Cardiovascular Techniques

Question 1

Basic structure of the vascular wall

Answer 1

• smooth muscle wraps around the vessels in circular fashion - goves tone
• perivascular nerves are in adventitia
• sympathetic nerve endings are in pervisacular nerves - constriction of underlyign smooth muscle
• endothelium, smooth muscle, perivascular nerves, elastic and fibrous tissue

Question 2

Vessel diameter and tone

Answer 2

Smooth muscle fibres contract, fibres get closer together, cause constriction and vice versa

Question 3

What is Isometric tension recording?

Answer 3

• measure tension generated by vascular wall while diameter of vessel remains constant
• large organ bath or wire myograph

Question 4

How do you set up large organ bath studies?

Answer 4

• FOR LARGE VESSELS
• mount segments as ring preparations on two fixed steel hooks
• keep in chamber with salt solution at 37 degrees with oxygen
• viable for over 6 hours
• very delicate technique - cant be too rough otherwise it wont act like it does in vivo

Question 5

How do you set up wire myograph studies?

Answer 5

• FOR SMALL VESSELS
• Mounted as ring on two fixed steel wires
• kept in chamber with salt solution at 37 degrees with oxygen
• viable for over 12 hours
• very similar to organ bath, measure isometric tension using two very thin wires connected to a transducer
• more stable temp than a big organ bath
• main problem is size - very small chamber (only slightly larger than 5p)

Question 6

How do we cause the vasocontraction?

Answer 6

• adding increasing conc of Methoxamine
• contracts small mesenteric artery
• EC50 at about 8uM
• Rmax of about 11mM

Question 7

Experimental design - Is methoxamine-induce contraction mediated by a1-adrenoceptors?

Answer 7

• make conc-contraction curves of methoxamine in presence and absence of an a1 antagonist (prazosin)
• see if there is still contraction with the antagonist
• Then test with a1 agonist and prazosin to check it works

Question 8

Experimental design - Is methoxamine as effective as the contractile agent endothelin?

Answer 8

• conc-response curves or both agents –> compare EC50 and Rmax
• use mesenteric arteries from other vascular regions and see if they have the same effect

Question 9

Experimental design - are contractions to a1-adrenoceptor agonist modified in hypertension?

Answer 9

• use vessels from control vs hypertensive patients - compare EC50 and Rmax values
• Use methoxamine or another a1 agonist
• isolate vessels from biopsys or small arteries from normotensive and hypertensive rats
• can use pre-existing hypertensive vessel, or induce it using AngII

Question 10

How do we induce the vasorelaxation?

Answer 10

• Carbachol
• NO - (L-arginine > NO > endothelium to muscle > activates Guanylyl cyclase > GTP to cGMP > relaxation)
• ACh (endothelium dependent - doesnt do the same when removed)

Question 11

Advantages of isometric tension recording

Answer 11

• examine function of small or large vessels
• viable for many hours if fresh
• detail investigation into mechanisms of action
• compare vascular functions in control vs disease states
• can be combined with other experimental techniques

Question 12

Disadvantages of isometric tension recording

Answer 12

• need skill to dissect and mount vessels
• isolation of vessels might remove influences from surrounding tissues
• difficult to study long-term changes using same vessels
• CANNOT MEASURE MYOGENIC CONTRACTIONS
• CANNOT MEASURE FLOW-INDUCED RELAXATIONS (no flow)

Question 13

How do we set up a pressure myograph?

Answer 13

• for small vessels - including arterioles
• keep in chamber in salt at 37 degrees with oxygen
• segments mounted on glass microcannulae - pressurised to appropriate transmural pressure
• MEASURING DIAMETER RATHER THAN TENSION
• isobaric because you can control the pressure

Question 14

How do we measure the diameter in isobaric diameter recordings?

Answer 14

• set the two pressures at either end of the chamber, there will be no flow - can look at diameter with no flow
• when we want to look at flow induced constriction or dilation, you can change these pressure gradient, causing increase in flow

Question 15

Looking at the myogenic response

Answer 15

• will happen in isolated tissue
• to do with transmural pressure (across wall)
• usually done in no flow setting
• start at low pressure, increase it step-wise
• at the beggining the vessel diamter will just be increasing, as you get higher, there will be a myogenic contraction
• used to conserve a constant blood flow

Question 16

What is shear stress?

Answer 16

Any newtonian fluid will cause a frictional force on the endothelium

Question 17

How do we look at flow-induced vasodilation?

Answer 17

• measure on pressure graph
• larger the diameter, the more relaxed the vessel is. When you apply a constrictor, it reduces the diameter, increasing pressure
• when you put flow into it, the shear stress causes vasodilation

Question 18

Advantages of isobaric diameter recording

Answer 18

• examine funciton of small vessels - including myogenic constriction and flow-induced dilation
• viable for many hours if fresh
• detailed investigation into mechanisms of action
• compare vascular functions in control vs disease
• can be combined with other experimental techniques

Question 19

Disadvantages of isobaric diameter recording

Answer 19

• dissection and mounting of vessels requires skill
• isolation of vessels may remove influences from surrounding tissues
• difficult to study longer-term changes in vascular function using same vessels

Question 20

Isolated vascular beds

Answer 20

• cannulate supply artery to vascular bed
• e.g. measure perfusion pressure under constant flow
• flow = perfusion pressure / vascular resistance
• if you keep flow constant, you can measure pressure, and changes in this will mean changes in vascular resistance

Question 21

What is Langendorff heart preparation?

Answer 21

• cannulate ascending aorta
• measure coronary perfusion pressure under constant flow
• viable for few hours
• can also examine LV contractions and HR if left beating naturally

Question 22

What is doppler flowmetry?

Answer 22

• measurement of blood flow in vivo with a stationary source and receiver
• measure doppler shift due to reflection by RBCs
• Doppler signal indicates amount of RBCs that flow through an area per unit time

Question 23

Laser doppler flowmetry

Answer 23

• for blood flow in arterioles or capillaries

- low power laser beam; passes through unbroken, non-pigmented tissue

Question 24

Ultrasound doppler flowmetry

Answer 24

• for blood flow in arteries

- high-frequency sounds waves for high velocity blood flow

Question 25

Advantages of doppler flowmetry

Answer 25

• examine changes in blood flow in vivo
• compare vascular functions in control vs disease
• can be adopted for non-invasive measurements
• allow long-term measurement of blood flow in same vessels

Question 26

Disadvantages of doppler flowmetry

Answer 26

• relative measurements of blood flow
• quality of signal depends on tissues and machines
• invasive procedures and anaesthesia may be necessary
• need experts to carry it out
• more difficult to do in vivo techniques

Question 27

Measuring arterial BP

Answer 27

• Cuff techniques

- Intra-arterial catheters

Question 28

SUMMARY

Answer 28

• isolated SMALL vessels - wall tension or diameter
• isolated vascular beds - perfusion pressure or flow
• laser or US doppler
• arterial catheter or cuff