Transcription
In this part of the lecture, we will briefly talk about hypoxia. But first, let's clarify another related term, hypoxemia. Hypoxemia sounds very similar, right? But it is a medical condition characterized by the low level of oxygen in blood, or more precisely, a low partial pressure of oxygen in the blood.
Hypoxia, on the other hand, refers to insufficient oxygen delivery to or utilization by the tissues. This means that a patient may have a normal or even high oxygen content in the blood, but the tissues can still suffer from hypoxia. From a perfusionist perspective, this is critically important because during perfusion, it is technically possible to ensure good oxygenation of the blood, but it is not always possible to control what is happening at the tissue level. Therefore, hypoxia is not only a respiratory problem. Very often, it is a problem of circulation and microcirculation.
In pathophysiology, hypoxia is viewed as a process, not a diagnosis. It is not a separate disease, but a dynamic state in which oxygen delivery or utilization no longer meets the needs of the cell. This process can develop acutely or chronically, locally or systemically, gradually or suddenly. In all cases, the result is the same, an energy crisis in the cell because the cell can no longer produce sufficient ATP.
Next, I will present the classification of hypoxia from a pathophysiological perspective, meaning that we will classify it based on the mechanism, not by the organ or by disease. Of course, it can be classified in other ways as well, but here we will focus on the pathophysiological approach.
The first type is hypoxic hypoxia. Sounds impressive, right? In this case, we are dealing with a reduced partial pressure of oxygen in the blood, which leads to a reduced amount of oxygen available. Classically, this develops due to respiratory disorders, some kind of lung pathology that impairs breathing, for example, and as a result, the amount of oxygen taken in is decreased. During cardiopulmonary bypass, this can usually be controlled quite well, but unfortunately, it is rarely the only type of hypoxia in complex patients. And by the way, this type of hypoxic hypoxia can also be observed in situations mentioned earlier, for example, when a person travels to high altitude regions. So, it can be observed also in kind of normal conditions.
Next is anemic hypoxia. The name itself already tells us what it means. The problem here is not the amount of oxygen, but rather the transport. Typically, the partial pressure of oxygen is normal, but the function or amount of hemoglobin is reduced or impaired. As a result, the patient has anemia, of course, either due to insufficient hemoglobin or a functional defect in hemoglobin. As I mentioned, in any case, the result is the same. The tissues do not receive enough oxygen. During cardiopulmonary bypass, we also observe that simply increasing blood flow does not always compensate for reduced oxygen carrying capacity, especially in patients with a limited microcirculatory reserve.
Then, let's move on, and next we have the most common type of hypoxia, stagnant hypoxia. In some sources may be also called circulatory hypoxia, but stagnant should be more frequent. It occurs when the circulation is unable to deliver sufficient oxygen to the tissues, even if the oxygen content in the blood itself is, again, adequate. So, here the problem lies in the circulation itself. This can develop in several situations. When cardiac output is reduced, when there is increased peripheral resistance, during microcirculatory collapse, or due to centralization of perfusion. By centralization of perfusion, I mean the body's adaptive protective mechanisms, where the peripheral resistance increases and blood flow is redirected preferentially to vital organs. So, we looked at these mechanisms at the beginning of the lecture. So, this is the type of hypoxia that results from cardiovascular pathology. And as I mentioned, stagnant hypoxia is the most common one and probably also the most clinically relevant form, especially for perfusionists in everyday practice. However, I must also add it is rarely present in a so-called pure form. Most often it occurs in combination with other types of hypoxia, because as usually in real life, well, things are not that simple. So, most oftenly you will have this hypoxia in combination with other types mentioned here in this lecture.
And finally, the last type, histotoxic hypoxia. In this case, oxygen reaches the tissues, everything seems normal, but the cells are unable to use it. Usually, the cause is mitochondrial dysfunction. Mitochondria are organelles where oxygen is normally used to produce ATP, and the most common cause here includes severe inflammatory states and sepsis, which is essentially a systemic inflammatory condition of the blood. The toxins involved here are often produced by the way within the body itself, not necessarily coming from external sources, but arising from cellular metabolic disturbances. This is a type of hypoxia where the numbers on the monitor may look perfect, yet the tissues continue to suffer. In such cases, even optimal perfusion cannot always resolve the problem, because the issue is at the cellular level. And as mentioned earlier, in clinical reality, it is very rare to encounter only one isolated type of hypoxia. Most often, it is stagnant hypoxia combined with other forms. In practice, perfusionists are essentially constantly dealing with hypoxia, even before it becomes clinically apparent.
And with that, we conclude this lecture section, and don't forget to properly learn the baroreceptor reflex, as it is the one mechanism which is working daily for us. See you in next lecture or practical. >> [groaning]