Although many medical professional, as well as scientists, use hypoxia and hypoxemia interchangeably, they do not mean the same. Hypoxemia is a condition where the oxygen content in the arterial blood is below normal while hypoxia is a failure of oxygen supply to tissues. Hypoxemia may be a cause of tissue hypoxia, but hypoxia and hypoxemia do not necessarily coexist.
What is Hypoxia?
Hypoxia is a failure of oxygen supply to tissues. Actual failure at the tissue level cannot be measured by direct laboratory methods. High serum level of lactate indicates the presence of a tissue hypoxia. Hypoxia and hypoxemia may or may not coexist. If there is increased delivery of oxygen into tissues, there will be no hypoxia at the tissue level even though there’s a lack of oxygen in the arterial blood. Increased cardiac output pumps more blood towards tissues; thus the net amount of oxygen delivered to tissues over a unit time is high. Some tissues can lower the oxygen consumption by stopping non-essential reactions. Therefore, what little oxygen delivered to tissues is adequate. On the other hand, if there is poor blood supply, low blood pressure, increased oxygen demand, and inability to utilize oxygen effectively at the tissue level, tissue hypoxia can occur even without hypoxemia. There are five major causes of tissue hypoxia; they are hypoxemia, stagnation, anemia, histotoxicity, and oxygen affinity. By far, hypoxemia is the commonest cause for tissue hypoxia.
What is Hypoxemia?
Hypoxemia is lack of oxygen content in the arterial blood. Oxygen content in the arterial blood is called arterial oxygen tension or oxygen partial pressure. The normal range of partial pressure of oxygen is from 80 to 100 mmHg. The blood oxygen level in arteries is directly related to the oxygen level in lungs. When we breathe in, normal atmospheric air enters the respiratory system. It flows through the trachea, bronchi, bronchioles, down to the alveoli. Alveoli have a rich capillary network surrounding them, and the barrier between air and blood is very thin. Oxygen diffuses from alveoli into the bloodstream until the partial pressures equalize. When the oxygen content in the air is low (high altitude), the amount of oxygen entering the blood stream goes down. Conversely, therapeutic oxygen increases the blood oxygen level. If there are no blockages, good perfusion and efficient utilization of oxygen at the tissue level, there will be no tissue hypoxia.
Stagnation Hypoxia: Cardiac output, blood volume, vascular resistance, venous capacitance, and systemic blood pressure directly affect tissue perfusion. Many organs have an auto-regulation mechanism. These mechanisms maintain the perfusion pressures of the organs stable throughout a wide range of varying systemic blood pressures. However, even when oxygenation of blood at lungs is efficient, if blood does not reach a particular organ due to atherosclerotic plaque formation or low blood pressure, tissue do not get enough oxygen. This is called stagnation hypoxia.
Anemic Hypoxia: Hemoglobin level below the normal for an age and sex is called anemia. Hemoglobin is the oxygen carrying molecule of blood. When hemoglobin level goes down, the oxygen carrying capacity of blood goes down. In severe anemia, the oxygen amount carried in the blood may not be sufficient to cope with intensive exertion. Therefore, tissue hypoxia develops.
Histotoxic Hypoxia: In histotoxic hypoxia, there is an inability of tissues to utilize oxygen. Cyanide poisoning, which interferes with the cellular metabolism, is a classic example of histotoxic hypoxia. In this case hypoxia can develop even without hypoxemia.
Hypoxia due to Oxygen Affinity: When hemoglobin binds oxygen tightly (oxygen affinity rises), it does not release oxygen at the tissue level. Therefore, the oxygen delivery to tissue goes down.