The term Stagnant Hypoxia can be problematic as Hypoxia so commonly refers to Arterial Hypoxia. There are terms such as hypoxic hypoxia. To make it clear I will use the term Stagnant Tissue Hypoxia although it is common practice to just assume the term Stagnant Hypoxia refers to the tissue as lacking oxygen and not the arterial blood.
Stagnant Tissue Hypoxia is where the tissues are hypoxic not due to a low oxygen content of the arterial blood but instead because the flow of the blood to the tissues is impaired. Stagnant tissue hypoxia is the usual cause of metabolic acidosis. It is useful to consider other causes of tissue hypoxia other than stagnant tissue hypoxia. These could also cause anaerobic metabolism and the accumulation of lactic acid.
These causes are generally further up the oxygen cascade.
1. Arterial Hypoxemia – The blood coming from the aorta has a low PaO2. This indicates that there is disease in the lungs or the airways. Stagnant hypoxia is a vascular or circulatory problem affecting the flow and distribution of blood. It is not a disease of the lungs and not it a problem of airway obstruction or airflow.
It could be that a low PaO2 could be due to a low barometric pressure. It is important to understand that barometric pressure affects the entire oxygen cascade down to the mitochondria. The blood in the capillaries could have a low PaO2 because of a low barometric pressure. Although the PaO2 of the capillary blood cannot be measured the physics of blood gasses determines that barometric pressure affects it.
2. Anemia – The blood has an inadequate amount of hemoglobin. Both Arterial Hypoxemia (low PaO2) and Anemia (low hemoglobin) result in arterial blood that is lacking in oxygen content.
3. Shock – Shock is often stated as inadequate perfusion of the tissues but the term shock is so thoroughly ingrained in the concept of inadequate perfusion due to low blood pressure that stagnant tissue hypoxia is useful as a different term. Hypotension is almost always labeled shock when the peripheral blood pressure is low enough. In theory shock is not just blood pressure but the use of the term in this way is so common that shock strictly in terms of blood pressure is the de facto use of the term. It is important to move away from the idea that the peripheral blood pressure as in that measured in the brachial artery can determine tissue perfusion. Blood pressure beyond the resistance arterioles could be used to determine flow to the tissues but is impossible to measure blood flow at this level. The resistance to flow is equally important to flow as is the pressure driving flow. The resistance between the capillary bed and the brachial artery is unknown. We can assume that it is normal but we could be wrong. The presence of lactic acidosis means that tissue perfusion is inadequate and the peripheral blood pressure could well be normal even elevated. Many people consciously or subconsciously believe that perfusion depends on a blood pressure that they can measure in the brachial artery and not on a blood pressure they can’t measure in the capillary bed. This ignores the effect of resistance. It is extremely difficult for clinicians, physicians, nurses, firefighters, etc to not practice in these erroneous assumptions.
4 Histotoxic hypoxia - This is when the cellular utilization of oxygen is poisoned by a substance such as cyanide. This causes aerobic metabolism to fail and leads to anaerobic metabolism and the accumulation of lactic acid. This is essentially impossible to distinguish from stagnate tissue hypoxia except to exclude the possibility of a tissue toxin like cyanide. Forensic investigators might have to consider the possibly of cyanide poisoning. Lactic acidosis is the key to knowing that stagnant tissue hypoxia is present. Cyanide poisoning could mimic it convincingly. There are other toxins that could be mentioned as poisoning the use of oxygen by mitochondria but cyanide toxicity is the purest example.