Respiratory Mechanics Flashcards
How is air flow generated?
by pressure gradients
What is Poiseuille’s Law equation? (using flow rate)
Q = πPr^4 / 8ηl Q = Flow rate P = pressure r = radius η = fluid viscosity l = length of tubing
What is Poiseuille’s Law equation? (using resistance)
R = 8ηl / πPr^4 R = resistance P = pressure r = radius η = fluid viscosity l = length of tubing
What is Poiseuille’s Law?
describes factors that affect resistance
What does halving tube diameter do to resistance?
increases resistance by 16 times
What components is blood pressure a product of?
Blood pressure = Cardiac Output (blood entering system) x Total Peripheral Resistance (size of tubes in system)
What happens to the airways as lung volume increases?
conductivity + dilation of the airways increases
What are the characteristics of small arteries + arterioles?
extensive smooth muscle - to regulate diameters + resistance to blood flow
What are the characteristics of veins + venules?
highly compliant - act as a systemic blood reservoir
Why does pressure in blood vessels fall across circulation?
viscous (frictional) pressure losses
Which blood vessels present the most resistance to flow?
small arteries + arterioles
Why is the relationship between blood pressure, cardiac output + resistance an approximation?
3 assumptions: • steady flow (which does not occur due to the intermittent pumping of the heart) • rigid vessels • right atrial pressure is negligible
What is regulation of flow achieved by?
variation in resistance in the vessels while blood pressure remains constant
What is MAP?
mean arterial pressure, also blood pressure
How is MAP calculated?
- MAP = diastolic BP + (systolic - diastolic BP)/3
* MAP = diastolic BP + (pulse pressure/3)
What 3 variables does resistance of a tube to flow depend on?
- fluid viscosity
- length of tube
- inner radius of tube
What does Poiseuille’s equation emphasise?
importance of arterial diameter as a determinant of resistance
How do small changes in vascular tone affect flow?
small changes in vascular tone = large changes in flow rate
What are the 2 types of fluid flow?
- laminar
* turbulent
What is laminar fluid flow?
- velocity is constant at any point
- flows in layers
- blood flows fastest closest to the centre of the lumen
What is turbulent fluid flow?
- erratic blood flow
- forms eddys
- prone to pooling
- associated w pathophysiological changes to endothelial lining of blood vessels
Where is blood pressure usually measured + why?
- measured on upper arm
* easily accessible + at heart-level
How is blood pressure measured?
slow deflation of blood pressure cuff causes TURBULENT FLOW - heard with sthetoscope
How can pulse pressure be calculated using BP measurements?
systolic BP - diastolic BP = pulse pressure (PP)
What is the ratio of systole to diastole?
1 : 3
What happens to transmural pressure + the airways during inspiration?
- transmural pressure is POSITIVE
* airways remain PATENT (open)
What effect does severe transmural pressure have on different sized airways?
- collapse in small-to-medium airways
* no collapse in large airways due to cartilage rings
What is compliance?
- tendency for tissue to distort under pressure
* change in volume / change in pressure
What is elastance?
- tendency for tissue to recoil to its original volume
* change in pressure / change in volume
Why do ventricular + aortic pressures differ?
- aortic valve closing
- ventricular pressure falls rapidly
- aortic pressure falls slowly
What explains the ventricular + aortic pressure differences?
elasticity of the aorta and large arteries which act to buffer the change in
pulse pressure
What is arterial compliance?
ability of the arterial wall to distend + increase volume w/ increasing transmural pressure
What happens to blood flow in the arteries during ejection?
blood enters arteries FASTER than it leaves them
How does pressure change when the aortic valve closes?
ejection ceases but due to elastic recoil of arteries, pressure falls slowly + there is diastolic flow in the downstream circulation
What is the Windkessel effect?
large central arteries (e.g. aorta) are elastic = acts as a reservoir during systole + stores some ejected blood forced out during diastole
What effect does reducing arterial compliance have on the Windkessel effect?
damping effect of Windkessel effect is reduced + pulse pressure increases
What is venous return facilitated by?
- skeletal muscle pumps in lower limbs
* respiratory muscle pumps in thorax
What is varscosity?
incompetent valves cause dilated superficial veins in the leg
What is oedema?
prolonged elevation of venous pressure (even with intact compensatory mechanisms) causes oedema in feet
What is an aneurysmal disease?
vessel walls weakening causing balloon-like distention = aneurysm
What effect do vascular aneurysms have on the radius of a blood vessel?
increases radius
What effect do vascular aneurysms have on the forces acting on a blood vessel?
for the same internal pressure, the inward force exerted by muscular wall must INCREASE
Why do aneurysms rupture?
force exerted by muscle wall must increase but muscle fibres have weakened - force needed can’t be produced so aneurysm will expand + rupture
What is the difference between venous + arterial compliance?
venous compliance = arterial compliance x 10-20 times
If arterial compliance decreases, what effect does this have on blood pressure?
- systolic increases
* diastolic decreases
What does increasing smooth muscle contraction do to venous volume + pressure?
- decreases venous volume
* increases venous pressure
How does ventilation differ across the lungs?
- less ventilation at the top
* more ventilation at bottom
Why is there less ventilation towards the top of the lungs?
- intrapleural pressure is MORE negative
- GREATER transmural pressure gradient
- alveoli larger + less compliant
Why is there more ventilation towards the bottom of the lungs?
- intrapleural pressure is LESS negative
- SMALLER transmural pressure gradient
- alveoli smaller + more compliant
How does perfusion differ across the lungs?
- lower flow rate towards top
* higher flow rate towards bottom
Why is there lower flow rate towards the top of the lungs?
- Lower intravascular pressure (gravity effect)
- Less recruitment
- Greater resistance
Why is there higher flow rate towards the bottom of the lungs?
- Higher intravascular pressure (gravity effect)
- More recruitment
- Less resistance
What varies more between base and apex; ventilation or perfusion?
perfusion
What does V/Q ratio stand for?
ventilation/perfusion ratio
What is V/Q ratio used for?
to assess to efficiency + adequacy of the matching of ventilation + perfusion
What is a V/Q ratio?
ratio of:
• amount of air reaching the alveoli per minute :
• amount of blood reaching the alveoli per minute
What are ventilation + perfusion the main determinants of?
blood O2 + CO2 concentration
How is V/Q ratio measured?
ventilation/perfusion scan
What can a V/Q mismatch cause?
type 1 respiratory failure
What is a typical V/Q ratio value of the lung as a whole?
0.8
How does V/Q ratio vary in the lung?
ratio value varies depending on position within lung
How would COPD affect compliance?
emphysemic breakdown of structural lung tissue INCREASES compliance
How does COPD affect resistance?
bronchitic swelling of the airways + mucus hyper-secretion INCREASES resistance to airflow
Why doesn’t resistance continue to increase as airways get smaller?
resistance to airflow depends on no. of parallel pathways present - many many alveoli vs. two bronchi…
What happens to airway resistance as lung volume increases?
airway resistance decreases
Why does airways resistance decrease as lung volume increases?
airways distend as lungs inflate + wider airways have lower resistance
