Patient - Haemodynamics(unfinished) Flashcards
Cardiac output depends on what 2 factors?
Autonomic nervous input
Venous filling pressure
CO=?
CO=Cardiac output=HRxSV
HR=heart rate
SV=stroke volume
What drives filling of ventricles during diastole?
Venous pressure
What does the aortic pressure resist during systole?
Aortic pressure resists flow out of left ventricle during systole
Define preload
The amount of stretch in the heart prior to contraction
i. e.
- Tension in expanded ventricle (due to venous filling)
- Greater stretch = myocytes need more energy (and oxygen) per contraction
Define afterload
The arterial pressure the ventricle must push against
-greater contractile force = more energy (and oxygen) use
What sets energy consumption?
Preload and afterload
What do both preload and afterload depend on?
The vascular system
Apart from transporting blood to all tissues, what else does the vascular system do? What is haemodynamics?
- interacts with cardiac function to control blood flows and pressures throughout the body
- This aspect - the physics of blood flow - is called haemodynamics
Define haemodynamics
The physics of blood flow
What is flow resistance?
The relationship between pressure and flow
How is flow resistance controlled?
By changing vessel diameter (by constriction and relaxation of vascular smooth muscle)
Define resistance in haemodynamics
R= Pressure/Flow
What can resistance be applied to?
An individual vessel/tube
or
An entire vascular branch e.g.
The peripheral circulation or the pulmonary circulation
What is Poiseuille’s Law?
R= change in P/F = 128/pi x nL/D^4 It describes flow resistance (R) of an individual vessel or tube where n=blood viscosity L=length D=diameter
What is flow resistance proportional to?
What is it inversely proportional to?
Proportional to blood viscosity and length
Inversely proportional to diameter
What is blood viscosity dependent on?
Haematocrit and protein content
Describe how the haemodynamics of resistance differs when in networks of vessels
- resistance is additive in series (sum of all section)
- resistance within a section (in parallel) is inversely proportional to the number of vessels (~1/n)
Is resistance lower in parallel or series? (of blood vessels)
R is lower in parallel
The smallest vessels have the most or greatest resistance to flow?
The greatest resistance to flow because pressure drops across each section from the aorta to the capillaries, but flow is the same
Lower resistance is due to the smaller diameter
What is total peripheral resistance?
Give the PRU formula
Also called systemic vascular resistance
1PRU = 100mmHg/100ml/s =~ (MAP-RAP)/CO
Where PRU= peripheral resistance unit
MAP= mean arterial pressure =~100mmHg
RAP= Right atrial pressure =~ 0 (relative to atmospheric)
What is the normal physiological range of PRU?
0.2-4 PRU (peripheral resistance units)
Total pulmonary resistance=
Pulmonary circulation is a low resistance (low pressure) system
0.12PRU
i.e. 10% of the TPR
Mean pulmonary artery pressure minus left atrial pressure~12mmHg=12PRU
Normal physiological range of the pulmonary resistance?
0.03 to 1 PRU
Vessel structure from inside to outside
1 - Endothelium (all vessels)
2 - Smooth muscle (arteries-thick, veins-thinner, not in capillaries)
3 - Connective tissue (adventitia), arteries and veins, not in capillaries
Increase in vascular smooth muscle tone causes..
reduction in diameter
Reduction in diameter causes..
greater flow resistance (only really significant in the smallest vessels, arterioles, capillaries, venules)
Greater flow resistance means
Reduced volume
increase in venous pressure and cardiac filling
Particularly important effect in large veins
Vessel diameter affected by balance between what 2 factors?
What do these factors affect?
2 factors = internal pressure and tension in vessel wall
They affect the volume of blood and resistance to flow
What is critical closing pressure
When arterial pressure falls to <20mmHg, tension in the artery wall collapses the vessel and shuts down flow
What is flow autoregulation?
If pressure surges, stretch-gated receptors in smooth muscle react to increase tone (acting to increase flow resistance and minimise changes in flow)
If pressure gets to low, what happens?
Reach critical closing pressure
Vessel will collapse
