00:00
So, moving on now we're
going to discuss a little bit
about acid-base disorders and I'm coming to
this really because of respiratory acidosis
and type II respiratory failure. One of the
major reasons why we do blood gases is not
just to measure the PO2 and the PaCO2 but
is actually to assess the acid-base balance
of the patient. And if you have lung problems,
there are two issues, you may have respiratory
acidosis and that is a situation that occurs
because when you underventilate the lung and
you're not able to get rid of the carbon dioxide
present in your blood, and that will lead
to a low pH, a high PaCO2., and as it becomes
compensated, as the body adjusts to the fact
that it is unable to get rid of the carbon
dioxide, then you will end up with the buffers
that compensate the acidosis, increasing and
we can see that in the blood gases by a high
bicarbonate level and by positive base excess.
The other respiratory problem that you get
with acid-base balance is respiratory alkalosis,
that is a problem of hyperventilation, you
breathing fast, and that blows off your carbon
dioxide and causes a respiratory alkalosis.
01:10
It is not a major problem, but it is one of
the reasons why you will feel fainting if
you breathe fast. And in fact, actually we often
create a little bit of respiratory alkalosis
in our patients when we're examining them because
we ask them to take deep breaths when we're
listening to their lungs. If they begin to
feel dizzy, that's reflecting a little bit
of respiratory alkalosis. I'm not going to discuss
metabolic acidosis
or metabolic alkalosis, I think the important
one there is in fact the metabolic acidosis
and we actually do see that in lung disease
but that's a consequence of the severe lung
disease causing hypoxia or if you have pneumonia,
septic shock, and that leading to metabolic
acidosis as the oxygen delivery to the muscles
and peripheral tissues is impaired.
01:54
So, type II respiratory failure. We've mentioned
type I, that's hypoxia with normal carbon
dioxide, type II is respiratory failure associated
with raised carbon dioxide. So it's defined
as a high carbon dioxide, greater than 6,
and, as I've just mentioned in the previous
slide the higher the CO2, the more likely
you are to have acidosis. Now that can be
compensated. So for example, you may get somebody
with an acute asthma attack, who is normally
has got a normal PCO2, comes in to a hospital
with a severe asthma attack, the CO2 is rising
because they've almost got status ashtmaticus
and may need ventilation. So they have a CO2
of 6, they will be quite acidotic because
it's an acute event and compensation has not
had time to occur. Now, in contrast, patients
with COPD who may have a degree of type II
respiratory failure chronically, they come
into hospital, they can have a CO2 of 10,
12 even higher than that, and that pH may
still be considerably higher than the patient
presenting with acute asthma and a CO2 of
only 7 and that's because of compensation.
03:08
And in that situation the COPD patients with
a bicarbonate of 35, approaching 40 perhaps.
03:16
The big problem with type II respiratory failure
is that oxygen can be dangerous and we see
this repeatedly in patients presenting with
COPD with an acute exacerbation, who have
been given a high flow oxygen on the way into
hospital. And that high flow oxygen unfortunately
leads to an increase in the respiratory acidosis.
The mechanism of that is that if you have
type II respiratory failure, your ventilatory
drive is actually taken over by the low oxygen
level in your blood, so having a low oxygen
keeps the ventilator drive going when you
have type II respiratory failure. And if you
give the patient too much oxygen and it comes
up too high, you lose that ventilatory drive.
The hypoxic drive for breathing is lost, and
as a consequence you actually create a situation
with a hypoventilation, the underventilation
of the lungs, becomes more of a problem. As
a consequence of that, the PCO2 will, PaCO2
will rise, and the pH will fall, and the respiratory
acidosis will worsen.
04:24
So the underlying principle for the treatment
of type II respiratory failure is to give
them controlled oxygen supplementation, but
aiming for a lower saturation than you would
normally in somebody with type I respiratory
failure.
04:37
A sort of safe PO2
of around 8 kPa where the risk of hypoxia
causing a cardiac respiratory arrest is much
lower than it would be, but still the POa2
is not in the normal range. And you do that
by giving the patient a controlled oxygen.
04:56
Importantly if patients are
receiving oxygen and they have type II respiratory
failure, you need to monitor what's happening
to the carbon dioxide level in the blood.
05:06
And to do that you have to repeat the blood
gases and that will tell you whether the oxygen
therapy is causing a rise in the CO2 and a fall
in the pH and therefore it needs to be titrated
downwards to a lower concentration of oxygen.