49. Forms and mechanisms of hypoxia. Ways of compensation – cyanosis

From greek.doctor

Hypoxia refers to a decreased supply or utilization of oxygen in tissues. There are four ways tissues can get too little oxygen, so we have four types of hypoxia.

Cyanosis

Cyanosis is the bluish discoloration of skin and mucosa that occurs in some types of hypoxia. Haemoglobin exists in two main forms, the form that has bound oxygen and the form that hasn’t, called reduced haemoglobin or deoxyhaemoglobin. Because the oxygen content of venous blood is lower than arterial blood can the level of deoxy-Hb increase in the veins and capillaries in some types of hypoxia.

Cyanosis occurs when the level of deoxy-Hb increases above 50 g/L. Polycythaemia can increase the risk for cyanosis.

Hypoxaemic hypoxia

When tissue hypoxia is caused by the arterial blood containing too little oxygen do we have hypoxaemic hypoxia. It’s characterized by low pO2, low saturation and low O2 content of the blood. It is often caused by the following:

  • High altitude
    • The inhaled air doesn’t contain enough oxygen
  • Alveolar hypoventilation
    • Enough air isn’t inhaled
  • V/Q mismatch
  • Right-to-left shunt
    • Venous blood is mixed with arterial blood

Because of the low O2 content in the blood will the deoxy-Hb level increase, causing cyanosis.

Hypoxaemic hypoxia can be compensated for in the short-term by increasing the cardiac output, which will increase perfusion. This can potentially cause high-output cardiac failure. 2,3-DPG levels will also increase, causing a right-shift in the dissociation curve and therefore making it easier for tissues to extract oxygen from blood. In the long-term will erythropoietin increase which causes polyglobulia. The resulting increased Hb content of blood means that the blood can carry more oxygen.

Ischaemic hypoxia

When hypoxia is caused by too little or too slow blood flow will we have ischaemic or stagnation hypoxia. The arterial oxygen content is normal, but the tissues aren’t perfused well enough, which causes the hypoxia.

The tissues will extract more oxygen from the blood than normal to compensate. This means that the difference in oxygen content between the arterial and venous blood increases, or in other words the ∆AV O2 increases.

Because the tissues will extract more oxygen from the blood than normally will the oxygen content of the venous blood decrease. This causes an increase in deoxy-Hb, causing cyanosis.

Ischaemic hypoxia can be caused by:

  • Generalized
    • Circulatory shock
    • Severe heart failure
    • Congenital heart formation (blue baby syndrome)
  • Regional
    • Cold environment
    • Venous thrombosis
    • Arterial obstruction

Ischaemic hypoxia is compensated for similarly to hypoxaemic hypoxia. 2,3-DPG increases in the short term, and in the long term will polyglobulia develop due to increased EPO.

Anaemic hypoxia

If the pO2 in blood is normal but the haemoglobin content is too low will the total arterial O2 content be too low. This means that the tissues will by hypoxygenated.

Because haemoglobin levels are low will it be very unlikely that the deoxy-haemoglobin levels will increase above 50 g/L, meaning that cyanosis mostly doesn’t develop in case of anaemic hypoxia (except when caused by methaemoglobinaemia).

Anaemic hypoxia can be caused by:

  • Anaemias
    • Iron-deficiency
    • Folate or B12-deficiency
    • Aplastic anaemia
    • Haemolytic anaemias
  • Methaemoglobinaemia
  • Abnormal haemoglobins
  • CO poisoning

The compensatory mechanisms for anaemic hypoxia are increased cardiac output and 2,3-DPG production and long-term left ventricular hypertrophy.

Histotoxic hypoxia

In histotoxic (or cytotoxic) hypoxia the arterial oxygen and haemoglobin contents are normal, but the tissues themselves can’t utilize the O2. This means that blood flows past the tissues without losing any significant amount of oxygen, resulting in that the venous blood and arterial blood has almost the same oxygen content! In other words is ∆AV O2 actually decreased, and an arterio-venous shunt has been formed.

Histotoxic hypoxia can be caused by:

  • Cyanide poisoning
  • Uraemia

Because the oxygen content of venous blood in this case is increased compared to normal will the level of deoxy-Hb actually decrease. This means that cyanosis can’t develop, and instead the patient can develop intensely pink skin (basically the opposite of cyanosis).