Haemodynamics

Question 1

what is haemodynamics?
how would u describe BP - constant?

Answer 1

Haemodynamics is the study of the physical and physiological principles governing the movement of blood through the circulatory system.

Blood pressure is, of course, not constant but pulsatile

Question 2

1. what is the definition for:

i) systolic BP
ii) diastolic BP

2. what is normal systolic and diastolic BP at brachial artery?
3. what are the marginal and definite hypertension thresholds for S&D BP?

Answer 2

i) systolic BP: is maximum pressure experienced in the arteries
ii) diastolic BP: minimum pressure in arteries

2. brachail artery BP: 120/80 mmHg
3. >140mmHg is systolic marginal hypertension
   * *>160mmHg** is definite intervention threshold

• *>90mmHg** is diastolic marginal hypertension
• *>100mmHg** definite intervention threshold

Question 3

what is pulse pressure?

how do u calculate MAP?

Answer 3

Pulse pressure is the difference between systole and diastole. As arteries are elastic, pulse pressure normally decreases slightly from aorta to brachial artery

Mean arterial pressure (MAP) is calculated as diastolic plus 1/3 pulse pressure

Question 4

how does pressure change in arterial tree? what does this mean anatomically?

what is pulse pressure like in aorta and large arteries? c.f arterioles?

Answer 4

aorta and conducting arteries: the high elasticity allows them to handle pressure well

elasticity changes in the blood vessels as you move further and further away from the heart, the pressure in which the vessels can handle drops.

pulse pressure is much the same in the aorta & the large arteries. The pulse disappears in the arterioles. Arteriolar constriction controls blood distribution

Question 5

what controls blood distribution?

Answer 5

blood distribution being controlled by arteriole constriction.

Question 6

why is there a difference between aortic pressure and aortic flow?

what does this mean?

Answer 6

flow is not proportional to pressure:

- get big aortic flow drop, but only a little pressure drop

means that during systole, more blood enters the aorta than leaves, so it distends

changes due to compliance

Question 7

what is compliance?

what is it due to?

why is useful?

Answer 7

definition: the ability of a vessel to respond to an increase in pressure by to distending or swell and increase the volume of blood it can hold, or with decreased pressure, a decrease in volume.

due to: due to elastin fibres in the arterial walls.

useful: 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.

Question 8

what happens to old people / smokers regarding compliance? why effect does this have on BP?

which curve on the graph is due to old people / young people?

Answer 8

= arteries lose 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. It can decrease the cardiac output as the isovolumetric phase of ventricular contraction is longer.

//

• *on graph:**
• in young people, curve shown on the diagram is steeper as there is a big increase in volume for a given change in pressure, yet this flattens out as you get older. This is due to the loss of elastic fibres and replace in collagen as mentioned above.

Question 9

why is a strong pulse in an elderly persons wrist actually bad?

Answer 9

(Pulse in an elderly person may seem very strong)

isn’t healthy due to the fact the reduction of pulse pressure is not happening as it travels from the aorta to distributing arteries/arterioles. This means the whole pressure found normally in elastic/conducting arteries is being transmitted to the wrist.

Question 10

what is the Windkessel effect?

Answer 10

The walls of the aorta and elastic arteries distend when the blood pressure rises during systole and recoil when the blood pressure falls during diastole. There is a thus net storage of blood during systole which discharges during diastole.

Without this distension and recoil they would easily become damaged when the pressure rises, and their ability to accommodate the rapid blood flow is beneficial for keep pressure high

Question 11

why is hypertension bad?

what is prehypertension? what are the values for high, pre-high and normal BP?

Answer 11

bad bc: such an important risk factor for vascular disease.
AND
a problem partly because the person is often unaware anything is wrong.

Prehypertension = a systolic pressure from 120 to 139 mm Hg or a diastolic pressure from 80 to 89 mm Hg

Question 12

what regulates blood flow if BP is high and steady?

* if pressure is constant, what is relationship to resistance? - high and constant BP ensures that local vasodilation does.. to local blod flow?

Answer 12

Blood flow through a particular organ can be regulated by relaxing or constricting its input arterioles.

If pressure is constant, flow is inversely proportional to resistance. A high and constant blood pressure ensures that local vasodilation is effective in increasing local blood flow

Question 13

what is Poiseuille’s Law - how does it relate to arterioles?

