When a hospital monitor alarms, displaying an abrupt drop in the oxygen reading, the shorthand term used by medical staff is “desat.” This word is an abbreviation for “desaturation,” which simply means a fall in the level of oxygen carried in the blood. While seeing this event can be alarming for patients and their families, it signals to the care team that the patient’s oxygen saturation has dropped below a pre-set threshold. The immediate goal of the staff is to quickly assess the patient and determine whether the drop is a technical glitch or a physiological event requiring intervention.
Understanding Blood Oxygen Saturation (SpO2)
The number displayed on the monitor, typically as a percentage, is the peripheral oxygen saturation, or SpO2. This percentage represents how much hemoglobin—the protein in red blood cells responsible for oxygen transport—is currently bound to oxygen molecules compared to its maximum capacity. It is an indirect, non-invasive measure of the oxygen content in the arterial blood. For a healthy adult, a normal SpO2 reading falls between 95% and 100%, indicating efficient oxygen transfer from the lungs to the bloodstream. When the SpO2 drops below 90%, the condition is medically termed hypoxemia. Sustained low oxygen levels can strain vital organs like the brain and heart, which is why a desaturation event triggers an immediate response from hospital personnel.
The Mechanism: How Desaturation Is Detected
The device used to monitor SpO2 and detect desaturation is the pulse oximeter, a small sensor placed most commonly on a finger, toe, or earlobe. This non-invasive device works by passing two different wavelengths of light—red and infrared—through the patient’s tissue. The principle relies on the fact that oxygen-carrying hemoglobin and deoxygenated hemoglobin absorb light differently. Oxygenated hemoglobin absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. By measuring the ratio of light absorption at these two wavelengths, the oximeter calculates the percentage of hemoglobin saturated with oxygen. The device focuses on the pulsatile flow of arterial blood to get an accurate reading, and this percentage is then displayed on the hospital monitor.
Common Reasons for Oxygen Drops
A desaturation alarm can be triggered by two main categories of causes: technical issues and actual physiological changes.
Technical Issues
Technical reasons are often the simplest to resolve. These include errors like patient movement, which can disrupt the sensor’s reading of the pulse. Poor circulation to the extremity, such as cold hands or a loose sensor probe, can also cause a false low reading because the light signal is inadequate.
Physiological Causes
Physiological causes represent a genuine drop in blood oxygenation and are more concerning. These alarms can result from a change in the patient’s breathing pattern, such as shallow breathing or a temporary pause in breathing (apnea). Underlying lung issues directly impair the lungs’ ability to transfer oxygen, including pneumonia, fluid accumulation in the lungs (pulmonary edema), or a collapsed lung (pneumothorax). Certain medications, such as pain relievers or sedatives, can also slow a patient’s respiratory rate, leading to a drop in SpO2.
Clinical Actions Following a Desaturation Event
When a desaturation alarm sounds, the medical team initiates a rapid sequence of actions. The first step is to physically assess the patient, checking their color, level of consciousness, and breathing effort. Staff then troubleshoot the equipment by checking the pulse oximeter probe’s placement and the quality of the signal waveform on the monitor. If the drop is confirmed as real, interventions begin quickly to restore the oxygen level to the target range.
Common actions to restore oxygen include:
- Repositioning the patient to open the airway.
- Encouraging deep breaths and coughing.
- Suctioning the airway if secretions are present.
- Administering supplemental oxygen, starting with a low flow through a nasal cannula and increasing as needed.
The nurse or respiratory therapist notifies the physician to address the underlying cause, ensuring the patient receives appropriate treatment for the respiratory event.