When a patient “desats,” medical professionals are using shorthand for desaturation, which signifies a sudden or significant drop in the amount of oxygen carried by the red blood cells. This event is a measurable decrease in the patient’s blood oxygen level, indicating the body is not effectively transferring oxygen from the lungs into the bloodstream. Because oxygen is necessary for all bodily functions, a desaturation event is treated as a serious physiological alarm in any clinical setting. Continuous monitoring of blood oxygen makes this a frequently used term in patient care.
Understanding Oxygen Saturation and Measurement
The amount of oxygen carried in the blood is known as oxygen saturation, or SpO2, which represents the percentage of hemoglobin molecules bound to oxygen. For a healthy person, a normal saturation level falls between 95% and 100%. A reading below 90% is considered low and a cause for concern, as it indicates the body’s tissues are not receiving sufficient oxygen.
This percentage is measured non-invasively using a device called a pulse oximeter, which is often clipped onto a fingertip or earlobe. The device uses a physical principle involving light absorption. Hemoglobin, the protein that transports oxygen in red blood cells, changes its light absorption properties depending on whether it is bound to oxygen (oxyhemoglobin) or not (deoxyhemoglobin).
The pulse oximeter shines two different wavelengths of light—red light and infrared light—through the patient’s tissue. Oxyhemoglobin absorbs more infrared light, while deoxyhemoglobin absorbs more red light. By measuring the ratio of light transmitted at these two wavelengths, the device calculates the percentage of hemoglobin saturated with oxygen. The resulting SpO2 number is the patient’s real-time oxygen saturation reading, allowing medical staff to detect a desaturation event instantly.
Common Causes of Desaturation
Desaturation can be caused by any condition that interferes with the body’s ability to move oxygen from the air into the circulating blood or to deliver that blood efficiently. These causes are broadly categorized into issues affecting the respiratory system, the circulatory system, or external factors. Respiratory issues are the most common reason for sudden desaturation, involving compromised airflow or gas exchange in the lungs.
Acute respiratory problems, such as an asthma attack, severe pneumonia, or a foreign body obstructing the airway, can cause rapid desaturation. In these cases, oxygen cannot reach the tiny air sacs in the lungs, called alveoli, where gas exchange occurs. Chronic conditions, like chronic obstructive pulmonary disease (COPD) or sleep apnea, lead to habitual low oxygen levels, with desaturation episodes occurring when breathing is further stressed.
Circulatory problems also contribute to desaturation, even if the lungs are healthy. A pulmonary embolism, a blood clot lodged in the lung arteries, blocks blood flow to a section of the lung, preventing oxygen pickup. Severe heart failure can lead to a backup of fluid in the lungs, known as pulmonary edema, which physically slows the diffusion of oxygen into the bloodstream.
External or environmental factors can also trigger desaturation. High altitude environments have a lower concentration of oxygen in the air, which can cause oxygen levels to drop even in healthy individuals. Smoke inhalation introduces carbon monoxide, which binds to hemoglobin much more readily than oxygen, creating a functional desaturation by preventing oxygen transport throughout the body.
Immediate Health Effects and Clinical Response
The consequence of desaturation is hypoxemia, a low level of oxygen in the blood, which leads to a lack of oxygen in the body’s tissues, or hypoxia. The brain and heart are particularly sensitive to this oxygen deprivation, and symptoms often manifest as confusion, agitation, or a decreased level of consciousness. Other physical signs include rapid breathing (tachypnea) and an increased heart rate as the body attempts to compensate. In severe cases, a bluish discoloration of the skin and mucous membranes, called cyanosis, may become visible.
The immediate clinical response to a desaturating patient is systematic and involves quickly verifying the pulse oximeter reading and starting interventions simultaneously. Medical staff first ensure the patient’s airway is open and then immediately administer supplemental oxygen, often using a high-flow mask to achieve an SpO2 target of 94–98% for most patients. Repositioning the patient, such as raising the head of the bed, is a simple, immediate action that can sometimes improve lung expansion and oxygenation.
The next step is a rapid assessment to determine and treat the underlying cause, following a structured approach to check airway, breathing, and circulation. This may involve listening to the lungs for signs of fluid or obstruction, or checking for signs of circulatory compromise. While stabilizing the patient’s oxygen level, the medical team works to address the root cause, such as using medication to open airways during an asthma attack or treating a severe infection like pneumonia.