Answer 13

This law states that the flow of liquid through a tube depends on the fourth power of the radius of the tube

At a fixed pressure differential between the two ends of a rigid tube, flow is proportional to (radius of tube)⁴ .
SO
if you keep pressure constant but double the radius you increase flow by 2⁴ , which is 16 times the previous level. In a rigid tube this equation applies to the flow at all times.

therefore with arterioles:

• Arterioles are SMALL, and often not much bigger than capillaries.
• Because of Poiseuille’s law, a decrease of diameter from say 25 µm to 20 µm could reduce flow to nearly half! Thus, the smooth muscle only has to constrict slightly to affect a significant reduction in flow.

Question 14

what is cardiac output?
what is stroke volume?
How do you measure cardiac output using stroke rate?

Answer 14

• *cardiac output:** the total blood flow out of the heart (litres/min)
• *stroke volume:** is the volume of blood pumped from the left ventricle per beat
• Cardiac output refers to the volume of blood the heart pumps out per minute, but stroke volume refers to the volume of blood pumped out of the left ventricle of the heart during each systolic cardiac contraction.*

heart is a pump, so its output will be the stroke volume multiplied by the heart rate Therefore, cardiac output is simply heart rate multiplied by stroke volume.

Question 15

what is value for cardiac output for healthy resting man?

Answer 15

• Cardiac output is approximately 5 litres per minute in a healthy resting man

Question 16

how can you measure cardiac output through a doppler ultrasound?

what is difference between ^ and ECG?

what is a Transoesophageal Doppler?

Answer 16

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 shift can then be used to calculate flow velocity

This measures blood in the aorta by bouncing sound waves off of blood coming into the aorta, whereas the echocardiography only measures heart size.

Transoesophageal Doppler:
An ultrasound probe is inserted into the oesophagus to mid-thoracic level. The probe measures blood velocity in the adjacent descending thoracic aorta. As the Doppler monitoring measures the velocity of blood, the method relies on a nomogram based on patient age, height and weight to convert the measured velocity into stroke volume and cardiac output.

Question 17

which factors effect HR and which affect stroke volume?

Answer 17

Question 18

how much ml per minute of blood does the following need?

brain, heart & kidneys?

what % do these three take up of overall cardiac output?

Answer 18

• Your BRAIN needs about 700 ml per minute of blood (14-15% resting Cardiac Output)
• Your HEART needs about 200 ml per minute of blood (4% resting cardiac output)
• Your KIDNEYS take about 1250 ml/min (25% resting cardiac output)
• These three organs take up about 40-45% C.O at rest.

Question 19

how many L (from initial 5L/min) is left after heart, brain and kidney have taken their share?

where does this go depend on?

Answer 19

After heart, brain and kidneys have had their share of the cardiac output only about 2.7-3.0 L/min is thus available for the rest of the body.

Where ^ goes depends on your 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

(so if u exercise after eating, blood isnt in the muscles, cuz gone to blood = throw up to get the food out so can be redirected :) )

Question 20

What mechanisms increase cardiac output during exercise?

Answer 20

- Cardiac output = stroke vol. x heart rate

• SO to increase CO, can increase both of SV (up about 1.5 times, from about 70 ml to 100ml) & HR (up to 2.5 x resting)
• 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 cardiac output of normally fit adult.

Question 21

in exercise, which factor increases cardiac output the most?

Answer 21

Note that most of the increase in C.O. in exercise is due to an increase in heart rate (from 72 to 190 i.e. 2.6 times) rather than stroke volume (from 70 to 100 i.e. 1.4 times)

Question 22

What controls agaisnt heart rate not increasing and increasing?

Answer 22

Due to diastole.

If you go too fast, you shorten diastolic filling time. If this becomes too short, there is not time for the atria/ventricles to fill/ The actual heart rate depends on the physical size of the heart.

Question 23

Which two factors influence the work of the heart?

Answer 23

• *1. Diameter of arterioles (main)
  2. Viscosity of blood**

Question 24

what influences viscosity of blood? (4)

what happens if viscosity is too high / low?

Answer 24

• *blood viscosity depends on:**
  1. (mainly) on the haematocrit (proportion of red cells in blood, normally ~45%

2. On the mechanical properties (mainly the deformability) of the red cells:
i) capillaries may be 5 µm or less and smaller than the diameter (7 µm) of the red cells.
ii) red cells have to bend or deform to get through capillaries
SO
-If the cells are too large or malformed (e.g. as in sickle cell disease) they clog up the capillary and oxygen delivery is compromised

3. red blood cell aggregation

4. plasma viscosity.

if viscosity is:
- too high: heart has to work much harder to pump the blood around the body.

