Hypoxemia: Causes, Symptoms and Treatment

The condition of hypoxemia refers to the low partial pressure of oxygen in the arterial blood. Hypoxemia is often confused with either anoxia, asphyxia, hypoxia or anemia. Although, these are in some way related to reduction in the levels of oxygen in the body, these are distinct medical conditions.

• Anoxia is the absence of oxygen supply in the body. This implies extremely low levels of oxygen in the body.
• Asphyxia is the absence of oxygen along with the accumulation of carbon dioxide.
• Hypoxia is the deficiency of oxygen in some specific part of the body.
• Anemia refers to a condition when oxygen content in the arterial blood is low and the partial pressures in the arterial blood are high.
• Hypoxemia refers to refers to a condition when oxygen content in the arterial blood is low as also the partial pressures in the arterial blood.

The arterial oxygen content can be given by the following equation:
Arterial Oxygen Content = (Hgb x 1.36 x SaO₂) + (0.0031 x PaO₂)
where,
Hgb is the hemoglobin
SaO₂ is the percentage of hemoglobin saturated with oxygen
PaO₂ is the partial pressure of arterial oxygen (95-100 is the normal range)

A person is said suffer hypoxemia when the pulmonary alveoli are starved of oxygen. In this condition, a substantial decrease is observed in the levels of PaO₂. In the case of patients suffering from severe hypoxemia, PaO₂ levels in the blood fall below 80.

Causes of Hypoxemia

Hypoxemia is usually triggered off by respiratory disorders. Additionally, hypoxemia may also be caused as a result of one or a combination of the following:

1. Hypoventilation: This refers to a condition when the oxygen (PaO₂) content is the blood decreases and a marked increase in the levels of carbon dioxide is observed. This lowered PaO₂ content can cause hypoxemia.
2. Low Inspired Oxygen: The FiO₂ content in the blood is called the fraction of inspired oxygen in the blood. A decrease in this fraction of inspired oxygen may cause hypoxemia.
3. Left to right shunt: This is another cause of hypoxemia. A left-to-right shunt is a condition when there is a transfer of blood from the left side of the heart to its right side. This may occur as a result of a hole in the following walls: the arterial walls, the wall separating the two upper chambers (left and right atrium) or the wall separating the two lower chambers (left and right ventricles) of the heart.
4. Ventilation-Perfusion mismatch: This is a condition in which an imbalance between the volume of gas expired by the alveoli (alveolar ventilation) and the pulmonary capillary blood flow is seen. This mismatch may cause hypoxemia.
5. Diffusion Impairment: In this condition, a marked reduction is seen in the oxygen movement from the alveoli to capillaries. This restricted movement may trigger hypoxemia.

Often, it is difficult to decide one single cause of hypoxemia in an acutely illnesses. It also becomes almost impossible to determine the extent of contribution of the causes of hypoxemia in such cases.

Symptoms of Hypoxemia

The symptoms of hypoxemia depend on the severity i.e. the amount by which the partial pressure has reduced.

1. Symptoms of mild hypoxemia:
   • Restlessness
   • Anxiety
   • Disorientation, confusion, lassitude and listlessness
   • Headaches
2. Symptoms of acute hypoxemia:
   • Cyanosis (Skin appearing bluish due to insufficient oxygen)
   • Cheyne-Stokes respiration (irregular pattern of breathing)
   • Increased blood pressure
   • Apnea (temporary cessation of breathing)
   • Tachycardia (increased rate of heartbeats, more than 100 per min)
   • Hypotension (abnormally low blood pressure, below 100 diastolic and 40 systolic. Here, as an effect of an initial increase in cardiac output and rapid decrease later.)
   • Ventricular fibrillation (irregular and uncoordinated contractions of the ventricles)
   • Asystole (severe form of cardiac arrest, heart stops beating)
   • Polycythemia (abnormal increase in RBCs. The bone marrow may be stimulated to produce excessive RBCs in case of patients suffering from chronic hypoxemia)
   • Coma

Treatment for Hypoxemia

Prone Positioning
The prone position refers to lying flat on the ground, with your face downwards. The prone position is known to increase oxygenation. It can be used as a treatment for mild hypoxemia. The effectiveness of prone positioning as a treatment of acute respiratory disorders is still under suspect.

Mechanical Ventilation
Mechanical ventilation is a mechanism by which it is possible to aid or substitute spontaneous breathing mechanically. Continuous Positive Airway Pressure (CPAP) is a device that provides mechanical ventilation. It is an effective way of treating severe hypoxemia. This is a device that forces a steady stream of air into the nasal passage. This flow, which is set at a pressure that can overcome obstructions, prevents the airway from closing. The pressure to be maintained, should be determined through careful observation.

Supplemental Oxygen Therapy (Oxygen therapy)
This form of treatment for hypoxemia includes administering oxygen to the patient, using oxygen concentrators, cylinders or tanks. However, it is crucial that the precise levels of oxygen in accurate amounts, be administered. Special care needs to be taken during supplemental oxygen therapy for infants. Supplemental oxygen therapy and CPAP are usually prescribed together as a treatment for hypoxemia. This is particularly efficient for treatment of hypoxemia caused due to hypoventilation.

Transfusion of packed RBCs
Packed red blood cells refers to the concentrate of red blood cells obtained after the removal of plasma in the blood. Packed red blood cells can be transfused as a treatment to patients suffering from hypoxemia. This is known to increase the oxygen-carrying capacity of the blood. Sufficient care should be taken during the blood transfusion to avoid infections. This form of treatment cannot be used in case of patients who develop polycythemia (which is characterized by abnormally high RBC count) as a result of chronic hypoxemia.

Increasing inspired oxygen
This form of treatment is an effective one for hypoxemia developed as a result of hypoventilation or due to the reduction in inspired oxygen.