Acute hypoxia is where we are, and -- or should I say acute hypoxemia.
Right, we always have to be a little bit more detailed. It’s a V/Q mismatch, improves supplemental oxygen.
So, this, for example, gives you examples such as pneumonia, COPD flares and PE.
Remember that no matter any time during a disease course of your lungs that supplemental oxygen is given,
but you always have to utilise caution because of that condition that we called your hypoxic drive.
But, if it’s COPD flares, then you are left with really not much of an option.
Something that you want to keep in mind and something that you very much want to understand
is the fact that what if your alveoli was to collapse.
This brings us to A L I, which stands for acute lung injury
or we have acute respiratory distress syndrome.
What these have in common is the fact that ultimately, the alveoli collapse.
And when your alveoli collapse, they’re no longer functional.
This is called a pulmonary shunt.
A shunt means that there is deoxygenated blood passing in the systemic circulation.
You should be familiar with the right-to-left shunting in the heart
that happens in cyanotic heart diseases, such as Tetralogy of Fallot.
So, why would you be so familiar with shunting in the heart and you’re not so familiar with the shunting in the lungs?
The only way that you can get shunting of your blood from the pulmonary arterial side,
which has normally a P O2 of approximately 40.
To get your P O2 of 40 to your pulmonary veins, you shunt across an alveoli that are no longer functional.
So, this is a intrapulmonary type of shunt.
Now, if there is an intrapulmonary type of shunt, if you were to give this patient oxygen,
if that alveoli are collapsed, how should you get a significant response?
You don’t get one. Now, the same thing can also be applied to intracardiac shunt.
For example, say that you had a right-to-left shunt at some point in time.
If you did, then you get deoxygenated blood into your left side, from your right side.
So you’ve shunted your lung and thus by giving oxygen, are you having a response that you expect?
No, you don't. Look for that particular clinical description.
What else may cause acute hypoxemia? Well, low FiO2. What does that mean?
Now be careful. You know me, at this point to be extremely, or try to be, as accurate as possible every single statement that I make.
And so, therefore, I’ve said earlier that FiO2 does not change on planet, Earth, right?
And by that I mean what?
Well, the fraction that you’re breathing in of oxygen on planet Earth is always going to be 0.2.
"So, Dr. Raj, why in the world are you seeing an FiO2 decreased here, especially in the sense of high altitude,
does not contradict with what you say?"
Well, you’re thinking it and I absolutely accept and welcome that question.
But, the fact is, if you’re going to high altitude, what is it that truly decreases? Your barometric pressure.
Now, you tell me, ambient air, what’s the simple equation of just ambient air?
It’s the fact that you take your barometric pressure and you multiple by what? FiO2.
So, at sea level, if it’s 760 that you’ve memorised, you multiple that by 0.2, what do you get? 160.
That’s normal. Sea level. FiO2 in totality.
However, if you take the same equation and you put it in to high altitude type of environment,
where now may be 250 times 0.2 then how low has that FiO2 dropped to?
Point is, you kept your 0.2 exactly the same. Ultimately, in high altitude, you have low FiO2.
Would that result in acute high altitude sickness? Oh, yes.
What else? Impaired diffusion. Now things become a little bit easier.
What this basically means is the fact that you’re not able to properly cross your barrier. What barrier?
The barrier between the alveoli and trying to get your oxygen into your pulmonary capillary.
Impaired diffusion, couple of important points that we already discussed.
A-a gradient widened or normal, please? Good, widened. What are some things that you wanna take in consideration for A-a gradient?
The age is a big one, right?
And the other big one is the fact that if you were to then increase your FiO2 in hospital setting.
Okay. Now, the other big point to test is going to be DLCO. Your DLCO laboratory examination
with the impaired diffusion would find a decrease DLCO.
Hypoventilation can cause hypoxia, but this occurs with (and due to) hypercapnea.
So, hypoventilation, if there isn’t enough breathing, please understand that you’re having a decrease in oxygen,
but that you also end up having an increase in carbon dioxide.
That’s not good. That displaces everything. Acute respiratory failure is my issue.
We’ve walked through some important points of acute hypoxemia. Let’s continue.
Now, the five possible physiologic causes of hypoxia.
Hypoxic respiratory failure is almost always caused by a V/Q imbalance or a shunt.
And by V/Q imbalance, we’re referring to something like perfusion defect,
ventilation defect and dealing with a diffusion defect in general.