So the question is how external PEEP can help in dynamic hyperinflation syndrome
we will try to answer this question in this short video here
and for that purpose we're going to have this graph: on the left side we have a
patient who has no auto-peep and on the right side we have a patient who has an
auto PEEP of 10 centimeter of water. Now notice on the left side if the patient
has no auto-peep in order for this patient to inhale he needs to create a
gradient.. so at the end of expiration the airway pressure would be zero, alveolar
pressure is zero and it's assumed that the pleural pressure is also zero
In order for this patient to create a gradient between the airway
pressure and the alveolar pressure, all what he has to do is .. contract
his inspiratory muscles to decrease the pleural pressure down to -5 cm H2O
by decreasing the pleural pressure to -5 cm H2O
the alveolar pressure will go down.. let's say it goes down to -4 or -5 cm H2O
this will create a gradient between the airway pressure and the alveolar pressure
and the patient will inhale. Now what happens if
the patient has auto-peep, let's assume that patient has auto-peep of 10 cm H2O
at the end of expiration the airway pressure will be zero
the alveolar pressure will be 10 cm H2O. Some of this PEEP
will be transmitted to the pleural space and it's assumed that 30 percent of this
is transmitted so the pleura pressure now is 3 cm H2O
In order to inhale, this patient has to create a gradient between the 0 and the 10
in order to do that the patient has to drop the alveolar pressure from 10 down to - 3 cm
he will need to generate a negative plural pressure of -13 cm H2O. In order to generate -13
centimeter of water you need to overcome this 3 cm H2O he will end up
generating a force that will decrease the pleural pressure by 16 cm H2O
so the work of breath for this patient will be tremendous
In spontaneously breathing patients, in order to inhale they need to create a greater
pressure gradient togenerate flow when auto-peep is present
Now let's see what happens if we add external PEEP to the circuit
So extrinsic PEEP will counteract the intrinsic PEEP and will decrease the work of breath for the patient
so let's see how at the end of expiration if external
peep is added, it will be headed to the whole circuit and then the tracheal
pressure will be at 10 centimeter of water, we already have an intrinsic
PEEP of 10 cm H2O in the alveoli, and as we said earlier some of
this pressure is transmitted to the plural pressure, now for this patient to
inhale he will need to create a gradient between the airway pressure
of 10 cm H2O and the alveolar pressure of 10 cm H2O
so all what he has to do is he needs to drop this pressure by 3 to 4 cm H2O
and he can drop that by contracting his inspiratory muscles
but all what he has to do is he needs to get that plural pressure to - 3 from +3
so he would need to generate only a force that will decrease the pleural pressure
by 6 cm H2O instead of 16 cm H2O as we saw in the earlier case
so by decreasing this pleural pressure to -3, the alveolar pressure
will go down from 10 to 7 and this will create a gradient between the
airway pressure and the alveolar pressure and the patient will inhale air
this way we will decrease the work of breathe of the patient
Of course in spontaneously breathing patient, you cannot do expiratory hold to check for the Auto-PEEP
and usually if you do that on the ventilator and you know the auto-peep of
let's say here 10 centimeter of water, the amount of extrinsic peep that you
want to add this to the circuit is somewhat between 70 to 80% of that
auto-peep. So if we have a patient on the ventilator you can measure
how much auto-peep he has and then you add.. if it is 10 centimeter you can add 7
or 8 centimeter of water to the circuit
but if the patient is spontaneously breathing
there is no way you can measure the Auto PEEP and at that time
you just add the extrinsic PEEP gradually you, increase the value
gradually, can you observe the work of the breath of the patient
by increasing the extrinsic PEEP
you should observe that the work of breathe is decreasing for that patient
as you go up on the extrinsic PEEP.
Now if you have esophageal monitor you will
be able to see the negative deflections or the excrusions the patient is doing
so this is the case where the peep is zero and you see how much negative
deflection, this is over the work of greed for the patient, so you can see how
much the patient is actually exerting himself to generate that flow and inhale
now by adding a PEEP of 10, you can see how these excrusions are decreased and
this means that the work of breath for this patient is decreased
I hope this explains how we benefit from adding extrinsic PEEP to patients who
have dynamic hyperinflation syndrome and auto-PEEP in relation to their work of breath
Thank you