-too low: not enough oxygen is transported.

Question 25

do rbc touch the endothelium? why ?

how else do rbc interact with capillary walls? what is a consequence of this?

Answer 25

Erythrocytes have a negative charge on the outside of their membrane. Endothelium also has a negative charge: thus, the erythrocytes do not physically touch the endothelial walls but are repelled by the electric charge

ALSO

• The edges of the erythrocytes do interact with **polypeptide chains protruding into the lumen of the capillary
• As the erythrocytes move along they deflect these chains which act toallow calcium to enter the endothelium.**
• This triggers formation of nitric oxide (NO), which relaxes and dilates the walls and acts as a local anticoagulant.

Question 26

what effect does rbc having greater diameter than capillaries have?

Answer 26

expect that the effective viscosity of the blood in the capillaries was 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.

Question 27

what happens to blood when it is stagnant?

name two situations when blood might become stagnant?

Answer 27

clots may form in the stagnant blood.

clots create risk of stroke (esp. due to atrial fibrillation) and DVT

Question 28

what is polycythaemia? what is absolute polycythaemia? what is relative polycythaemia?

Answer 28

Polycythaemia is a disease state in which the haematocrit increases. It can be due to an excessive production of red blood cells, absolute polycythaemia, or to a decrease in the volume of plasma, relative polycythaemia

Question 29

what is leplaces law?

how does this relate to arterioles and capillaires?

Answer 29

Laplace’s law states that the pressure that an elastic wall can withstand depends on the tension produced in the walls by their elasticity, divided by the radius (diameter) of the vessel

The smaller the radius of a vessel, the greater the pressure that a given wall strength can withstand. Thus, small diameter arterioles only need thin walls to withstand normal arterial pressures

Because capillaries have such a small diameter, they can withstand fluid pressures of >20 mm Hg using only the small tension generated by the basement membrane. They have no smooth muscle in their walls, as any form of muscle would impede the exchange of fluids and gases.

Question 30

what is an atheroma?

what effect do atheromas have on BV ability to withold pressure?

Answer 30

An atheroma is a fatty deposit on the inside of an artery.

If an artery wall is weakened or gets a tear, its radius slightly increases and so the balancing pressure (from Laplaces law) that the elastic tissue generates is less.

The wall will therefore balloon out, and this will further reduce the effectiveness to withstand the pressure. Eventually, an aneurysm may occur

Question 31

what is an aneurysm?

where do they usually occur?

why do they occur?

Answer 31

aneurysm: a bulge in a blood vessel caused by a weakness in the blood vessel wall

location: usually at branch points. often occur in abdominal aorta or in the cerebral arteries (due to lots of branches)

why?:

• highly associated with **ATHEROMA formation
• The narrowed lumen (diameter) of the artery in the atheroma plus turbulent flow canproduce local ‘spikes’ of high pressure in the artery.** (especially if the person has high blood pressure anyway).
• often a local INFLAMMATION reaction in the atheroma which causes destruction of elastin fibres in the artery wall, weakening it and making it more prone to stretch when there is a blood pressure spike.

get a vicisous circle: more the aneurysm inflates, the weaker it becomes, so it inflates more

Question 32

how can u detect aneurysm? (3)

how can u get rid of cerebral aneursym?

Answer 32

1. incidentally during MRI or angiography,
2. rupture causing subarachnoid brain haemorrhage,
3. present with symptoms of mass effect on neural structures.

Cerebral aneurysms can be treated by clipping or coiling.

Question 33

which graph shows high compliance and low compliance?

Answer 33

Question 34

a high blood pressure ensures that local X of arterioles is effective in Y local blood flow?

Answer 34

a high blood pressure ensures that local dilation of arterioles is effective in increasing local blood flow?

Question 35

what is crucial for a low total peripheral resistance in arterial tree?

high viscosity is particularly damaging to which circulation?

Answer 35

flexibility of erythrocytes is crucial for low total peripheral resistance in aterial tree

high viscosity is particularly damaging to pulmonary circulation: where BP is low, so viscosity needs to be in normal range