Characteristics of the four classical hypoxias compared with the normal state.
The panels above present the major features of the four classical hypoxias compared with the normal condition.
Stagnant Hypoxia: Insufficient blood flow is the problem, so that oxygen delivery (blood flow X O2 conc.) is lower than normal. Arterial PO2, % saturation, and O2 vols % are normal. In order to meet the normal O2 demands, more than the normal amount of O2 must be removed from each ml of blood perfusing tissues. Thus, venous PO2, % saturation, and O2 content must become lower than normal. Thus, the (a-v) O2 differences for both partial pressure and concentration must be higher than normal.
Hypoxic Hypoxia: Inadequate oxygenation of arterial blood is the problem resulting in lower than normal PO2, % saturation, and O2 content. The cause might be a low ventilation / perfusion, diffusion problem, limited O2 availability (high altitude), etc. Since the arterial values start out lower than normal, the venous saturation values must also be lower than normal. However, since we are on the steep part of the HbO2 dissociation curve, the change in PO2 does not have to be as much as normally (here PO2 fall from 50 to 20 mmHg (difference= 30), instead of the normal fall of from 100 to 40 (difference= 60). The (a-v) O2 concentration difference is about normal.
Histotoxic Hypoxia: The arterial blood has normal values but the metabolic system's ability to use O2 is abnormal (e.g. - Cyanide, as in Jonestown, 1978). Thus, O2 remains relatively unused and venous oxygen saturation remain abnormally high. Both the a-v O2 differences for partial pressures and concentration are very low.
Anemic Hypoxia: The problem is not enough hemoglobin (Hb) to carry O2. Both arterial PO2 and % saturation are normal but O2 concentration is lower than normal (here shown at about 1/2 the normal value of 20 vols %). As a consequence, the limited amount of Hb that is available must be unloaded of its O2 to a greater extent than usual. This can only happen if venous PO2 falls lower than normal along with the % saturation and O2 content. Thus the (a-v) partial pressure difference becomes higher than normal. The (a-v) concentration difference is about normal since both the arterial and venous O2 contents are lower than normal to the same extent. Carbon monoxide can be a cause of anemic hypoxia, since it ties up Hb and prevents oxygen transport.
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