Hypoxemia is a condition defined by an abnormally low level of oxygen in the arterial blood. This occurs when the body’s respiratory system fails to effectively transfer oxygen from the lungs into the bloodstream. Since oxygen is the primary fuel source for nearly every cell, low blood oxygen levels can quickly impair the function of vital organs, including the heart and the brain, leading to life-threatening complications.
Recognizing the Signs of Low Blood Oxygen
The initial signs of low circulating oxygen include shortness of breath, medically known as dyspnea. The body attempts to compensate for the oxygen deficit by increasing the breathing rate (tachypnea). This rapid breathing is often accompanied by an accelerated heart rate (tachycardia) as the heart tries to circulate the limited oxygen more quickly.
As oxygen deprivation becomes more severe, symptoms progress to affect the central nervous system. Individuals may experience headaches, confusion, or disorientation. A visibly objective sign of severe hypoxemia is cyanosis, a bluish tint appearing on the lips, fingernails, or skin. This discoloration results from a high concentration of deoxygenated hemoglobin in the blood near the surface.
Underlying Conditions That Disrupt Oxygen Exchange
Hypoxemia results from five primary mechanisms that disrupt the gas exchange process: ventilation-perfusion (V/Q) mismatch, shunting, diffusion impairment, hypoventilation, and low inspired oxygen.
Ventilation-Perfusion (V/Q) Mismatch
The most common cause of hypoxemia is a V/Q mismatch, which occurs when the balance between air reaching the alveoli (ventilation) and blood flowing through the capillaries (perfusion) is uneven. An increased V/Q ratio occurs when an area is ventilated but not perfused, such as with a pulmonary embolism (blood clot). Conversely, a decreased V/Q ratio occurs when conditions like pneumonia or pulmonary edema fill the alveoli with fluid, preventing air from reaching the perfused capillaries.
Shunting and Diffusion Impairment
A right-to-left shunt is a mechanism where blood moves from the right side of the heart to the left without passing through the lungs’ capillaries. Because this shunted blood bypasses gas exchange entirely, supplemental oxygen often provides less benefit compared to a V/Q mismatch. Diffusion impairment occurs when the membrane separating the air sac from the blood vessel thickens, slowing oxygen transfer. Lung scarring (pulmonary fibrosis) is a typical cause, and this problem is often worse during exercise.
Hypoventilation and Low Inspired Oxygen
Alveolar hypoventilation means the total amount of fresh air moving into the lungs is insufficient. This can be caused by conditions that weaken respiratory muscles or depress the brain’s breathing drive, such as drug overdoses, severe neuromuscular diseases, or chest wall abnormalities. Finally, a low partial pressure of inspired oxygen can cause hypoxemia even in a healthy person, such as when ascending to high altitudes where atmospheric pressure is lower.
How Doctors Measure Oxygen Levels
Doctors use two primary methods to determine a patient’s oxygen status. The most common and non-invasive technique is pulse oximetry, which measures the percentage of hemoglobin carrying oxygen, known as oxygen saturation (\(\text{SpO}_2\)). This device clips onto a finger or earlobe and provides a reading within seconds. For a healthy adult, a normal \(\text{SpO}_2\) level typically ranges from 95% to 100%.
The definitive diagnosis is obtained through an Arterial Blood Gas (ABG) analysis, the gold standard for measuring blood oxygen. This invasive test requires drawing a blood sample directly from an artery, usually in the wrist. The ABG measures the partial pressure of oxygen (\(\text{PaO}_2\)), the technical value used to define hypoxemia. A normal \(\text{PaO}_2\) range is 75 to 100 millimeters of mercury (mm Hg); a value below 60 mm Hg constitutes hypoxemia.
Correcting Hypoxemia
The immediate goal of treatment is to quickly raise blood oxygen levels using supplemental oxygen therapy. This increases the concentration of oxygen in the air a patient breathes. For mild hypoxemia, oxygen is delivered through low-flow systems like a nasal cannula. The flow rate is adjusted to maintain a target saturation level, usually between 92% and 98%.
For moderate to severe cases, higher-flow devices are used, such as simple face masks or non-rebreather masks. A non-rebreather mask uses a reservoir bag and one-way valves to ensure the patient inhales a very high percentage of oxygen. If oxygen levels remain low despite using a mask, advanced respiratory support may be necessary. This includes non-invasive ventilation, such as BiPAP, which delivers pressurized air through a fitted mask to keep airways open.
In the most severe instances of respiratory failure, mechanical ventilation may be required, where a machine breathes for the patient through a tube placed in the windpipe. While these acute interventions stabilize the patient, the long-term management of the hypoxemia requires addressing the underlying cause. For example, pneumonia is treated with antibiotics, and a pulmonary embolism is treated with blood thinners.