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okay now let's go to talk through the
different amounts of ventilation that a
person has so the classic formula for
ventilation
this is minute ventilation or total
ventilation is simply taking the
breathing frequency times the tidal
volume if we want to account for a viola
or ventilation now we need to account
for dead space so here we have the
volume of air that reaches out of the
mouth
it's the breathe in frequency times the
tidal volume minus the dead space so a
velar ventilation accounts for dead
space while minute ventilation does not
alveolar ventilation is more important
in pulmonary medicine than minute
ventilation but minute ventilation is
easier for us to measure because a dead
space is a little bit hard to measure so
let's take this example here of a
fourteen-year-old girl with a history of
asthma she does a number of pulmonary
function tests after taking a short
active beta adrenergic agonist a great
example this is albuterol her tidal
volume is 400 milliliters her breathing
frequency is 10 breaths per minute so if
you were calculating her minute
ventilation you simply take 10 times 400
and that would yield four thousand
milliliters per minute if you wanted to
calculate our alveolar ventilation we're
going to have to account for dead space
so in this case you'll take her tidal
volume 400 minus the dead space volume
which is 200 which would yield 200
multiply that by 10 which is a breathing
frequency and you get 2,000 milliliters
per minute so Avalar ventilations will
always be less than minute ventilations
but ovular ventilation tells you the
amount of gas that actually gets to the
point where gas exchange can occur
so there are ways to calculate this in a
more complex manner and this is shown
with this particular formula but we
don't have to worry about this formula
to the same extent because we just want
to make sure that the relationship
between a viola ventilation and co2 is
inverse of each other and I'll show you
this from some teeter-totter examples if
a velar ventilation goes up co2 has to
go down the alternate is also occurs as
the co2 goes up alveolar ventilation
rate goes down and where you can use
this clinically is all you have to do is
either measure the co2 or measure
alveolar ventilation and you'll be able
to predict the other so if someone has a
high co2 in their blood you
automatically know that they're a velar
ventilation rate is low and the inverse
of that is also true if you see that co2
is low you automatically know that
alveolar ventilation rate is high
ventilation also relies on lung
compliance so compliance is the inverse
of stiffness or how stiff the lungs are
normally you have a nice compliant long
however it can change in certain disease
states such as interstitial pulmonary
fibrosis in this case there is scar
tissue that forms in the lungs
increasing the amount of collagen fibers
and making the tissue less pliable or
less stretchable and therefore its
compliance has decreased there are other
clinical conditions such as emphysema
and an EM buzina there's an increase in
compliance and this is done because the
emphysema process pathophysiology breaks
down elastin fibers and therefore there
it's less it's increased its compliance
and it's less stiff you can think of it
as more floppy so if you want to think
of these three examples part of the best
way to do that
is to think of its overall compliance
now with compliance there are changes
that occur even in the lung itself
depending upon how much stretch is
produced by particular forces so in this
case gravity can cause a decrease in
lung compliance in top portions of the
lung there's an increase in compliance
at the base of the lung so if you're a
visual person I think the easiest way to
think about compliance is think about
two rubber bands if the rubber band is
not stretched it has higher compliance
if you've already fully stretched a
rubber band it is hard to stretch it to
a greater degree in this case its
compliance has decreased when it's
already pre stretched now why is this
important for overall ventilation
because ventilation has a greater
propensity to go to areas that have high
compliance so an area that is highly
compliant such as the base of the lung
will get more ventilation than areas
that are lower compliant such as the
apex of the law if we want to look at
this in terms of pathophysiology if you
have fibrotic areas of the lung with low
compliance ventilation or air flow will
not want to go to those areas but will
in fact go to others
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