3 hemodynamics 2ndyears
TRANSCRIPT
Hemodynamics
Physics of Blood flow in the circulation
Circulatory System
• Heart:
Has 2 collecting chambers - (Left, Right
Atria)
Has 2 Pumping chambers - (Left, Right
Ventricles)
Circulation Schematic
Lungs Tissues
Left Side of Heart
Right Side of Heart
A V
V A
Pulmonary Vein
Pulmonary Artery
Aorta
Sup. & Inf. Vena Cava
Mitral Valve
Pulmonary Valve
Aortic Valve
Tricuspid Valve
Heart Valves
• Atrioventricular (A-V) valves - separate Atria from Ventricles
• Bicuspid (Mitral) - Left Side
• Tricuspid - Right Side
• Semi-Lunar Valves - separate ventricles from Arteries
Opening, Closing of Valves - Depends on Pressuredifferences between bloodin adjacent areas
Heart Sounds
• ‘Lubb’ (1st sound) - Closure of A-V valves• ‘Dupp’ (2nd sound) - Closure of S-L valves
Caused by Turbulence on closing.
Anything extra ’Murmur’ (swishing of blood)
Could be due to:• Stenosis of Valves (calcification)• Valves not closing properly
(Incompetence, Insufficiency)
Increases Pressure on heart
Blood Vessels
• Arteries• Capillaries• Veins
Systemic Pathway:Left Ventricle Aorta Arteries Arterioles
of Heart Capillaries
Venules Veins Right Atrium of Heart
Blood
• Composition:– Approx 45% by Vol. Solid Components
» Red Blood Cells (12m x 2 m)
» White Cells
» Platelets
– Approx 55% Liquid (plasma)» 91.5% of which is water
» 7% plasma proteins
» 1.5% other solutes
Blood Functions
• Transportation
of blood gases, nutrients, wastes
• Homeostasis (regulation)
of Ph, Body Temp, water content
• Protection
As a Result …….
• Blood behaves as a simple Newtonian Fluid when flowing in blood vessels
i.e. Viscous stresses Viscosity, strain rate
dy
du y u(y)
No slip at wall
• Viscosity of Blood = 3 3.5 times of water
• Blood acts as a non-newtonian fluid in smaller vessels (including capillaries)
Cardiac Output• Flow of blood is usually measured in l/min• Total amount of blood flowing through the
circulation = Cardiac Output (CO)
Cardiac Ouput = Stroke Vol. x Heart Rate
= 5 l/min
Influenced by Blood Pressure & Resistance
Force of bloodagainst vessel wall
•Blood viscosity•Vessel Length•Vessel Elasticity•Vasconstriction / Vasodilation
with water retention with dehydration, hemorrage
Overall
• Greater Pressure Greater Blood
Differences Flow
• Greater Resistance Lesser Blood Flow
Driving force for blood flow is pressure created by ventricular contraction
Elastic arterial walls expand and recoil
continuous blood flow
Blood PressureBlood Pressure
Blood pressure is highest in the arteries (Aorta!) and falls continuously . . .
Systolic pressure in Aorta: 120 mm Hg
Diastolic pressure in Aorta: 80 mm Hg
Diastolic pressure in ventricle: ?? mm Hg
Ventricular pressure difficult to measure
arterial blood pressure assumed to indicate driving pressure for blood flow
Arterial pressure is pulsatile
useful to have single value for driving pressure: Mean Arterial Pressure
MAP = diastolic P + 1/3 pulse pressure
Pulse Pressure = systolic pressure - ??
= measure of amplitude of blood pressure wave
MAP influenced by
• Cardiac output
• Peripheral resistance
MAP CO x Rarterioles
• Blood volume– fairly constant due to homeostatic mechanisms
(kidneys!!)
BP too low:BP too low:
• Driving force for blood flow unable to overcome gravity
O2 supply to brain
Symptoms?
