change in Hemodynamic state

2. All current cardio- and vasoactive drugs consist of these components and/or their mixtures. Each commercially-produced cardio- and vasoactive drug has a specific vector on the hemodynamic map below.

3. There is no pharmacological agent available to perform volume expansion; volume expansion can only be accomplished by intake of fluids - orally or intravenously.

4. A change of level of any of these modulating agents always changes both MAP and SI, and, subsequently, the CI level.

Change in Hemodynamic state• As a human body produces changes in hemodynamic and

perfusion modulators to adjust the hemodynamic state in order to respond to a varying oxygen demand, a clinician treating any hemodynamic disorder (abnormal hemodynamic state) can affect therapeutically only the levels of the three hemodynamic modulators and of the perfusion modulator.

• volume-reducing drugs (diuretics), • positive and negative inotropes, • vasodilators and vasoconstrictors, • positive and negative chronotropes.

Possible Hemodynamic States

Hypertension Hyper/hypo Hyper/Normo Hyper/hyper

Normotension NormohypoNormo/Normo

Normo/hyper

Hypotension Hypo/hypo hypo/Normo Hypo/hyper

BLOOD FLOW

Hypodynamic Normodynamic Hyperdymamic

PRESSURE

General features of the cardiovascular system

120

Pulmonary and systemic circulations

• Cardiac output and heart rate of the two circuits are equal, so stroke volumes are the same.

• Despite this, all pressures are higher in the systemic (peripheral) circuit. This shows that the vessels of the circuits are very different. The systemic circuit has much higher resistance and much lower compliance than the pulmonary circuit.

• The lower pressures mean that the work of the right ventricle is much lower.

• In addition, the lower capillary pressure protects against the development of pulmonary edema

NORMAL BLOOD PRESSURE IN DIFFERENT PORTIONS OF CIRCULATORY SYSTEM

• Local arteriolar dilation decreases arteriolar resistance, which increases flow and pressure downstream (more pressure and more flow get downstream).

• Local arteriolar constriction increases arteriolar resistance, and flow and pressure decrease downstream

Types of blood vessels

1. Windkessel vessels/Distensible vessels- aorta ,pulmonary artery and their large branches.

2. Resistance vessels– arterioles ,metarterioles and pre capillary sphincter.

3. Exchange vessels—capillaries4. Capacitance vessels- venules and veins5. Shunt vessels or Thoroughfare vessels/A-V

shunts

WINDKESSEL (DISTENSIBLE) VESSELS (large arteries)

• Highly elastic• Windkessel means elastic reservoir• During systole they distend withstanding the

high systolic pressure • Once systole is over they recoil contributing to

the diastolic pressure

COMPLIANCE OF BLOOD VESSELS• The compliance or capacitance of a blood vessel

describes the volume of blood the vessel can hold at a given pressure. Compliance is related to distensibility and is given by the following equation:

where C ,Compliance (mL/mm Hg) ,V Volume (mL), P Pressure (mm Hg)

• The equation for compliance states that the higher the compliance of a vessel, the more volume it can hold at a given pressure.

• Compliance is essentially how easily a vessel is stretched;(and remain so)

• If a vessel is easily stretched, it is considered very compliant. The opposite is noncompliant or stiff.

• Elasticity is the inverse of compliance. A vessel that has high elasticity (a large tendency to rebound from a stretch) has low compliance.

BLOOD DISTRIBUTION IN DIFFERENT PARTS OF CIRCULATORY SYSTEM

Blood Volume

• The largest blood volume in the cardiovascular system is in the systemic veins.

• The second largest blood volume is in the pulmonary system.

• Both represent major blood reservoirs.• The systemic veins and the pulmonary

vessels have very high compliance compared to the systemic arteries; this is primarily responsible for the distribution of blood volume.

CHARACTERISTICS OF SYSTEMIC VEINS

• Systemic veins are about 20 times more compliant than systemic arteries.

• Veins also contain about 70% of the systemic blood volume and thus represent the major blood reservoir.

• In the venous system, then, a small change in pressure causes a large change in venous volume

Example-Hemorrhage

• Cause venous pressure to decreases. • Because veins are very compliant vessels, this

loss of distending pressure causes a significant passive constriction of the veins and a decrease in blood stored in those veins.

• The blood removed from the veins will now contribute to the circulating blood volume (cardiac output), a compensation for the consequences of hemorrhage.

Volume loading (infusion of fluid)

• Increases venous pressure. The increased pressure distends the veins; this is a passive dilation.

• The volume of fluid stored in the veins increases, which means that some of the infused volume will not contribute to cardiac output.

• The large volume and compliant nature of the veins act to buffer changes in venous return and cardiac output.

• Because of the high compliance of veins, large increases of pressure occur mainly with substantial increases of volume, as in congestive heart failure, or with massive sympathetic activity that reduces compliance.

