111114 diffusion Flashcards
what is in the diffusion rate formula?
pressure difference
area
solubility
distance
square root of molecular weight
clinically, which factors might contribute to diffusion impairment in the lung?
SA
distance (thickness)
pressure gradient
what do the chemoreceptors sense?
the DISSOLVED gases, like oxygen. so if placed bag over head vs give oxygen/CO, both cases the person would die but the placed bag case would cause a more traumatic condition b/c you’re not getting the normal diffusion of oxygen.
role of hemoglobin in the pulmonary capillary?
it takes up the O2, so it creates that pressure gradient for O2 to dissolve in the blood. so much more volume of O2 crosses the respiratory membrane.
why is O2 more susceptible to diffusion impairment than CO2?
solubility-CO2 is 20x more soluble
solubility of O2
0.03 ml/L/mm Hg
normal stroke volume
70 mL
what is the normal oxygen requirement in a normal adult at rest?
250 mL O2/min (this is diffusion rate)
this is how much diffuses across to the capillary per minute
how many mL of O2 are removed on average from 100mL of blood passing through systemic capillaries?
5 mL
what is diffusing capacity-the eqn?
diffusion rate/(alveolar partial pres-mean capillary partial pressure)
why is CO used to calculate DL (diffusion capacity)?
there isn’t any CO dissolved in blood b/c of high affinity for hemoglobin, so you don’t have to measure the mean capillary pressure
there is always a gradient for CO to pass since barely any of it is dissolved in blood
methods of diffusion capacity?
steady state
single breath
steady state method for measuring DL-advantages/disadvantages?
more natural
easier for pts with lung disease
can be used during exercise
sometimes more invasive
what is the single breath methods for measuring diffusion capacity?
exhale to residual volume, full vital capacity inspiration of trace CO and reference gas, 10 second breath hold, then maximal expiration
the reference gas is not taken into the blood
what are advantages/disadvantages of single breath method?
rapid testing and immediate results
more difficult to perform–pts with lung disease may find it hard to take a deep breath and hold of 10 sec
values are higher than steady state DL
normal diffusion capacity values for males
low 30s in ml/min/mm Hg
normals will be different for different age, height, gender
how does exercise affect DL?
increases by factor of 3
in abnormal lung that takes 0.75 seconds for the PO2 to increase from 40 to 100mm Hg, what happens to DL?
decreases
you don’t get hypoxemic at rest. HOWEVER, there’s no reserve for exercise
why is CO2 not vulnerable as much to diffusion impairment?
more soluble
more carefully controlled (by central chemoreceptors vs peripheral chemoreceptors for O2). central chemoreceptors are very sensitive for CO2
diffusion capacity is affected by
membrane diffusing capacity
perfusion problems
ventilatory problems
in restrictive lung disease, what is DL/VA
normal (DL is decreased and VA is also decreased)
obstructive lung disease–what is DL/VA?
low (DL is decreased, VA is normal of even greater than normal)
emphysema
loss of elastic recoil-won’t deflate as well
so now need to use chest to compress. however, when you compress alveoli will compress airways at the same time. so as you’re pushing air out, you’re also squeezing the airway shut. so then you try and try–balloon pops
emphysema is a problem mainly during expiration
V/Q mismatch
shunt
or
dead space