BP too high:BP too high:
• Weakening of arterial walls - AneurysmWeakening of arterial walls - Aneurysm
Risk of rupture & hemorrhageRisk of rupture & hemorrhage
Cerebral hemorrhage: ?Cerebral hemorrhage: ?
Rupture of major artery: Rupture of major artery:
Auscultation of brachial artery with stethoscope
Laminar flow vs. turbulent flow
BP estimated by SphygmomanometryBP estimated by Sphygmomanometry
Principles ofPrinciples of SphygmomanometrySphygmomanometry
Cuff inflated until brachial artery compressed and blood flow stopped what kind of sound?what kind of sound?
turbulent flow
Slowly release pressure in cuff:
Pressure at which . . .
. . . sound (= blood flow) first heard:
. . . sound disappeared:
• Pressure can be stated in terms of column of fluid.
Pressure Units
mm Hg cm H2O PSI ATM
50 68 0.9 0.065
100 136 1.9 0.13
200 272 3.8 0.26
300 408 5.7 0.39
400 544 7.6 0.52
Pressure = Height x Density
or P = gh
If Right Atrial pressure = 1 cm H2O in an open column of
blood
Pressure in feet = 140 cm H2O
Rupture
Venous Valves
Density of blood= 1.035 that of water
Incompetent venous valves Varicosities
Actual Pressure in foot= 4-5 cm H2O
Pressures in the circulation
• Pressures in the arteries, veins and heart chambers are the result of the pumping action of the heart
• The right and left ventricles have similar waveforms but different pressures
• The right and left atria also have similar waveforms with pressures that are similar but not identical
1. The LV pressure begins to rise after the QRS wave of the ECG
2. Pressure rises until the LV pressure exceeds the aortic pressure
The blood begins to move from the ventricle to the aorta
3. As blood enters the aorta, the aortic pressure begins to rise to form the systolic pulse
4. As the LV pressure falls in late systole the aortic pressure falls until the LV pressure is below the aortic diastolic press.
5. Then the aortic valve closes and LV pressure falls to LA pressure
•The first wave of atrial pressure (the A wave) is due to atrial contraction•The second wave of atrial pressure (the V wave) is due to ventricular contraction
Normal Pressures
• RV and pulmonary systolic pressure are 12-15 mm Hg
• Pulmonary diastolic pressure is 6-10 mm Hg• LA pressure is difficult to measure because access
to the LA is not direct
• The severity of AS is determined by the pressure drop across the aortic valve or by the aortic valve area
• The high velocity of blood flow through the narrowed valve causes turbulence and a characteristic murmur AS can be diagnosed with a stethoscope
AS produces a pressure gradient between the aorta and LV
i.e. For blood to move rapidly through a narrowed aortic valve orifice, the pressure must be higher in the ventricle
Pressure Measurement
• Accurate pressure measurements are essential to understanding the status of the circulation
• In 1733 Steven Hales connected a long glass tube directly to the left femoral artery of a horse and measured the height of a column of blood (8 feet, 3 inches) to determine mean BP
• Direct pressure measurements are made frequently in the cardiac catheterization laboratory, the ICU and the OR
• A tube is inserted into an artery and connected to an electrical strain gauge that converts pressure into force that is sensed electrically
• The output of the transducer is an electrical signal that is amplified and recorded on a strip chart
• For correct pressure measurements the cannula and transducer must be free of air, the cannula should be stiff and short
Cardiac Output (CO)Measurement
• The measurement of blood flow through the circulation is usually done clinically using either the Fick method
• The Fick method states that the cardiac output is equal to the oxygen consumption divided by the arterial-venous oxygen difference
CO = Oxygen consumption / A-V O2
• The measurement is done by determining the oxygen consumption using respiratory gas measurements and the O2 content of arterial and mixed venous blood
• The mixed venous blood sample is obtained from a PA with a catheter
• The arterial sample can be drawn from any artery