The Four Hypoxias

I have found that the easiest and best-understood method of addressing immediately life-threating conditions is to learn, understand and be able to intervene on the four types of hypoxia.

Hypoxia:

Hypoxia is a very general term that refers to a decrease in the amount of oxygen that is available to supply the cells and tissues of the body.  Hypoxia ultimately disrupts the intracellular oxidative process and impairs cellular function leading to cellular death.

Hypoxia differs from another term “Hypoxemia” which refers directly to the decrease in arterial oxygen tension, the PaO2.  Unfortunately, if the clinician narrows his/her focus on only the PaO2 it

There are four types of hypoxia that can be easily identified. The types of hypoxia can be easily remembered if one recognizes the effects that they have human body. Additionally, most causes of death can be directly attributed to one of these four types of hypoxia.

Stagnant Hypoxia:

Stagnant hypoxia which is probably most frequently seen in the intensive care unit can be described as a condition which results in a reduction in the total cardiac output, a decrease in blood flow to various tissues, a collection or pooling blood in particular areas of the body, or other restriction which slows, decreases the amount of, or halts blood flow. Stagnant hypoxia ultimately interferes with oxygenation by interfering with the bodies’ oxygen carriers, hemoglobin. The hemoglobin is unable to freely pass to one or more areas of the body. In these patients, you may or may not see an alteration in overall respiratory status. This is to say, that the patient may not present with tachypnea. An example of stagnant hypoxia would be if one would extend their finger and placement tourniquet on the proximal end of the finger occluding blood flow. The tissues in the distal end of the finger would then be suffering from stagnant hypoxia. The clinician may also cause stagnant hypoxia via intragenic means. One example of this could be if the patient is receiving continuous positive pressure ventilation and the clinician sets the ventilator with high pressures the subsequent increase in intrathoracic pressure may result in narrowing or occluding vasculature.

Possible Causes of Stagnant Hypoxia:

1. Pulmonary Embolism

2. Cardiac Failure

3. Cardiac Arrest

4. Stroke

5. Hyperventilation

6. Extreme  Temperature

7. Positive Pressure Ventilation

8. Compartment Syndrome

Specific Clinical Features:

•   PO2-Areterial 95mmHg; Venous 25mmHg

•   %O2 Saturation- Arterial 97%; Venous 45%

Hypemic Hypoxia

The post operative, intensive care or trauma clinician most frequently sees this type hypoxia. This is seen when there is a reduction in the oxygen carrying capacity of the blood. This is to say that there is a lack of hemoglobin, which exists in the body. When there is a decrease in blood there is a decrease in oxygen-carrying capacity and therefore the tissues and cells of the body ultimately suffer for this lack of oxygen. We are all well aware that when patient presents to the trauma room status post gunshot wounds to the tour so we can expect that even though the patient may have an oxygen saturation of 100%. We understand that the patient because of blood loss is ultimately in hypoxic state. What do we do? We replace this lost volume with packed red blood cells increasing the oxygen carrying capacity of the body and we transport the patient to the operating room where bleeding can be controlled surgically. A patient may also present to the emergency room after being involved in a home fire. The patient's carbon monoxide levels may be significantly increased. As we all well know, carbon monoxide has a high affinity to hemoglobin than does oxygen.  The space on hemoglobin used for the carrying of oxygen is occupied by carbon monoxide displacing oxygen to form carboxyhemoglobin.  In this case the patient has enough oxygen in the body but the amount of available hemoglobin is markedly less

In the previous scenarios I have stated that the patient may present with an oxygen saturation level of 100%. This begs the question does the nurse, respiratory therapist, or physician have to place oxygen on the patients? The simple answer here is yes! The patient is still hypoxic on a cellular level and therefore find providing the patient with more oxygen we increase free oxygen availability.

Possible Causes of Hypemic Hypoxia:

1. Anemia

2. Hemorrhage

3. Carbon Monoxide

4. Drugs (i.e. Nitrates)

5. Hemoglobin anomalies

Specific Clinical Features:

•   PaO2 - Arterial 95mmHg; Venous 40mmHg

•   Decrease in hemoglobin.

Hypoxic Hypoxia:

The easiest of the four hypoxias to remember!  Simply, it is hypoxia resulting from the lack of oxygen!  This can be caused by a decrease in space used for the exchange of oxygen or a decrease in the availability of oxygen.  Therefore if ambient air at sea level has a oxygen content of 21% and because of the partial pressure of the atmosphere at sea level an individual is able to utilize all (or 100%) of that 21% of oxygen, giving the individual a SpO2 of 98%.  Conversely if that same individual begins to climb Mount Everest when approximately 22,000 feet above sea level is reached, although there is still 21% oxygen in the ambient air (this is a great test question) the partial pressure has changed, now the individual has a SpO2 of 60%.

Another example could be the patient, who, because of pneumonia presents to the emergency department with a SpO2 of 72% on room air.  There is still an ambient oxygen level of 21%, however because of disease process the lung walls begin to thicken and now the area that is used for gas diffusion is less, resulting in hypoxia.

Possible Causes of Hypoxic Hypoxia:

1. Low PO2 in inspired air- high altitude.

2. Decreased pulmonary ventilation- airway obstruction, paralysis of respiratory muscle, narcotics

3. Defect in exchange of gases through the membrane.

4. A-V shunts, cyanotic heart diseases.

Specific Clinical Features:

•   PO2 - Arterial 40mmHg; Venous 2mmHg

•   %O2 Saturation - Arterial 75%; Venous 45%

Histotoxic Hypoxia

Last but not least, there is histoxic hypoxia.  In histoxic hypoxia there is no lack of available oxygen, and there is no pathologic condition that interferes with the diffusion of oxygen into the bloodstream.  In fact the issue is not getting oxygen to the hemoglobin, instead it is getting oxygen off the hemoglobin.

When a patient presents to the emergency room with signs and symptoms of hypoxia (increase in respiratory rate, drowsiness, disorientation, increase in heart rate or blood pressure, confusion or coma) status postindustrial fire, we immediately begin to assess the patient.   We note the following:

1.     RR: 50, HR: 119, BP: 168/100, SpO2 97%

2.     No carbon noted in the oral pharynx

3.     GCS 11, poor short term memory

4.     Hgb: 11.5,  carboxyhemaglobin:

5.     PaO2 95mmHg, PvO2 91 mmHg

6.     Skin: Peripheral & central cyanosis note.

7.     Intercostal, clavicular retractions with tracheal tugging.

8.     States “I can’t see its all blurry”.

9.     Chest X-Ray clear, with hyperinflation noted.

What lab test would you order? (Imagine you have the time to get any lab you want wait for the results and never compromise the patient’s health and safety).  Grab a cyanide level.  Cyanide acts by inhibiting the cytochrome oxidase enzyme system ultimately interfering with the body’s ability to use oxygen.

Possible causes of Histotoxic Hypoxia:

1. Cyanide Poisoning.  

2. Alcohol or drug ingestions.      

3. Metabolic disorders.     

4. Carbon monoxide can also be a hypemic as well as a histoxic hypoxia.

Specific Clinical Features:

•   PO2 - Arterial 95mmHg; Venous 90mmHg

•   %O2 Saturation - Arterial 97%; Venous 96%

Treatment of Hypoxia & Conclusion:

The treatment of any of the four types of hypoxia must be to provide oxygen, immediately followed by treatment of the underlying cause.  This may be intubation, administration of packed red blood cells, or an antidote for a particular toxin.  By having an understanding of the types of hypoxia and their underlying cause the clinician can better treat the patient in a rapid and efficient manner.