Cardiovascular Techniques Flashcards
Basic structure of the vascular wall
- 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
Vessel diameter and tone
Smooth muscle fibres contract, fibres get closer together, cause constriction and vice versa
What is Isometric tension recording?
- measure tension generated by vascular wall while diameter of vessel remains constant
- large organ bath or wire myograph
How do you set up large organ bath studies?
- 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
How do you set up wire myograph studies?
- 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)
How do we cause the vasocontraction?
- adding increasing conc of Methoxamine
- contracts small mesenteric artery
- EC50 at about 8uM
- Rmax of about 11mM
Experimental design - Is methoxamine-induce contraction mediated by a1-adrenoceptors?
- 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
Experimental design - Is methoxamine as effective as the contractile agent endothelin?
- 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
Experimental design - are contractions to a1-adrenoceptor agonist modified in hypertension?
- 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
How do we induce the vasorelaxation?
- Carbachol
- NO - (L-arginine > NO > endothelium to muscle > activates Guanylyl cyclase > GTP to cGMP > relaxation)
- ACh (endothelium dependent - doesnt do the same when removed)
Advantages of isometric tension recording
- 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
Disadvantages of isometric tension recording
- 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)
How do we set up a pressure myograph?
- 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
How do we measure the diameter in isobaric diameter recordings?
- 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
Looking at the myogenic response
- 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
What is shear stress?
Any newtonian fluid will cause a frictional force on the endothelium
How do we look at flow-induced vasodilation?
- 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
Advantages of isobaric diameter recording
- 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
Disadvantages of isobaric diameter recording
- 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
Isolated vascular beds
- 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
What is Langendorff heart preparation?
- 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
What is doppler flowmetry?
- 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
Laser doppler flowmetry
- for blood flow in arterioles or capillaries
- low power laser beam; passes through unbroken, non-pigmented tissue
Ultrasound doppler flowmetry
- for blood flow in arteries
- high-frequency sounds waves for high velocity blood flow
Advantages of doppler flowmetry
- 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
Disadvantages of doppler flowmetry
- 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
Measuring arterial BP
- Cuff techniques
- Intra-arterial catheters
SUMMARY
- isolated SMALL vessels - wall tension or diameter
- isolated vascular beds - perfusion pressure or flow
- laser or US doppler
- arterial catheter or cuff
