“Desatting” is medical shorthand for oxygen desaturation, a measurable drop in the amount of oxygen carried by red blood cells. Peripheral oxygen saturation (SpO2) is a fast, non-invasive measurement used to assess a person’s respiratory status. Monitoring this level is standard in nearly all healthcare settings because a sudden or sustained drop indicates the body is struggling to maintain adequate oxygen delivery. This decline can quickly compromise organ function and signals an underlying medical problem.
Understanding Oxygen Saturation
Oxygen saturation refers to the percentage of hemoglobin molecules in the arterial blood that are bound to oxygen. Hemoglobin is a protein inside red blood cells that functions as the vehicle for transporting oxygen from the lungs to the body’s tissues. Each hemoglobin molecule has the capacity to bind up to four oxygen molecules.
The measurement reflects the fraction of oxygen-saturated hemoglobin compared to the total amount of hemoglobin available. Maintaining a high saturation level is important because nearly all the oxygen transported in the blood is attached to hemoglobin. If the saturation falls, the body’s cells, which require oxygen to produce energy, begin to receive an insufficient supply. This state of low blood oxygen is medically termed hypoxemia, which can progress to hypoxia, or low oxygen levels in the body’s tissues.
Monitoring and Defining Low Readings
Oxygen saturation is measured using a device called a pulse oximeter, which is typically clipped painlessly onto a finger or earlobe. This device shines light through the tissue and determines the SpO2 level by analyzing the light absorption characteristics of oxygenated versus deoxygenated hemoglobin. The reading is one of the most frequently assessed measurements in medicine, often considered a “fifth vital sign.”
For a healthy individual at sea level, a normal SpO2 reading is between 95% and 100%. A reading that falls below 90% is considered a concerning desaturation event, indicating hypoxemia, and may require supplementary oxygen. The medical term “desatting” also refers to a rapid and significant drop from a patient’s established baseline, even if the final number remains above the 90% threshold. For instance, a drop from 98% to 92% may be a significant event requiring attention, especially if it is recurrent.
When monitoring a patient during sleep, physicians may use the Oxygen Desaturation Index (ODI), which tracks the number of times per hour the blood oxygen level drops by a certain percentage, often 3% or 4%, from the baseline. An ODI of fewer than five events per hour is considered normal, but a high index points toward a condition like sleep apnea. Such frequent, cyclic drops in oxygen can place significant strain on the cardiovascular system over time.
Common Causes of Desaturation
Desaturation can occur due to a variety of reasons that interfere with any step of the oxygen delivery process, from inhaling air to blood circulation. The most common causes involve conditions that affect the lungs’ ability to exchange gases, such as Chronic Obstructive Pulmonary Disease (COPD), asthma, or an acute infection like pneumonia. These respiratory problems restrict airflow or damage the tiny air sacs, called alveoli, where oxygen transfer occurs.
A condition known as ventilation-perfusion mismatch is a frequent physiological mechanism for desaturation, occurring when the amount of air reaching the lungs does not match the amount of blood flow passing through. This can be caused by a pulmonary embolism, which is a blood clot in the lung’s artery, or by collapsed lung tissue. Sedative medications or neurological conditions can also cause desaturation by slowing down the breathing rate, leading to hypoventilation.
Desaturation can also be caused by issues in the circulatory system, such as severe heart failure, where the heart cannot pump blood effectively. Environmental factors can also cause desaturation, most notably at high altitudes where the atmospheric pressure is lower. This lower pressure reduces the partial pressure of oxygen, making it more difficult for oxygen to diffuse into the bloodstream, which can lead to acute mountain sickness.