• Similarly, substantial decreases of central venous pressure occur with large loss of volume.

• An exception is the effect of posture, which can lower central venous pressure, even though blood volume has not changed.

• This is because gravity causes blood to pool in the dependent veins.

The actual venous return to the heart is determined by the venous pressure gradient.

Cross-Sectional Area

Velocity of the Bloodstream

• Velocity, as relates to fluid movement, is the distance that a particle of fluid travels with respect to time, and it is expressed in units of distance per unit time (e.g., cm/sec).

• Flow, is the rate of displacement of a volume of fluid, and it is expressed in units of volume per unit time (e.g., cm3/sec).

• In a rigid tube, velocity (v) and flow (Q) are related to one another by the cross-sectional area (A) of the tube

Total cross sectional area

Factors influencing velocity• Cross sectional area of segment• Phase – Systolic phase ↑ velocity Diastolic phase ↓ velocity Viscosity - ↑viscosity ↓ velocity ↓ viscosity ↑ velocity

Applied physiology Velocity decreases in heart failure.

Velocity of circulation

• In most arterial locations, the dynamic component will be a negligible fraction of the total pressure.

• However, at sites of an arterial constriction or obstruction, the high flow velocity is associated with a large kinetic energy, and therefore the dynamic pressure component may increase significantly.

• Hence, the pressure would be reduced and perfusion of distal segments will be correspondingly decreased.

Q. The greatest pressure decrease in the circulation occurs across the arterioles because(A) they have the greatest surface area(B) they have the greatest cross-sectional area(C) the velocity of blood flow through them is

the highest(D) the velocity of blood flow through them is

the lowest(E) they have the greatest resistance

Q. A 25 year old graduate student while going for her lectures on her power bike skids off the road and sustains a fracture to her right leg. The fractured leg is bleeding profusely. At the ER, her blood pressure is determined to be low. Homeostatic mechanisms in stabilizing the blood pressure will include increases in total peripheral resistance. The site of highest resistance in the vasculature is in the;

A. ArteriolesB. VenulesC. CapillariesD. Large arteriesE. Veins

Q. 12 A healthy 32-year-old woman participates in a clinical study. Her blood volume is 5,200mL. Images are obtained to determine the volume of blood in various vessels in various body positions at rest and during exercise. While lying supine, which of the following vascular structures will most likely contain the largest portion of the total blood volume in this woman?

A. The left ventricleB. The right ventricleC. The pulmonary vasculatureD. Veins and venulesE. Vena cavaeF. CapillariesG. Arterioles

BLOOD FLOW

• Quantity of blood that passes a given point of circulation in a given period of time.

Units= ml/min • Normal blood flow is – streamline or

laminar(Silent) • Random flow in a vessel - Turbulent flow• In laminar flow , the velocity of flow is greater in

the center than the outer edges .

DEMONSTRATION OF LAMINAR & TURBULENT BLOOD FLOW

Axial streaming and flow velocity

• Laminar flow is flow in layers.

• Laminar flow occurs throughout the normal cardiovascular system, excluding flow in the heart.

• The layer with the highest velocity is in the center of the tube.

• Turbulent flow is non layered flow.

• It creates murmurs. These are heard as bruits in vessels with severe stenosis.

• It produces more resistance than laminar flow.

CRITICAL VELOCITY • The maximal velocity at which the flow becomes

turbulent .• Expressed in REYNOLDS NUMBER .• R= PDV / • P= Density of blood (1), D = diameter of vessel , V=

Velocity of blood flow (cm/sec) , = viscosity in poises• When number is 2000 – TURBULENCE occurs . Velocity , Cross section eg; Stenosis Velocity , Viscosity eg; Anaemia

IN THE CLINIC – turbulent flow

• Usually accompanied by audible vibrations, detected with a stethoscope .

• When the turbulence occurs in the heart, the resultant sound is termed a murmur; when it occurs in a vessel, the sound is termed a bruit.

• E.g- In severe anemia, (1) the reduced viscosity of blood and (2) the high flow velocities associated with the high cardiac output .

• Blood clots, or thrombi, are more likely to develop in turbulent than in laminar flow.

Viscosity• Viscosity is a property of a fluid that is a

measure of the fluid’s internal resistance to flow.

• Viscosity is the frictional resistance in between the laminae of the flowing fluid .

• Frictional resistance is due to red cells and plasma proteins.

• The greater the viscosity, the greater the resistance.

• The prime determinant of blood viscosity is the hematocrit.

A MD 2 student is performing experiments on blood flow in various vessels. She came to the conclusion that the velocity of blood flow is slowest in the capillaries. The most likely reason for this is:

A.Capillaries have the smallest cross-sectional area

B. Capillaries have the largest cross-sectional areaC. Decreased in blood viscosity in the capillariesD.Single stream of blood flowE. Decreased in turbulence