Haemodynamics Notes

Haemodynamics

Hemodynamics describes the relationship between blood flow, blood pressure, hydraulic resistance. The sole function of the circulatory system is transportation.

Flow

Difference in volume/difference in time-Units: Litres/min

Flow is a rate : another way of measuring flow is by velocity x area = cm3/seconds- so branching increases the cross sectional area but decreases the velocity

-Darcy’s law: Flow is proportional to pressure difference, the proportionality coefficient is hydraulic conductance (ease of blood flow) the reciprocal of hydraulic conductance is hydraulic resistnace

The flow through systemic circulation is measured by the cardiac output

-one way of measuring cardiac output = stroke volume (70ml) x Heart rate (70bpm)

- also calculated by the pressure difference (mean aortic pressure minus central venous pressure) and systemic resistance is total peripheral resistance

According to darcy’s law, Cardiac output (roughly 5l / minute)

CO = (Mean arterial pressure – central venous pressure ) / Total peripheral resistance

(MAP - CVP) = (CO x SVR) if rearranged CO = [(MAP - CVP) / SVR]

-central venous blood pressure is close to atmospheric pressure = 0 so equation can be simplified to C0=Arterial pressure/total peripheral resistance

-mean arterial pressure is determined by cardiac output x total peripheral resistance

Blood pressure- driving force for constant blood flow

-blood pressure is measured by the height it can drive a coloumn of liquid-

-Force = Area x height x density of the liquid in the coloumn (p) x gravitational acceleration (g)

Volume x density = mass

Pressure = area x height x density x gravity / area

= Height (h) x gravity (g) x density (p)

-units of blood pressure mmHg (millimeters of mercury)- if measuring aterial pressure or cmH20- used to measure venous pressure and kPA

Measuring arterial blood pressure

-blood pressure is measured using a blood pressure gauge- Sphygomomanometer- this measures aterial blood pressure in millimeters Hg

-inextensible cuff containing an inflatable bag is wrapped around the arm and the bag is inflated to a pressure level above the expected systolic pressure

-this occludes (squeezes the vessel shut) the underlying brachial artery and halts blood flow downstream and the pressure in the cuff is measured by mercury

-a stethoscope is placed over the brachial artery in the hollow of the elbow- when the brachial artery is fully occluded- no sound is heard as no blood is flowing but when the cuff pressure is gradually lowered a sequence of sounds are found

-when the cuff pressure falls below systolic pressure the artery open briefly during each systole- there are spurts of blood and these vibrate the artery wall- creating a dull tapping sound Korotkoff sound- the pressure at which the Korotkoff sound first appears is the systolic pressure

-as the cuff pressure is lowered the Korotoff sounds grow louder because the spurts of blood grow stronger

-when the cuff pressure is lowered further the Korotkoff sounds diminish- diastolic pressure

-blood pressure is not constant over time

-Normal systemic aterial pressure: systolic 120mmHg, diastolic 80mmHg

Mean arterial pressure

-mean pressure is halfway between systolic and diastolic pressures in the brachial artery

Mean brachial artery pressure= Pressure (diastole) + Pressure (systolic)- Pressure (diastolic)

3

-mean pressure is around 90mmHg

-Factors that change the mean arterial pressure is cardiac output and totoal peripheral resistance

Mean blood pressure = cardiac output x Total peripheral resistance

The aterial pressure pulse

-it is the pressure felt when feeling a pulse and it is caused by the difference between systolic and diastolic pressures -if the resting blood pressure (systolic/diastolic) pressure is 120/80- the pulse pressure is 40 mmHg

-the pulse which creates pressure waves is transmitted on the surface of blood vessels is transmitted faster than blood and it decays over distance (aorta-highest, arterioles, capillaries, veins)

-blood is incompressible, blood ejected into the proximal aorta causes distension and pushes the blood forward previously occupying the space- the displaced blood then distends the wall downstream and displaces the blood ahad

-factors governing the pulse pressure = stroke volume/arterial compliance

-stroke volume- minus the runoff during the ejection phase –greater the stroke volume the bigger the pulse pressure

-arterial stiffness (elastance) – 1/compliance –greater the stiffness of the artery wall the bigger the pulse pressure- arterial stiffness increases with mean pressure, ejection and age

-the proximal pulse is transmitted rapidly to the peripheral arteries- pulse travels -4/5 ms in young people and 10-15m/s in elderly. However pulse transmission velocity is much faster than blood velocity 0.2m/s

Factor affecting arterial pulse pressure : arterial compliance

-compliance is measure of distensibility- arterial compliance is defined as the increase in blood...

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